1. Power Switching Devices | Basic concepts of Power Electronics. State Merits and Demerits of Power Electro: Power electronics is the branch of electrical engineering that deals with the process and control of the flow of electric energy for supplying voltage and current for user load. Italso deals with the conversion of electric power by wave-shaping of voltage, current or both us- ing fast switching of power semiconductor device. The conversion technique requires the switch- ing-on and switching off of these devices. ‘These electronics circuit are used to generate control signal that control these power semiconduc tor devices. The result of measurement is expressed by a number representing the ratio of the un- known quantity to the adopted unit of measurement. Power electronics made up of three words power, electronics and control. Control deals with the steady-state and dynamic characteristics of closed-loop systems. Power deals with the static and rotating power equipment for the generation, transmission, and distribution of electric energy. Electronics deals with the solid-state devices and circuits for signal processing for desired control, Consumer’ Load Modulator, Reference Figure 1.1 Generalized Block Diagram of Power Electronics System The figure 1.1 shows the general block diagram of power electronic systems. The power input to the power modulator is usually from the electric energy source at the line frequency 50 Hz or 60 Hz, The phase angle between input voltage and current depend on the control of power modulator. The processed output is given to the consumer’s load. The feedback system compares the output of power modulator with the desired reference power and the error between these two is fed back to the power modulator asa control signal. In the power modulator, the control signal interacts with the input power and to minimize the er- ror. Thus, the proper power output is fed to the consumer's load. Systems Merits: Low power loss Low maintenance Small size and welght Fast dynamic response High efficiency High reliability Long life Lower cost Demerits: Tendency to generate harmonics Converters are operated at low Input power factor under certain operating conditions Power Electronics Controllers have low overload capacity Difficult to regenerate power in power electronic converter systems Hardik Lakhani, EE Department Power Electronics (3140915) 11. Power Switching Devices | Power Electronics Converters ‘Types of converters | Functions Commutation Applications Rectifiers Fixeda.c, voltage to fixed | Line D.C. Drives, Electric traction, (controlled or variable dc.voltage Power supply, HVDC system, and Battery charging, Lighting uncontrolled) systems etc, ‘AG Voltage Fixed a.c. voltage to vari-_| Line ‘AC. Drives, fan regulators, Controllers able ac. voltage Induction and resistance heating and control, static reactive power compensa- tion, power supplies, lamp dimmers etc. D.C. choppers Fixed dic, voltage to vari-_ | Forced/Load D.C, Drives, Electric traction, able dc, voltage regenerative drives, regulated dic, power supply, D.C. static switch etc. Inverters Fixed dc, voltage to fixed | Line/Forced/Load | A.C. Drives, electric traction, (controlled or variable a.c. voltage slip energy recovery, Power and and frequency supplies, HVDC transmission uncontrolled) and transformer ete. Cycloconverters Fixedfrequencyac.to | Line ‘AG. Drives, Blectrie traction, variable frequency a.c. rolling mills, heating conver- ters for furnace ete, Classification of Power Semiconductor Devices Power Semiconductor Devices can be classified in to three groups according to their degree of control- lability. The groupings are: (a) Diodes Diodes are uncontrolled rectifying devices. Their on and off state are controlled by power cire (b) Thyristors Turn-on controlled by gate signal and they remain latched in the on-state but must be turn-off by the power circuit, (SCR, LASCR, ASCR, TRIAG, GTO, MCT, IGCT, RCT, SITH) (© Controllable Switches ‘These are turn-on and off by gate signal. (BJT, MOSFET, IGBT, SIT) ‘+ The Power Semiconductor switching devices can also be classified under following basis: a) Uncontrolled Turn-on and Turn-off b) Controlled turn-on and uncontrolled turn-off ©} Controlled turn-on and turn-off ¢) Continuous gate signal requirement ©) Pulse gate requirement £) Bipolar voltage-withstanding capability g) Unipolar voltage-withstanding capability h) Bidirectional current capability 1) Unidirectional current capability Power Diodes Power Semiconductor Diodes are like low-power p-n junction diodes called signal diodes. But, for high- er power application power diodes are used. + The symbol of the Power diode is same as signal level diode. The power diode is a two terminal, two- layer, p-n semiconductor device. The two terminals of diode are called anode and cathode, Hardik Lakhani, EE Department Power Electronics (3140915)1. Power Switching Devices | Darshan + The voltage, current and power ratings of power diodes are much higher than the signal diodes, ‘Symbol Construction ; * 5 ca m joi Cathode \ 0 Ae s © 0 » A.) | |. Anade metalization Epitaxial iayer 1 (Neo) (Drift region) 1 (Nae) ‘Substrate (cathode) i bs Cathode metalization * Power diodes are more complex in structure and in operation than signal diodes. ‘+ This complexity arise due to low-power device are modified in order to make them sultable for high power application. Reverse Saturation Current (ls) Two important characteristic of power diode is: (i) V-I Characteristics (ii) Reverse Recovery Characteristics (@V-1 Characteristics + When anode is positive with respect to cathode, diode is said to be forward biased. Initially diode current is zero with increase of source Vs from zero value. + From Vs = 0 to cut-in voltage, forward diode current is very small. That cut-in voltage is known as threshold voltage. Beyond that threshold voltage, current rises rapidly and diode is said to be con- duct, When diode conduct, the forward voltage drops in order of 0.7 to 1Y. * When cathode is positive with respect to anode, diode is said to be reverse biased. In reverse bi- ased condition, a smalll reverse current called leakage current is flow that is in microampere or Mil- liampere. ‘+ The leakage current increase slowly with reverse voltage until breakdown reach, At this break- down voltage, diode is turned on in the reverse direction. If current become very high, then they destroyed the diode. The breakdown of diode is avoided by operating the diode below specified re- petitive reverse voltage VRRM. Characteristic of diode is shown in fig (ii) Reverse Recovery Characteristics * Whenever the diode is switched off the current decays from IF to zero and further continues in re verse direction owing to the charges stored in the space charge region and the semiconductor re- gion, + Thisreverse current attainsa peak IRR and again start approaching zero value and finally the diode is off after time trr. This time is defined as reverse recovery time and is defined as time between the Hardik Lakhani, EE Department Power Electronics (3140915)1. Power Switching Devices | Darshan instant forward current reaches zero and the instant the reverse current decays to 25% of IRR. AF ter this time the diode is said to attain its reverse blocking capability. Applications: Asa rectifier diode For voltage clamping Asavoltage multiplier Asa freewheeling diode Thyristor (or SCR - Silicon Controlled Rectifier) * Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. * This device is the solid-state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. ‘+ Infact, SCR (Silicon Controlled Rectifier) is a trade name given to the thyristor by General Electric Company. ‘+ The device has terminals Anode(A), Cathode(K) and the Gate(G). The Gate terminal(G) is attached to the p-layer nearer to the Cathode(K) terminal. + Basically, SCR is a three-terminal, four-layer semiconductor device consisting of alternate layers of p-type and n-type material. Hence it has three pn junctions J1, J2 and J3. The figure shows an SCR with the layers p-n-p-n. * Ithas three basic terminals, namely the anode, cathode and the gate mounted on the semiconductor layers of the device. * A thyristor is a four-layer, 3 junction p-n-p-n semiconductor device consisting of at least three p-n junctions, functioning as an electrical switch for high power operations. Symbol & Construction Cathode (C) Ge) Cathode a I" 4 Ls, » " No= 10 em i ‘i nm JI" ote d 4 , Ne=10%m?———p, posoum ato Anode (4) V-I Characteristic: * A detailed study of the characteristics reveal that the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. Reverse Blocking Mode: * Reverse Blocking Mode of SCR is that operational mode in which it offers high impedance for current flow and hence do not conduct. An SCR in reverse blocking mode behaves as if an open switch. Hence this mode is also known as OFF state of SCR. Hardik Lakhani, EE Department Power Electronics (3140915) 41. Power Switching Devices | * As clear from the V-I curve, the anode to cathode voltage in is negative in this mode. This means that anode terminal is made negative with respect to cathode. This leads to reverse biasing of the SCR. Consequently, junction J1 and J3 are reversed biased while the junction J2 is forward biased. ‘The device behaves as if two diodes are connected in series with reverse voltage applies across them. © Asmall leakage current of the order of mili or micro ampere flows thorough the SCR in this mode. If the reverse voltage is increased, then at some critical voltage an avalanche breakdown takes place at reverse biased junctions 1 and J3 which leads to sudden increase in reverse current, * This critical reverse voltage is called Reverse Breakdown Voltage. VBR represents this reverse breakdown voltage in the V-I characteristics. It can be seen that, there is a sharp increase in reverse current at this voltage. © This increased reverse current may result in more losses in the SCR which in turn may damage the SCR. Therefore the reverse voltage across the SCR terminals should not exceed reverse breakdown voltage during its operation, * When the reverse voltage is less than VBR, SCR offers high impedance in the reverse direction and hence do not conduct. This is the reason, in reverse blocking mode; an SCR may be treated as an ‘open switch. | Forward Conduction (on state) Latching current Holding current Jiao i$ VBo / Forked." Fonbard leakage Blocking Current Reverse leakage current Characterisites : \Vgo = Forward breakover voltage \Vpp = Reverse breakover voltage |= Gate Current Forward Blocking Mode: * Forward Blocking Mode is that operational mode of SCR in which it does not conduct even though it is forward biased. The term forward biased SCR implies that its anode terminal is positive with respect to cathode terminal, * In this mode, the junction J1 and J3 are forward biased but junction J2 is reverse biased. A small leakage current, called the forward leakage current as shown in the V-I characteristics of SCR in this mode. As the forward leakage current is small, SCR offers high impedance. Therefore an SCR can be treated as an open switch even in forward blocking mode. Forward Conduction Mode: * As we have seen that in Forward Blocking mode, even through the SCR is forward biased, it does not conduct. But the good thing is that, in forward blocking mode, junction J1 and J3 are forward bi- ased and J2 is reversed biased. This means, there are two possibilities for making SCR to conduct in this mode: * Increase the anode to cathode voltage to such an extent which leads to avalanche breakdown of the reverse biased junction J2. + Apply positive gate pulse between gate and cathode terminal. * When the forward biasing voltage is increased then at some critical voltage VBO, an avalanche breakdown take place at reverse biased junction J2. This critical voltage is known as Forward Brea kover Voltage. Hardik Lakhani, EE Department Power Electronics (3140915) 51. Power Switching Devices | @ Darshan * Since junction J1 and J3 are already forward biased, an avalanche breakdown at J2 will result in sudden increase in anode current in forward direction, Switching or ON OFF or Dynamic Characteristics of SCR or Thyristor %o Gate pulse Vand ig t + Turn ON time of SCR: A forward biased thyristor can be turned on by applying a positive voltage between gate and cathode terminal. But it takes some transition time to go from forward blocking mode to forward conduction mode. This transition time is called turn on time of SCR. + Delay time of SCR: Delay time of SCR can be defined as the time taken by the gate current to increase from 90% to 100% from its final value, * Rise time of SCR: Rise time of SCR in the time taken by the anode current to rise from 10% to 90% of its final value. * Spread time of SCR: It is the time taken by the anode current to rise from 90% to 100% ofits final value. It depends upon the cross-sectional area of cathode. + Turn OFF time of SCR: It can be defined as the interval between anode current falls to zero and device regains its forward blocking mode. On the basis of removing carrier charges from the four layers, turn off time of SCR can be divided into two time regions; Reverse recovery time an Gate recovery time. + Reverse recovery time: Itis the interval in which change carriers remove from J1 , and J3 junction. At time t1 , anode current falls to zero and it will continue to increase in reverse direction with same slope (di/dt) of the forward decreasing current, This negative current will help to sweep out the carrier charges from junction [1 and J3 . At the time t2 carrier charge density is not sufficient to maintain the reverse current hence after t2 this negative current will start to decrease. The value of current at t2 is called reverse recovery current. Due to rapid decreasing of anode current, a reverse spike of voltage may appear across the SCR, Total recovery time t3 ~ t1 is called reverse recovery time. After that, device will start to follow the applied reverse voltage and it gains the property to block the forward voltage. * Gate recovery time: After sweeping out the carrier charges from junction [1 and J3 during reverse recovery time, there remain trapped charges in J2 junction which prevent the SCR from blocking the forward voltage. These trapped charges can be removed by recombination only and the interval in which this recombination is done, called gate recovery time. + Latching current: This is the rating of current, below which the SCR can’t be turned on even the gate signal is applied. That means this much anode current must rise to turn on the device. The gate pulse must be continuous until anode current is greater or equal to latching current of thyristor otherwise the device will fail to be turned on. + Holding current: This is the rating of current, below which anode current must fall to turn off the device. Hardik Lakhani, EE Department Power Electronics (3140915) 61. Power Switching Devices | @ Darshan * Gate triggering current: This is the value of gate current below which device cannot be turned on. This value of current specified ata forward break down voltage. + Gate triggering voltage: This is the value of minimum gate voltage that must be a acquired by the gate circuit. for proper turn on of the SCR. This voltage value is also specified at a particular. * Anode voltage rating: This rating gives us a brief idea about withstanding power of a thyristor in forward blocking made in the absence of gate current. + Peak working forwarding blocking or forward OFF state voltage: It specifies the maximum forward voltage (positive voltage that applied across anode and cathode) that can be withstand by the SCR at the time of working. + Peak repetitive forward blocking voltage: It specifies the peak forward transient voltage that a SCR can block repeatedly or periodically in forward blocking mode. This rating is specified at a maximum allowable junction temperature with gate circuit open. + Peak non-repetitive or surge forward blocking voltage: It is the peak value of the forward tran- sient voltage that does not appear periodically. This type of over voltage generated at the time of switching operation of circuit breaker. + Peak working reverse voltage: It is the maximum reverse voltage (anode is negative with respect to cathode) which can be withstand by the thyristor repeatedly or periodically. It is nothing but peak negative value of the AC sinusoidal voltage. + Peak repetitive reverse voltage: It is the value of transient voltage that can be withstand by SCR in reverse bias at maximum allowable temperature, + Peak non-repetitive reverse voltage: It implies the reverse transient voltage that does not ap- pear repetitively. Though this voltage value is 130% of VBR, it lies under reverse break over vol: tage, VBR. + Forward on-state voltage drop: This is the voltage drop across the anode and cathode when rated current flows through the SCR at rated junction temperature, Generally this value is lie between 1 to 15 volts, + Finger voltage: Minimum value of voltage which must be applied between anode and cathode for turning off the device by gate triggering. Generally this voltage value is little more than normal ON state voltage drop. MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Symbol Construction J wd MOSFET: N-Channel MOSFET: P-Channel Enhancement Type Enhancement Type Diffused Channel P-Type Substrate MOSFET: N-Channel MOSFET: P-Channel Depletion Type Depletion Type: Structure of N-channel MOSFET ‘+ The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices. + The MOSFET is a four-terminal device with source(S), gate (G), drain (D) and body (B) terminals. ‘The body of the MOSFET is frequently connected to the source terminal so making it a three- terminal device like field effect transistor. The MOSFET is very far the most common transistor and can be used in both analog and digital circuits. Hardik Lakhani, EE Department Power Electronics (3140915) z1. Power Switching Devices | ‘The working of MOSFET depends upon the MOS capacitor. The semiconductor surface at the below oxide layer which is located between source and drain terminal. It can be inverted from ptype to n-type by applying a positive or negative gate voltage respectively. When we apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward with the substrate. The depletion region populated by the bound negative charges which are associated with the acceptor atoms. The electrons reach channel is formed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel, Instead of positive voltage if we apply negative voltage, a hole channel will be formed under the oxide layer. P-Channel MOSFET: The P- Channel MOSFET has a P- Channel region between source and drain. It isa four terminal device such as gate, drain, source, body. ‘The drain and source are heavily doped p+ region and the body or substrate is n-type. The flow of current is positively charged holes. When we apply the negative gate voltage, the electrons present under the oxide layer with are pushed downward into the substrate with a repulsive force. The depletion region populated by the bound positive charges which are associated with the donor atoms. The negative gate voltage also attracts holes from p+ source and drain region into the channel region. N-Channel MOSFET: The N-Channel MOSFET has a N- channel region between source and drain It is a four terminal device such as gate, drain , source , body. This type of MOSFET the drain and source are heavily doped n+ region and the substrate or body is P- type. The current flows due to the negatively charged electrons. When we apply the positive gate voltage the holes present under the oxide layer pushed downward into the substrate with a repulsive force. The depletion region is, populated by the bound negative charges which are associated with the acceptor atoms. The elec- trons reach channel is formed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if voltage is applied between drain and source the current flows freely between the source and drain and the gate voltage controls the electrons in the chan- nel. Instead of positive voltage if we apply negative voltage a hole channel will be formed under the oxide layer. Characteristic: f Vos, _ CubOF Region Vest Vos decreases i.e. Vos < Vr Vose Vass Locus‘6t Pinch-Off Voltage ® ©) p-channel depletion type MOSFET (a) Transfer characteristics (b) Output characteristics ‘The MOSFET works by electronically varying the width of a channel along which charge carriers flow (electrons or holes). The charge carriers enter the channel at source and exit via the drain. The width of the channel is controlled by the voltage on an electrode is called gate which is located between source and drain. It is insulated from the channel near an extremely thin layer of metal oxide. ‘The MOSFET can function in two ways: Depletion mode and Enhancement mode. In depletion mode, when there is no voltage on the gate, the channel shows its maximum. conductance, As the voltage on the gate is either positive or negative, the channel conducti ecreases. Hardik Lakhani, EE Department Power Electronics (3140915) 8Darshan 1. Power Switching Devices ‘+ Inenhancement mode, when there is no voltage on the gate the device does not conduct. More is the voltage on the gate, the better the device can conduct, Applications Itis used asa switch. IGBT (Insulated Gate Bipolar Transistor) Symbol Construction Collector 0 Saal Emitter ‘+ IGBT has PMOSFET like input characteristics and Power BJT like output characteristics and hence symbol is also an amalgamation of the symbols of the two parent devices. The three terminals of IGBT are Gate, Collector and Emitter. IGBT is known by various other names also, such as- Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), Conductively Modulated Field Effect Transistor (COMFET), Insulated Gate Transistor (IGT) ‘* The structure of IGBT is very much similar to that of PMOSFET, except one layer known as injection layer which is p unlike n substrate in PMOSFET. This injection layer is the key to the superior characteristics of IGBT. Other layers are called the drift and the body region. The two junctions are labeled J1 and J2 . Figure above shows the structure of n-channel IGBT. Characteristic: k ‘Active region Saturation region (On region) Cut-off (forward blocking) region IGBT has a combination of BJT and MOSFET characteri Gate behavior like MOSFET - easy to turn on and off. Low losses like BJT due to low on-state Collector-Emitter voltage VCE = (2-3V).. Ratings: Voltage: VCE < 6000V, Current 2500A currently available. Good switching capability (up to 100 kttz) for newer devices. ‘ics. Compromises include: Hardik Lakhani, EE Department Power Electronics (3140915) 91. Power Switching Devices @ Darshan + Typical application, IGBT is used at 20-50 kz * For very high-power devices and applications, frequency is limited to several kHz, * Very popular in new products; practically replacing BJT in most new applications. * “Snubber less” operation is possible. Most new IGBTs do not require snubber Advantages © Lower gate drive requirements © Low switching losses * Small snubber circuit requirements + High inputimpedance * Voltage controlled device * Enhanced conduction due to bipolar nature * Better safe operating area Bidirectional Switches DIAC and TRIAC 1, DIAC —Op— ‘ NS : * The DIAC is basically a two-terminal parallel-inverse combination of semiconductor layers that permits triggering in either direction, There is no control terminal on this device. Physically it looks like a regular semiconductor diode. + The DIAC is a two-way breakover triggering device, Note that neither terminal is referred to as the cathode. Instead, there is an anode 1 (or electrode 1) and an anode 2 (or electrode 2). When anode 1 is positive with respect to anode 2, the semiconductor layers of interest are pIn2p2 and n3. For anode 2 positive with respect to anode 1, the applicable layers are p2n2p1 and ni, Characteristic: Blocking State Negative Half-cyele * As the voltage is increased form zero in either direction, a small amount of leakage current occurs, as shown in characteristics curve. «When breakover voltage is reached in either direction, DIAC starts to conduct Hardik Lakhani, EE Department Power Electronics (3140915) 101. Power Switching Devices @ Darshan © The device is rated by voltage and current. © When both forward and reverse break-over voltages are almost same value, itis known as symmetrical DIAC. Applications: * Itcan be used in an oscillator circuit. It can be used in an oscillator circuit. * Itis low power triggering device. 2, TRIAC * Triacisa three terminal AC switch which is different from the other silicon controlled rectifiers in the sense that it can conduct in both the directions that is whether the applied gate signal is positive or negative, it will conduct. Thus, this device can be used for AC systems as a switch. «The TRIACis a two-way SCR with one gate terminal. ‘Symbol Construction mara mT2 MTt | 6 © Thisisa three terminal, four-layer, bi-directional semiconductor device that controls AC power. * ADIAC-TRIAC combined package is called Quadrac. * Two SCRs are connected in inverse parallel with gate terminal as common. Gate terminal is connected to both the N and P regions due to which gate signal may be applied which is irrespective of the polarity of the signal. Hence, the device is bilateral. Characteristic: SCR but it isapplicable to both positive and negative triac voltages. * It can be triggered with positive or negative polarity of gate pulses. It requires only a single heat sink of slightly larger size, whereas for SCR, two heat sinks should be required of smaller size. * When the device gets turned on, a heavy current flows through which may damage the device, hence in order to limit the current, a current limiting resistor should be connected externally to it. By applying proper gate signal, firing angle of the device may be controlled. The gate triggering circuits should be used for proper gate triggering. Hardik Lakhani, EE Department Power Electronics (3140915) W1. Power Switching Devices @ Darshan Itrequires single fuse for protection. They are not much reliable compared to SCR. Applications: They are used in control circuits. Itis used in High power lamp switching. It is used in AC power control. Two transistor analogy of SCR * Itisa multi-layer semiconductor device, hence the “silicon” part of its name. It requires a gate sig- nal to turn it “ON’, the “controlled” part of the name and once “ON" it behaves like a rectifying di- ode, the “rectifier” part of the name. In fact, the circuit symbol for the SCR suggests that this device acts like a controlled rectifying diode. A A le ° A 2 ee ie Pale) © I, D | I @ o , © Two-transistor analogy of SCR (a) schematic diagram; (b) and (c) two-transistor model * The principle of thyristor operation can be explained with the use of two-transistor model. + Asshown in Fig. Junctions J1 - J2 and J2 - J3 can be considered to constitute PNP and NPN trans tors separately. © From Fig. (c), in the off-state ofa transistor, collector current IC is related to emitter current IE as Ie = ale + leno «Where «ris the common-base current gain and ICRO is the common-base leakage current of collec- tor-base junction of transistor + For transistor Ql in Fig. 2.3¢, «Emitter current le = anode current la and Ie = collector current le: * Therefore, for Q1, Ter = Gla + Tenor ~ @) Where, ay = common — base current gain of Q, and [e901 = common — base leakage current of Qy © Similarly, for transistor Q2 Teg =e Honor soxacicccaree (2) Where, , = common — base current gain of Qz Icgor = comtmon — base leakage current of Qz and I, = emitter current of Q) «The sum of two collector currents given by equations (1) and (2), is equal to the external circuit current La entering at anode terminal A, Ta = Ter + Hep sssneesnnnnsee (3) Te = 81 Tea + Levon +2 he + Lep02 wrens (4) ‘© When gate current is applied, then J, = 1, + [,, By substituting value of J, in equation (4), Hardik Lakhani, EE Department Power Electronics (3140915) 121. Power Switching Devices | @ Darshan Ta = %1 ta + Iepor +%2 (la + Ip) + Iewo2 Scalp tlenov4len02 ale Tt) “@) + For silicon transistor, current gain a is very low at low emitter current, With an increase in emitter current, «build-up rapidly, © With gate current J, = 0, and with thyristor forward biased, (c+) is very low and forward lea- kage current somewhat more than (Icio1 + Ieno2) flows. + If by some means, the emitter current of two component transistors can be increased so that («+2%,) approaches unity, then [, would tend to become infinity thereby turning-on the device, + Actually, external load limits the anode current to a safe value after the thyristor begins conduc tion, The methods of turning-on a thyristor, in fact, are the methods of making (=«,+0) to ap- proach unity, Thyristor Turn-On Methods + With anode positive w.rt. cathode, a thyristor can be turned on by any of following methods: a) Forward voltage triggering b) Gate triggering ©) av/at triggering d) Temperature triggering ¢) Light triggering + These methods of turning-on a thyristor are discussed below Forward voltage triggering * At forward breakdown voltage, thyristor changes from off-state to on-state characterized by low voltage across thyristor with large forward current. As other junctions (J1, J3) are already forward biased, breakdown of junction J2 allows free movement of carries across three junctions and as a result, large forward anode-current flows. As stated before, this forward current is limited by the load impedance. + In practice, the transition from off-state to on-state obtained by exceeding Vpo is never em- ployed as it may destroy the device. * The magnitudes of forward and reverse breakdown voltages are nearly the same and both are tem perature dependent. In practice, it is found that Van is slightly more than Vso. Therefore, for- ward breakover voltage is taken as the final voltage rating f the device during the design of SCR applications. * After the avalanche breakdown, junction J2 looses its reverse blocking capability. Therefore, if the anode voltage is reduced below Vio, SCR will continue conduction of the current. The SCR can now be turned off only by reducing the anode current below a certain value called holding current. Gate triggering © Turning on of thyristors by gate triggering is simple, reliable and efficient, it is therefore the most usual method of firing the forward biased SCRs. © When turn-on of a thyristor is required, a positive gate voltage between gate and cathode is ap- plied. With gate current established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate current. Higher the gate current, lower is the forward brea- kover voltage. * When positive gate current is applied, gate P layer is flooded with electrons from the cathode, This is because cathode N layer is heavily doped as compared to gate P layer. As thyristor is forward bi- ased, some of these electrons reach junctions J2. As a result, width of depletion layer around june- tion J2 is reduced. This causes the junction J2 to breakdown at an applied voltage lower than for- ward breakover voltage Vso. If magnitude of gate current is increased, more electrons will reach junction J2, as a consequence thyristor will get turned on at a much lower forward applied voltage. + If gate current is reduced to zero before the rising anode current attains a value, called the latching current. The gate pulse width should be chosen to ensure that anode current rises above the latch- Hardik Lakhani, EE Department Power Electronics (3140915) 13@ Darshan 1. Power Switching Devices ing current. Thus, latching current may be defined as the value of anode current which it must at- tain during turn-on process to maintain conduction when gate signal is removed. * Once the thyristor is conducting, gate loses control. The thyristor can be turned-off only if the for- ward current falls below a low-level current called the holding current, Thus holding current may be defined as the minimum value of anode current below which it must fall for turning-off the thy- ristor. The latching current is higher than the holding current, * Note that latching current is associated with turn-on process and holding current with turn-off process, It is usual to take latching current as two to three times the holding current. In industrial applications, holding current is almost taken as zero, dv/dt triggering © The dv/dt Triggering is the technique in which SCR is turned ON by changing the forward bias vol- tage with respect to time. dv/dt itself means rate of change of voltage w.r.t time. * swe have discussed earlier in this post, junction J2 is reversed biased in a forward blocking mode of SCR. A reversed biased junction may be treated asa capacitor due to presence of space charges in the vicinity of reversed biased junction. Let us assume its capacitance to be ‘C’ farad, The charge on capacitor, voltage across the capacitor and capacitance are related as below: Qa differentiating both sides wat time, we get, dQ av e () but current / = “2 ar av '=¢(@) * Thus, the current through the reversed biased junction J2 is directly proportional to (dv/at). There- fore, if the rate of rise of forward voltage ic. (dv/dt) is high, the charging current I will also be high. ‘This charging current acts like gate current and turns ON the SCR or thyristor even though the gate current is zero. If should be noted that, itis rate of rise of voltage which is responsible for turning the SCR ON. It is independent of magnitude of voltage. The voltage may be low, but the rate of its rise should be high enough to turn SCR ON. Temperature triggering * During forward blocking mode, most of the applied voltage appears across reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the junction tempera- ture, * With increase in temperature, leakage current through junction J2 further increases. This cumula- tive process may turn on the SCR at some high temperature, Light triggering + For light triggered SCRs, a recess is made in the inner p-layer. When this recess is irradiated, free charge carriers are generated, The pulse of light of appropriate wavelength is guided by optical fi- bres for irradiation. If the intensity of this light thrown on the recess exceeds a certain value, for- ward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR), * The LASCRshave been used in high-voltage dc. transmission systems. Firing Circuits or Gate Triggering Circuits for Thyristors ‘+ An SCR can be switched from off-state to on-state in several ways, ‘The gate triggering is the most common method of turning on the SCRs, because this method lends itself accurately for turning on the SCR at the desired instant of time. © The gate triggering is an efficient and reliable method. Main features of firing circuit ‘+ The most common method for controlling the onset of conduction in an SCR by means of gate voltage control. The gate control circuit is also called firing, or triggering circuit. Hardik Lakhani, EE Department Power Electronics (3140915) 41. Power Switching Devices | + These gating circuits are usually low-power electronic circuits. A firing circuit should fulfil the following two functions: Shieides ase’~ JU pam vs Ac Oe Pulse Pulse L geusioime” é oat generator amplitier EE R oo = )Pulse = transtormer DC power supply use SCR = transtormer NY Controt ‘circuit Driver circuit Power circuit A general layout of firing circuit of SCRs ‘+ If power circuit has more than one SCR, the firing circuit should produce gating pulses for each SCR at the desired instant for proper operation of the power circuit. eg. in a single-phase semi- converter using two SCRs, the triggering circuit must produce one firing pulse in each half cycle and in a three-phase full converter using six SCRs, gating circuit must produce one trigger pulse af- ter every 60° interval. ‘© The control signal generated by a firing circuit may not be able to turn-on an SCR. It is therefore common to feed the voltage pulses to a driver circuit and then to gate-cathode circuit. A driver cir- cuit consists of a pulse amplifier and a pulse transformer. ‘* Delay Angle or Firing Angle * “The delay angle or firing angle is the value of ‘wt’ at which a thyristor is turned on”, It is denoted by aand measured w.rt. the zero-crossing instant of the ac supply voltage. * The thyristor can be switched on by > aslow rising rectified ac signal > asharp single pulse > aconstant magnitude de signal > atrain of high-frequency pulses ‘+ The thyristor switches on as soon as applied gate voltage exceeds the critical gate triggering vol- tage level, at the switching angle a. The ideal switching signal for a thyristor should have an ade- quate amplitude of current for sufficient duration with a short rise time. * In short, a thyristor conducts when it is properly biased and the trigger source of a trigger circuit supplies the required minimum gate voltage and gate current. Resistance Triggering Circuit (R-triggering circuit or R-D triggering circuit) + Itoffers simple and economical firing circuit. * It suffers from a limited range of firing angle control (0° to 90°), great dependence on temperature and difference in performance between individual SCRs. * Figure shows the most basic resistance triggering circuit. R2 is the variable resistance, R is the sta- bilizing resistance. * In case R2 is zero, gate current may flow from source, through load, R1, D and gate to cathode. This current should not exceed maximum permissible gate current gm. © Rican be found from the relation, Von \, FS lym or Ry = 1 Ri =], lym Where Vis the maximum value of source voltage Hardik Lakhani, EE Department Power Electronics (3140915) 151. Power Switching Devices @ Darshan * Hence, the function of R1, is to limit the gate current to a safe value as R2 is varied. * Resistance R should have such a value that maximum voltage drop across it does not exceed maxi: mum possible gate voltage Vgm. = | | Yes Vinsinwt a<90" + Asresistances R1, R2 are large, gate trigger circuit draws small current. * Diode D allows the flow of current during positive half cycle only. «As the firing angle control is from 0° to 90°, the half wave power output can be controlled from 100% (for a= 0°) down to 50% (for a = 90°). RC half wave triggering circuit ‘Vo: oa} R D2 Or Mn sin wt oF Ss c (nee) + The limited range of firing angle control by resistance firing circuit can be overcome by RC firing circuit, * As shown in Fig, by varying the value of R, firing angle can be controlled from 0° to 180°. In the negative half cycle, capacitor C charges through D2 with lower plate positive to the peak supply Hardik Lakhani, EE Department ] Power Electronics (3140915) 161. Power Switching Devices | voltage Vm at wt = -90°, + After wt = -90°, source voltage vs, decreases from -Vm at wt = -90° to zero at wt = 0°. During this period, capacitor voltage ve, may fall from -Vm at wt = -90° to some lower value -oa at wt = 0° as shown in Fig. * As the SCR anode voltage passes through zero and becomes positive, C begins to charge through variable resistance R from the initial voltage -oa, When capacitor charges to positive voltage equal to gate trigger voltage Vgt, SCR is fired and after this, capacitor holds to a small positive voltage. * Diode D1 is used to prevent the breakdown of cathode to gate junction through D2 during the nega- tive half cycle. * Inthe range of power frequencies, 1 7 = Period of ac line Frequency in seconds RC full wave triggering circuit + Asimple RC trigger circuit giving full wave output voltage is as shown in Fig. * Diodes D1-D4 form a full wave diode bridge, In this circuit, the initial voltage from which the capa- itor G charges is almost zero, The capacitor C is set to this low positive voltage by the clamping ac tion of SCR gate. © When capacitor charges to a voltage equal to Vgt, SCR triggers and rectified voltage vd appears across load as v0. The value of RC is calculated by the relation, RO> soe Br 2 o The value of Ris given by R < TRIAC firing circuit by using DIAC * In Fig. below R is variable, whereas R1 is fixed. When R is zero, R1 protects the diac and triac gate from full supply voltage. * RZ limits the current when diac turns on. The value of C and Rare so selected as to give a firing an- gle range of nearly 0° and 180°. ‘+ In practice, a triggering angle range of 10° to 170* is only possible by the firing circuit. Hardik Lakhani, EE Department Power Electronics (3140915) 71. Power Switching Devices * Variable R controls the charging time of capacitor Cand hence the firing angle of the triac. * When R is small, the charging time constant equal to (R1+R)C, is small. Therefore, source voltage charges capacitor C to diac trigger voltage earlier and firing angle for triac is small. Likewise when Ris high, firing angle of triac is large. Vo ‘* Asynchronized UJT trigger circuit is as shown in Fig. Diodes D1-D4, rectify ac to de. R1 lowers Vdc toa suitable value for the Zener diode and UJT. Zener diode Z functions to clip the rectified voltage to a standard level Vz. ‘This voltage Vz, is applied to the charging circuit RC. Currentil, charges capacitor C ata rate determined by R. Voltage across capacitor is marked by ve. When voltage vc reaches the unijunction threshold voltage, the E-B1 junction of UJT breaks down and the capacitor C discharges through primary of pulse transformer sending a current i2 as shown in Fig. As the current i2 is in the form of pulse, windings of the pulse transformer have pulse vol- tages at their secondary terminals. Hardik Lakhani, EE Department Power Electronics (3140915) 181. Power Switching Devices | © Darshan © Pulses of the two secondary windings feed the same in-phase pulse to two SCRs of afull-wave cir cuit. SCR with positive anode voltage would turn on. ‘+ The firing angle can be controlled up to about 150°, This method of controlling the output power by varying charging resistor R is called ramp control, open-loop control or manual control. Ramp and Pedestal triggering sion of synchronized UJT triggering. + Fig. shows the ramp and pedestal triggering cir- cuit of two SCRs connected in antiparallel for con- trolling power in an ac load. © This trigger circuit can also be used for triggering “z.4 the thyristors in a single-phase semi converter or “* a single-phase full converter. ‘+ The various waveforms are shown in Fig. + Ramp and pedestal triggering is an improved ver- Pulse Transformer in Firing Circuit ro voltage RL 1 t 12 | Lt tree transformer 6 t e — Hardik Lakhani, EE Department ] Power Electronics (3140915) 19Darshan 1. Power Switching Devices * Pulse transformers are used quite often in firing circuits for SCRs and GTOs, © This transformer has usually two secondaries. The turn ratio from primary to two secondaries is or 1:1:1, * These transformers are designed to have low winding resistance, low leakage reactance and low inter-winding capacitance. «The advantages of using pulse transformers in triggering semiconductor devices are: > the isolation of low-voltage gate circuit from high-voltage anode circuit > the triggering of two or more devices from the same trigger source * Asquare pulse at the primary terminals of a pulse transformer may be transmitted at its sec- ondary terminals as a square wave or it may be transmitted as a derivative of the input wave- form. + Asshown in Fig. the advantages of this arrangement are: > There need not be a variable strength pulse generator since the pulses may be of the same amplitude and strength of the generated pulses may be increased simply by vary- ing the dc bias voltage. > The operation of the circuit becomes independent of the pulse characteristics since the only role the pulse plays is to turn-on or turn-off the transistor. Therefore, there is no effect of pulse distortion on the working of the circuit. * In practices exponentially decaying pulses as showm in Fig. are preferred due to the following reasons: ¥ This pulse waveform is suitable for injecting a large charge in the gate circuit for reliable turn on. > The duration of this pulse is small; therefore, no significant heating of the gate circuit is observed, > The size of the pulse transformer is reduced. Cosine Firing Scheme synch. v, Transf. vy 2 TCompara-|_V3_| Clock pulse stor 1 generator | Ps F compara-|_ve_|ciock puise| aa ‘er J+ tor 2 Jgenerator2 tEem Ee va -Eem’ © Cosine firing scheme for thyristors in single phase converters is shown in Fig. + The synchronizing transformer steps down the supply voltage to an appropriate level, + The input to this transformer is taken from the same source from which converter circuit is ener- gized. + The output voltage v1 of synchronizing transformer is integrated to get cosine wave v2. +The de control voltage Ec varies from maximum positive Ecm to maximum negative Fem, so that Hardik Lakhani, EE Department Power Electronics (3140915) 201. Power Switching Devices | @ Darshan firing angle can be varied from 0 to 180°. * The cosine wave v2 is compared in comparators 1 and 2 with Ec and -Ec. + When Ec is high as compared to v2, output voltage v3 is available from comparator 1. Same is true for comparator 2.S0 the comparators 1 and 2 give output pulses v3 and v4 respectively. + Itis seen that firing angle is governed by the intersection of v2 and Ec. + When Ec is maximum, firing angle is zero, Thus, Vm COS & = Ey = cor ( Fe) . a= cor? (E) where Vom, = maximum value of cosine signal v, * Fora single-phase full converter, average output voltage is V, =—*cosa * This shows that cosine firing scheme provides a linear transfer characteristic between the av- erage output voltage ¥, and the control voltage Ec. This scheme, on account of its linear trans- fer characteristic, improves the closed-loop response of the converter system. This feature has made the cosine firing scheme quite popular in industrial applications. Gate drive circuit for MOSFET ‘+ MOSFET, being a voltage controlled device, does not require a continuous gate current to keep it in the ON state, + However, it is required to charge and discharge the gate-source and the gate-drain capacitors in cach switching operation. + The switching times of a MOSFET essentially depends on the charging and discharging rate of these capacitors. * Therefore, if fast charging and discharging of a MOSFET is desired at fast switching frequency the gate drive power requirement may become significant Voo Vp Logic level gate pulse (a) (0) + To turn the MOSFET on the logic level input to the inverting buffer is set to high state so that tran- sistor Q3 turns off and QI turns on, ‘+ The top circuit of Fig (b) shows the equivalent circuit during turn on. Note that, during turn on QL remains in the active region. The effective gate resistance is RG + R1 / (81 + 1). Where, B1 is the de current gain of QI + To turn off the MOSFET the logic level input is set to low state. Q3 and QZ turns on whole Q1 turns off. The corresponding equivalent circuit is given by the bottom circuit of Fig. (b) + The switching time of the MOSFET can be adjusted by choosing a proper value of RG. Reducing RG will incase the switching speed of the MOSFET, However, caution should be exercised while i creasing the switching speed of the MOSFET in order not to turn on the parasitic BJT in the MO: FET structure inadvertently. Hardik Lakhani, EE Department Power Electronics (3140915) 211. Power Switching Devices | © Darshan Gate drive circuit for IGBT ~ Totem pole rene Nes Stpliier ps R Bel Ct" im 3-3 Ve ToIGBT To1cat T Gute oT Gate E E Tum on equivalent circuit Tum off equivalent circuit &) * The logic level gate drive signal is first opto-isolated and fed to a level shifting comparator. This stage converts the unipolar (usually positive) out put voltage of the opto-isolator to a bipolar (£Vgg) signal compatible to the IGBT gate drive levels. ‘+ The output of the comparator feeds a totem pole output amplifier stage which drives the IGBT. The equivalent circuit of the gate drive during turn on and off are shown in Fig (b). ‘* IF Vec| >|Vgg| then both Qi and Q2 will operate in the active region and reasonably constant value of B1 & B2 of these two transistors can be used for analysis purpose. ‘+ These equivalent circuits along with the model of the IGBT input MOSFET can be used to analyze the switching performance of the device. Conversely, for a desired switching performance a suita- ble gate drive circuit can be designed * The gate drive circuit of an IGBT should ensure fast and reliable switching of the device. In particular, it should; Apply maximum pemnissible VgE during ON period. Apply a negative voltage during off period. Control dig'dt during turn ON and tum off to avoid excessive Electro magnetic interference EMD. © Control dvee/dt during swi © Minimize switching loss. © Provide protection against short cirouit fault. to avoid IGBT latch up. Comparison among SCR, IGBT and MOSFET, Characteristic TThyrstor ier MOSFET ‘Symbol # a A Gate Control Variable | Current Voltage Vorage Control of Gate or Base rene Gate has control Gate has control Drive Power Minimal Minimal On State Voltage Drop <2Vv Medium (3.3V) High (4-10V) Voltage Range (5kv) (2-3.5kV) {akv) Current Range aka (500A 244) (1504) Power Range S6MVA ‘AMvA 100kvA Switching Losses Medium Low Very Low Hardik Lakhani, EE Department Power Electronics (3140915) 221. Power Switching Devices | D Darshan Characteristic “Thynistor iat ‘MOSFET ‘Conduction Losses Low High High Upper Frequency (Hz) ‘400 —10k 50k aM ‘Switching Frequency (He) | 100-1k 160k 1G. ‘Switching Time (us) 20 0s 7 ‘On Sstate Resistance (Ohm) _| 0.25m—2.24m ‘com os Gate Drive Circuitry ‘simple Simple. Snubber Unpolarizad Non-essential Non-assential ‘Temperature Coefficient | Negative Fi Porte @ High Carer) Positive Cost Reliability High igh (Uiited in Thermal Efficiency “Applications ‘AC TO DC converters, AC | DC to AC converters, AC Motor | DC Choppers, Low Power UPS, Voltage Controllers, Drives, UPS, Choppers, SMPS | SMPS, Brushless DC Motor Electronic Circurt Drives Breakers Hardik Lakhani, BE Departinent Power Blectronies (3140915) 23