INTRODUCTION Power Electronics

Application of Power Electronics In early days, control of the electric power was achieved with electric machinery. Power electronics have revolutionized the concept of power control for power conversion and for control of electrical motor drives. Power electronics combine power, electronics, and control.

Control deals with the steady-state and dynamic characteristics of closed-loop system. Power deals with static and rotating power equipment. Electronics deals with the solid-state devices and circuits for signal processing to meet the desired control objectives.

Therefore, power electronics is defined as the applications of solid-state electronics for control and conversion of electric power. Power electronics is based on switching of the power semiconductor devices. It covers a variety of switching circuits.

History of Power Electronics The history of power electronics began with introduction of the mercury arc rectifiers in 1900. Devices which were based on the mercury arc valve technology were used until 1950. The first electronic revolution began in 1948 with the invention of the silicon transistor at Bell Labs.

Most of today's advanced electronic technologies are based on the transistor concept. The next breakthrough was invention of Thyristor (SCR) in 1956, which is a PNPN triggering transistor.  The second revolution began in 1958 with development of the commercial thyristor by GE.

That was the beginning of a new era of power electronics.

Power Semiconductor Devices Since the first thyristor was developed in 1957, there have been tremendous advances in the power semiconductor devices. Until 1970, the conventional thyristors had been exclusively used for power control applications. Since 1970 many types of power semiconductor devices were developed.

Control Characteristics The power semiconductor devices can be operated as switches by applying a control signals to gate.

Power semiconductor switching devices can be classified on the basis of: Uncontrolled turn on and off (diodes) Controlled turn on and uncontrolled turn off (SCR) Controlled turn on and off (BJT, MOSFET, GTO, IGBT) Continuous gate signal requirement (BJT, MOSFET, IGBT)

MOSFET, GTO) Pulse gate requirement (SCR, GTO) Bipolar voltage-withstanding capability (SCR, GTO) Unipolar voltage withstanding capability (BJT, MOSFET, GTO) Bidirectional current capability (TRIAC) Unidirectional current capability (SCR, GTO, BJT, MOSFET, DIODE)

Characteristics and Specification of Switches There are many types of power switching devices. Each has its own advantages and disadvantages for an application.

Ideal Switches In the on-state: carry high forward current, low forward voltage drop, and low resistance In the off-state: withstand a high voltage, low leakage current, and high resistance During turn-on and turn-off process instantaneously turn on and off

Low gate power for turn on and off Controllable turn on and off Turn on and off require a small pulse High dv/dt & di/dt Low thermal impedance

Sustain any fault current (i2t) Equal current sharing for parallel operation Low price

Characteristics of Practical Devices During the turn-on and turn-off process a practical device requires: a finite delay time  rise time  storage time fall time

Types of Power Electronic CKTs For control of electric power or power conditioning, the conversion of electric power from one form to another is necessary. Switching characteristics of the power devices permit this conversion.

Power electronics circuits can be classified into six types: Diode rectifiers Ac-dc converters (controlled rectifier) Ac-ac converters (ac voltage controllers) Dc-dc converters (dc choppers) Dc-ac converters (inverters) Static switches

Design of Power Electronics Equipment The design is divided into four parts: Design of power circuits Protection of power devices Determination of control strategy Design of logic and gating circuits

In the chapters that follow, we will describe various types of power electronic circuits. In analysis, the power devices are assumed to be ideal switches. The effect of circuit resistance and source inductance is ignored. Ignoring these parameters will simplify the design steps, but it is very useful to understand operation of the circuit and establish the control strategy.

Determining the RMS Value The RMS value of current should be known for determination of conduction losses and current rating of the device. The RMS value of a current waveform is: Also:

Peripheral Effects Operations of power converters are mainly based on the switching of power semiconductor devices. As a result, converters introduce current and voltage harmonics into the supply system and on the output of the converters.  

These can cause problems of distortion of the output voltage, harmonic generation into the supply system, and interference with the communication and signaling circuits. Therefore, it is normally necessary to introduce filters on the input and output of a converter system to reduce the harmonic level.  

The following figure shows the block diagram of a generalized power converter.  

LECTURE 2 (Ch. 2) POWER SEMICONDUCTOR DIODES AND CKTS ECE 452 Power Electronics 33

Introduction Power semiconductor diodes play a significant role in power electronic circuits. A diode acts as a switch to perform various functions, such as switches in rectifiers and freewheeling in switching regulators. Power diodes can be assumed as ideal switches for most applications. 34

However, practical diodes differ from the ideal characteristics and have certain limitations. Power diodes are similar to pn-junction signal diodes, but they have larger power, voltage, and current handling capabilities. 35

Semiconductor Basics A pure silicon material is known as an intrinsic semiconductor with resistivity that is too low to be an insulator and too high to be a conductor. The resistivity can be changed by doping, which involves a single atom of the added impurity per over a million atoms of silicon. 36

indium and we have a vacant location called hole. With different impurities, power devices are produced from various structures of n-type and p-type semiconductor layers. n-Type: Silicon is doped with phosphorus, arsenic, or antimony and we have a loose electron. p-Type: Silicon is doped with boron, gallium, or indium and we have a vacant location called hole. 37

In a p-type material holes are majority carriers and electrons are minority carriers. In an n-type material electrons are majority carries and holes are minority carriers. An applied electric field can cause a current to flow in an n-type or p-type material. 38

Diode Characteristics A power diode is a two-terminal pn-junction device. 39

When the anode potential is positive with respect to cathode, the diode is said to be forward biased and the diode conducts. A conducting diode has a relatively small forward voltage drop across it. When the cathode voltage is positive with respect to the anode, the diode is said to be reverse biased. 40

The v-i characteristics of a diode is shown below: Under reverse-biased conditions, a small reverse current (leakage current) flows. The v-i characteristics of a diode is shown below: 41

VT is a constant called thermal voltage and is given by: This characteristic can be expressed by an equation known as Schockley diode equation: VT is a constant called thermal voltage and is given by: 42

The thermal voltage at room temperature (25 oC) is: Where: The thermal voltage at room temperature (25 oC) is: 43

At a specified temperature, the leakage current is a constant for a given diode. The diode characteristic of the previous figure can be divided into three regions: Forward-biased region: VD > 0 Reverse-biased region: VD < 0 Breakdown region: VD < -VBR 44

Forward-Biased Region The diode current ID is very small if the diode voltage is less than a threshold voltage (0.7 volts). The diode fully conducts if VD is higher than the threshold voltage. Then the Schockley diode equation can be simplified with about 2% error. 45

Reverse-Biased Region If VD is negative and |VD| >> VT, the exponential in the diode equation becomes negligible, and the diode current ID equation will be reduced to: 46

Breakdown Region In the breakdown region, the reverse voltage is high, and it exceeds a specified voltage known as the breakdown voltage VBR. The reverse current increases rapidly with a small change in reverse voltage beyond VBR. 47

The operation in the breakdown region can be destructive, if the power dissipation is above the rating of the diode. The power dissipation level is specified in the manufacturer's data sheet. 48

Reverse Recovery Characteristic The current in a forward-biased junction diode is due to the net effect of majority and minority carriers. When the diode is in a forward conduction mode, and its forward current is reduced to zero due to a voltage reversal, the diode continues to conduct due to the minority carriers. 49

This time is called the reverse recovery time of the diode. The minority carriers require a certain time to recombine with opposite charges and to be neutralized. This time is called the reverse recovery time of the diode. The reverse recovery time, trr, is measured from the initial zero crossing of the diode current to 25% of the peak reverse current. 50

trr consists of two components, ta and tb: 51

The ratio of tb/ta is known as the softness factor, SF. For practical purposes, we need to be concerned with the total recovery time trr and the peak value of the reverse current. The peak reverse current IRR can be expressed as: 52

The reverse recovery charge QRR is the amount of charge carriers that flow across the diode in the reverse direction due to changeover from forward conduction to reverse blocking condition. Its value is determined from the area enclosed by the path of the reverse recovery current. 53

Equating the two IRR equations will yield: Therefore: Equating the two IRR equations will yield: 54

If tb is negligible as compared to ta, which is usually the case, then the above equation will be reduced to: Finally, 55

It can be concluded that the reverse recovery time trr and the peak reverse current IRR depend on QRR and di/dt. The peak reverse recovery current, reverse charge, and the softness factor are all of interest to the circuit designer. These parameters are commonly included in the specification sheets of diodes. 56

LECTURE 13 (Ch. 5) DC-DC CONVERTERS ECE 452 Power Electronics 57

Step-Up Converter with Resistive Load A step-up converter with a resistive load is shown below. 58

The current equation for mode 1 is: At the end of mode 1 at t = kT, 59

When the Switch S1 is opened (Mode 2), the current will flow through R and L. Then: 60

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Performance Parameters The power semiconductor devices require minimum time to turn on and turn off. Therefore, the duty cycle k can only be controlled between kmin and kmax. The switching frequency of the converter is also limited. 62

The load ripple current depends inversely on the chopping frequency. The frequency should be as high as possible to reduce the load ripple current, and to minimize the size of any additional series inductor in the load circuit. 63

Therefore, the performance parameters of the step-down and step-up converters are as follows: Ripple current Maximum switching frequency Continuous or discontinuous current Minimum value of inductance to maintain continuous current Output current and voltage THD Input current THD 64

Converter Classifications Depending on the direction of the voltage and current flows, converters can be classified into five types: First quadrant converter Second quadrant converter First and second quadrant converter Third and fourth quadrant converter Four-quadrant converter 65

Forward Motoring Forward Braking Reverse Motoring Reverse Braking 66

First Quadrant Converter The load current flows into the load. Both the load voltage and the load current are positive. 67

This is a single-quadrant converter, and is operated as a rectifier. 68

Second Quadrant Converter The load current flows out of the load. The load voltage is positive and the load current is negative. 69

This is a single-quadrant chopper, operated as an inverter. 70

A second quadrant converter is shown in the following figure, where the battery E is a part of the load and may be the back emf of a dc motor. 71

When the switch is turned on, the voltage E drives current through inductor L and the load voltage VL becomes zero. When the switch is turned off, the energy stored in the inductor will be returned to the supply Vs. 72

First & Second Quadrant Converter The load current is either positive or negative. The load voltage is always positive. 73

This is a two-quadrant converter.   The first and second quadrant converters can be combined to form this converter. 74

S1 and D4 operate as a first quadrant converter. S4 and D1 operate as a second quadrant converter. Care must be taken to ensure that the two switches are not fired together; otherwise, the supply Vs will be short-circuited.  This converter can operate either as a rectifier or as an inverter. 75

Third & Fourth Quadrant Converter The load voltage is always negative. The load current is either positive or negative.   76

This converter can operate as a rectifier or as an inverter (negative two-quadrant operation). S3 and D2 operate to yield a negative voltage and a negative load current. 77

When S3 is closed, a negative current flows through the load. When S3 is opened, the load current freewheels through D2. S2 and D3 operate to yield a negative voltage and a positive load current. 78

When S2 is closed, a positive load current flows.   When S2 is opened, the load current freewheels through D3. The polarity of E should be reversed for this circuit to yield negative voltage and positive current. 79

Four Quadrant Converter The load voltage and load current can be either positive or negative.   This is known as a four-quadrant converter. 80

One first and second quadrant converter and one third and fourth quadrant converter can be combined to form this converter. 81

For operation in the 3rd and 4th quadrant, the direction of the battery E must be reversed. 82

LECTURE 14 (Ch. 5) DC-DC CONVERTERS ECE 452 Power Electronics 83

Switching Mode Regulators DC converters can be used as switching-mode regulators to convert a dc voltage, normally unregulated, to a regulated dc output voltage. The regulation is normally achieved by PWM at a fixed frequency. The elements of switching-mode regulators are shown in the following figure. 84

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Control voltage ve is obtained by comparing the output voltage with its desired value. The vcr can be compared with a sawtooth voltage vr to generate the PWM control signal. 86

There are four basic topologies of switching regulators: Buck regulators Boost regulators Buck-Boost regulators Cuk regulators 87

Buck Regulators In a buck regulator, the average output voltage Va is less than the input voltage Vs. 88

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The buck regulator is simple and requires only one transistor. The di/dt of the load current is limited by the inductor L. The input current is discontinuous and a smoothing input filter is required. The buck regulator provides a one polarity output voltage and unidirectional output current. 90

Boost Regulator In a boost regulator, the output voltage is greater than the input voltage. 91

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Due to a single transistor, it has a high efficiency. A boost regulator can step up the output voltage without a transformer. Due to a single transistor, it has a high efficiency. The input current is continuous; however, a high peak current has to flow through the power transistor. 93

A much higher current will flow through the filter capacitor. The output voltage is very sensitive to changes in duty cycle k and it might be difficult to stabilize. The average output current is less than the average inductor current by a factor (1-k). A much higher current will flow through the filter capacitor. 94

The end result is that the L and C are much larger than those of a buck regulator. 95

Buck-Boost Regulator A buck-boost regulator provides an output voltage which may be less than or greater than the input voltage. The output voltage polarity is opposite to that of the input voltage. 96

k Va = [-k/(1-k)]Vs 0.1 -0.111111111 0.2 -0.25 0.3 -0.428571429 0.4 -0.666666667 0.5 -1 0.6 -1.5 0.7 -2.333333333 0.8 -4 0.9 -9 97

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The regulator is also known as an inverting regulator. Thus, a buck-boost regulator provides output voltage polarity reversal without a transformer. The regulator is also known as an inverting regulator. 99

Limitations of Single Stage Conversion The regulators use only one transistor. They employ a one stage conversion, and require inductors or capacitors for energy transfer. Due to current-handling limitation of a single transistor, the output power of these regulators is small, tens of watts. 100

At a higher current, the size of these components increases, with increased component losses, and the efficiency decreases. In addition, there is no isolation between the input and output voltage, which is highly desirable. 101

For high power applications, multistage conversions are used: A dc voltage is converted to ac by an inverter. The ac output is isolated by a transformer and then converted to dc by a rectifier. 102

Comparison of Regulators There is no change in the position of the main switch for the buck and buck-boost regulators. There is no change in the position of the main switch for the boost and Cuk regulators. 103

In section 5.8 we showed the voltage equation for the regulators with the assumptions that there were no resistances associated with inductors and capacitors. However, such resistances though small may reduce the gain significantly. 104

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The magnetic loss increases with the square of frequency However, a higher frequency reduces the size of inductors for the same value of ripple current and filtering requirement. The design of dc-dc converters requires a compromise among switching frequency, inductor size, capacitor size, and switching losses. 106

LECTURE 15 (Ch. 6) PULSE-WIDTH MODULATED INVERTERS ECE 452 Power Electronics 107

Introduction DC-to-AC converters are known as inverters. The function of an inverter is to change a dc input to a symmetrical ac output voltage of desired magnitude and frequency. The output voltage can be fixed or variable at a fixed or variable frequency. 108

A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. If the dc voltage is fixed, a variable output voltage is obtained by varying the inverter gain (PWM). The inverter gain is defined as the ratio of the ac output voltage to dc input voltage. 109

The output voltage waveforms of ideal inverters should be sinusoidal. However, the waveforms of ideal inverters are nonsinusoidal and contain harmonics. For high power applications, low distortions are required. 110

Inverters can be broadly classified into single-phase and three-phase inverters. Inverters use controlled turn-on turn-off devices, such as BJTs, MOSFETs, IGBTs, and GTOs. 111

Principle of Operation The principle of single-phase inverter operation can be explained with the following figure. 112

Note that the inverter consists of two choppers. The rms output voltage is: The rms value of the fundamental component is: 113

Performance Parameters The output of practical inverters contains harmonics. The quality of an inverter is evaluated in terms of the following performance indices. 114

Harmonic Factor of nth Harmonic This is a measure of individual harmonic contribution defined as: where Vo1 is the rms value of the fundamental component and Von is the rms value of the nth harmonic. 115

Total Harmonic Distortion This is the measure of closeness in shape between a waveform and its fundamental component. 116

Distortion Factor THD gives the total harmonic content, but it does not indicate the level of each harmonic component. If a filter is used at the output of inverters, the higher order harmonics are attenuated more effectively. 117

The distortion factor indicates the amount of harmonic distortion that remains in a particular waveform after the harmonics being subjected to a second-order attenuation (divided by n2). Therefore, DF is a measure of effectiveness in reducing unwanted harmonics without having to specify the values of the load filter. 118

Lowest Order Harmonic It is that harmonic component whose frequency is closest to the fundamental one, and its amplitude is equal or greater than 3% of the fundamental component. 119

Single-Phase Bridge Inverter A single-phase bridge inverter is shown below. 120

The rms output voltage is Vo = Vs. 121

Three-Phase Inverters Three-phase inverters are used for high power applications. Three-single phase inverters can be connected in parallel to form a three-phase inverter. This arrangement will require 12 transistors, 12 diodes, and three single-phase transformers. 122

The gating signals of the single phase inverters should be 120 degrees with respect to each other. The transformer primaries are isolated from each other, while the secondaries may be connected in wye or delta. 123

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A three-phase output can be obtained from a configuration of six transistors and six diodes. Two types of control signals can be applied to the transistors: 180o conduction or 120o conduction 125

180 degree Conduction For this mode of operation, each device conducts 180 degrees.   The sequence of firing is: 123, 234, 345, 456, 561, 612. The gating signals are shifted from each other by 60 degrees. 126

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120 degree Conduction In this mode, each transistor conducts for 120 degrees. The sequence of firing is: 61, 12, 23, 34, 45, 56, 61. 128

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LECTURE 16 (Ch. 6) PULSE-WIDTH-MODULATED INVERTERS ECE 452 Power Electronics 130

Three-Phase Inverters Three-phase inverters are used for high power applications. Three-single phase inverters can be connected in parallel to form a three-phase inverter. This arrangement will require 12 transistors, 12 diodes, and three single-phase transformers. 131

The gating signals of the single phase inverters should be 120 degrees with respect to each other. The transformer primaries are isolated from each other, while the secondaries may be connected in wye or delta. 132

133

A three-phase output can be obtained from a configuration of six transistors and six diodes. Two types of control signals can be applied to the transistors: 180o conduction or 120o conduction. 134

180 degree Conduction For this mode of operation, each device conducts 180 degrees.   The sequence of firing is: 123, 234, 345, 456, 561, 612. The gating signals are shifted from each other by 60 degrees. 135

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120 degree Conduction In this mode, each transistor conducts for 120 degrees. The sequence of firing is: 61, 12, 23, 34, 45, 56, 61. 137

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Voltage Control of Single-Phase Inverters In industrial applications, it is required to control the output voltage of inverters. There are various techniques to vary the inverter gain. The most efficient method of controlling the gain is to incorporate PWM control within the inverters. 139

The commonly used techniques are: Single-pulse-width modulation Multi-pulse-width modulation Sinusoidal pulse-width modulation Modified sinusoidal pulse-width modulation Phase-displacement control 140

Single Pulse-Width Modulation There is one pulse per half-cycle, and its width is varied. 141

The modulation index is: The rms output voltage is: The following figure shows the harmonic profile with variation of the modulation index M. 142

The dominant harmonic is the third. DF increases significantly at a low output voltage. 143

Multiple-Pulse-Width Modulation The harmonic content can be reduced by using several pulses in each half-cycle of output voltage. This type of modulation is also known as uniform-pulse-width modulation (UPWM). 144

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The number of pulses per half cycle is found from: where mf is defined as frequency modulation ratio. The rms output voltage is: 146

The following figure shows the harmonic profile against the variation of modulation index, and p=5.   147

Sinusoidal Pulse-Width Modulation Instead of maintaining the width of all pulses the same, the width of each pulse is varied in proportion to amplitude of a sine wave. This kind of modulation is known as SPWM. 148

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The rms output voltage is: The DF and LOH are reduced significantly, as shown below. 150

This type of modulation eliminates all harmonics less than 2p-1. 151

Modified Sinusoidal Pulse-Width Modulation This utilizes a different method of modulation. 152

The harmonic profile is shown below. 153

Phase-Displacement Control Voltage control can be obtained by using multiple inverters and summing the output voltages of individual inverters. A single-phase full-bridge inverter can be perceived as the sum of two-bridge inverters. 154

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A 180 degrees phase displacement produces “c”. A delay angle of α produces “e”. Then the rms output voltage is: 157

LECTURE 17 (Ch. 7) THYRISTORS (SCRs) ECE 452 Power Electronics 158

Introduction Thyristors are one of the most important types of power semiconductor devices. They are operated as bistable switches, operating from nonconducting state to conducting state. Thyristors can be assumed as ideal switches for many applications. 159

The thyristor will turn off when the current is brought to zero. In reality, thyristors exhibit certain characteristics, and they have some limitations. Conventional thyristors are designed without gate-controlled turn-off capability. The thyristor will turn off when the current is brought to zero. 160

Gate turn-off thyristors (GTO) are designed to have controlled turn-on and controlled turn-off capability. Thyristors have lower on-state conduction losses and higher power handling capability compared to transistors. However, transistors have higher switching speeds and lower switching losses. 161

Thyristor Characteristics A thyristor is a four-layer semiconductor device of pnpn structure with three pn junctions. The following figure shows the thyristor symbol and the sectional view of the three pn junctions. 162

However, the junction J2 is reverse-biased. When the anode voltage is positive with respect to cathode, the junctions J1 and J3 are forward-biased. However, the junction J2 is reverse-biased. Only a small leakage current flows from anode to cathode. Thyristor is in forward blocking or off state. 163

If the anode-to-cathode voltage VAK is increased to a sufficiently large value, the reverse-biased junction will break. This is known as the avalanche breakdown and the corresponding voltage is called forward breakover voltage VBO. 164

The device will be in a conducting state or on-state. Since J1 and J3 are already forward biased, there will be free movement of carriers across all three junctions, resulting in a large forward anode current. The device will be in a conducting state or on-state. The voltage drop would be due to the ohmic drop in four layers and it is small, typically 1V. 165

In the on-state, the current is limited by the external impedance. The current must be greater than the latching current in order for the device to conduct; otherwise, the device will go into the blocking mode as the anode-cathode voltage is reduced. 166

The holding current is in the range of milliamperes. If the current is reduced below a value which is known as the holding current the thyristor will go into the blocking state. The holding current is in the range of milliamperes. 167

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This is like two series-connected diodes being reverse-biased. When the cathode voltage is positive with respect to the anode, the junctions J1 and J3 are reversed-biased and J2 is forward-biased. This is like two series-connected diodes being reverse-biased. The reverse leakage current is known as the reverse current IR. 169

A thyristor can be turned on by increasing the forward voltage beyond VBO, but such a turn on can destroy the thyristor. In practice, the forward voltage is maintained below VBO and the thyristor is turned on by applying a positive voltage between its gate and cathode. 170

This was shown on the previous figure by dashed lines. Once a thyristor is turned on by a gating signal, the device continues even if the gate signal is removed. Therefore, a thyristor is a latching device. 171

Two Transistor Model of Thyristor The latching action can be demonstrated by using a two-transistor model of thyristor. 172

The common-base current gain is defined as: The collector current IC of a thyristor is related to the emitter current IE and the leakage current of the collector-base junction, ICBO. The common-base current gain is defined as: 173

Therefore, 174

Substituting (2) into (1) and solving for IA: But, Substituting (2) into (1) and solving for IA: 175

If , then the denominator will approach 0 and IA will be infinite. The α varies with the emitter current, and the variation is shown below. If , then the denominator will approach 0 and IA will be infinite. Consequently, the thyristor will turn on. 176

Under transient conditions, the capacitances of the pn junctions will influence the characteristics of the thyristor. 177

If a thyristor is in a blocking state, a rapidly rising voltage applied across the device would cause high current flow through the junction capacitors. If the dv/dt is large, ij2 would be large and this would result in increased leakage current ICBO1 and ICBO2. 178

The high value of the leakage currents may cause α1 + α2 to approach unity and turn the device on. This large current through the junction capacitors may damage the device. 179

Thyristor Turn-On A thyristor is turned on by increasing the anode current. This is accomplished in one of the following ways. 180

Thermals If the temperature of a thyristor is high, there will be an increase in the number of electron-hole pairs. This will increase α1 and α2 and the thyristor may be turned on. 181

Light If light is allowed to strike the junction, the electron-hole pairs will increase and the thyristor may be turned on. The light activated thyristors (LASCR) are turned on by allowing light to strike the silicon wafer. 182

High Voltage If the forward anode-to-cathode voltage is greater than the VBO, the thyristor will turn on. dv/dt If the rate-of-rise of the anode-cathode voltage is high, the charging current of the capacitive junctions will turn on the thyristor 183

Gate Current If a thyristor is forward biased, the injection of the gate current by applying a positive gate voltage between the gate and the cathode terminals would turn on the thyristor. The following figure shows the effects of the gate current on forward blocking voltage. 184

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The following points should be considered in designing the gate control circuit. The gate signal should be removed after the thyristor is turned on. A continuous gating signal would increase the power loss in the gate junction. While the thyristor is reversed biased, there should be no gate signal; otherwise, the thyristor may fail due to an increased leakage current. 186

The width of gate pulse tG must be longer than the time required for the anode current to rise to the holding current value IH. In practice, the pulse width is made more than the turn-on time ton of the thyristor. 187

Thyristor Turn-Off A thyristor which is in the on-state can be turned off by reducing the forward current to a level below the holding current. In a line-commutated converter circuit where the input voltage is alternating, a reverse voltage appears across the thyristor immediately after the forward current goes through the zero value. 188

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A forced-commutated circuit is shown next. The turn-off time tq is the minimum value of time interval between the instant when the on state current has decreased to zero and the instant when the thyristor is capable of withstanding forward voltage without turning on. A forced-commutated circuit is shown next. 190

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LECTURE 18 (Ch. 7) THYRISTORS (SCRs) ECE 452 Power Electronics 192

Thyristor Types Thyristors are manufactured by diffusion. The anode current requires a finite time to propagate to the whole area of the junction. This is from the point near the gate when the gate signal is applied. Manufacturers use various gate structures to control the di/dt, turn-on time, and turn-off time. 193

Based on physical construction, turn-on and turn-off behavior, thyristors are classified into nine categories: Phase-controlled thyristors (SCRs) Bidirectional phase-controlled thyristors (BCTs) Fast-switching thyristors (SCRs) Light-activated silicon-controlled rectifiers (LASCRs) Bidirectional triode thyristors (TRIACs) Reverse-conducting thyristors (RCT) 194

Gate-turn-off thyristors (GTOs) FET-controlled thyristors (FET-CTHs) MOS-controlled thyristors (MCTs) Emitter turn-off (control) thyristors (ETOs) Integrated gate-commutated thyristors (IGCTs) Static induction thyristors (SITHs) 195

Phase-Controlled Thyristors This type of thyristors operates at the line frequency and is turned off by natural commutation. The turn-off time is of the order of 50 to 100 μs. They are used for low-speed switching applications, also known as converter thyristor. 196

The dv/dt is about 1000 V/μs and the di/dt is 500 A/μs. The on-state voltage varies from 1.15 V for 600V to 2.5 V for 4000 V SCRs, and 1.25 V for 1200 V and 5500 A thyristor. The dv/dt is about 1000 V/μs and the di/dt is 500 A/μs. 197

BCTs It is a new concept for high power phase control. It combines advantages of having two antiparallel thyristor in one package. 198

The maximum voltage rating is as high as 6.5 kV at 1.8 kA. The maximum current rating is 3 kA at 1.8 kV. A BCT has two gates; one for turning on the forward current, and one for turning on the reverse current. This thyristor is turned on with a pulse current to one of its gates. It is turned off if the anode current falls below the holding current. 199

Fast-Switching Thyristors The thyristors are used in high-speed switching applications with forced commutation. They have fast turn-off time, in the range of 5 to 50 μs. The on-state forward voltage drop is inversely proportional to the turn-off time. 200

This type of thyristor is also known as an inverter thyristor. They have high dv/dt of 1000 V/μs and high di/dt of 1000 A/μs. The on-state forward voltage is about 1.7 V for a 2200 A, 1800 V SCR. 201

Light-Activated SCR This device is turned on by direct radiation of light on the silicon wafer. The gate structure is designed to provide sufficient gate sensitivity for triggering from practical light sources. The LASCRs are used in high-voltage and high-current applications such as HVDC and SVC. 202

LASCR offers complete electrical isolation between the light triggering source and the switching device of a power converter, which floats at a potential as high as a few hundred kV. The voltage rating can be as high as 4 kV at 1500 A with light triggering of less than 100 mW. The typical di/dt is 250 A/μs and the dv/dt could be as high as 2000 V/μs. 203

Bidirectional-Triode Thyristors A TRIAC can conduct in both directions.   It is normally used in ac phase control applications such as ac voltage controllers. It can be considered as two SCRs connected in antiparallel with a common gate connection. 204

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Since a TRIAC is a bidirectional device, its terminals cannot be designated as anode or cathode. If MT2 is positive with respect to MT1, the TRIAC can be turned on by applying a positive gate signal between G and MT1. If MT2 is negative with respect to MT1, the TRIAC can be turned on by applying a negative gate signal between G and MT1. 206

Reverse-Conducting Thyristors In many choppers and inverter circuits, an antiparallel diode is connected across an SCR. This is to allow the reverse current flow due to an inductive load. The diode clamps the reverse blocking voltage of the SCR to 1 or 2 V under the steady-state conditions. 207

It is also called an asymmetrical thyristor (ASCR). An RCT is a compromise between the device characteristics and circuit requirements. It is also called an asymmetrical thyristor (ASCR). The forward blocking is as high as 2000 V and the current rating goes up to 500 A. 208

Gate-Turn-Off Thyristors A gate-turn-off thyristor (GTO) like an SCR can be turned on by applying a positive gate signal. GTO can be turned off by a negative gate signal. GTOs have certain advantages over SCRs: Elimination of the commutating components Reduction of electromagnetic noise due to elimination of the commutation chokes 209

A large initial gate trigger pulse is required to turn on a GTO. Faster turn-off times Improved efficiency of the converters  A large initial gate trigger pulse is required to turn on a GTO. Once the GTO is turned on, forward gate current must be continued for the whole conduction period (1% of the turn on pulse). 210

A GTO requires a relatively high negative current pulse to turn off. It also has a higher on-state voltage of 3.4 V for a 550 A and 1200 V device. 211

FET-Controlled Thyristor A FET-CTH device combines MOSFET and a thyristor. 212

It has a high switching speed, high di/dt, and high dv/dt. If a sufficient voltage is applied to the gate of the MOSFET (3 V), a triggering current for the thyristor is generated. It has a high switching speed, high di/dt, and high dv/dt. This device can be turned on like conventional thyristors, but it cannot be turned off by gate control. 213

MOS Turn-Off Thyristor It is a combination of a GTO and a MOSFET. It overcomes limitations of the GTO turn-off ability. 214

The main drawback of a GTO is that they require a high pulse current drive circuit for the low impedance gate. The typical amplitude of the current for the gate circuit is about 35% of the main current. The MTO provides the same functionality as the GTO but uses a gate drive that needs to turn on a MOS transistor on and off. 215

It can handle 10kV and up to 4 kA. It can be used in power applications from 1 to 20 MVA. The MTO is turned on similar to conventional SCRs by applying a current pulse to its gate. To turn off the MTO, a gate pulse voltage is applied to the MOSFET gate. Turning on the MOSFET shorts the emitter-base of the npn transistor and turns the SCR off by stopping the latching process. 216

Emitter Turn-Off Thyristors ETO is a MOS-GTO hybrid device that combines the advantages of GTO and MOSFET. ETO has two gates: one normal gate for turn-on and one with a series MOSFET for turn-off. 217

ETO is turned on by applying a positive voltage to gates 1 and 2. High power ETOs with current and voltage ratings of 4 kA and 6 kV have been demonstrated. ETO is turned on by applying a positive voltage to gates 1 and 2. A negative voltage to the gate will turn off the device. 218

IGCT The internal structure and equivalent circuit of an IGCT are similar to that of GTO. It is turned on by applying a current to the gate. It is turned off by applying a fast rising and high gate current. 219

MOS-Controlled Thyristor A MOS-Controlled Thyristor (MCT) combines the features of a regenerative four-layer thyristor and a MOS-gate structure. 220

The gate structure can be represented by a p-channel MOSFET and an n-channel MOSFET. An MCT has: Low voltage drop during conduction Fast turn-on time (0.4 μs) and fast turn-off time (1.25 μs) Low switching losses Low reverse voltage blocking capability High gate input impedance 221

Static Induction Thyristor The characteristics of a SITH are similar to those of a MOSFET. A SITH is normally turned on by applying a positive gate voltage like normal thyristors, and is turned off by application of a negative voltage to its gate. 222

SITH has low on-state resistance or voltage drop. A SITH has fast switching speeds and high dv/dt and di/dt capabilities. The switching time is in the order of 1 to 6 μs. The voltage rating is as a high a 2500 V and the current rating is limited to 500 A. 223

Series Operation of Thyristors For high voltage applications, two or more thyristors can be connected in series. Due to manufacturing, the characteristics of thyristors of the same type are not identical. The following figure shows the off-state characteristics of two thyristors. 224

225

For the same off-state current, their off-state voltage differ. The voltage sharing is accomplished by connecting resistors across each thyristor. 226

For equal voltage sharing, the off-state currents differ as shown below. 227

228

Let there be ns thyristors in the string. The off-state current of thyristor T1 is ID1, and that of other thyristors are equal (if they share equal voltages) such that: ID2 = ID3 = IDn & ID1 < ID2 Since T1 has the least off-state current, T1 will share higher voltage (if they don’t share equal voltages). 229

The value of resistor R is calculated from: Where, will be maximum when is maximum or when is zero. 230

During the turn-off, the differences in stored charge cause differences in the reverse voltage sharing. 231

The thyristor with the least recovery charge, or reverse recovery time, will face the highest transient voltage. It will become necessary to use an RC network across each thyristor, and R limits the discharge current. The same RC network is used for both transient voltage sharing and dv/dt protection. 232

The transient voltage across the thyristor is obtained from: 233

A derating factor which is normally used to increase the reliability of the string is defined as: 234

LECTURE 19 (Ch. 7) THYRISTORS ECE 452 Power Electronics 235

Parallel Operation of Thyristors When thyristors are connected in parallel, the load current is not shared equally. The current unbalance increases the junction temperature of the SCR carrying the higher current and decreases its internal resistance. This increases its current sharing and may damage the thyristor. 236

A small resistance connected in series will force equal current sharing. 237

Magnetically coupled inductors can assure equal current sharing during the transient period. 238

di/dt Protection A thyristor requires a minimum time to spread the current conduction uniformly throughout the junction. If the rate-of-rise of anode current is very fast compared to the spreading velocity of a turn-on process, a hot spot will occur. 239

As a result of the excessive temperature, the device may fail. Therefore, in practical circuits the device must be protected against high di/dt. 240

Let us consider the following circuit. Dm will conduct when thyristor T1 is off. If T1 is fired when Dm is still conducting, di/dt can be very high. 241

In practice, the di/dt is limited by adding a series inductor Ls. Then: 242

dv/dt Protection If the switch S1 is closed at t = 0, a step voltage will be applied across the thyristor T1. The dv/dt may be high enough to turn on the device. 243

The dv/dt can be limited by connecting a capacitor Cs across T1. When the thyristor T1 is turned on, the discharge current of capacitor is limited by resistor Rs. 244

The circuit dv/dt can be found approximately from: The RC circuit is known as a snubber circuit, and the voltage across the thyristor will rise exponentially. The circuit dv/dt can be found approximately from: 245

The value of Rs is found from the discharge current ITD. The snubber circuit can be designed based on the known value of the dv/dt for a device.   The value of Rs is found from the discharge current ITD. 246

The load can form a series circuit with the snubber network as shown below. 247

We can show that the damping ratio of the second order circuit will be: where Ls is the stray inductance. 248

To limit the peak voltage overshoot across the T1, a damping ratio in the range of 0.5 to 1 is used. The L is typically high, and Rs should be high and Cs should be small to retain the desired value of damping ratio. A large value of Rs will reduce the discharge current, and a low value of Cs will reduce the snubber losses. 249