UNIVERSITY OF TECHNOLOGY

Transcription

1 UNIVERSITY OF TECHNOLOGY Third Year DEPARTMENT OF ELECTRICAL ENGINEERING Electronics Engineering Section AC Machine and Power Electronics Module-II: Power Electronics: Power electronics devices and characteristics Controlled and uncontrolled rectifier Inverters DC-DC converters Textbooks: 1. H.C. Gerhard Henneberger, Electrical Machines. 2. M. Rashid, Power electronic handbook.

2 1.1 INTRODUCTION TO POWER ELECTRONICS Power electronics is an enabling technology, providing the needed interface between the electrical source and the electrical load, as depicted in Fig. 1. The electrical source and the electrical load can, and often do, differ in frequency, voltage amplitudes and the number of phases. The power electronics interface facilitates the transfer of power from the source to the load by converting voltages and currents from one form to another, in which it is possible for the source and load to reverse roles. The controller shown in Fig.1 allows management of the power transfer process in which the conversion of voltages and currents should be achieved with as high energy-efficiency and high power density as possible. Adjustable-speed electric drives represent an important application of power electronics. Fig.(1) 1

3 1.2 APPLICATIONS OF POWER ELECTRONICS: Some Applications of Power Electronics: 1. Aerospace: Space shuttle power supplies, satellite power supplies, aircraft power systems. 2. Commercial: Advertising, heating, air-conditioning, central refrigeration, computer and uninterruptible power supplies, elevators. 3. Industrial: Arc and industrial furnaces, blowers and fans, pumps and compressors, industrial lasers, transformer-tap 'changers, rolling mills, welding. 4. Residential :Air-conditioning, cooking, lighting, space heating, refrigerators, electric-door openers, dryers, fans, personal computers, vacuum cleaners, washing and sewing machines, food mixers. 5. Telecommunication: Battery chargers, power supplies ( dc and UPS ). 6. Transportation: Battery chargers, traction control of electric vehicles, electric locomotives, street cars~ trolley buses subways, automotive electronics. 2

4 7.Utility systems: High voltage dc transmission (HVDC), excitation systems, VAR compensation, static circuit breakers, fans and boiler-feed pumps, supplementary energy system (solar, wind) ADVANTAGES AND DISADVANTAGES OF POWER ELECTRONIC CONVERTERS High efficiency due to low loss in power-semiconductor devices. High reliability of power-electronic converter systems. Long life and less maintenance due to the absence of any moving parts. Fast dynamic response of the power-electronic systems as compared to electromechanical converter systems. Small size and less weight result in less floor space and therefore lower installation cost. Mass production of power-semiconductor devices has resulted in lower cost of the, converter equipment Systems based on power electronics. 3

5 However, suffer from the following disadvantages; Power-electronic converter circuits have a tendency to generate harmonics in the supply system, as well as in the load circuit. In the load circuit, the performance of the load is influenced, for example, a high harmonic content in the load circuit causes commutation problems in dc machines, increased motor heating and more acoustical noise in both dc and ac machines. So steps must be taken to filter these out from the output side of a converter. In the supply system, the harmonics distort the voltage waveform and seriously influence the performance of other equipment connected to the same supply line. In addition, the harmonics in the supply line can also cause interference with communication lines. It is, therefore necessary to insert filters on the input side of a converter. Ac to dc and ac to ac converters operate at a low input power factor under certain operating conditions. In order to avoid a low p.f, some special measures have to be adopted. 4

6 Power-electronic controllers have low overload capacity. These converters must, therefore, be rated for taking momentary overloads. As such, cost of power electronic controller may increase. The advantages possessed by power electronic converters far outweigh their disadvantages mentioned above. As a consequence, semiconductor-based converters are being extensively employed in systems, where power flow is to be regulated. As already stated, conventional power controllers used in many installations have already been replaced by semiconductor-based power electronic controllers POWER SEMICONDUCTOR DEVICES Silicon controlled rectifier (SCR) was introduced first in Since then, several other power semiconductor devices have been developed. All these semiconductor devices are enumerated below along with their ratings. Power diodes are available up to 3000 V, 3500 A, 1 khz. Thyristors have ratings up to 6000V,3500 A, 1 khz. 5

7 SITHs (static induction thyristors) can operate up to 4000 V,2200 A,20 khz. GTOs (gate-turn off thyristors) have ratings of 4000V,3000 A, 10 khz. MCTs (MOS controlled thyristors) can work up to 600 V, 60 A, 20 khz. Triacs have power ratings of 1200V, 300 A, 400 Hz. BJTs are used up to power ratings of 1200 V, 400 A, 10 khz. Power MOSFETs (metal oxide semiconductor field effect transistors) and SITs (static induction transistors) have relatively low range of 1000 V,50 A and 1200 V, 300 A respectively. Both these devices can, however, operate satisfactorily up to a frequency range of 100 khz. IGBTs (insulated gate bipolar transistors) are available up to 1200 V, 400 A and 20 khz. Based on turn-on and turn-off characteristics and gate signal requirements, the power semiconductor devices can be classified as under: 6

8 (a) Diodes: These are uncontrolled, rectifying devices. Their on and off states are controlled by powersupply. (b) Thyristors: These have controlled turned-on by a gate signal. After thyristors are turned-on, they remain latchedin on-state due to internal regenerative action. (c) Controllable switches: These devices are turned-on and turned-off by the application of control signals. The devices which behave as controllable switches are BJT, MOSFET, GTO,SITH, IGBT, SIT and MCT. SCR, GTO, SITH and MCT require pulse-gate signal for turning them on ; once these devices are on, gate pulse is removed. But BJT, MOSFET, IGBT and SIT require continuous signal for keeping them in turn-on state. 7

9 1.5 TYPES OF POWER ELECTRONIC CONVERTERS A power electronic system consists of one or more power electronic converters. A power electronic converter is made up of some power semiconductor devices controlled by integrated circuits. The switching characteristics of power semiconductor devices permit a power electronic converter to shape the input power of one form to output power of some other form. Static power converters perform these functions of power conversion very efficiently. Power electronic converters (or circuits) can be classified into six types as under: 1.Diode Rectifiers: A diode rectifier circuit converts ac input voltage into a fixed dc voltage. The input voltage may be singlephase or three phase. Diode rectifiers find wide use in electric traction, battery charging, electroplating, electrochemical processing, power supplies, welding and uninterruptible power supply (UPS) systems. 2. Ac to dc converters (Phase-controlled rectifiers): These convert constant ac voltage to variable dc output voltage. These rectifiers use line voltage for their commutation as such these are also called line-commutated or naturally-commutated ac to 8

10 dc converters. Phase-controlled converters may be fed from 1- phase or 3-phase source. These are used in dc drives, chemical industries, excitation systems for synchronous machines etc. 3. DC to dc converters ( DC Choppers) : A dc chopper converts fixed dc input voltage to a controllable dc output voltage. The chopper circuits require forced, or load, commutation to turn-off the thyristors. For lower power circuits, thyristors are replaced by power transistors. Classification of chopper circuits is dependent upon the type of commutation and also on the direction of power flow. Choppers find wide applications in dc drives, subway cars, trolley trucks, battery-driven vehicles etc. 4. DC to ac converters (inverters) : An inverter converts fixed dc voltage to a variable ac voltage. The output may be a variable voltage and variable frequency. These converters use line, load or forced commutation for turning-off the thyristors. Inverters find wide use in induction-motor and synchronous-motor drives, induction heating, UPS, HVDC transmission etc. At present, conventional thyristors are also being replaced by GTOs in high power applications and by power transistors in low-power applications. 9

11 5. AC to ac converters: These convert fixed ac input voltage into variable ac output voltage. These are of two types as under: (a) AC voltage controllers (AC voltage regulators) : These converter" circuits convert fixed ac voltage directly to a variable ac voltage at the same frequency. AC voltage controller employs two thyristors in ant parallel or a triac. Turn-off of both the devices is obtained by line commutation. Output voltage is controlled by varying the firing angle delay. AC voltage controllers are widely used for lighting control, speed 'control of fans, pumps etc. (b) Cycloconverters : These circuits convert input power at one frequency to output power at a different frequency through onestage conversion. Line commutation is more common in these converters, though forced and load commutated cycloconverters are also employed. These are primarily used for slow-speed large ac drives like rotary kiln etc. 6. Static switches: The power semiconductor devices can operate as static switches or contactors. Static switches possess many advantages over mechanical and electromechanical circuit breakers. Depending upon the input supply, the static switches are called ac static switches or dc static switches. 10

12 2. Power Semiconductor Diodes Power semiconductor diodes play a significant role in power electronics circuits. A diode acts as a switch to perform various functions. Such as switches in rectifiers, freewheeling in switching regulators, charge reversal of capacitor and energy transfer between components, energy feedback from the load to the power source, and trapped energy recovery. Power diodes can be assumed as ideal switches for most applications but practical diodes differ from the ideal characteristics and have certain limitations. The power diodes are similar to pn-junction signal diodes. However, the power diodes have larger power, voltage, and current handling capabilities than that of ordinary signal diodes. The frequency response (or switching speed) is low compared to signal diodes. 2.1 Diode Characteristics A power diode is a two terminal pn junction device. The two terminals of diode are called anode and cathode. Fig.(2.1) shows the sectional view of pn-junction and diode symbol. 11

13 Fig. 2.1.(a)p-n junction (b)diode symbol When the anode potential is positive with respect to the 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: and the magnitude of this drop would depend on the manufacturing process and junction temperature. When the cathode potential is positive with respect to the anode, the diode is said to be reverse biased. Under reveres biased conditions, a small reveres current (also known as leakage current) in the range of micro- or mill ampere flows and this leakage current increases slowly in magnitude with the reverse voltage until breakdown or avalanche voltage is reached. At this breakdown voltage, diode is turned on in the reversed direction. If current in the reversed direction is not limited by a series resistance, the current will become quite high to destroy the diode. The reverse avalanche breakdown of a diode is 12

14 avoided by operating the diode below specified peak repetitive reverse voltage VRRM. Fig.2.1(c) illustrates diode characteristics where VRRM and cut-in voltage are shown. Diode manufacturers also indicate the value of peak inverse voltage (PIV) of a diode. This is the largest reverse voltage to which a diode may be subjected during its working. PIV is the same as VRRM. Fig.2.1(c) V-I characteristics of diode. The V-I characteristics shown in fig. 2.1c can be expressed by an equation known as schockley diode equation, and it is given by ௦ ವ ഓ (1) 13

15 Where = current through the diode, A = diode voltage with anode positive with respect to cathode, V ௦=leakage (or reverse saturation) current, typically in the range 10-6 to A n= empirical constant known as emission or ideality factor, whose value varies from 1to 2. The emission coefficient n depends on the material and the physical construction of the diode. For germanium diodes, n is considered to be 1, for silicon diodes, the predicted value of n is 2, but for most practical silicon diodes, the value of n falls in the range 1.1 to 1.8. ఛ is constant called thermal voltage and it is given by ఛ (2) where q=electron charge: coulomb(c) T=absolute temperature in Kelvin (K=273+ C) K=Boltzmann s constant: J/K At a junction temperature of 25 C, Eq. (2) gives ఛ ଵ.ଷ ଵ ଶଷ (ଶଷ ଶହ) ଵ.ଶଶ ଵ ଵଽ 14

16 The power diodes are now available with forward current ratings of 1 A to several thousand amperes and with reverse voltage ratings of 50 V to 3000 V or more. Example (1): The forward voltage drop of a power diode is V D =1.2 at I D =300A. Assuming that n=2 and V τ saturation current I s. =25.8mV, find the Solution we can find the leakage (or saturation) current I s from ௦ ವ ഓ ௦ ଵ.ଶ (ଶ ଶହ. ଵ షయ ) Which gives I s = *10-8 A 2.2 Reverse Recovery Characteristics The current in a forward biased junction diode is due to the net effect of majority and minority carriers. Once a diode is in a forward conduction mode and then its forward current is reduced to zero (by applying a reverse voltage), the diode continues to conduct due to minority carriers which remain 15

17 stored in the pn-junction and the bulk semiconductor material. The minority carriers require a certain time recombine with opposite charges and to be neutralized. This time is called the reverse recovery time of diode. Fig.2.2 shows two reverse recovery characteristics of junction diodes. Fig. (2.2): Reverse recovery characteristics The soft recovery type is more common. The reverse recovery time is denoted as t rr and is measured from the initial zero crossing of the diode current to 25% of maximum (or peak) reverse current, I RR. t rr consists of two components, t a and t b. t a is due to charge storage in the depletion region of the junction and represents the time between the zero crossing and the peak reverse current, I RR. t b is due to charge storage in the bulk semiconductor material. The ratio t b /t a is known as the softness 16

18 factor, SF. For practical purposes, one needs be concerned with the total recovery time t rr and the peak value of the reverse current I RR (3) The peak reverse current be expressed in reverse di/dt as ௧ (4) Reverse recovery charge Q RR, is the amount of charge carries 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. భ మ భ మ భ మ (5) ଶ ௧ (6) Equating eq.(4) to eq. (6) gives ଶ ௧ (7) If t b is negligible as compared to t a, which is usually the case, t rr t a, and eq.(7) becomes ଶ ௧ (8) 17

19 and (9) The storage charge is dependent on the forward diode current I F. If a diode is in reverse biased condition, a leakage current flow due to the minority carriers. Then the application of forward voltage would force the diode to carry current in the forward direction. However, it requires a certain time known as forward recovery (or turn - on) time before all the majority carriers over the whole junction can contributed to the current flow. If the rate of rise of the forward current is high and the forward current is concentrated to a small area of the junction, the diode fail. Thus the forward recovery time limits the rate of the forward current and the switching speed. Exsample2: The reverse recovery time of a diode is trr=3 and the rate of fall of the diode current is di/dt =30A/ s. Determine (a) the storage charge Q RR and (b) the peak reverse current I RR. Solution 18

20 (a) The storage charge Q RR is ଵ ଶ ଶ ଶ (b) the peak reverse current I RR is ௧ 2.3 Power Diode Types Depending on the recovery characteristics and manufacturing techniques, the power diodes can be classified into three categories 1. Standard or general-purpose diodes 2. Fast-recovery diodes 3. Schottky diodes General purpose diodes The general-purpose rectifier diodes have relatively high reverse recovery time, typically 25 s. And are used in low-speed applications, where recovery time is not critical (diode rectifiers and converters for a low input frequency up to 1-kHz applications and line-commutated converters) 19

21 These diodes cover current ratings from less than 1A to several thousand of amperes, with voltage ratings from 50 V to around 5kV. Fast-recovery diodes The fast-recovery diodes have low recovery time, normally less than 5 s. They are used in dc-dc and dc-ac converter circuits, where the speed of recovery is often of critical importance. These diodes cover current ratings from less than 1A to hundreds of amperes, with voltage ratings from 50V to around 3kV. Schottky Diodes This class of diodes use metal-to-semiconductor junction for rectification purposes instead of pn-junction. Schottky diodes are characterized by very fast recovery time and low forward voltage drop. Rectified current flow is by majority carriers only and this avoids the turn-off delay accompanied with minority carrier recombination. Their reverse voltage ratings are limited to about 100 V and forward current ratings vary from 1 A to 300 A. Applications of Schottky diodes include high-frequency instrumentation and switching power supplies. 20

22 3.POWER TRANSISTORS Power transistors have controlled turn on and turn off characteristics. The transistors, which are used as switching elements, are operated in the saturation region, resulting in a low on state voltage drop. The switching speed of modern transistors is much higher than that of thyristors and they are extensively employed in dc dc and dc ac converters, with inverse parallel connected diodes to provide bidirectional current flow. However, their voltage and current rating are lower than those of thyristors and transistors are normally used in low to medium power applications. The power transistor can be classified broadly into five categories; 1.Bipolar junction transistors (BJTs). 2.Metal-oxide-semiconductor field-effect transistors (MOSFETs). 3.Insulated gate bipolar transistors (IGBTs). 4.Static induction transistors (SITs). 5.COOLMOS 21

23 3.1 Bipolar junction transistors (BJTs) A bipolar transistor is formed by adding a second p or n region to a pn junction diode. With two n regions and one p region, two junction are formed and it is known as an NPN transistor, as shown in fig.(3.1-a). With two p regions and one n region, it is called as an PNP transistor, as shown in fig.(3.1-b).the three terminals are named as collector, emitter, and base. A bipolar transistor has two junctions, collector base junction (CBJ) and base emitter junction (BEJ). Fig.(3.1-a): NPN transistor Fig.(3.1-b): PNP transistor The term' bipolar ' denotes that the current flow in the device is due to the movement of both holes and electrons. Steady-state Characteristics Although there are three possible circuit configurations common collector, common emitter, and common based, the 22

24 common emitter configuration is generally used in switching applications. The typical Input characteristics of base current I B against base-emitter voltage V BE, are shown in fig.(3.2-b). When collector-emitter voltage V CE2 current decreases as shown in Fig.(3.2-b). is more than V CE1, base Fig.(3.2-a):npn-transistor circuit diagram (b)input characteristics (c)output characteristics. Figure(3.2-c) shows the typical output characteristics of collector current I c against collector emitter voltage V CE. There are three operation regions of a transistor: cutoff, active, and saturation. In cutoff region, the transistor is off or the base current is not enough to turn it on and both junctions are reverse biased. In active region, the transistor acts as an amplifier, where the base current is amplified by a gain and the collector emitter voltage decreases with the base current. The CBJ is reverse 23

25 biased, and the BEJ is forward biased. In the saturation region, the base current is sufficiently high so that the collector emitter voltage is low, and the transistor acts as switch. Both junction (CBJ and BEJ) are forward biased. 3.2Metal oxide semiconductor field effect transistors (MOSFETs). A power MOSFET is a voltage controlled device and requires only small input current. The switching speed is very high and the switching times are of the order of nanoseconds. A power-mosfet has three terminals called drain, source and gate in place of the corresponding three terminals collector, emitter and base for BJT. As its operation depends upon the flow of majority carriers only, MOSFET is a unipolar device. The control signal, or base current in BJT is much larger than the control signal (or gate current) required in a MOSFET. This is because of the fact that gate circuit impedance in MOSFET is extremely high, of the order of 10 9 ohm. This large impedance permits the MOSFET gate to be driven directly from microelectronic circuits. BJT suffers from second breakdown voltage whereas MOSFET is free from this problem. Power 24

26 MOSFETs are now finding increasing applications in low-power high frequency converters. The symbol and basic circuit diagram for n-channel power MOSFET is shown in Fig. 3.4 where voltages and currents are as indicated. Fig. 3.4 (c) shows typical transfer characteristic for n-channel power MOSFET. It is seen that there is threshold voltage V GST below which the device is off. The magnitude of V GST is of the order of 2 to 3 V. (a) (b) (c) Fig. (3.4) Figure 3.5 shows the output characteristic of an n channel enhancement MOSFET. There are three regions of operation: (1) cutoff region, where V GS V T ; (2) pinch off or saturation region, where V DS V GS V T ; and linear region, where V DS 25

27 V GS - V T. the pinch off occurs at V DS = V GS - V T. in the linear region, the drain current I D varies in proportion to the drain source voltage V DS. Due to high drain current and low drain voltage, the power MOSFETs are operated in the linear region for switching actions. In the saturation region, the drain current remains almost constant for any increase in the value of V DS and the transistors are used in this region for voltage amplification. It should be noted that saturation has opposite meaning to that for bipolar transistors. Fig. (3.5): Output characteristic of enhancement type MOSFET 26

28 3.3Insulated gate bipolar transistors (IGBTs) IGBT is a new development in the area of power MOSFET technology. This device combines into it the advantages of both MOSFET and BJT. So an IGBT has high input impedance like a MOSFET and low-on-state power loss as in a BJT. Further, IGBT is free from second breakdown problem present in BJT. IGBT is also known as metal-oxide insulated gate transistor (MOSIGT), conductively-modulated field effect transistor (COMFET) or gain-modulated FET (GEMFET). It was also initially called insulated gate transistor (IGT). circuit symbol, static and transfer characteristics are shown 27

29 Applications IGBTs are widely used in medium power applications such as dc and ac motor drives, UPS systems, power supplies and drives for solenoids, relays and contactors. IGBT converters are more efficient with less size as well as cost, as compared to converters based on BJTs. At present, the state of the art IGBTs are available up to 1200 V, 500 A. 28

30 4. Thyristor A Thyristor is one of the most important types of power semiconductor devices. Thyristors are very widely used in power electronic circuit. They are operated as a bistable switches, operating from nonconducting to conducting stable Thyristor characteristic A thyristor is a four layer semiconductor device of pnpn structure with three pn-junctions. It has three terminals: anode, cathode, and gate. Fig.(4.1) shows the thyristor symbol and the sectional view of three pn-junctions. Thristors are manufactured by diffusion. Fig. (4.1): thyristor symbol and three pn- junctions. 29

31 When the anode voltage is made positive with respect to the cathode, the junctions J 1 and J 3 are forward biased. The junction J 2 is reverse biased, and only a small leakage current flows from anode to cathode. The thyristor is then said to be in the forward blocking or off state condition and the leakage current is known as off- state current I D. if the anode to cathode voltage V AK is increased to a sufficiently large value, the reverse biased junction J 2 will break. This is known as avalanche breakdown and the corresponding voltage is called forward break down voltage V BO. Since the other junctions J 1 and J 3 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 then in a conducting state or onstate. The voltage drop would be due to the ohmic drop in the four layers and it is small, typically, 1V. The anode current must be more than a value known as latching current I L, in order to maintain the required amount of carrier flow across the junction; otherwise, the device will revert to blocking condition as the anode to cathode voltage is reduced. Latching current I L is the minimum anode current 30

32 required to maintain the thyristor in the on state immediately after a thyristor has been turned on and the gate signal has been removed. A typical V I characteristic of a thyristor is shown in fig.(4.2). Fig. (4.2): Thyristor V I characteristic Once thyristor conducts, it behave like a conducting diode and there is no control over the device. The device will continue to conduct because there is no depletion layer on the junction J 2 due to free movements of carriers. However, if the forward anode current is reduced below a level known as the holding current I H, a depletion region will develop around junction J 2 31

33 due to the reduced number of carriers and the thyristor will be in the blocking state. The holding current is in order of mill amperes and is less than the latching current I L. current I H thyristor in the on state. Holding is the minimum anode current to maintain the When the cathode voltage is positive with respect to the anode, the junction J 2 is forward biased, but the junctions J 1 and J 3 are reverse biased. The thyristor will be in the reverse blocking state and reverse leakage current known as reverse current, I R, would flow through the device. 4.2 Thyristor Turn On A thyristor is turned on by increasing the anode current. This can be accomplished in one of the following ways. Thermals if the temperature of a thyristor is high, there will be an increase in the number of electron hole pairs, which would increase the leakage currents, and the thyristor may turned on. This type of turn on may cause thermal runaway and is normally avoided. 32

34 Light if light is allowed to strike the junctions of the thyristor, the electron hole pairs will increase; and the thyristor may be turned on. The light activated thyristors are turned on by allowing light to strike the silicon wafers. High voltage if the forward anode to cathode voltage is greater than the forward breakdown voltage V FB, sufficient leakage current will flow to initial regenerative turn on. This type of turn on may be destructive and should be avoided. dv/dt if the rate of rise of the anode cathode voltage is high, the charging current of the capacitive junctions may sufficient enough to turn on the thyrastor. A high value of charging current may damage the thyrastor ; and the device must be protected against high dv/dt. The manufacturers specify the maximum allowable dv/dt of thyristors. Gate current If a thyristor is forward biased, the injection of gate current by applying positive gate voltage between the gate and be 33

35 cathode terminals would turn on the thyristor. As the gate current is increase, the forward blocking voltage is decreased as shown in fig.(4.2). The following points should be considered in designing the gate control circuit: 1. The gate signal should be removed after the thyristor is turn on. A continuous gating signal would increase the power loss in the gate junction. 2. While the thyristor is reversed biased, there should be no gate signal; otherwise, the thyristor may fail due increased leakage current. 3.The width of gate pulse t G must be longer than the time required for the anode current to rise to the holding current value I H. in practical, the pulse width t G is normally made more than the turn on time t on of the thyristor. 34

36 Example (4.1) The capacitance of reverse biased junction J 2 in a thyristor is C j2 =20 pf and can be assumed to be independent of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16mA. Determine the critical value of dv/dt. Solution ଶ ଶ critical value of dv/dt from ൫ మ ൯ ௧ we can find the ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଷ ଵଶ 4.3 Protection If the current in a thyristor rises at too high a rate, that is, high di/dt, the device can be destroyed. Some inductance must be present or inserted in series with the thyristor so that di/dt is below a safe limit specified by the manufacturer. 35

37 A thyristor may turn on (without any gate pulse) if the forward voltage is applied too quickly. This is known as dv/dt turn-on and it may lead to improper operation of the circuit. A simple R C snubber normally used to limit the dv/dt of the applied forward voltage. Snubber circuits purpose: Limit rate of rise of voltage across a switching device. Limit rate of rise of current flowing through a switching device Modify the switching trajectory Simple snubber circuits Turn-off snubber circuit dv/dt is shown in fig

38 When the device turns-off, capacitor voltage is charged to Vs by R L. Value of the capacitor may be chosen to limit the rate of rise of dv/dt, then the voltage across the capacitor is ௦ Rated of change of V c with time is ௧ ಽ ௦ ௧ ಽ Maximum rate of change of V c occurs at t = 0 ௫ ௦ Minimum value of C to limit dv/dt to a specified value is given by ௦ ௫ If a thyristor with a turn-off snubber circuit has a maximum dv/dt rating of 35 V s -1 and supplies a load resistance of 10 Maximum step change in voltage is 350 V, the minimum value of C is ௦ ௫ 37

39 Effect of adding the capacitor in the turn-off snubber circuit is shown in fig. below Area under power (=v*i) curve with time represents switching energy loss in the device. Turn-on snubber circuit is shown in fig.4.4. Inductor prevents thyristor current increasing instantaneously Inductor current increases from 0 A to Vs/R L exponentially Rated of change of I l with time is ௧ ಽ 38

40 ௦ ௧ ಽ Maximum rate of change of I l occurs at t = 0 ௫ ௦ Minimum value of L to limit di l /dt to a specified value is given by ௦ ௫ Effect of adding the inductor in the turn-on snubber circuit is shown in fig. below Problems with simple snubber circuits are: When device is turned on, C is effectively shorted out by the device and an unlimited current can flow 39

41 When the device is turned off, the high di/dt in the inductor implies an unlimited voltage across it which is in series with the device and the supply. Modified snubber circuits 4.4 Gate Turn Off Thyristors (GTOs) The gate turn-off thyristor (GTO) incorporates many of the advantages of the conventional thyristor and the high-voltage switching transistor. It is a PNPN device that can be triggered into conduction by a small positive gate-current pulse, but also has the capability of being turned-off by a negative gate-current 40

42 Pulse, (normally1/4 or 1/5 of anode current), For example, a 4000 V, 3000 A device may need -750 A gate current to turn it off. This facility allows the construction of inverter circuits without the bulky and expensive forced commutating components associated with conventional thyristor circuitry. The GTO also has a faster switching speed than the regular thyristor, and it can withstand higher voltage and current than the power transistor or MOSFET. The GTO is a three-terminal device with anode, cathode and gate terminals. The various circuit symbols are shown in Figure 4.3. The two-way arrow convention (Figure 4.5 (i)) on the gate lead distinguishes the GTO from the conventional thyristor. Fig. (4.5): GTO symbols 41

43 4.5 Triac The triac is a three terminal semiconductor for controlling current in either direction. Below is the schematic symbol for the triac. Notice the symbol looks like two SCRs in parallel ( opposite direction) with one trigger or gate terminal. The main or power terminals are designated as MT1 and MT2. (See the schematic representation below) When the voltage on the MT2 is positive with regard to MT1 and a positive gate voltage is applied, the left SCR conducts. When the voltage is reversed and a negative voltage is applied to the gate, the right SCR conducts. Minimum holding current, I H, must be maintained in order to keep a triac conducting. 42

44 A triac operates in the same way as the SCR however it operates in both a forward and reverse direction. To get a quick understanding of its operation refer to its characteristic curve and compare this to the SCR characteristic curve. It can be triggered into conduction by either a PLUS (+) or MINUS (-) gate signal. Obviously a triac can also be triggered by exceeding the break over voltage. This is not normally employed in triac operation. The break over voltage is usually considered a design limitation. One other major limitation, as with the SCR, is dv/dt, which is the rate of rise of voltage with respect to time. A triac can be switched into conduction by a large dv/dt. Typical applications are in phase control, inverter design, AC switching, relay replacement, etc. 43

45 4. Thyristor A Thyristor is one of the most important types of power semiconductor devices. Thyristors are very widely used in power electronic circuit. They are operated as a bistable switches, operating from nonconducting to conducting stable Thyristor characteristic A thyristor is a four layer semiconductor device of pnpn structure with three pn-junctions. It has three terminals: anode, cathode, and gate. Fig.(4.1) shows the thyristor symbol and the sectional view of three pn-junctions. Thristors are manufactured by diffusion. Fig. (4.1): thyristor symbol and three pn- junctions. 29

46 When the anode voltage is made positive with respect to the cathode, the junctions J 1 and J 3 are forward biased. The junction J 2 is reverse biased, and only a small leakage current flows from anode to cathode. The thyristor is then said to be in the forward blocking or off state condition and the leakage current is known as off- state current I D. if the anode to cathode voltage V AK is increased to a sufficiently large value, the reverse biased junction J 2 will break. This is known as avalanche breakdown and the corresponding voltage is called forward break down voltage V BO. Since the other junctions J 1 and J 3 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 then in a conducting state or onstate. The voltage drop would be due to the ohmic drop in the four layers and it is small, typically, 1V. The anode current must be more than a value known as latching current I L, in order to maintain the required amount of carrier flow across the junction; otherwise, the device will revert to blocking condition as the anode to cathode voltage is reduced. Latching current I L is the minimum anode current 30

47 required to maintain the thyristor in the on state immediately after a thyristor has been turned on and the gate signal has been removed. A typical V I characteristic of a thyristor is shown in fig.(4.2). Fig. (4.2): Thyristor V I characteristic Once thyristor conducts, it behave like a conducting diode and there is no control over the device. The device will continue to conduct because there is no depletion layer on the junction J 2 due to free movements of carriers. However, if the forward anode current is reduced below a level known as the holding current I H, a depletion region will develop around junction J 2 31

48 due to the reduced number of carriers and the thyristor will be in the blocking state. The holding current is in order of mill amperes and is less than the latching current I L. current I H thyristor in the on state. Holding is the minimum anode current to maintain the When the cathode voltage is positive with respect to the anode, the junction J 2 is forward biased, but the junctions J 1 and J 3 are reverse biased. The thyristor will be in the reverse blocking state and reverse leakage current known as reverse current, I R, would flow through the device. 4.2 Thyristor Turn On A thyristor is turned on by increasing the anode current. This can be accomplished in one of the following ways. Thermals if the temperature of a thyristor is high, there will be an increase in the number of electron hole pairs, which would increase the leakage currents, and the thyristor may turned on. This type of turn on may cause thermal runaway and is normally avoided. 32

49 Light if light is allowed to strike the junctions of the thyristor, the electron hole pairs will increase; and the thyristor may be turned on. The light activated thyristors are turned on by allowing light to strike the silicon wafers. High voltage if the forward anode to cathode voltage is greater than the forward breakdown voltage V FB, sufficient leakage current will flow to initial regenerative turn on. This type of turn on may be destructive and should be avoided. dv/dt if the rate of rise of the anode cathode voltage is high, the charging current of the capacitive junctions may sufficient enough to turn on the thyrastor. A high value of charging current may damage the thyrastor ; and the device must be protected against high dv/dt. The manufacturers specify the maximum allowable dv/dt of thyristors. Gate current If a thyristor is forward biased, the injection of gate current by applying positive gate voltage between the gate and be 33

50 cathode terminals would turn on the thyristor. As the gate current is increase, the forward blocking voltage is decreased as shown in fig.(4.2). The following points should be considered in designing the gate control circuit: 1. The gate signal should be removed after the thyristor is turn on. A continuous gating signal would increase the power loss in the gate junction. 2. While the thyristor is reversed biased, there should be no gate signal; otherwise, the thyristor may fail due increased leakage current. 3.The width of gate pulse t G must be longer than the time required for the anode current to rise to the holding current value I H. in practical, the pulse width t G is normally made more than the turn on time t on of the thyristor. 34

51 Example (4.1) The capacitance of reverse biased junction J 2 in a thyristor is C j2 =20 pf and can be assumed to be independent of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16mA. Determine the critical value of dv/dt. Solution ଶ ଶ critical value of dv/dt from ൫ మ ൯ ௧ we can find the ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଶ ଷ ଵଶ 4.3 Protection If the current in a thyristor rises at too high a rate, that is, high di/dt, the device can be destroyed. Some inductance must be present or inserted in series with the thyristor so that di/dt is below a safe limit specified by the manufacturer. 35

52 A thyristor may turn on (without any gate pulse) if the forward voltage is applied too quickly. This is known as dv/dt turn-on and it may lead to improper operation of the circuit. A simple R C snubber normally used to limit the dv/dt of the applied forward voltage. Snubber circuits purpose: Limit rate of rise of voltage across a switching device. Limit rate of rise of current flowing through a switching device Modify the switching trajectory Simple snubber circuits Turn-off snubber circuit dv/dt is shown in fig

53 When the device turns-off, capacitor voltage is charged to Vs by R L. Value of the capacitor may be chosen to limit the rate of rise of dv/dt, then the voltage across the capacitor is ௦ Rated of change of V c with time is ௧ ಽ ௦ ௧ ಽ Maximum rate of change of V c occurs at t = 0 ௫ ௦ Minimum value of C to limit dv/dt to a specified value is given by ௦ ௫ If a thyristor with a turn-off snubber circuit has a maximum dv/dt rating of 35 V s -1 and supplies a load resistance of 10 Maximum step change in voltage is 350 V, the minimum value of C is ௦ ௫ 37

54 Effect of adding the capacitor in the turn-off snubber circuit is shown in fig. below Area under power (=v*i) curve with time represents switching energy loss in the device. Turn-on snubber circuit is shown in fig.4.4. Inductor prevents thyristor current increasing instantaneously Inductor current increases from 0 A to Vs/R L exponentially Rated of change of I l with time is ௧ ಽ 38

55 ௦ ௧ ಽ Maximum rate of change of I l occurs at t = 0 ௫ ௦ Minimum value of L to limit di l /dt to a specified value is given by ௦ ௫ Effect of adding the inductor in the turn-on snubber circuit is shown in fig. below Problems with simple snubber circuits are: When device is turned on, C is effectively shorted out by the device and an unlimited current can flow 39

56 When the device is turned off, the high di/dt in the inductor implies an unlimited voltage across it which is in series with the device and the supply. Modified snubber circuits 4.4 Gate Turn Off Thyristors (GTOs) The gate turn-off thyristor (GTO) incorporates many of the advantages of the conventional thyristor and the high-voltage switching transistor. It is a PNPN device that can be triggered into conduction by a small positive gate-current pulse, but also has the capability of being turned-off by a negative gate-current 40

57 Pulse, (normally1/4 or 1/5 of anode current), For example, a 4000 V, 3000 A device may need -750 A gate current to turn it off. This facility allows the construction of inverter circuits without the bulky and expensive forced commutating components associated with conventional thyristor circuitry. The GTO also has a faster switching speed than the regular thyristor, and it can withstand higher voltage and current than the power transistor or MOSFET. The GTO is a three-terminal device with anode, cathode and gate terminals. The various circuit symbols are shown in Figure 4.3. The two-way arrow convention (Figure 4.5 (i)) on the gate lead distinguishes the GTO from the conventional thyristor. Fig. (4.5): GTO symbols 41

58 4.5 Triac The triac is a three terminal semiconductor for controlling current in either direction. Below is the schematic symbol for the triac. Notice the symbol looks like two SCRs in parallel ( opposite direction) with one trigger or gate terminal. The main or power terminals are designated as MT1 and MT2. (See the schematic representation below) When the voltage on the MT2 is positive with regard to MT1 and a positive gate voltage is applied, the left SCR conducts. When the voltage is reversed and a negative voltage is applied to the gate, the right SCR conducts. Minimum holding current, I H, must be maintained in order to keep a triac conducting. 42

59 A triac operates in the same way as the SCR however it operates in both a forward and reverse direction. To get a quick understanding of its operation refer to its characteristic curve and compare this to the SCR characteristic curve. It can be triggered into conduction by either a PLUS (+) or MINUS (-) gate signal. Obviously a triac can also be triggered by exceeding the break over voltage. This is not normally employed in triac operation. The break over voltage is usually considered a design limitation. One other major limitation, as with the SCR, is dv/dt, which is the rate of rise of voltage with respect to time. A triac can be switched into conduction by a large dv/dt. Typical applications are in phase control, inverter design, AC switching, relay replacement, etc. 43

60 Forced Commutation Large class of power electronic circuits which operate from DC source (choppers and inverters), natural commutation is not possible. Where self-commutating devices such as power transistors, power MOSFETs, IGBTs and GTO thyristors are used, turn-off can be achieved by control of device base or gate conditions In applications where thyristors are required for reasons of rating, then achievement of controlled turn-off required use of external circuits. This is referred to as forced commutation Function of a forced commutation circuit: First to reduce current through conducting thyristor to zero Then to maintain reverse voltage across this thyristor for a duration equal to or greater than the thyristor turn-off time, to re-establishes the blocking state 44

61 Forced commutation circuits Commutation by an external voltage source 1) External voltage source reduces forward current through thyristor to zero: When switch closes, external voltage source is more preferred route for load current than thyristor; hence thyristor current falls to zero. Once thyristor turn-off is complete, switch can be opened to let load current fall to zero. 2) Switch realized using bipolar transistor: Acceptable from power dissipation point of view to use bipolar transistor to switch off high power thyristor because bipolar transistor is dissipating power only for very short interval when thyristor is being switched off 45

62 3) Alternatively, pulse transformer can be used to place appropriate voltage across thyristor. Commutation circuit using a capacitor 1. Reverse voltage can be applied using capacitor Assume that capacitor is initially charged in direction shown ( +). When switch (S) is closed, capacitor provides load current in preference to thyristor because of its higher voltage. This forces thyristor forward current below level of holding current. Reduction of forward current to zero takes place almost instantaneously. Capacitor discharges through load, maintaining necessary reverse voltage across thyristor to complete turn-off 46

63 before its voltage falls to zero and it then recharges in reverse direction. In practice, additional circuitry provides required initial condition voltage on capacitor and initiates discharge. 2. Second thyristor T2 acts as auxiliary switch: Fig. a Fig. b Fig. c 47

64 T 1 is the main load thyristor, T 2 is the auxiliary thyristor to switch the capacitor charge across T 1 for turnoff, and the inductor L is necessary to ensure the correct polarity on capacitor C. When the battery is connected, no current flows as both thyristors are off. For correct operation, capacitor C must first be charged by firing thyristor T 2, giving the simple equivalent circuit of fig.b, except capacitor C has no initial charge on it; hence an exponential decay current of initial value E/R flows in the branches shown. After time has elapsed, the capacitor C will be charged ideally to the battery voltage E, but in practice when the charging current falls below thyristor T 2 holding level, current will cease. Firing thyristor T 1 connects the battery to the load, as is clear in the equivalent circuit of fig.c. At the same time, an oscillation starts between the inductor L and capacitor C which continues for one half cycle as the diode prevents reverse current flow, hence the charge on capacitor C is reversed from that shown in fig. c. to that shown in fig. b. Now firing T 2 clearly puts the charge on capacitor C across thyristor T 1, turning it off, and giving the conditions shown 48

65 in fig. b, where the current in the thyristor T 2 and the load will have an initial value of 2E/R. 3. Simple Parallel - capacitor chopper: The simple circuit shown in above figure avoids the use of an inductor. The principle is that firing thyristor T 1 connects the battery to the load R 1 and at the same time enables the capacitor C to charge via resistor R 2. Firing thyristor T 2 places the charge on capacitor C across T 1, turning it off. Thyristor T 2 will remain on with current flow via R 2, capacitor C oppositely charging via R 1. Firing thyristor T 1 now connects the battery to the load R 1 and at the same time turns thyristor T 2 off by placing capacitor C across it. 49

66 The disadvantage of this simple circuit is the loss in the resistor R 2, as it carries current throughout the load off period. The loss can be minimized by making R 2 large compared to R 1, but this will lengthen the charging time of the capacitor, hence limiting the rate at which the load can be switched. 50

67 Single and Three Phase AC/DC Converters Fig (1) shows the input and the outputs of AC to DC converters. The input is single phase or three phase AC supply normally available from the mains. The output is the controlled DC voltage and current. The AC to DC converters include diode rectifiers as well as controlled rectifiers. The controlled rectifiers mainly use SCRs. Since the input is AC supply, the SCRs are turned off natural commutation. Hence external commutation circuits are not required. Hence AC to DC converters are also called as line (supply) commutated converters. These converters are used for DC drives, uninterruptible power supplies ( UPS), high voltage DC transmission (HVDC) systems. 51

68 1- Single phase diode rectifiers Rectification is the process of conversion of alternating input voltage to direct output voltage. As stated before, a rectifier converts ac power to dc. In diode based rectifiers, the output voltage cannot be controlled. In this section, uncontrolled single phase rectifiers are studied. The diode is assumed ideal has no forward voltage drop (V d =0). 1-1 Single phase half wave uncontrolled rectifier This is the simplest type of uncontrolled rectifier. It is never used in industrial applications because of its poor performance. Its study is, however, useful in understanding the principle of rectifier operation. In a single phase half wave rectifier, for one cycle of supply voltage, there is one half cycle of output, or load, voltage. The load on the output side of rectifier may be R, RL or RL with a freewheeling diode.these are now discussed briefly. Single phase half wave uncontrolled rectifier with R load The circuit diagram of a single half wave rectifier is shown in fig. (2-a). The waveforms of s o o and d are sketched in 52

69 fig.(2-b). For a resistive load, output current i o has the same waveform as that of the output voltage when diode conducts. o. Diode voltage is zero Fig. (2) Single phase half wave diode rectifier with R load (a) circuit diagram and (b) waveforms D 0 ݏ When diode is forward biased,it is therefore turned on v ܦ =0 (ideal) v= ݏ v (1) (2) 53

70 (3) (4) (5) (6) The efficiency of a rectifier (7) The form factor (FF), which is a measure of the shape of output voltage (8) The ripple factor (RF), which is a measure of the ripple content (9) 54

71 Example(1): The rectifier in fig.(2) has a purely resistive load of R. Determine a) the efficiency, b) the form factor, c) the ripple factor. Solution: 55

72 Single phase half wave uncontrolled rectifier with RL load A single phase half wave rectifier feeding RL load is shown in fig. ( 3-a ). Current continues to flow-even after source voltage v ݏ has become negative; this is because of the presence of inductance L in the load circuit. Voltage v = i has the same wave shape as i. Inductor voltage v ܮ = v ݏ v is also shown. The current i flows till the two areas A and B are equal. Area A represents the energy stored by L and area B the energy released by L. It must be noted that mean value of voltage v ܮ across inductor L is zero. Fig.(3) Single phase half wave diode rectifier with RL load (a) Circuit diagram and (b) waveforms 56

73 When i =0 at reverse bias across diode D as shown. At, v ܮ = 0, v = 0 and voltage v ݏ appears as, voltage v ܦ across diode jumps from zero to sin where >. Here = is also the conduction angle of the diode. (10) (11) RL load with freewheeling diode Performance of single phase half wave diode rectifier with RL load can be improved by connecting a freewheeling diode across the load as shown in fig.(4-a). Output voltage is v =v ݏ for 0. At =, source voltage v ݏ is zero, but output current i is not zero because of L in the load circuit. Just after =, as v ݏ tends to reverse, negative polarity of v ݏ reaches cathode of FD through conducting diode D, whereas positive polarity of v ݏ reaches anode of FD direct. Freewheeling diode FD, therefore, gets forward biased. As a result, load current i is immediately transferred from D to FD as v ݏ tends to reverse. After =, diode current i ݏ =0 and it is subjected to reverse voltage with PIV equal to. 57

74 Fig.(4) Single phase half wave diode rectifier with RL load and freewheeling diode (a) circuit diagram and (b) waveforms After ߨ= ωt, current freewheels through circuit RL and FD. The energy stored in L is now dissipated in R. When energy stored.ߨ 2 >ߚ= t in L = energy dissipated in R, current fails to zero at The effects of using freewheeling diode are as under: 1- It prevents the output voltage from becoming negative. 2- As the energy stored in L is transferred to load R through FD, the system efficiency is improved. 3- The load current waveform is more smooth, the load performance is therefore improved. 58

75 fig.(4-b). The waveforms for the v ݏ, v,i,v ܦ,i ݏ and i fd are drawn in (12) (13) Single phase half wave uncontrolled rectifier with RE load Single phase half wave rectifier with load resistance R and load E is shown in fig.(5-a). Fig.(5) Single phase half wave diode rectifier with RE load 59

76 The diode would not conduct at =0 because diode is reverse biased until source voltage v ݏ equals E. When sin 1 =, diode D starts conducting and turn-on angle 1 is given by (14) The diode now conducts from = 1 to =( 1 ), i.e. conduction angle for diode is ( 2 1 ) as shown in fig.(5-b). During the conduction period of diode, the voltage equation for the circuit is R (15) 60

77 Example (2): The battery voltage in fig. 5 is E=12 V and its capacity is 100Wh. The average charging current should be I dc =5A. the primary input is V p =120V, 60 Hz, and the transformer has turn ratio of n=2:1. Calculate (a) the conduction angle delta of the diode, (b) the current limiting resistance R, (c) the power rating P R of R, (d) the charging time h o in hours, (e) the rectifier efficiency, and (f) the PIV of the diode. 61

78 1-2 Single phase bridge uncontrolled rectifier With R load A single phase bridge rectifier employing diodes is shown in fig.(6-a).when point (a) is positive with respect to point (b), diodes D 1, D 2 conduct together so that output voltage is v b. Each of the diodes D 3 and D 4 is subjected to a reverse voltage of v ݏ as shown in fig.(6-b), When point (b) is positive with respect to point (a), diodes D 3, D 4 conduct together and output voltage is v a. Each of the two diodes D 1 and D 2 experience a reverse voltage of v ݏ as shown. Fig.(6) Single phase bridge rectifier (a) circuit diagram and (b) waveforms 62

79 (16) (17) (18) (19) Example(3): The rectifier in fig.(6) has a purely resistive load of R. Determine a) the efficiency, b) the form factor, c) the ripple factor. Solution 63

80 The performance of a bridge rectifier is improved compared to that a half wave rectifier 64

81 Single phase bridge uncontrolled rectifier with RL load Fig.(7) Single phase bridge diode rectifier with RL load When L is very high the change of current and the ripple can be assumed to be zero. The load current can be assumed to be pure dc or the load current will be approximately pure dc. (20) (21) 65

82 2-1 Three phase half bridge uncontrolled rectifier Fig. (8) Three phase half bridge rectifier (a) circuit diagram (b) waveform In this case only one diode is conducting at any given instant, that one which is connected to the phase having the highest instantaneous value. This results in the load voltage v L having the waveform shown in fig.(8-b), which is the top of successive phase voltages. While v 1 is the most positive phase, diode D 1 conducts but, directly v 2 become more positive than v 1, the load current commutates (transfers) from diode D 1 to diode D 2. 66

83 For usual case the load is inductive with L very high therefore it can be assumed to be continues level. 2-2 Three phase bridge uncontrolled rectifier (22) (23) 67

84 A three phase bridge rectifier is commonly used in high-power applications and it is shown in fig(9). This is a full wave rectifier.it can operate with or without a transformer and gives six-pulse ripples on the output voltage. The diodes are numbered in order of conduction sequences and each one conducts for (120 ). The conduction sequence for diodes is 12, 23, 34, 45, 56 and 61. The pair of diodes which are connected between that pair of supply lines having the highest amount of instantaneous line-toline voltage will conduct. When v is the most positive phase diode D 1 conducts and during this period first v is the most negative with diode D 6 conducting until v becomes more negative when the current in diode D 6 commutates to diode D 2. The load voltage follows in turn six sinusoidal voltages during one cycle, these being (v v ), (v v ), (v v ), (v v ), (v v ), (v v ), all having the maximum value of the line voltage that is 3 times the phase voltage. The line voltage can be found by V XN, V YN. (24) 68

85 (25) (26) Example (4): A three phase bridge rectifier has a purely resistive load of R. Determine a) the efficiency, b) the form factor, c) the ripple factor. Solution: 69

86 3- Single phase thyristor rectifiers (controlled rectifier) Controlled rectifiers are basically AC to DC converters. The power transferred to the load is controlled by controlling triggering angle of the devices. Fig.(10) shows this operation. Fig. (10) Principle of operation of a controlled rectifier The triggering angle (α) of the devices is controlled by control circuit. 70

87 3-1 Single phase half wave controlled rectifier R load Fig.(11) Single phase half wave thyristor circuit with R load An SCR can conduct only when anode voltage is positive and a gating signal is applied. The SCR will be off until a pulse is supplied to the gates which turn the SCR on. For resistive load, current i o is in phase with v o. Firing angle is therefore measured between the supply zero and the instant of pulse. 71

88 (27) The maximum value of Vmean occurs at α = 0 (28) (29) (30) (31) RL load A single phase half wave thyristor circuit with RL load is shown in fig.(12-a). At,ߙ= t thyristor is turned on by gating signal. The load voltage v at once becomes equal to source voltage v ݏ as shown. But the inductance L forces the to rise gradually. After some time, i load, or output, current i,ߨ= t reaches maximum value and then begins to decrease. At v is zero but i is not zero because of the load inductance L. After,ߨ= t SCR is subjected to reverse anode voltage but it will not be turned off as load current i is not less than the holding current. At some angle <ߚ,ߨ i reduces to zero and SCR is turned off as it is already reverse biased. 72

89 Fig. (12) Single phase half wave circuit with RL load (32) (33) (34) 73

90 3-2 Single phase bridge controlled rectifier Fig.(13) Single phase full controlled bridge rectifier (35) 74

91 3-3Three phase half wave controlled rectifier Fig. (14) Three phase half wave controlled circuit thyristors 75

92 From fig.(14-b) for (0 < <30 ) (36) From fig.(14-c&d) for (37) 76