Power Transistors & GTO Thyristor

Power Transistors & GTO Thyristor

Student Workbook

92920-J0 Edition 2 Ê>||Æ0J0Ä%#LË

3092920J00503

SECOND EDITION

Second Printing, March 2005

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THIS PAGE IS SUPPOSE TO BE BLANK Table of Contents

Unit 1 – Introduction to the Circuit Board..................................................................................1

Exercise 1 – Transistor and Thyristor Identification...................................................................3 Exercise 2 – Overview of the Circuit Blocks..............................................................................5

Unit 2 – Driver and Load Circuit Blocks.....................................................................................7 Exercise 1 – Familiarization with the Driver Circuit Block .....................................................10 Exercise 2 – Familiarization with the Load Circuit Block........................................................12

Unit 3 – Basic Operation of Power Transistors and GTO Thyristors ....................................15 Exercise 1 – Operations of Power Bipolar Transistors .............................................................19 Exercise 2 – Operations of Power MOSFETs and IGBTs........................................................20 Exercise 3 – Operations of GTO Thyristorsh ...........................................................................22

Unit 4 – Principles of Power Switching Circuits .......................................................................25 Exercise 1 – Switching Time and Conduction Voltage Drop...................................................29 Exercise 2 – Switching Power in an Inductive Load ................................................................31 Exercise 3 – Free-Wheeling Diode Recovery Time .................................................................34 Exercise 4 – Losses in Electronic Power Switches...................................................................37

Unit 5 – Bipolar Transistor and GTO Thyristor Switches ......................................................39 Exercise 1 – The Bipolar Power Transistor ..............................................................................44 Exercise 2 – The Darlington Power Transistor .........................................................................46 Exercise 3 – The GTO Thyristor...............................................................................................48

Unit 6 – The Power MOSFET and IGBTs.................................................................................51

Exercise 1 – The Power MOSFET............................................................................................57 Exercise 2 – The IGBT .............................................................................................................59 Exercise 3 – The Ultra-Fast IGBT ............................................................................................62

Appendix A – Safety ................................................................................................................. A-ii

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Introduction

This Student Workbook provides a unit-by-unit outline of the Fault Assisted Circuits for Electronics Training (F.A.C.E.T.) curriculum. The following information is included together with space to take notes as you move through the curriculum. ♦ The unit objective ♦ Unit fundamentals ♦ A list of new terms and words for the unit ♦ Equipment required for the unit ♦ The exercise objectives ♦ Exercise discussion ♦ Exercise notes The Appendix includes safety information.

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Power Transistors & GTO Thyristor Unit 1 – Introduction to the Circuit Board

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UNIT 1 – INTRODUCTION TO THE CIRCUIT BOARD

UNIT OBJECTIVE At the completion of this unit, you will be able to locate and identify the major components on the Power Transistors and GTO Thyristor circuit board using information presented in the exercises.

UNIT FUNDAMENTALS In power electronics, different types of electronic switches are encountered. In a first category, driving signals can only be used to turn on the switching (or reverse blocking) semiconductors (ex: thyristor and TRIAC). These semiconductors can be studied using the model 91011 Thyristor and Power Control Circuits circuit board. A second category includes power semiconductors known as self-commutated devices, since they can be either turned on or turned off through control signals. In this category one finds the power transistors: NPN bipolar, Darlington, MOSFET and IGBT. One also finds the GTO thyristor that is mainly used in high power applications. In the following exercises we will cover the basic fundamentals of power transistors and GTO thyristors and give an overview of the Power Transistors and GTO Thyristor circuit board.

NEW TERMS AND WORDS IGBT - Insulated Gate Bipolar Transistor - a combination of bipolar transistors and a MOSFET. GTO thyristor - Gate Turn Off Thyristor - a semiconductor that acts as a thyristor, but that can be turned off by applying a large negative pulse to its gate. switching time - the time taken by an electronic switch to activate or interrupt the current flow in a circuit (turn-on or turn-off the circuit power). Driver - an amplification circuit designed to provide a high-current control signal to an electronic switch.

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc @ 1A) multimeter


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 1 – Transistor and Thyristor Identification

EXERCISE OBJECTIVE When you have completed this exercise, you will be able to locate and identify each type of power transistor and the GTO thyristor in its associated circuit block. You will verify the information presented in this exercise by visual observations.

DISCUSSION • This exercise presents the most widely-used controlled turn-on and turn-off (self-

commutated) power semiconductors of the industry: the bipolar transistor, the Darlington transistor, the MOSFET transistor, the IGBT and the GTO thyristor.

• The NPN bipolar power transistor was, for a long time, the most commonly used switch for average power applications. In conduction, it can switch currents up to 400 A and support voltages of 1000 V when off.

• Note that the PNP bipolar transistor is practically not used in power electronics since it can not operate with very high power and it is more difficult to fabricate than the NPN transistor. Only the NPN bipolar transistor will be studied in this course.

• To obtain a higher gain from the bipolar transistor, two transistors must be combined in a Darlington type assembly.

• To obtain switches easier to control, manufacturers will increase the gain still further using three bipolar transistors in a Darlington assembly. However, this new device has very poor switching speed.

• To improve the switching time manufacturers add diodes and resistors. This configuration allows us to obtain a case made of three bipolar transistors having a total hFE gain of 750, that can support 400 A in conduction and 1000 V when turned off.

• The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor allows the control of medium-power. It can switch a current of 70 A and support a voltage of 500 V at frequencies exceeding 20 kHz with the control signal of a simple logic gate.

• Note that the P-channel MOSFET transistor is practically not used in power electronics since it can not operate with very high power and it is more difficult to fabricate than the N-channel transistor. Only the N-channel MOSFET transistor will be studied in this document.

• The most commonly used self-commutated semiconductor is actually the IGBT (Insulated Gate Bipolar Transistor). This transistor is an assembly of field effect transistors and bipolar transistors. This gives a transistor that can switch currents up to 500 A and voltages of 1200 V using only the power of a logic gate. Moreover, for comparable power, IGBTs are less expensive than MOSFET and Darlington transistors.


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• IGBTs can be provided with or without a diode between the collector and the emitter. • To turn semiconductor switches on and off under very high power, the GTO thyristor should

be used. For example, a GTO can switch a current of 850 A and support a voltage of 4500 V. • The POWER TRANSISTOR AND GTO THYRISTOR circuit board is divided into 8 circuit

blocks. • Five of them contain power transistors and one contains a GTO thyristor. • The DRIVER (DR) circuit block and the LOAD (Z) circuit block complete the circuit board.

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 2 – Overview of the Circuit Blocks

EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the basic functions of the POWER TRANSISTORS AND GTO THYRISTOR circuit board. Using a multimeter, you will have studied the different circuits on the circuit board.

DISCUSSION • The POWER TRANSISTOR AND GTO THYRISTOR circuit board is divided into eight

circuit blocks, including the DRIVER (DR) and the LOAD (Z). • The DRIVER (DR) circuit block is used to provide the control signals for the different

electronic switches. It can be linked to a square wave generator through the two terminals located on either side of the GEN symbol.

• The LOAD (Z) circuit block can be modified to allow the realization of typical load configurations.

• In each of the six electronic switch circuit blocks, there is a +15 V power supply that allows the circuit to be turned on.

• The MOSFET, IGBT and ULTRA-FAST IGBT circuit blocks are similar, except for transistor Q1, which differs in each case.

• In each of these three circuit blocks, resistor R1 is placed between the DR symbol and the gate of the transistor, and resistor R2 is used for measuring the current flowing through the electronic switch.

• There is also a fast zener diode, called "voltage suppressor" that is designed to clip the transient voltage peak, and a push-button switch allowing temporary removal of the diode for test purposes.

• Two different types of IGBT are presented on the circuit board. • The first type of IGBT, dedicated to general applications (motor drive, emergency power

supply, etc.), has a weak on-state voltage. • The second type, the ULTRA-FAST IGBT, presents a smaller switching time but has the

disadvantage of a higher on-state voltage. This second type of IGBT will be used in higher frequency applications.

• The BIPOLAR TRANSISTOR, DARLINGTON TRANSISTOR and GTO THYRISTOR circuit blocks are quite similar except for electronic switch Q1 that is specific to each circuit block.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Power Transistors & GTO Thyristor Unit 2 – Driver and Load Circuit Blocks

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UNIT 2 – DRIVER AND LOAD CIRCUIT BLOCKS

UNIT OBJECTIVE At the completion of this unit, you will be able to use the DRIVER (DR) and LOAD (Z) circuit blocks.

UNIT FUNDAMENTALS

The POWER TRANSISTORS AND GTO THYRISTOR circuit board is made up of six self-commutated semiconductor switches, A DRIVER (DR) circuit block and a LOAD (Z) circuit block.


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The DRIVER (DR) circuit block can be configured according to different needs. It is used to provide the control signals necessary to study any of the six semiconductor switches. The LOAD (Z) circuit block can also be configured for different needs. It allows the study of the different semiconductor switching circuits included on the circuit board.

NEW TERMS AND WORDS optocoupler transistor driver - circuit including both an optocoupler which allows electrical isolation by coupling input to output using a light source and detector, and a driver circuit. electrical isolation - Indicates that there is no electrical conduction between two sections of a circuit. free-wheeling diode - A diode used to prevent voltage surge in an inductive circuit by allowing current flow to continue when an electronic switch is turned off. reverse recovery time - Time needed for the diode to recover its blocking capacity when the current is reversed in this one.

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc @ 1A) multimeter oscilloscope, dual trace square wave generator


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 1 – Familiarization with the Driver Circuit Block

EXERCISE OBJECTIVE When you have completed this exercise, you will be able to use and configure the DRIVER (DR) circuit block to provide the appropriate control signal to a semiconductor switch circuit.

DISCUSSION • The DRIVER (DR) circuit block includes an optocoupler transistor driver (the integrated

circuit), a 0 to +10 V variable power supply and a 0 to -10 V variable power supply. • The DRIVER (DR) output is located between the terminal A and the common. • Using jumpers, the DRIVER (DR) circuit block can be configured to supply a dc voltage

varying from 0 to +10 V or 0 to -10 V. It can also provide a square wave signal of 0/+15 V or -15/+15 V.

• An optocoupler transistor driver is a circuit used to provide the control signal necessary to properly operate the power semiconductor while ensuring electrical isolation between the power section and the driving circuit.

• In high power circuits, operating generally under high voltage levels, electrical isolation is quite important to avoid damage to the driving circuit by the high voltage.

• The use of the optocoupler transistor driver also allows the control of numerous devices with the same driving circuit.

• To obtain a square-wave control signal at the optocoupler transistor driver output, a square wave generator must be connected on the driver circuit input (GEN).


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 2 – Familiarization with the Load Circuit Block

EXERCISE OBJECTIVE When you have completed this exercise, you will be able to use the various components of the LOAD (Z) circuit block. You'll also be able to configure the load to obtain either a pure resistive load or a resistive inductive load used with or without a free-wheeling diode.

DISCUSSION • In the LOAD (Z) circuit block, there are two load resistors, one resistor for current

measurements, one inductor, and three free-wheeling diodes. • Resistor R3, used for current measurements, has a value of 1Ω and is always added to the

other resistors in the load circuit. • Resistors R1 and R2 both have a resistive value of 10Ω. They can be configured in three

different ways. • Resistor R1 can be used alone to obtain an 11Ω (10Ω + 1Ω) load. • With R1 in parallel with R2 , a load of 6Ω (5Ω + 1Ω) is obtained. • Finally, using R1 and R2 in series, a total load of 21Ω (20Ω + 1Ω) is obtained. • Inductor L1 is always present in the load circuit. However, it can be short-circuited using a

jumper. • Each of the three diodes (CR1, CR2 and CR3) has a very specific role. These three diodes are

of different technologies and only one of them should be used at a time. • A correctly-placed jumper allows selection of one of these diodes. The selected diode will be

used as a free-wheeling diode and will provide an alternative path for the inductive current when the semiconductor switch is turned off.

• In fact, all the semiconductor switches found on the circuit board have the ability to stop the current flowing through them. Since the load can be inductive and power shut-off in an inductive circuit creates high voltage surges damaging for the semiconductors, a means should be found to eliminate them.

• The free-wheeling diode provides another path for the inductive current and thus allows the voltage surges to be avoided.

• Diode CR1 is a general purpose technology diode and is designated as "General Purpose". This diode isn't optimized to have a fast reverse recovery time(trr).

• Diode CR2 is optimized to provide a reverse recovery time described as FAST and diode CR3 is optimized to provide an ULTRA-FAST reverse recovery time.

• Once the LOAD (Z) circuit block is configured, it can be connected to any of the five power transistor circuit blocks, or to the GTO THYRISTOR circuit block.

• The two output terminals of the LOAD (Z) circuit block are identified by the letters B and C. As seen previously, these letters are used as guides for the connections with the semiconductor switch circuit blocks.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Power Transistors & GTO Thyristor Unit 3 – Basic Operation of Power Transistors and GTO Thyristors

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UNIT 3 – BASIC OPERATION OF POWER TRANSISTORS AND GTO THYRISTORS

UNIT OBJECTIVE At the completion of this unit, you will be able to turn on and turn off the various power transistors and the GTO thyristor. You will be able to switch the different semiconductor types using a simple circuit including a dc source and a pure resistive load.

UNIT FUNDAMENTALS

You have seen in the F.A.C.E.T. course on Thyristor Power Control Circuits that the thyristor (SCR) and the TRIAC can be turned on using a control signal, but that the same control signal cannot be used to turn them off. These semiconductors turn off when the current decreases to 0 A. For example, if a thyristor or a TRIAC is used with a dc power supply, the current will never decrease to 0 A, thus, the thyristor or the TRIAC will always stay on.


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However, if you use any of the power semiconductors of the POWER TRANSISTORS AND GTO THYRISTOR circuit board, it is possible to turn them off with a control signal even if they are used with a dc supply. Having the possibility to control the turn-off of an electronic switch is really useful for creating a large number of self-commutated converters.

In power electronics, the transistor is used as a controllable switch. The power transistor will then be either in the saturation region (weak voltage across its terminals and large current flowing through it, meaning relatively weak dissipated power)

or in the cutoff region (large voltage across its terminals and very weak current flowing through it, meaning negligible dissipated power).


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However, the power transistor should not be operated in its linear region in order to avoid a large current flow through the device in addition to a large voltage on its terminals. This operating mode involves large power dissipation in the transistor that will generally result in the destruction of the device. This last consideration applies to all the semiconductor switches used in power electronics.

NEW TERMS AND WORDS bipolar transistor gain - The bipolar transistor gain is defined using the transistor in its linear region. In this operating mode, the collector current intensity is the result of the multiplication of the base current with the bipolar transistor gain. When the transistor is in the saturation (on) or cutoff (off) region, the relationship is no longer valid. gate threshold voltage - The lowest gate-to-source (gate-to-emitter) voltage at which the MOSFET (the IGBT) begins to conduct. CMOS - A transistor type mainly used in logic gates that has the particularity of consuming little current and operating at a 15 V logic level. logic gate - Circuit made of transistors used as switches that allows one or many functions of boolean logic to be performed. holding current - The minimum current necessary to maintain conduction in a GTO thyristor, a thyristor, or a TRIAC, when no current circulates in the gate.

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc @ 1A) multimeter oscilloscope, dual trace


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 1 – Operations of Power Bipolar Transistors

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to switch on and off the bipolar and the Darlington transistors using the 0 to +10 V power source.

EXERCISE DISCUSSION • The bipolar and Darlington transistors operate in their saturation region when a current IB

sufficiently high circulates from the base towards the emitter. • This current IB should be higher than the current IC circulating between the collector and the

emitter divided by the bipolar transistor gain (hFE). • For example, a transistor having a gain hFE of 20 and a current IC of 100 A should have a

current IB above 5 A to be in saturation. • If the transistor has to support an overload of 200 A, a base current over 10 A must be

provided. Thus, the driving circuit has to be sufficiently powerful. If an operating voltage of 10 V is used, the power supply will have to provide 100 W or more.

• Finally, to turn off a bipolar or a Darlington transistor, the current IB has to be interrupted.

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 2 – Operations of Power MOSFETs and IGBTs

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to switch on and off the power MOSFET and the two IGBTs found on the circuit board using the 0 to +10 V power source.

DISCUSSION • Power MOSFETs and IGBTs are the easiest self-commutated power semiconductors to

control. • To turn them on, a gate voltage (referenced to the source for the power MOSFET, and to the

emitter for the IGBT) greater than the gate threshold voltage (VGS(th) for the MOSFET and VGE(th) for the IGBT) and less than the maximum voltage of the device must be applied. Typically, a voltage of 10 to 15 volts will be adequate to turn on most MOSFETs and IGBTs.

• To turn off these same devices, a gate voltage below VGS(th) (or VGE(th)) but not higher than the maximum negative voltage is necessary. Typically, zero volts will be applied to turn off MOSFETs and IGBTs.

• The connection between the gate and the source (the emitter in the case of the IGBT) can be considered as a simple capacitor that should be charged to turn on the transistor and discharged to turn it off. This voltage can be delivered by a simple CMOS logic gate.

• We saw in the first unit, that a power MOSFET can control currents up to 70 A in conduction and support 500 V when turned off, while an IGBT operates with currents up to 500 A and can support 1200 V. Controlling such power with the small power provided by a logic gate is quite amazing.

• However, to obtain short switching times, the MOSFET and IGBT must be controlled with a driving circuit dedicated to this purpose. These circuits are optimized to rapidly apply the appropriate gate voltage to turn on the transistor, and to rapidly return the gate voltage to 0 V to turn it off.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 3 – Operations of GTO Thyristorsh

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to switch on and off the power GTO thyristor using the 0 to 10 V positive power source and the 0 to -10 V negative power source.

DISCUSSION • As seen previously, the GTO thyristor is the controllable switch that can be used with the

highest power level. Considering this fact, it is important to clearly understand its operation. • To turn on a GTO thyristor, a positive current pulse should circulate from the gate to the

cathode (IG). A current will then begin to flow from the anode to the cathode (IA), the same as for the diode and the thyristor.

• To turn off the GTO thyristor, a negative current pulse must be applied to the gate. • It is not necessary to maintain the flow of gate current for the GTO thyristor to remain on.

However, if the current IA flowing from the anode to the cathode decreases below a certain threshold (holding current) the GTO will turn off.

• To avoid this situation, which can result in severe complications in a power circuit, gate current must be maintained during the whole period the GTO thyristor must stay on.

• In most power electronics circuits, large reverse voltages cannot be applied to the electronic switch. Numerous transistors can be damaged by a reverse voltage greater than 15 V.

• The GTO thyristor is the only controllable electronic switch that can support a reverse voltage as large as the forward voltage. For example, some GTO thyristors withstanding a forward voltage of 1000 V can also support a reverse voltage of 1000 V.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Power Transistors & GTO Thyristor Unit 4 – Principles of Power Switching Circuits

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UNIT 4 – PRINCIPLES OF POWER SWITCHING CIRCUITS

UNIT OBJECTIVE In this unit, you will study the behavior of the current and the voltage across a bipolar transistor during conduction and switching. You will learn the role of the free-wheeling diode and how to choose the diode type appropriate for this function. You will finally learn how power losses occur in a bipolar transistor.

UNIT FUNDAMENTALS The self-commutated power electronic switches can be used in numerous power converter topologies. The POWER TRANSISTORS AND GTO THYRISTOR circuit board was not designed for the study of various converter topologies, but rather to allow the observation of the different characteristics of the power transistor and the GTO thyristor used in a simple buck chopper setup. While the operating principles of power converters is not the purpose of this course, a brief explanation of buck chopper operation will allow for a better comprehension of power switching.

The buck chopper is the dc equivalent, to the ac step-down transformer. It can convert a high dc voltage to a lower dc voltage while maintaining excellent performance.


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For example, a transformer can be used to convert a supply of 120 Vac at 1 A to a supply of 12 Vac at 10 A. In the same way, a buck chopper can be used to convert a supply of 120 Vdc at 1 A to a supply of 12 Vdc at 10 A.

Moreover, the buck chopper has a transformation ratio that can be controlled electronically. Thus, a transformation ratio of 10:1 can be changed to a ratio of 2:1 and provide a supply of 60 Vdc at 2 A. One then obtains a 0 to 120 Vdc variable power supply that can be controlled electronically. The transformation ratio of the buck chopper is directly related to the percentage of time the electronic switch is conducting (duty cycle). In other words, the output voltage of the chopper equals the input voltage multiplied by the percentage of time the switch conducts.


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In the following exercises, a duty cycle of 50 % (2:1) will be used, providing an output voltage of 7.5 V, one-half the input voltage of 15 V.

NEW TERMS AND WORDS duty cycle - Percentage of time that an electronic switch is conducting. turn-on delay time - The time interval between the application of an input pulse turning on the transistor and the beginning of transistor turn-on. current rise time - The time necessary for the current to rise from 10% to 90% of its peak amplitude, when the transistor turns on. turn-off delay time - The time interval between the application of an input pulse turning off the transistor and the beginning of transistor turn-off. current fall time - The time necessary for the current to decrease from 90% to 10% of its peak amplitude when the transistor turns off. buck chopper - Circuit topology that includes a diode and a self-commutated electronic switch. The circuit allows the conversion of a fixed dc voltage at its input to a variable dc voltage at its output. The output voltage can range from 0 V to the full input voltage.

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc at 1A) square wave generator oscilloscope, dual trace,


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 1 – Switching Time and Conduction Voltage Drop

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to describe the behavior of a bipolar transistor when it turns on or turns off, and during conduction.

DISCUSSION • To visualize what is happening in a commutating electronic device, we will study a circuit

formed by a bipolar transistor, a load resistor and a fixed dc power supply. • To simplify the study, the resistive load, the power supply and the connecting cable won't be

considered as inductive. However, it is important to note that, in practice, these components are inductive and influence circuit behavior.

• The electronic switches used in power electronics do not exhibit ideal behavior. • The response time after the application of a turn-on or turn-off pulse is not instantaneous.

When a base current is applied to turn on a transistor and there is no current flowing in the transistor and load resistor, the transistor doesn't respond before a certain time. This time interval is called turn-on delay time (td(ON)).

• The transistor will then progressively begin to conduct for a few microseconds. The current will increase in the load and the transistor up to a value equal to VCC/R. The time interval during which the current increases to maximum amplitude is called the current rise time tr.

• While this current IC increases in the transistor collector, the voltage (VCE) across the transistor will decrease from VCC to a weak on-state voltage VCE(ON), as shown in the figure.

• The voltage across the transistor can be approximated from the following equation VCE = VCC - RIC • If the base current is interrupted, the transistor does not respond before a certain time called

the turn-off delay time (td(OFF)). It will then cease to conduct and the current IC flowing through it progressively decreases for a few microseconds until it stops completely. The figure also shows that voltage VCE increases progressively during the turn-on delay time (tf).

• Set the oscilloscope controls so you can observe the transistor turning on.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 2 – Switching Power in an Inductive Load

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to switch the current in an inductive load and you will understand the purpose of the free-wheeling diode.

DISCUSSION • You have seen in the previous exercise how to switch the current in a pure resistive load.

However, for different reasons, pure resistive loads are rarely used in power electronics. • High power resistors are generally inductive. They are usually located far from the command

circuit and so, require long connecting cables that are also inductive. • Moreover, the abrupt current interruptions in a load create large undesirable electromagnetic

emissions. It is desirable to filter the current with a smoothing inductor placed in series with the load resistor.

• Knowing that it is not possible to interrupt the current in an inductor without creating a large voltage surge, it is necessary to understand how to switch an inductive load while avoiding this problem.

• When a transistor switching an inductive load begins to conduct, current IC increases exponentially, as shown in the figure, and the voltage VCE decreases in a few microseconds.

• The current rise is slower than in the case of a resistive load since the inductor opposes fast current variations.

• Notice that a weak voltage can still be observed across the transistor when it is in conduction (VCE(ON)).

• If the transistor ceases to conduct, current IC decreases and this fast current drop will induce a voltage surge across the transistor. This surge is caused by the energy stored in the inductor that has to be released when the current is stopped.

• The inductor value and the current flowing through it before transistor turn-off determines the quantity of energy stored in the inductor. The higher the energy, the longer the voltage surge.

• The amplitude of this surge is a function of the transistor current cut-off rate. The faster the transistor interrupts the current, the higher the surge.

• This voltage surge can cause destruction of the transistor. On the POWER TRANSISTORS AND GTO THYRISTOR circuit board, protective circuits limiting the surges were placed in order to avoid transistor damage.


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• To switch an inductive load while avoiding voltage surges, a diode has simply to be placed in parallel with the load.

• In this new circuit configuration, when the transistor turns off and interrupts the load current (IL), it can now flow through diode (ID), called the free-wheeling diode. There is therefore no current cut-off in the inductor and, thus, no voltage surge.

• Observe the behavior of the currents in the transistor (IC), in the load (IL) and in the diode (ID), and also voltage VCE when the transistor turns off.

• The voltage VCE increases rapidly to a voltage of VCC + 0.7 V. When this voltage is reached, the diode starts to conduct. The current flow is then transferred from the transistor to the diode. One then observes zero current in the transistor (IC) and full load current in the diode (ID).

• It is important to note that the voltage across the load is approximately VCC when the transistor conducts. The current IL is then increasing in the load.

• When the transistor is off, the voltage across the load is equal to the diode on-state voltage, and so, the current IL is decreasing in the load.

• One can observe that the load current oscillates between IMAX and IMIN. • As explained previously, it is desirable to smooth this current. To decrease the amplitude of

the current oscillation, either the switching frequency, or the inductor value can be increased. • However, if the inductor value is increased, so will be the cost of the circuit. You will also

see in the next exercises that, if the switching frequency of the transistor is increased, it can then overheat.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


34

Exercise 3 – Free-Wheeling Diode Recovery Time

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to describe the behavior of the free-wheeling diode during transistor switching. You will know of the different diode technologies used in power electronics and understand the criterion used to select the more convenient diode type for an application.

DISCUSSION • You have seen in the previous exercise, that the use of a free-wheeling diode is essential

when you operate with an inductive load. It was also explained that, in high power, nearly all loads can be considered as inductive due to the inductive property of long connecting cables.

• It is important to clearly understand free-wheeling diode operation to ensure proper selection of an appropriate diode type.

• In conventional electronics, you have seen that the diode is a semiconductor junction of P type and N type. The P type junction is connected to a terminal called anode and the N type junction is connected to a second terminal called cathode.

• When the anode voltage is higher than the cathode voltage, the current circulates from the anode towards the cathode.

• When the anode voltage is less than the cathode voltage, the diode is turned off and doesn't allow current to flow.

• In power electronics, this simplified explanation of the diode behavior doesn't completely explain the circuit's operation.

• To turn on the diode, the explanation seen previously is always valid. • However, the condition necessary to block the diode is no longer a function of the voltage

but rather a function of the current. • In power electronics, the diode is considered blocked after the current has been reversed for a

sufficient amount of time. • The time required for the diode to restore its blocking capacity after the reversal of current is

called "reverse recovery time" (trr). • This figure shows how the current and the voltage evolve when the diode is turned off. • The current is negative for a time trr and the voltage (VAK(ON)) remains positive, equal to

the direct on-state voltage, during all this time. • When the diode restores its blocking capacity, after time trr, the voltage is reversed across the

diode.


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• The amplitude of the negative current peak in the diode can become very high and cause overheating of the commutating transistor. As a matter of fact, when the diode is used as a free-wheeling diode in a buck chopper setup, the reverse current peak is added to the load current that must be supported by the transistor.

• Consider this figure of a buck chopper made up of a transistor, a free-wheeling diode, an inductive load and a dc power supply.

• Suppose that the circuit has been operating for a certain time and that the load current has some ripple.

• At the moment just preceding transistor turn-off, current is circulating through the free-wheeling diode.

• This figure shows the evolution of the currents and the voltages in the diode and the transistor when the transistor turns off. It can be clearly seen that the reverse current peak in the diode involves the appearance of a very large current peak in the transistor.

• Moreover, the current peak in the transistor occurs when the transistor supports the full supplied voltage across its terminals. The power dissipated by the transistor that is the result of the current and the voltage multiplication, will then be very large at each switching point.

• If a diode having a large trr is used, the current peak will be higher and will last longer. • The reverse recovery time is mainly a function of the technology used in manufacturing the

diode.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 4 – Losses in Electronic Power Switches

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to describe how power losses occur in a bipolar transistor when it is switching, as well as when it is simply in conduction. You will also learn about the influence of switching frequency on the losses.

EXERCISE DISCUSSION • It was seen in the previous exercises that the bipolar transistor is not an ideal switch, and that

switching is not instantaneous. Unfortunately, this non-ideal behavior causes power losses that can result in transistor overheating.

• The dissipated power in a transistor is mainly the result of the multiplication between the voltage VCE across its terminals and the current IC flowing through the collector.

• The power dissipated in the transistor when it is conducting is the result of the multiplication of the weak on-state voltage (VCE(ON)) (generally 1 to 5 V) with the current IC. This type of loss is known as the conduction loss.

• Suppose we have a voltage VCE(ON) of 1 V and a current IC of 10 A. During conduction, you will obtain a power dissipation of 10 W.

• While the transistor is turning off, the current decreases for a time tf and the voltage increases. This results in a variation of the power dissipated in the transistor during time tf.

• For example, for a voltage VCE of 50 V when the transistor is turned off and a current of 10 A when it is turned on, one obtains at the half-way point, a maximum power dissipation of approximately 125 W (25V x 5A).

• When the transistor is turned off, there is no significant power dissipation since the collector current is negligible.

• When the transistor is turning on, another variation in the power dissipation can also be observed since the current rises for a time tr while the voltage across the transistor falls to VCE(ON). As for the turn-off, a maximum power dissipation of approximately 125 W can be observed.

• However, since the rise time tr is smaller than the fall time tf, there will be less power dissipation at turn-on than at turn-off.

• The power losses occurring during transistor turn-on and turn-off are known as switching losses.


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• In the case of a real free-wheeling diode, the reverse recovery time of the diode must be considered when the transistor is turned on. During the additional delay, a current peak through the transistor occurs at the same moment that it has to support the full supplied voltage on its terminals. An excess amount of dissipated power must then be included in the switching losses.

• The slower the switching times (large tf and tr), the larger the power dissipation, thus the switching losses. Moreover, the larger the diode reverse recovery time, the larger again will be the power dissipated.

• It is necessary to have fast switching electronic switches as well as free-wheeling diodes with a small reverse recovery time, if one desires to minimize power losses in an electronic power circuit.

• The operating frequency of the electronic switches also influences the power losses. At low frequency, conduction losses contribute mainly to transistor losses.

• Inversely, at high frequency, switching losses are the main source of losses.

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

Power Transistors & GTO Thyristor Unit 5 – Bipolar Transistor and GTO Thyristor Switches

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UNIT 5 – BIPOLAR TRANSISTOR AND GTO THYRISTOR SWITCHES

UNIT OBJECTIVE At the completion of this unit, you will be able to describe switching operation of the bipolar transistor, the Darlington transistor and the GTO thyristor and you will know how to improve their switching times. You will know the role of the accelerating circuit and the effects on the driving signal when components of this circuit are modified.

UNIT FUNDAMENTALS

In considering the turning on of an electronic switch, it is important to realize that the more intense the driving signal, the faster the switch will turn on, and the more quickly the load current will reach its maximum value. There is also a reduction of the switching time when a more intense current is used as a control signal. The figure shows for a resistive load, the turn-on switching times of the bipolar transistor when two different base current intensities are applied. The higher that the base current will be, the faster the turn-on will be. However, if a large control current is maintained during conduction, large power dissipation will occur in the driving circuit. It is advantageous to have an intense control current when the switch is turning on, but not during conduction.


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The addition of an RC circuit at the base (the gate for the GTO thyristor) of the switch allows the creation of positive current peaks that will accelerate turn-on without causing large power dissipation during the conduction phase.

This same RC circuit also produces negative current peaks when the control signal is interrupted and, thus, it improves the turn-off switching time. If the intensity of this negative current peak is increased, turn-off will be much faster. One way to obtain this higher negative current is to use a bipolar rather than a unipolar control source.


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With a voltage (VDR) differential from +15 V to -15 V, a higher negative current peak will be delivered by the RC circuit. A shorter turn-off switching time will result.

The accelerating circuit can be modified to obtain a different value of driving current.

For example, if the resistance of the RC circuit is decreased, current peaks will be higher.


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If the capacitance of this same RC circuit is increased, peaks will be larger.

Finally, if the resistance limiting the driving current is increased, the current will be weaker.

NEW TERMS AND WORDS None

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc @ 1A) square wave generator oscilloscope, dual trace


43

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


44

Exercise 1 – The Bipolar Power Transistor

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to identify different ways of improving bipolar transistor switching. You will know the role of the various circuit components that aid in switching. You will be able to describe the relationship between the base current (IB) intensity and the overload capacity of the bipolar transistor.

DISCUSSION • As seen in previous exercises, the bipolar transistor can be turned-on or turned-off by

applying or interrupting the current at its base. This change of state doesn't occur instantaneously and mainly depends on the intensity and the form of the base current (IB).

• The addition of an RC circuit at the bipolar transistor base allows the creation of positive and negative current peaks that will help accelerate turn-on and turn-off without causing large power dissipation during the conduction phase.

• With a bipolar, rather than a unipolar control source, higher negative current pulses will be obtained.

• This increase of negative current peaks will result in faster turn-off switching times. • You have seen, in power electronics, that a bipolar transistor is held in conduction by

applying a base current sufficiently large to obtain saturation. However, if the base current is increased too much, it will require a bigger power supply for the accelerating circuit and high losses will result in the transistor base.

• It is desirable to have a base current as weak as possible, but one which will hold the transistor in saturation all across the load current (IC) range.

• In order to allow the transistor to resist overload, the current (IC) range can fluctuate from 0 A up to about two to five times the nominal current.

• Therefore, it is necessary to provide an accelerating circuit able to deliver a base current (IB) of at least two to five times that required to obtain the nominal load current.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


46

Exercise 2 – The Darlington Power Transistor

EXERCISE OBJECTIVE At the completion of this exercise, you will be able to identify some ways to improve Darlington transistor switching. You will also be able to describe the effects of base current on the overload capacity of the Darlington transistor.

DISCUSSION • The primary interest of the Darlington transistor in power electronics is its current gain,

which is much higher than that of the bipolar transistor. This high gain is obtained by cascading two or more bipolar transistors in a Darlington setup.

• The current gain of such a setup is approximately equal to the current gain product of the transistors in the circuit.

• The Darlington presented in this figure has a total gain of about 1000, which is the product of the transistor Q1 and the transistor Q2 gains. The small gain of the first transistor (20) is increased by a factor of 50.

• Therefore, a weaker driving current IB will be required to obtain the same load current IC, as well as a less powerful driving circuit.

• However, the Darlington setup has also negative effects caused by the cascade of many bipolar transistors. One of these is the significant increase in the leakage current.

• A bipolar transistor without current at its base is in the turn-off state, thus open between the collector and the emitter. In spite of this turn-off state, a weak current succeeds in flowing from the collector towards the emitter. It is called the leakage current ICEO.

• In the case of a single bipolar transistor, this leakage current is negligible. But in the case of the Darlington transistor, having two or many transistors in cascade allows this weak current to be significantly amplified.

• This default can be corrected by adding resistors between bases and emitters. They are known as base-emitter shunt resistors.

• Note that the addition of these resistors partly reduces the total current gain. The base-emitter shunt resistor of transistor Q2 has the highest influence on the gain loss.

• Another negative effect of the Darlington setup is the increase in the on-state voltage (VCE(ON)). In a Darlington, the total collector-emitter voltage VCE is the sum of the collector-emitter voltage VCE1 of the first transistor Q1 and the base-emitter voltage VBE2 of the second transistor Q2.


47

• The resulting on-state voltage is then higher than that of the bipolar transistor. • Finally, one of the major problems with this setup is the necessity of applying a strong

negative current pulse to each of the transistors forming the Darlington in order to accelerate its turn-off.

• To apply negative pulses on the second transistor (Q2), a diode must be connected between the base and the emitter of the transistor Q1.

• In this way, each transistor will have the advantage of the negative current pulses and the Darlington turn-off switching time will be significantly reduced.

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 3 – The GTO Thyristor

EXERCISE OBJECTIVE At the completion of this exercise, you will have a better understanding of GTO thyristor behavior. You will be able to apply the gate current necessary to correctly turn-on and turn-off the GTO.

EXERCISE DISCUSSION • As seen previously, the GTO thyristor is a self-commutated semiconductor that can be turned

on by applying a positive current pulse at its gate, • As seen previously, the GTO thyristor is a self-commutated semiconductor that can be turned

on by applying a positive current pulse at its gate, and turned off by applying a negative current pulse.

• Moreover, it is preferable to maintain the gate current during the conduction period to avoid turn-off of the GTO thyristor if current IA decreases below the threshold of the holding current.

• The GTO thyristor has an on-state voltage (VAK(ON)) higher than that of the bipolar and the Darlington transistors. Therefore, it has a greater power dissipation during conduction than these two transistors.

• The minimum amplitude of the positive current pulse (IG) required to turn on the GTO thyristor is a physical specification typical to each GTO and it is independent of the current to be controlled (IA).

• Generally, a one ampere current is sufficient to turn on most power GTOs. • However, it is different in the case of the minimum amplitude of the negative current pulse

since this is dependent on the current IA that must be interrupted. • Typically, to control a given current IA, a negative pulse of at least 20 to 25 % of its

maximum amplitude must be injected at the GTO gate. • For example, to interrupt a current IA of 100 A, the negative current pulse must have a

minimal amplitude of about -20 to -25 A. • Unfortunately, GTO thyristors are impossible to turn off when the current IA exceeds a

certain limit. Even if a higher reverse current is applied at its gate, the GTO remains in conduction.

• It is thus necessary to install a safety device to interrupt the current in the GTO if a problem occurs. The safety system will protect the GTO against over currents that could result from a short-circuit.

• There is two types of GTO thyristor: the "reverse blocking" type and the "anode short" type. • The "reverse blocking" GTO is as well able to block a high reverse voltage as a high forward

voltage. • The "anode short" GTO cannot block a reverse voltage higher than about 15 V, but it can

switch at a higher frequency.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Power Transistors & GTO Thyristor Unit 6 – The Power MOSFET and IGBTs

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UNIT 6 – THE POWER MOSFET AND IGBTS

UNIT OBJECTIVE At the completion of this unit, you will be able to describe switching operation of the Power MOSFET, the IGBT and the Ultra-Fast IGBT. You will know how the switching times of these devices can be optimized.

UNIT FUNDAMENTALS

It was seen previously that a bipolar transistor is essentially a device controlled by a current. This current must circulate from the base towards the emitter to allow collector current to flow. The Darlington transistor and the GTO thyristor are also devices controlled by current.

The MOSFET and the IGBT are fundamentally different since they are controlled by a voltage that must be applied to their gate to turn them on.


52

The connection between the gate and the source of a MOSFET (emitter in the case of an IGBT) can be considered as a simple capacitor that must be charged or discharged to control the turn-on or turn-off of the device.

For turn-on, this gate-source capacitance CGS (gate-emitter capacitance CGE for the IGBT) must be charged using a supply that gives a voltage above the threshold voltage of the device (V(DR) > VGS(th) for MOSFET or V(DR) > VGE(th) for IGBT).

When the capacitor is charged (VGS = V(DR)), the device is considered turned-on and no current circulates in the gate.


53

To turn-off the device, the capacitor must be discharged.

When the capacitor is discharged (VGS = 0 V), the device is considered turned-off.

Contrary to a bipolar transistor, the power gain of a MOSFET or an IGBT is extremely high. A large load current ID (IC for the IGBT) can circulate through a MOSFET without any current IG flowing through its gate; it is only necessary to maintain the load at capacitance CGS (CGE for the IGBT). The power required to control a MOSFET or an IGBT is then very small, and the driving circuit very simple.


54

The gates of MOSFETs and IGBTs cannot generally support a voltage above +/- 20 V. Since the gate has a near infinite impedance (capacitance), there is a large risk that a simple electrostatic discharge can seriously damage the gates of these devices. Therefore, care must be taken when such devices are handled, installed or tested in a circuit.

Since components can easily be accessed on the circuit board and in order to limit damages that could result from electrostatic surge, a Zener diode has been placed between the gate and the source of the MOSFET (emitter for IGBTs). The push-button switch placed in series with the Zener diode is there strictly to temporarily remove the diode in order to allow verification of the device using the diode test function of a multimeter.

Generally, the Zener diode can be found in high power circuits using a MOSFET because abrupt and large changes of the voltage VDS will produce, via the drain-gate capacitance CDG, voltage surges between the gate and the source. These voltage surges VGS can rise above the threshold voltage VGS(th) and turn on temporarily the device. In some cases, positive and negative voltage surges can even exceed the limit of +/- 20 V and thus, damage the device.


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For example, a 300 V change of the drain-source voltage VDS can cause a gate-source voltage (VGS) peak about 50 V. This same phenomenon exists for the IGBT and also requires gate protection.

NEW TERMS AND WORDS static drain-source on-state resistance - The static drain-source on-state resistance is the dc resistance between the drain and source terminals with a specified gate-source voltage applied to bias the device to the on state. current densities - The current flowing through a surface, divided by the cross-sectional area of that surface. It is expressed in amperes per square meter, or in the more usual SI multiple of amperes per square millimeter.

EQUIPMENT REQUIRED F.A.C.E.T. base unit POWER TRANSISTORS AND GTO THYRISTOR circuit board power supply (15 Vdc @ 1A) square wave generator oscilloscope, dual trace


56

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 1 – The Power MOSFET

EXERCISE OBJECTIVE At the completion of this exercise, you will know the behaviour of the power MOSFET during switching operation. You will be able to explain how MOSFET switching can be improved. You will also understand why ripples can be observed on the drain-source voltage when the transistor turns off.

DISCUSSION • MOSFET switching times are determined primarily by the device capacitances. • The gate structure has capacitance CDG to the drain and CGS to the source. The MOSFET

also has a capacitance CDS between the drain and source. • To clearly understand the behaviour of the MOSFET, it is important to understand the role of

these three capacitances. • However, note that the device data sheets typically specify Ciss, Coss and Crss because these

capacitances can be most readily measured. These values are related to the interelectrode capacitances by the relationships:

Ciss = CDG + CGS (in parallel) Coss = CDG + CDS (in parallel) Crss = CDG


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 2 – The IGBT

EXERCISE OBJECTIVE At the completion of this exercise, you will know the behaviour of the IGBT during switching operation. You will be able to explain how IGBT switching can be improved. You will also be able to visualise the difference between MOSFET and IGBT switching, principally at turn-off.

DISCUSSION • The IGBT, Insulated Gate Bipolar Transistor, is a switching transistor with a device

operation and structure similar to that of an Insulated Gate Field Effect Transistor, more commonly known as a MOSFET.

• As MOSFETs, IGBTs are voltage controlled devices, they only require voltage on the gate to maintain conduction through the device.

• IGBTs have higher current densities than comparable bipolar transistors, while at the same time having simpler gate-drive requirements than the familiar power MOSFET.

• IGBTs are manufactured in voltage and current ratings extending well beyond what is normally available in power MOSFETs. For exemple, at the high power end, devices with a voltage rating of 1200 V and current rating of 600 A are available.

• In general, the IGBT offers clear advantages in high voltage (>300 V), high current (1-3 A/mm2 of active area), and medium speed (to 10-20 KHz).

• The circuit symbol generally used for the IGBT is shown in Figure a). It is similar to that of an npn bipolar transistor with an insulated gate terminal in place of the base.

• The equivalent circuit of the IGBT can be depicted quite accurately by a pnp bipolar transistor, where the base current is controlled by a MOSFET and limited by a variable base resistor. The conductivity of the base resistor is increased (modulated) when the IGBT is turned on.

• It is possible to enhance the IGBT model by a more complex equivalent circuit. • The IGBT consists of a pnp bipolar transistor driven by an n-channel MOSFET in a pseudo-

Darlington configuration. • The JFET supports most of the voltage and allows the MOSFET to be a low voltage type,

and consequently have a low RDS(on). • As it is apparent from the equivalent circuit, the voltage drop VCE(on) across the IGBT is

the sum of two components: a diode drop across the p-n junction and a voltage drop across the driving MOSFET.


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• Since the IGBT is a MOS gate device, it has three characteristic capacitances Cies, Coes, and Cres. These capacitances are specified in the data sheet because they are most readily measured. They can be used to determine the IGBT junction capacitances CCG, CGE and CCE.

Cies = CCG + CGE (in parallel) Coes = CCG + CCE (in parallel) Cres = CCG • The switching speeds of IGBTs are higher than those of bipolar power transistors. The

switching performance at turn-on is very similar to that of power MOSFETs, but turn-off times are longer. Therefore, the maximum switching frequencies with IGBTs fall between those of bipolar power transistors and power MOSFETs.

• During turn-off, the initial fall in current is steep, similar to that of the power MOSFET. • But this is followed by a long "tail" during which the decay takes place relatively slowly. • Typically, the tail starts around 25% of the on-state current. • During the tail, the IGBT supports the load voltage while the tail current is flowing. • This causes increased switching power loss, and therefore limits the switching frequency. • Like for a MOSFET, selecting the proper series gate resistor RG for IGBT gate drive is very

important. The value of the gate resistor has a significant impact on the dynamic performance of the IGBT.

• The IGBT is turned on and off by charging and discharging the gate capacitance. A smaller gate resistor will charge/discharge the gate capacitance faster, thus reducing, principally at turn-on, the switching times and switching losses.


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NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


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Exercise 3 – The Ultra-Fast IGBT

EXERCISE OBJECTIVE At the completion of this exercise, you will know the behaviour of the Ultra-Fast IGBT during switching. You will also be able to visualise the difference between IGBT and Ultra-Fast IGBT switching, principally at turn-off.

DISCUSSION • Different families of IGBTs with different crossover frequencies have been created to

maximize operation of the device for different applications. • Standard IGBTs have been optimized for voltage drop and conduction losses and have the

lowest voltage drop per unit of current density among the IGBTs. • Fast IGBTs offer a combination of low switching and low conduction losses that closely

matches the switching characteristics of many popular bipolar transistors. • Ultra-Fast IGBTs have been optimized for switching losses and have the lowest switching

losses per unit of current density among the IGBTs. These devices have switching speeds that are comparable to those of power MOSFETs in practical applications. They can operate comfortably at 50 KHz in square wave switching.

• At high voltages (>300 V), designers can use the IGBTs to replace MOSFETs, with their much higher die sizes, in existing off-line power conversion applications and significantly cut component cost without impacting overall power system performance.

• Due to the higher usable current density of IGBTs, an IGBT with a die two sizes smaller can usually handle two to three times more current than the typical MOSFET it replaces.

• For example, at high voltage (>300 V) and low frequency (<5 KHz), a single Standard IGBT (600 V, 31 A@100oC) can replace six paralleled high-power MOSFETs included in a single Pak Module (600 V, 30A@100oC). This single IGBT saves in parts costs and board space.

• The gate drive requirement for IGBTs is similar to that for MOSFETs, and may even be simpler due to smaller die size and input capacitance.

• The maximum switching frequency for IGBTs is limited mainly by the total switching loss, principally during the non-ideal turn-off commutation due to the current tail.

• In a typical 80 to 100 KHz power supply application, one might expect to find that for a MOSFET about 75% of the losses would be conduction losses due to the device's RDS(on).

• For an Ultra-Fast IGBT in the same application, about 70% of the losses would be due to switching losses, primarily during turn-off. Thus, improvements in the turn-off characteristics of IGBTs would be of greatest leverage in reducing overall losses.

• A new IGBT with a faster switching speed has been created recently by International Rectifier under the name WARP Speed IGBT.


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• The fast switching speed of this device results principally in a major improvement in turn-off characteristics. This includes a reduction in current tail effects and total switching energy losses.

• WARP Speed IGBTs have lowered Eoff losses to about half of the value of Ultra-Fast IGBTs. This roughly 50% improvement virtually doubles the frequency range of IGBTs without significant impact on power losses.

• With switching speeds up to 150 KHz, the WARP Speed series of IGBTs have switching characteristics that are very close to those of power MOSFETs, without sacrificing the inherently superior conduction characteristics and higher usable current densities of IGBTs.

• The power switches discussed so far can be summarized in a comparative table that may be useful in placing them in the proper perspective.

NOTES ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________


64

APPENDIX A – SAFETY

Safety is everyone’s responsibility. All must cooperate to create the safest possible working environment. Students must be reminded of the potential for harm, given common sense safety rules, and instructed to follow the electrical safety rules. Any environment can be hazardous when it is unfamiliar. The F.A.C.E.T. computer-based laboratory may be a new environment to some students. Instruct students in the proper use of the F.A.C.E.T. equipment and explain what behavior is expected of them in this laboratory. It is up to the instructor to provide the necessary introduction to the learning environment and the equipment. This task will prevent injury to both student and equipment. The voltage and current used in the F.A.C.E.T. Computer-Based Laboratory are, in themselves, harmless to the normal, healthy person. However, an electrical shock coming as a surprise will be uncomfortable and may cause a reaction that could create injury. The students should be made aware of the following electrical safety rules. 1. Turn off the power before working on a circuit. 2. Always confirm that the circuit is wired correctly before turning on the power. If required,

have your instructor check your circuit wiring. 3. Perform the experiments as you are instructed: do not deviate from the documentation. 4. Never touch “live” wires with your bare hands or with tools. 5. Always hold test leads by their insulated areas. 6. Be aware that some components can become very hot during operation. (However, this is not

a normal condition for your F.A.C.E.T. course equipment.) Always allow time for the components to cool before proceeding to touch or remove them from the circuit.

7. Do not work without supervision. Be sure someone is nearby to shut off the power and provide first aid in case of an accident.

8. Remove power cords by the plug, not by pulling on the cord. Check for cracked or broken insulation on the cord.

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Power Transistors & GTO Thyristor

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