UNIVERSITY OF NAIROBI SCHOOL OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
PROJECT NO. 100THREE PHASE AC TO DC CONVERTER USING MODERN SWITCHES
ByMAINA JOHN MWIRIGI
F17/28057/2009
SUPERVISOR: DR.OGABA
EXAMINER: MR AHMED SAYYID
PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF BACHELOR OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING OF THE UNIVERSITY OF NAIROBI
SUBMITTED ON: FRIDAY 24TH APRIL 2015
CERTIFICATION
This report has been submitted to the Department of Electrical and Information Engineering, University of Nairobi with my approval as supervisor:
Project Supervisor: DR OGABA
Signature:
Date:
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DECLARATION OF ORIGINALITY
NAME OF STUDENT: MAINA JOHN MWIRIGI
REGISTRATION NUMBER: F17/28057/2009
COLLEGE: Architecture and Engineering
FACULTY/SCHOOL/INSTITUTE: Engineering
DEPARTMENT: Electrical and Information Engineering
COURSE NAME: Bachelor of Science in Electrical and Electronic Engineering
TITLE OF WORK: THREE PHASE AC TO DC CONVERTER USING MODERN SWITCHES
(i) I understand what plagiarism is and I am aware of the university policy in this regard.(ii) I declare that this final year project report is my original work and has not been
submitted elsewhere for examination, award of a degree or publication. Where other people’s work or my own work has been used, this has properly been acknowledged and referenced in accordance with the University of Nairobi’s requirements.
(iii) I have not sought or used the services of any professional agencies to produce this work(iv) I have not allowed, and shall not allow anyone to copy my work with the intention of
passing it off as his/her own work.(v) I understand that any false claim in respect of this work shall result in disciplinary
action, in accordance with University anti-plagiarism policy.
Signature: ……………………………………………………………………………………
Date: …………………………………………………………………………………………
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ACKNOWLEDGEMENTFirst at all, I would like to thank God the Almighty for his bless towards myself. Without his blessing I might not able to complete my final year project entitle “ three phase ac-dc converter using modern switches”.
I would also like to take this opportunity to thank all the people who had assisted me directly and indirectly in completing the project. My first gratitude goes to Dr. Ogaba, my supervisor for the project whom had given support, advice and guidance I might need. He had been guiding me from the start of the project until the final touch of the thesis write up. With his helps, I had learned many things regarding the project, as well as extra knowledge that I believe I would not have this sort of opportunity elsewhere. The project would obviously not be successful without him.
Last but not least, I would like to thank my beloved parents who had given me a lot of moral support while I was struggling with this project. Once again to all, thank you very much.
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DEDICATIONTo my parents Francis Maina and Lydia Wambui , brother Evanson Mwangi and sisters for always wishing me the best in life.
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TABLE OF CONTENTSCERTIFICATION.............................................................................................iiDECLARATION OF ORIGINALITY...................................................................iiiACKNOWLEDGEMENT..................................................................................ivDEDICATION.................................................................................................vTABLE OF FIGURES.....................................................................................viiiABSTRACT...................................................................................................ixCHAPTER 1: INTRODUCTION........................................................................10
1.1 Background...........................................................................................101.2 Objectives.............................................................................................101.3 Project Report Outline..............................................................................10
CHAPTER 2: LITERATURE REVIEW...............................................................122.1 Technical Background..............................................................................122.2 Rectification..........................................................................................122.3 Three Phase Rectifier...............................................................................14
2.3.1 Vienna Rectifier Principal...................................................................................................14
2.3.2 Operation of Vienna Rectifier.............................................................................................15
2.3.3 Semiconductor Components in the Vienna Rectifier..........................................................19
2.4 Diode...................................................................................................192.5 IGBT...................................................................................................212.6 Power loss............................................................................................24
CHAPTER 3: SYSTEM DESIGN.......................................................................263.1 Vienna rectifier set up..............................................................................263.2 MOSFETS............................................................................................283.3 Depletion-Enhancement MOSFET and Enhancement MOSFET..........................303.4 MOS Transistor Theory and Applications......................................................31
CHAPTER 4: SYSTEM IMPLEMENTATION......................................................354.1 Vienna Rectifier Topology Using MOSFETS.................................................354.2 Switching topology for higher power...........................................................364.3 Low profile power module integration..........................................................38
CHAPTER 5: RESULTS..................................................................................395.1 Circuit.................................................................................................395.2 Analysis...............................................................................................405.3 Input Phase Potential...............................................................................425.4 Component Selection...............................................................................42
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5.4.1 MOSFET Selection.............................................................................................................42
5.4.2 Diode Selection...................................................................................................................43
5.5 Power loss calculation..............................................................................435.6 MOSFET Current and Voltage Waveform.....................................................43
5.4.1 Output Power Calculation...................................................................................................44
5.4.2 Output Capacitor Size.........................................................................................................45
CHAPTER 6: CONCLUSION...........................................................................476.1 Future Work..........................................................................................47
REFERENCES..............................................................................................48
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TABLE OF FIGURESFigure 2.1 Bridge Rectifiers...........................................................................13Figure 2.2 Vienna Rectifier........................................................................14Figure 2.3 Vienna rectifier equivalent schematic diagram..........................................15Figure 2.4 Conduction path for phase A.............................................................16Figure 2.5 Conduction path for phase A.............................................................16Figure 2.6 Conduction path for phase A.............................................................17Figure 2.7 Conduction path for phase A.............................................................17Figure 2.8 The current path for two switching position..........................................18Figure 2.9 The current path for two switching position..........................................18Figure 2.10 Diode circuit symbol and Cross-sectional view of a pn-junction diode.......20Figure 2.11 The i-v characteristic of a pn-junction diode (a) the idealized (b) the actual. 21Figure 2.12 Reveres recovery.........................................................................21Figure 2.13 The n-channel IGBT circuit symbols and Cross-sectional view of an IGBT..................................................................................................22Figure 2.14 The i-v characteristic of an IGBT.....................................................23Figure 2.15 Turn-on voltage and current waveforms of an IGBT and Turn-off voltage and current waveforms of an IGBT...........................................................................24Figure 3.16................................................................................................26Figure 3.17................................................................................................27Figure 3.18 Current & Voltage Waveforms................................................27Figure 3.19 IGBTs Current Waveforms.............................................................28Figure 3.20 The structure of depletion-enhancement n-channel and p-channel MOSFET29Figure 3.21The structure of enhancement n-channel and p-channel MOSFET.............29Figure 3.22 Symbol of depletion-enhancement MOSFET......................................29Figure 3.23 Symbol of enhancement MOSFET...................................................30Figure 24 Transfer characteristic of DE MOSFET................................................31Figure 3.25 Transfer characteristic of E MOSFET...............................................32Figure 26 Ideal Operating Points......................................................................33Figure 3.27 Operating point of IGBT................................................................34Figure 4.28 Vienna Rectifier Topology Using MOSFETS......................................35Figure 4.29 Switching topology for higher power................................................36Figure 4.30 Single 1x 35 kW PFC unit using 3 branch modules...............................37Figure 4.31 Parallel operation of 2x 17.5 kW PFC stages with phase-shifted driver......37Figure 5.32 Sinusoidal voltage wave................................................................39Figure 5.33 System Circuit Design............................................................40Figure 5.34 Waveforms when MOSFETs are conducting..........................................41Figure 5.35 Simulation Waveforms.....................................................................41Figure 5.36 MOSFET Current and Voltage Waveform.............................................43Figure 5.37 Output Voltage signal.......................................................................45Figure 5.38 Power off transition from zero full load.............................................46
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ABSTRACTThree phase Ac to Dc conversion of electric power is widely employed in adjustable speed drives (ASDs), uninterruptible power supply(UPSs),HVDC systems and utility interfaces with non conventional energy sources such as electroplating welding units etc, battery charging for electric vehicles and power supplies for telecommunication systemsThe purpose of this project is to successfully convert three phase ac to dc conversion of electric power using modern switches . Rectifiers are used as stand-alone units feeding single and multiple dc loads and as input stages of ac systems because of their virtually unlimited output power and fine controllability. AC/DC line-commutated converters are used as the most usual choice for applications. This is due to simplicity of the circuits requiring a minimum number of active and passive componentsThyristors are the main line-commutated power switches. In this project we use systems built on thyristors and transistors known as controlled rectification because their dc output can be changed where rectification process are quite varied
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CHAPTER 1: INTRODUCTION
1.1 BackgroundPower electronic converters can be found wherever there is a need to modify a form of electrical energy (i.e. change its voltage, current or frequency). The power range of these converters is from some milli watts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With classical electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Thus, the main metric of power electronics becomes the efficiency. The first very high power electronic devices were mercury arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts. Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches. Advent of silicon-controlled rectifiers, abbreviated as SCRs, led to the development of a new area of application called the power electronics. Prior to the introduction of SCRs, mercury-arc rectifiers were used for controlling electrical power, but such rectifier circuits were part of industrial electronics and the scope for applications of mercury- arc rectifiers was limited. Once the SCRs were available, the application area spread to many fields such as drives, power supplies, aviation electronics, high frequency inverters and power electronics originated.
1.2 Objectives
The first objective or the purpose of this project is to design a circuit to convert ac input voltage to constant dc output voltage. The second objective is to develop hardware of the system where the output voltage will be in dc and can be used by students in the lab to study
1.3 Project Report OutlineThis project report is basically split into five chapters. The first chapter is dedicated to the introduction. The introduction provides a simple overview of the AC converters by providing background information. Relevant citations have been given and main emphasis of the project statement established.The second chapter is composed of the literature review. The literature review also gives a technical background on the first subheading. Secondly, previous research done by others is clearly stated and comparison is done. The relevance of previous research to this work is also identified. The third part of the literature review gives aim of the project and a final conclusion.
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The third chapter provides the background theory of various AC converters taken into account. It also combines analysis by showing the derivation of the transfer function . The fourth chapter discusses different conversion mechanisms and provides a detailed analysis of three phase rectifiers. Finally; the fifth chapter is dedicated to results and simulations. It also gives conclusion and future work.
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CHAPTER 2: LITERATURE REVIEW
2.1 Technical Background
This chapter presents an overview of rectifier. A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC). Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other component
2.2 Rectification
AC/DC converters serve as rectifiers. They convert ac to dc in a number of industrial, domestic, agricultural, and other applications. Rectifiers are used as stand-alone units feeding single and multiple dc loads and as input stages of ac systems because of their virtually unlimited output power and fine controllability. Their speed of response is usually adequate to handle electromechanical transients occurring in motor drives and power suppliers.AC/DC line-commutated converters or, as they also called, converters with natural commutation or passive rectifiers, are the most usual choice for applications, where a single-phase and three-phase supply is available. This is due to simplicity of the circuits requiring a minimum number of active and passive components. Thyristors are the main line-commutated power switches. The term “line-commutated” describes the type of commutation, i.e. the transfer of current from one conducting element to the next, as a function of the mains voltage. To turn on a thyristor, an injection of a current pulse into its gate is required.In low-power applications, vehicle, medicine, and household devices, where there is no ac supply or where reactive current and harmonics caused by a line commutation would be unreachable, it is accepted to employ forced commutated converters having a more complex circuitry and sometimes involving higher losses. A special situation exists also with dc and ac loads, where the response of a line-commutated converter may be insufficient to cope with the stringent dynamic and energy efficiency demands and where an additional converter supplied by a dc link and operated with a high switching frequency is necessary. For these purposes active rectifiers are developed.The basic rectifier topologies are given in the circuit diagrams of Fig2.1 where the load is presented by the dc motors. The systems built on diodes are called uncontrolled rectifiers, and those built on thyristors and transistors are known as controlled rectifiers because their dc output can be changed. The rectification processes are quite varied accordingly , there are differenttypes of rectifying circuits
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Figure 2.1 Bridge Rectifiers
- midpoint (M) and bridge (B) rectifiers- single-phase (M1, M2, B2) and three-phase (M3, B6) rectifiers- half-wave (1 pulse per supply period) and full-wave (2, 3, 6 pulses) rectifiers
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2.3 Three Phase Rectifier
There various methods of rectifying ac current into dc current .In this project we shall major in vienna rectifier
2.3.1 Vienna Rectifier PrincipalThe Vienna rectifier was introduced in 1993 by Prof. Johann W. Kolar [10]. It is a threephase, three levels and three switch rectifier; it is kind of PWM (Pulse Width Modulation)rectifier with controlled output voltage. The topology of the Vienna Rectifier is a combinationof a boost DC\DC converter with a three-phase diode bridge rectifier. Fig. 2.2 illustrates thisrectifier circuit
Figure 2.2 Vienna Rectifier
As Fig. 2.2 shows, the output capacitor is split in two parts with two equal values (C1 and C2).Across each capacitor, two voltage sources +V0/2 and -V0/2 exist which detect the output voltage of the circuit. Therefore three different voltages (+V0/2, 0, -V0/2) are available. The DC bus voltage is assumed to be a constant dc voltage [11] and can be connected to a conventional six switch or other type of inverter [12].The input current for each phase is defined by the voltage applied across the corresponding inductor LN; the input voltage of the rectifier is determined by the switching state and the input current direction. The input inductors (LN) charge when the switch is on and the current increase in the inductor, and when the switch is off the inductors discharge through thepositive or negative diode depending on the current flow direction. The existence of an input inductor creates a current source at the input while the capacitors create output voltages. In other words, the Vienna rectifier may be considered as a diode–transistor matrix connecting the input current sources with output voltages [13]. Fig. 2.3 shows an equivalent schematic diagram for the Vienna rectifier.
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Figure 2.3 Vienna rectifier equivalent schematic diagram
The Vienna rectifier like all the other converters in power electronics has advantages and disadvantages. The pros of this converter are given below,• Has continuous sinusoidal input current• No need for a neutral wire• Low number of IGBTs used• Low manufacturing cost• Reduce blocking voltage stress on power semiconductor (in comparison with two levelconverters it becomes half because there is only one switch existing on one phase legof the rectifier. This result would be true if the neutral point is completely balanced;however the neutral operating point operating imbalance, the capacitor voltage willexceed half of dc-link voltage and in this case, the voltage stress on the switches willincrease, therefore the neutral point is an important topic in three system levelconverter [14]).• Reduction in switching loss of the power semiconductors by almost 40% [15].• Wide voltage range [15].• Higher efficiency.• Boosting ability.• Production of three levels of voltage with two equal DC voltages.On the other hand, acting as a unidirectional active AC\DC converter and lack of regeneration can be considered as cons. In unidirectional converters power flows in one direction from AC side to DC side and it cannot act as an inverter.
2.3.2 Operation of Vienna Rectifier
The Vienna rectifier has three switches, and by choosing their (ON\OFF) state considering the polarity of the phase current in each phase, the voltage for each phase will be determined. So, the phase voltage is depending on the direction of phase current and switch position. In this topology, the midpoint N is considered as reference point with zero voltage. Therefore, the phase voltage is described as:
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L di/dt=E-V ……………………………………………………. (2.5)
when the phase current is positive,
V={+Vo2¿¿ S=0 and v =0 , s=1…………………….. (2.6)
and when the phase current is negative, where s=0 corresponds to off state and s=1 to the on state. The figure below shows behavior of phase A. Phases B and C behave the same pattern
Figure 2.4 Conduction path for phase A
a) Line current is positive and SA is ON
Figure 2.5 Conduction path for phase A
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b) Line current is positive and SA is OFF
Figure 2.6 Conduction path for phase A
c) Line current is negative and SA is ON
Figure 2.7 Conduction path for phase A
d) Line current is negative and SA is OFF
If the line current is positive and switch SA is ON (Fig. 2.4); the current pass through the switch and the phase voltage (VAN) become zero. If the polarity of the current remains as before but the control switch SA is off, the current flows through diode D11, the voltage VAN is +V0/2, Fig. 2.4(b) illustrates this case. Similarly, the voltage VAN can determined if the line current is negative and switch SA is ON or OFF, the current path for these two cases is illustrated in Fig. 2.4
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The current path for two example switching position 110 and 001 are shown in figure below
Figure 2.8 The current path for two switching position
Figure 2.9 The current path for two switching position
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If it assumed that the current for phase A is positive and it is negative for both phase B and C. Then eight different switching positions can be considered, given the results shown in table below:
SA SB SC VAN VBN VCN
0 0 0 +Vo/2 -Vo/2 -Vo/20 0 1 +Vo/2 -Vo/2 00 1 0 +Vo/2 0 -Vo/20 1 1 +Vo/2 0 01 0 0 0 -Vo/2 -Vo/21 0 1 0 -Vo/2 01 1 0 0 0 -Vo/21 1 1 0 0 0
Eight different switching combinations
2.3.3 Semiconductor Components in the Vienna Rectifier
2.4 Diode
Diodes are the simplest devices among the entire power semiconductors. The pn-junction as shown in Fig. 2.7 is the basic building block for all the other power semiconductors. Generally, it consists of the three layers with different doping and thickness. The first layer is a heavily doped n-type, n+ that formed the cathode. On the top of that, is a layer of lightly doped n-type, n- that is called drift region and finally a heavily doped p-type, p+ layer on top, is forming the anode.
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Figure 2.10 Diode circuit symbol and Cross-sectional view of a pn-junction diode
The diode‘s cross sectional area depends on the amount of current that the device is designed to carry. Diodes always conduct the current in one direction, from the anode to the cathode. As the i-v characteristic (Fig. 2.8) shows, when the applied voltage over the diode is greater than a definite forward threshold voltage (about 1 V), the diode starts to conduct and the current grows linearly. When the diode is reversed biased or the voltage across the diode is less than the forward threshold voltage, then the diode blocks the current. In the ideal case, the forward voltage drop can be considered zero and the ideal characteristic curve is obtained.
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Figure 2.11 The i-v characteristic of a pn-junction diode (a) the idealized (b) the actual
In normal operation, the diode works in the both forward biased region which is called conduction region and the reverse blocking region. In the later region, a small leakage current flows as the voltage becomes more negative until the reverse voltage reaches the breakdown voltage of the diode. When the diode is switched off, a negative current occurs as shown in Fig. 2.11. This negative current sweeps out excess carriers in the diode and allows it to block a negative polarity voltage. This process takes a short time which is called reverse recovery time.The reverse recovery current Irr and reverse recovery time trr of the diode increases with increasing carrier lifetime
Figure 2.12 Reveres recovery
2.5 IGBT
The insulating gate bipolar transistor IGBT is one of the power semiconductor devices, which are created by the combination of the bipolar junction transistor BJT and the MOSFET. Each of these power semiconductors has particular characteristics; the combination of them can make a good trade off of both. For example, BJTs have low voltage drop in the on state but have longer switching times, especially at turn-off. The MOSFETs can be turned on and off fast due to small gate capacitance, but their on-state conduction losses are higher incomparison with BJTs at the same rating [23]. Therefore, the combination gives low
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conduction loss and fast switching, which is the unique character of the IGBT.The diagram below shows a vertical crossection for n- channel IGBT and its symbol.The structure is quite similar to the MOSFET .The p + layer forms the drain while n+ layer forms source:
Figure 2.13 The n-channel IGBT circuit symbols and Cross-sectional view of an IGBT
The figure above shows the i-v characteristic of the n-channel IGBT. In the forward direction it looks similar to the BJTs, but with a difference being in the way it is controlled. The IGBTs arecontrolled with input voltages, the gate–source voltage, while the input current is thecontrolling parameter for the BJTs. The reverse blocking voltage can be as large as theforward blocking voltage if the structure is made without n+ layer. However, if the n+ layer is used in the construction, the breakdown voltage is lowered significantly.
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Figure 2.14 The i-v characteristic of an IGBT
The figure below shows the switching characteristic for the IGBTs, which are similar to the MOSFETs. The power dissipation appears during the turn-on switching period. The faster turn-on switching helps to reduce that power dissipation. The IGBT’s turn-off switching curves are shown in Fig. 2.14 and there is a difference in the drain current waveform compares with the MOSFET. In the IGBTs, the drain current consists of two time intervals.During the first time interval, tfi1, the rapid drop corresponds to the turn-off of the MOSFET section of the IGBT. The second time interval, tfi2, corresponds to the BJTs and is due to the stored charges in the n- drift region.
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Figure 2.15 Turn-on voltage and current waveforms of an IGBT and Turn-off voltage and current waveforms of an IGBT
2.6 Power loss
Losses in power electronics are important features that determine the efficiency. Among
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different types of losses in the power electronic switches, only switching losses andconduction losses are considered in this project and other types of losses such as: gate driver losses, capacitor losses and snubber’s losses are ignored. The losses can thus be written as
P loss = Psw + Pcond………………………………..(2.10)
where P loss is the total power loss in the converter, Psw is the switching loss in the IGBT or diode and Pcond is the conduction loss in the IGBT and diode.Conduction losses can be calculated by the use of a forward voltage drop component (Von) and an on-state resistor (ron).
V(IGBT/MOSFET)ON =V(IGBT/MOSFET) + I(IGBT /MOSFET)ron
Pcond =1/T∫ v (IGBT /MOSFET ) on i(IGBT / MOSFET )
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CHAPTER 3: SYSTEM DESIGN
In this chapter the topology of the Vienna rectifier is investigated and component selection is described
3.1 Vienna rectifier set up
As mentioned in Chapter 2, the Vienna rectifier consists of only three IGBTs. If all threeswitches are in the OFF state, the functionality is the same as for a diode rectifier and the output power result is exactly the same as for the diode rectifier (2.5 MW output power). When switching of the IGBTs applied to the circuit, the Vienna rectifier is activated. The switches will operate with low switching frequency. In this application since we aim to obtain lower power loss, therefore there is only one switching for each half cycle. The IGBTs canturn-on at the beginning or at the end of the pulse, but since the phase voltage should lag thephase current, the switches are turned on at the beginning of the pulse. Fig. 3.4 explainsswitching pattern clearly.
Figure 3.16
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Figure 3.17
Figure 3.18 Current & Voltage Waveforms
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Figure 3.19 IGBTs Current Waveforms
3.2 MOSFETS
The metal oxide semiconductor field effect transistor MOSFET a voltage control current device. It differs from junction field effect transistor JFET that it has no pn junction structure. It has a metal gate, which insulates the conducting channel with silicon oxide SiO2. In the modern design, metal gate has been replaced by either p+ or n+ polysilicon. There are two types of MOSFET namely depletion-enhancement DE and enhancement E types. Figure() and () show the difference between the types. The DE type has a narrow channel adjacent to the gate connecting the drain and source of the transistor. It can operate in either depletion mode or enhancement mode. The mode of operation is like the JFET. The E type does not have a narrow connecting channel. It operates by forming a conducting channel of the same type like the source and drain. The channel is formed either by attracting electron or depleting away electron to form an n-channel or p-channel connecting the source and the drain. MOSFET not only can be used to design amplification circuit. It can also be used as a capacitor and a resistor. This capability makes the VLSI design simpler because there is no need to use other element for capacitor and resistor in the design.
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Figure 3.20 The structure of depletion-enhancement n-channel and p-channel MOSFET
Figure 3.21The structure of enhancement n-channel and p-channel MOSFET
The symbol of both depletion-enhancement and enhancement MOSFET types are shown in Fig (3.20) and (3.21) respectively below. Note that in at for most cases, by design the substrate is connected to the source.
Figure 3.22 Symbol of depletion-enhancement MOSFET
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Figure 3.23 Symbol of enhancement MOSFET
3.3 Depletion-Enhancement MOSFET and Enhancement MOSFET
The transfer characteristic of depletion-enhancement DE and enhancement E MOSFETs are shown in Fig. (3.22) and (3.23) respectively. The characteristic curve of DE type is same as the JFET except there is an additional enhancement part, where the curve extends to positive VGS, attract either hole or electron. In depletion mode, electron in n-channel or hole in p-channel is depleted away. In enhancement mode, electron is attracted in n- channel and hole in p-channel
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Figure 24 Transfer characteristic of DE MOSFET
The equation for the transfer characteristics of depletion-enhancement DE MOSFET is same the JFET which is:
I D=I DSS ¿
Where IDSS is normally given by data sheet. If the design parameters are given then IDSS is defined as :
I DSS=12∪nCOX
WL
V GS (OFF )2
3.4 MOS Transistor Theory and Applications
Where L is channel length, W is width of the gate, Cox is capacitance of oxide per unit area, and μn is the effective mobility of electron, which has a nominal value 650cm 2/V-s at 25 o C. Enhancement MOSFET uses only channel enhancement. If the drain and source are n-type, the gate is biased with positive voltage to attract electron from the substrate near the oxide to form an n-type conducting channel. Likewise, E MOSFET type has p-type drain and source, the gate is biased with negative voltage to form a p-type conducting channel.
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Figure 3.25 Transfer characteristic of E MOSFET
There is a minimum gate-to-source voltage to be applied before conducting channel can be formed. This gate-to-source voltage is called threshold voltage and is denoted as VGS(th).The threshold voltage VGS(th)is used to overcome the flat band potential, surface potential, fixed charge, and oxide capacitance Cox of the gate before inversion can be occurred. A typical I D-DS characteristic curve of an n-channel MOSFET for the selected GS is shown in Fig.(3.25). There are three regions - the cut-off region, the triode region (or almost linear region) and saturation region (operation region). The saturation region is the useful region for an amplifier operation, where the other regions are good for switch operation. The curve also shows that the output impedance is infinite, which is not true in reality. We shall discuss how to determine the finite output resistance.
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Figure 26 Ideal Operating Points
The output power of the converter depends on angle between back EMF and phase voltage therefore, in order to extract the maximum power; the best operating point for Vienna rectifier should be found. Mostly the voltage is lagging current hence IGBT turns on at the beginning of phase current .The diagram below shows operating point of IGBT
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Figure 3.27 Operating point of IGBT
In order to calculate the dc-link voltage, phase angle and other required parameters to find the best operating point for the Vienna rectifier, the transformation from Cartesian co-ordinations, to Polar co-ordinations, Fast Fourier analysis (FFT) has been used.
P=3/2 Eicosᵩ
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CHAPTER 4: SYSTEM IMPLEMENTATION
4.1 Vienna Rectifier Topology Using MOSFETS
It is a unidirectional three phase, three-level PWM pulse rectifier system with only three turn-off power semiconductors.
Figure 4.28 Vienna Rectifier Topology Using MOSFETS
The technical and economic advantages of the system compared to alternative concepts. The main advantages are:
Only one turn-off power semiconductor (MOSFET or IGBT) per phase, Three level characteristic of the bridge branches, resulting in a low voltage stress on the
power semiconductors, Continuous, uninterrupted input current with comparably low switching frequency
components, Two divided output voltages that may be loaded asymmetrically High reliability, as no bridge or output voltage short circuit can occur, even in the case of a
control failure
The main disadvantage of the system is that it cannot invert power into the mains, which would be required in a motor drive solution, for example, with a suitable PWM control of the power transistors Si (i = R, S, T), a pulse-shaped three-phase rotating current system Uu,i can be created at the
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System’s input. Its voltage difference to the mains voltage Un,i (which shows a fully sinusoidal characteristic) occurs across the input inductors and affects the most sinusoidal current consumption of the system.
4.2 Switching topology for higher power
For higher powers (the current limit is at an output power of about 10kW), the higher losses require parallel connection of the switches Si of the switch topologies.
Figure 4.29 Switching topology for higher power
The topology in Fig. (4.29) above is preferable, as the center point diodes DM becomes surplus to requirements. This requires, however, not three but six gate driver circuits.The functioning of the two systems is equivalent; the switches Si+ and Si- are operated by the same control signal si .For the practical implementation, two (circuit technology) alternatives were considered:
a) Single 1x 35 kW PFC unit, using 3 branch modules, e.g., in the Micro semi SP4 module:
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Figure 4.30 Single 1x 35 kW PFC unit using 3 branch modules
b) Parallel operation of 2x 17.5 kW PFC stages with phase-shifted driver, 2 'full bridge modules
Figure 4.31 Parallel operation of 2x 17.5 kW PFC stages with phase-shifted driver
This option has a number of advantages namely:
The common mode suppressor choke Lcm of option A is surplus to requirements The phase shift of 180° in switching frequency of the drive signals of the two partial
systems partially compensates the ripple of the choke currents in the mains current No parallel connection of COOLMOSTM 1) transistors in the power module is required
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Modular design: may be scaled down to17.5 kW output power by removing one of the two paralleled PFC stages.
A disadvantage is the increased hardware requirement resulting from the additional current transducers and driver amplifiers. However, the positive features outweigh the negatives.
4.3 Low profile power module integration
The electrical configuration of the 3-phase Vienna rectifier is built from a significant number of individual devices from different technologies (6 fast recovery diodes, 6 MOSFETs, 3 thyristors and 3 standard rectifier diodes). The circuit configuration using discrete devices would spread over a very wide area to achieve the complete power function. Interconnecting all the power semiconductors together is not only complex from a layout point of view but also introduces high parasitic wiring inductances and resistances that increase the voltage stress of the power semiconductors and limits their switching speed. The consequent long wires between devices generate inevitable parasitic ringing or oscillations that are difficult to cancel and/or damp out and these may affect the stability and EMC performance of the system. Therefore it makes eminent sense to integrate the whole power function inside a single package to achieve the power conversion module of the 3- phase Vienna Rectifier according to characteristics without sacrificing speed. This results in less stress on the switches and generates much less switching noise. The quiet behavior of the diodes makes it easier to meet EMI/RFI noise emissions limits.The moderate forward voltage provides a good balance between switching and conduction losses. With avalanche rating characteristics the DQ diodes contribute to a more rugged system. All MOSFET devices as well as the SCR’s are provided with command signals taken directly from the dice such that the corresponding Kelvin connections do not interfere with the power circuit to further reduce the amount of noise in the control circuit. This enhances the quality of the control signals, improves the safety of operation and facilitates filtering operation without affecting the speed of the system. With platinum doping, the DQ diodes offer less degradation of performance at elevated temperatures such that they can be specified up to 175°C maximum junction temperature. Consequently the state of the art super junction MOSFET devices can operate at their full speed without significant de-rating on the switched current up to junction temperatures of 125°C.On the other hand, the highly doping level of the base region of COOLMOSTM transistors allows higher temperature operation than with conventional silicon transistors having the same blocking voltage. COOLMOS devices could be safely operated up to 200°C. It has also to be noted that both the COOLMOS transistors and DQ diodes exhibit very low leakage current as temperature increases, thus minimizing the power losses and improving the reliability over the entire junction temperature range varying from -55°C to 175°C.\
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CHAPTER 5: RESULTSIn order to ascertain the conversion of AC to DC current, simulations were carried out for the
various types of converters. The simulation software used was PSIM. It also important to note
that PSIM provides a tool for technical computing and has technology to model simulate and
analyze complicated circuits especially in power electronic. It’s a useful tool to evaluate
interaction between components
The design parameters used in the design were as follows
Capacitor (66u)Inductor (3.15 m)Resistive load (1500 ohms)Mosfet (MOS1,MOS2,MOS3)Ac voltage source (V1,V2,V3)
A sinusoidal voltage was provided as input of the vienna rectifier. The wave form below were obtained from simulation
Figure 5.32 Sinusoidal voltage wave
5.1 Circuit
The circuit was designed and various component inserted in psim as shown below:
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e
Figure 5.33 System Circuit Design
5.2 Analysis
As mentioned in chapter 4 that the switches are turned on at the beginning of the pulse when MOSFETs are conducting the potential voltage becomes zero, otherwise the potential voltage is equal to Vdc/2The figure below shows it clearly:
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Figure 5.34 Waveforms when MOSFETs are conducting
The theory of the Vienna rectifier was explained in chapter 3. There it was described that the output voltage is a function of both switching state and current direction. The waveforms below are taken from simulation confirmed this theory:
T1 T2 T3 T4
Figure 5.35 Simulation Waveforms
During the interval T1, the mosfet is conducting the phase current is positive then current passes through D11 and D14. Then, the phase potential voltage becomes zero.During the time interval t2, the mosfet is in its OFF state, and then the phase current passesthrough D11 and D1. In this case, the phase potential voltage is equal to the positive dc-linkvoltage.At t3, the direction of the current is changed. While the mosfet is conducting, two diodes, D12and D13, are also conducting, and during t4 the MOSFET is not conducting, then both D12 and D2is a suitable path for the phase current and the potential voltage is equal to the negative dc linkvoltage.
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5.3 Input Phase Potential
MOS 2 MOS 2 MOS 3 V1N V 2N V 3N1 1 0 0 0 + V dc/21 0 0 0 - V dc/2 + V dc/21 0 1 0 - V dc/2 00 0 1 +V dc/2 - V dc/2 00 1 1 + V dc/2 0 00 1 0 + V dc/2 0 - V dc/21 1 0 0 0 - V dc/21 0 0 0 + V dc/2 - V dc/21 0 1 0 + V dc/2 00 0 1 - V dc/2 + V dc/2 00 1 1 - V dc/2 0 00 1 0 - V dc/2 0 + V dc/2
5.4 Component Selection
5.4.1 MOSFET SelectionThe maximum voltage across each mosfet is half of dc link voltage. However in order to fulfill the maximum current and voltage for each switch,the number of this module should be connected to each other in series or parallel.The relation between Iref and Irated (rms) is approximately:
I rated(rms)=Iref/2
So the number of module in parallel is
N parallel=max|Irms|/Irated(rms)
And the needed number of modules in series can be found as
N series =V dc/V ref
Where Iref module reference current, Vref module reference voltage (usually a little bit more than half the rated module voltages)
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5.4.2 Diode SelectionEach diode has the specific rms current value due to different conduction path.Based on these values the diode module is selected. A number of this module should be connected together in series or fulfill the required voltage and current for diode.
I rated(rms)=Iref/2
The number of module in parallel is,
N parallel = max|Irms |/ Irated(rms)
Since the selected module has two diodes one module is enough .And the needed number of modules in series can be found as:
N series = Vdc/Vref
5.5 Power loss calculation
In this section, the conduction and the switching loss calculation for both the IGBT and thediode is investigated. (It is assumed that the temperature is 125°c for all calculations).
5.6 MOSFET Current and Voltage Waveform
Figure 5.36 MOSFET Current and Voltage Waveform
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The MOSFET turns on only twice during one line period and it conducts for 90 degrees (duty cycle=0.5) during half a period. The conduction loss is done using the forward voltage drop and on state current as well as on state resistance. Turn on can be neglected and the switching loss only consist of turn off switching lossThe conduction loss and switching loss of each component is calculated separately as shown below:
P cond P sw P sw P sw P lossMOSFET 889.5635 43.357 1014.7 1058.1 1947.7D1 556.46 - - 81.45 637.92D11 1050.7 - - 0 1050.7D12 1043.6 - - 0 1043.6D13 494.24 - - 81.30 575.54D14 494.18 - - 81.45 575.63D2 549.29 - - 81.30 630.59Total loss for one leg 6561.7
5.4.1 Output Power Calculation Output power of rectifier is calculated based on the following relations,
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Simulations of the output voltage are shown below:
Figure 5.37 Output Voltage signal
The high-frequency components of the PWM switching frequency visible on the psimare attenuated by the EMC filter to below the current limits. Sudden load jumps are regulated within approximately50 ms. In case of a power-ON transition from zero to rated power the voltage dip is about 100V, as shown aboveA high dynamism of the regulation is required in particular to avoid overvoltages as a result ofsudden load shedding, as these may cause voltage overload of components
5.4.2 Output Capacitor Size
The aim of this part is to calculate the size of the output capacitors for the Vienna rectifier. Atfirst, it is assumed that the limitation for the output voltage ripple is 5%. Therefore themaximum voltage across the each capacitor is calculated by below equation
And the output power simulation results are shown below
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Figure 5.38 Power off transition from zero full load
The above shows power off transition from zero full load. There is a voltage overshoot of about 100 V.
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CHAPTER 6: CONCLUSION
In this report, it is shown and explained how it is possible to use a Vienna rectifier to convert ac to dc current to be connected to an HVDC grid. The system is simulated in psim softwareIn Chapter 4, the Vienna rectifier setup has been explained and the analytical relation between electrical power and dc-link voltage has been proposed.It can be seen from the simulation in Chapter 5, that the Vienna rectifier has low losses due toa very low switching frequency. The mosfet are turned-on only twice during the one lineperiod. The results show that the Vienna rectifier has higher efficiency in comparison with theconventional mosfet converter and lower efficiency in comparison with the diode rectifier. Thepure diode rectifier that works with the same generator can only provide half theconventional IGBT converter provides as output power.
6.1 Future Work
The main focus of this thesis has been on the behavior of the Vienna rectifier with an ac iput . The Vienna rectifier has been simulated in the psim software with a low switching frequency. It might be interesting to design a real source of power such as PMSG and connect the Vienna rectifier to it.It could be interesting to extend this thesis by designing an output filter and connect theVienna rectifier to the DC/DC converter. In this work, only the losses in the switches are considered, that could be a continuing this work, to investigate the losses in the cables and the thermal calculation in the rectifier.It is also possible to study the control topology for the Vienna rectifier with a PMSG.Finally, a cost evaluation for system could be achieved and the result could be compared withthe diode rectifier and the conventional MOSFET converter.
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