IFB270Advanced Electronic Circuits

Chapter 11: Thyristors

Prof. Manar MohaisenDepartment of EEC Engineering

Korea University of Technology and Education (KUT)

Review of the Precedent Lecture● To introduce several concepts on capacitance in amplifiers

● To introduce the Miller’s Theorem● To introduce the Miller s Theorem

● To discover the decibel as a unit to measure power or gain

● To investigate the low-frequency amplifier response

● To investigate the high-frequency amplifier response

● To analyze the frequency response of the multistage amplifier

K d● Keywords

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Lecture Objectives● Introduce the four layer diode

● Introduce the silicon controlled rectifier (SCR)● Introduce the silicon-controlled rectifier (SCR)

● Introduce the light-activated SCR (LASCR)

● Introduce several applications of the SCR/LASCR

● Introduce the diac and triac

● Introduce the Unijunction Transistor (UJT) and Programmable UT

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The Four Layer Diode● Structure of the 4-layer diode (a.k.a. Shockly diode and SUS)

● The pnpn structure can be seen as a pnp transistor and an npn transistor● The pnp transistor (Q1) consists of layers 1 ~ 3● The pnp transistor (Q1) consists of layers 1 3● The npn transistor (Q2) consists of layers 2 ~ 4

● When the Anode (A) is positively biased with respect to the Cathode (K)● The emitter-base (Q1) and the base-emitter (Q2) junctions become forward-biasedThe emitter base (Q1) and the base emitter (Q2) junctions become forward biased● The base-collector (common for both Q1 and Q2) becomes reverse-biased

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The Four Layer Diode – contd.● Characteristic curve

● Two regions● Forward-blocking region: The diode is in the off state (high input resistance an open● Forward blocking region: The diode is in the off state (high input resistance, an open

circuit)■ The anode current is less than the switching current (IS)

● Forward-conduction region: The diode is in the on state (a short circuit)■ The current is larger than the holding current (IH)

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The Four Layer Diode – contd.● Characteristic curve – contd.

● Two critical currents● Switching current: The current at which the diode switches from the forward-blocking● Switching current: The current at which the diode switches from the forward blocking

region (off state) to the forward-conduction region (on state)● Holding current: The anode current below which the diode switches from the forward-

conduction region to the forward-blocking region

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The Four Layer Diode – contd.● Characteristic curve – contd.

● The forward-breakover voltage (VBR(F))● For VAK varies from 0 to VBR(F) the anode current gradually increases● For VAK varies from 0 to VBR(F), the anode current gradually increases● After VAK = VBR(F),

■ VAK drops suddenly, ■ The anode current increases (higher than IH), ■ and the 4-layer enters the forward-conduction region

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The Four Layer Diode – contd.● Example 11-1

● VAK = 20 V, in the forward-blocking region IA = 1 uA,● Find the diode resistance● Find the diode resistance

AKAK

A

20 V 20M1 AVR I μ= = = Ω

● Example 11-2p● The 4-layer diode is on (forward-conduction region)● VAK dropped to 0.9 V● Find IAA

A20V 0.9V 19.1mA1k

SS

RR

S

VI I R

−= = = =Ω

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The Four Layer Diode – contd.● An application

● When switch is closed, the capacitor charges through R up to VBR(F)

● The diode switches to the on state (forward-conduction region; small resistance)● The diode switches to the on state (forward-conduction region; small resistance)● The capacitor rapidly discharges through the diode

● The diode enters the forward-blocking region when IA < IH (holding current)● This operation is repeated leading the shown voltage across the capacitor● This operation is repeated leading the shown voltage across the capacitor

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The Silicon-controlled Rectifier (SCR)● Structure

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The SCR – contd.● SCR equivalent circuit

● Similar to the 4-layer diode with the exception that it has the gate terminal

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The SCR – contd.● Turning the SCR on

● VG = 0● IG = 0 and Q2 is off therefore Q1 is off and IA = 0● IG 0 and Q2 is off, therefore Q1 is off and IA 0

● Triggering the gate: VG ≠ 0 (IG ≠ 0)● IB2 turns Q2 on, making a path for IB1, thus turning Q1 on● Even if the trigger is removed, Q2 will remain on, due IB2Even if the trigger is removed, Q2 will remain on, due IB2

● Note that VAK must be positive!

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The SCR – contd.● Turning the SCR on – contd.

● Without IG, the SCR can be switched on/off by controlling VAK

● Same operation of the 4-layer diode● Same operation of the 4 layer diode● However, as IG increases, the VAK required to turn the SCR on decreases● Also, high values of VAK will not damage the device if the current is controlled

● Nonetheless this situation must be avoided to keep control of the SCR● Nonetheless, this situation must be avoided to keep control of the SCR

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The SCR – contd.● Turning the SCR off

● Even though the trigger is removed, the SCR remains in the on state● The anode current must drop below IH to in order to turn off the SCR● The anode current must drop below IH to in order to turn off the SCR

● There are two methods to turn the SCR off● Anode current interruption: This can be done by either a momentary series

interruption (open circuits) or parallel switching arrangement (short circuit)( ) g g ( )● Forced commutation: An external circuit is used to generate a current opposite to the

anode current in direction where the summation is below the holding current

14Korea University of Technology and Education (KUT)anode current interruption forced commutation

The Light-activated SCR (LASCR)● Operation

● Similar to the SCR with the exception that it can be triggered by light● The LASCR is most sensitive to light when the gate terminal is open● The LASCR is most sensitive to light when the gate terminal is open

● Therefore, if necessary a resistor is placed from the gate to the cathode to reduce the sensitivity

● Circuit example

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SCR Applications● Application I: On-off control of current

● Closing SW1 triggers the SCR and it switches to the on state● SW1 can be opened but the SCR will remain in the on state● SW1 can be opened but the SCR will remain in the on state● The load will still have a high current

● Closing SW2 switches the SCR to the off state● The SCR is short circuitedThe SCR is short circuited● The anode current is reduced below the holding current and the SCR enters the off

state

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SCR Applications – contd.● Application II: Half-wave power control

● This application is used in ac power control for● Lamp dimmers electric heaters electric motors● Lamp dimmers, electric heaters, electric motors, …

● R2 is used for triggering the SCR via the diode● The triggering can be at any instant of the positive half cycle

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SCR Applications – contd.● Application II: Half-wave power control – contd.

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SCR Applications – contd.● Show the voltage waveform across the SCR

● In relation to the load current for 180, 45, and 90 degrees conditions.

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SCR Applications – contd.● Sawtooth generator

● When the switch is closed● The capacitor charges until triggering the SCR● The capacitor charges until triggering the SCR● Then, the capacitor rapidly discharges via the SCR

● This is repeated to generate the sawtooth signal shown below

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The Diac and Triac● The diac

● The top and bottom layers contain both n and p materials● The right side of the stack is a pnpn structure with same characteristics of the 4-layer● The right side of the stack is a pnpn structure with same characteristics of the 4-layer

diode● The left side is an inverted 4-layer diode with an npnp structure

● The diac, therefore, conducts the current in both directions

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The Diac and Triac – contd.● The diac (equivalent circuit)

● Modeled as four transistors (a)● The bias in (b) makes Q1 and Q2 forward-biased Q3 and Q4 reverse biased● The bias in (b) makes Q1 and Q2 forward-biased, Q3 and Q4 reverse biased

● The pnpn structure is in operation● The bias in (c) makes Q1 and Q2 reverse-biased, Q3 and Q4 forward-biased

● The npnp structure is in operation● The npnp structure is in operation

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The Diac and Triac – contd.● The Triac

● The triac can be simply considered as two SCRs connected in parallel● Therefore it does not require a breakover voltage to turn it on● Therefore, it does not require a breakover voltage to turn it on

● The gate can trigger the triac to switch to the on state● Then, the voltage across the triac defines the direction of the current flow

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The Diac and Triac – contd.● Triac characteristic curves

● Note that the breakover voltage decreases as the gate current increases● Same as the SCR● Same as the SCR

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The Diac and Triac – contd.● Triac’s bilateral operation

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The Diac and Triac – contd.● Applications

● Triac as a control device (average power control to a load)

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The Diac and Triac – contd.● Applications – contd.

● D1 conducts in the positive half-cycle, R1 sets the trigger point● D2 conducts in the negative half-cycle R1 sets the trigger point● D2 conducts in the negative half-cycle, R1 sets the trigger point

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The Silicon-controlled Switch● Structure of the SCS

● Same as that of the SCR with the exception that it has two gates; the cathode gate and the anode gateg● Therefore, SCS is a four-terminal thyristor

● The two gates are used to trigger the SCS on and off● Unlike the SCR, where the gain is only used for triggering it on, g y gg g

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The Silicon-controlled Switch – contd.● Operation of the SCS (the on state)

● The SCS can be turned on using either gate terminal● A positive pulse on the cathode gate turns the SCS on● A positive pulse on the cathode gate turns the SCS on

■ Turns Q2 on, which provides a path for the base current of Q1 and turn it on● A negative pulse on the anode gate turns the SCS on

■ Turns Q1 on, which provides base current of Q2 and turn it on

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The Silicon-controlled Switch – contd.● Operation of the SCS (the off state)

● The SCS can be turned off using either gate terminal● A negative pulse on the cathode gate turns the SCS off● A negative pulse on the cathode gate turns the SCS off

■ Turns Q2 off, which blocks the base current of Q1 which turns off as a result● A positive pulse on the anode gate turns the SCS off

■ Turns Q1 off, which blocks the base current of Q2 which turns off as a result

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The Silicon-controlled Switch – contd.● Operation of the SCS (the off state)

● In this method, the BJT is used to reduce the anode current below the holding current.● (a) series interruption, when the BJT is off, the anode current = 0 and the SCS is

turned off● (b) parallel interruption, when the BJT is on, the anode current = 0 and the SCS is

turned off

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The Unijunction Transistor● Structure of the UJT

● It has three terminals; Emitter, Base 1, and Base 2● Note that that UJT has different characteristics due to its special structure● Note that that UJT has different characteristics due to its special structure

● Only one pn junction unlike the BJT and the FET

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The Unijunction Transistor – contd.● Equivalent circuit of the UJT

● A diode that represents the junction, two resistances; r’B1 and r’B2● r‘B2 varies inversely with IE● r B2 varies inversely with IE

● The interbase resistance

' ' 'BB B1 B2r r r= +

● Voltage divider

'B1

'B1

BB'BB

rrV Vr

⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

=

● Standoff ratio'B1'BB

rr

η =

BB⎝ ⎠

● The point at which the junction is forward-biased● VEB1 must satisfy

BB

BBEB1 pnV V Vη= +

● The equality point is referred to as the peak-point voltage

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BBEB1 pnV V Vη+

The Unijunction Transistor – contd.● The characteristic curve of the UJT

● Fixed VBB

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The Programmable Unijunction Transistor● Structure of the PUT

● It is more similar to an SCR than to a UJT● As a difference with the SCR the anode-to-gate voltage can be used to turn on and● As a difference with the SCR, the anode-to-gate voltage can be used to turn on and

turn off the device

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Lecture Summary● Introduced the four layer diode

● Introduced the silicon controlled rectifier (SCR)● Introduced the silicon-controlled rectifier (SCR)

● Introduced the light-activated SCR (LASCR)

● Introduced several applications of the SCR/LASCR

● Introduced the diac and triac

● Introduced the Unijunction Transistor (UJT) and Programmable UT

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