ower MOSFet Power MOSFETS are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current of the order of nanoamperes. Hence the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents. The circuit symbol of a power MOSFET is shown in Fig. 30.5. Power MOSFETs are majority carrier devices, and there is no Drain minority carrier storage time. Hence they have exceptionally fas rise and fall times. They are essentially resistive devices when turned on, while bipolar transistors present a more or less co It VcEst) over the normal operating range. Power dissipation in MOSFETs is Id-Rpston), and in bipolars it is ICVcEsat). At low currents, therefore, a power MOSFET may have a lower conduc- Gate tion loss than a comparable bipolar device, but at higher cur ents, the conduction loss will exceed that of bipolars. Also, the ncreases An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In MOSFETS, Rps on) increases with temperature, Source and thus the current is automatically diverted away from the hot FIGURE 30.5 Power MoSFEt circuit symbol. pot.The drain body junction appears as an antiparallel diode (Source: B. K. Bose, Modern Power electronics between source and drain. Thus power MOSFETs will not sup- Evaluation, Technology and Applications, p 7@ port voltage in the reverse direction. Although this inverse diode 1992 IEEE is relatively fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as 100 ns. Since MOSFETs cannot be protected by fuses, n electronic protection technique has to be used. With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional MOSFETS. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating within its specification range at all times, its chances for failing catastrophically are minimal. However, if its absolute naximum rating is exceeded, failure probability increases dramatically. Under actual operating conditions, a MOSFET may be subjected to transients- either externally from the power bus supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the absolute maximum ratings. Such conditions are likely in almost every application, and in most cases are beyond a designers control. Rugged devices are made to be more tolerant for over-voltage transients. Ruggedness is the ability of a MOSFet to operate in an environment of dynamic electrical stresses, without activating any of the parasitic bipolar junction transistors. The rugged device can withstand higher levels of diode recovery dv/dt and static dv/dt. Insulated-Gate Bipolar Transistor (IGBT The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic (low saturation voltage)of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFet, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure f an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Power MOSFET Power MOSFETs are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors. The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current of the order of nanoamperes. Hence the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents. The circuit symbol of a power MOSFET is shown in Fig. 30.5. Power MOSFETs are majority carrier devices, and there is no minority carrier storage time. Hence they have exceptionally fast rise and fall times. They are essentially resistive devices when turned on, while bipolar transistors present a more or less constant VCE(sat) over the normal operating range. Power dissipation in MOSFETs is Id2 RDS(on), and in bipolars it is ICVCE(sat). At low currents, therefore, a power MOSFET may have a lower conduction loss than a comparable bipolar device, but at higher currents, the conduction loss will exceed that of bipolars. Also, the RDS(on) increases with temperature. An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In MOSFETs, RDS(on) increases with temperature, and thus the current is automatically diverted away from the hot spot. The drain body junction appears as an antiparallel diode between source and drain. Thus power MOSFETs will not support voltage in the reverse direction. Although this inverse diode is relatively fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as 100 ns. Since MOSFETs cannot be protected by fuses, an electronic protection technique has to be used. With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional MOSFETs. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating within its specification range at all times, its chances for failing catastrophically are minimal. However, if its absolute maximum rating is exceeded, failure probability increases dramatically. Under actual operating conditions, a MOSFET may be subjected to transients — either externally from the power bus supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the absolute maximum ratings. Such conditions are likely in almost every application, and in most cases are beyond a designer’s control. Rugged devices are made to be more tolerant for over-voltage transients. Ruggedness is the ability of a MOSFET to operate in an environment of dynamic electrical stresses, without activating any of the parasitic bipolar junction transistors. The rugged device can withstand higher levels of diode recovery dv/dt and static dv/dt. Insulated-Gate Bipolar Transistor (IGBT) The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic (low saturation voltage) of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current FIGURE 30.5 Power MOSFET circuit symbol. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 7. © 1992 IEEE.)
that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum nction temperature of the device under all conditions for guaranteed reliable operation. The on- tate voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied In general, IGBTs can be classified as punch EMITTERN GATE EMITTER N+ GATE through(PT) and nonpunch-through(NPT) struc- Ires, as shown in Fig. 30.6. In the PT IGBT, an N uffer layer is normally introduced between the P+ substrate and the N-epitaxial layer, so that the whole N- drift region is depleted when the device is blocking N-Base or Epitaxial Drift Regis the off-state voltage, and the electrical field shape Base or Epitaxial Drift Regi gion is close to a rectangular punch-through IGBT, a better trade-off between the Substrate forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V. COLLECTOR High voltage IGBTs are realized through non punch-through process. The devices are built ona n- wafer substrate which serves as the n- base drift region. Experimental NPT IGBTs of up to about 4 KV have been reported in the literature. NPT IGBTs more robust than PT IGBTs particularly under short circuit conditions. But NPT IGBTs have a higher for ward voltage drop than the PT IGBTs The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of parallel-connected IGBTs are (1)on-state current unbalance, caused by Va(sat) distribution and main FIGURE 30.6 Nonpunch-through IGBT,(b)Punch- circuit wiring resistance distribution, and(2)current through IGBT, (c)IGBT equivalent circuit unbalance at turn-on and turn-off, caused by the switching time difference of the parallel connected devices and circuit wiring inductance distribution. The NPT IGBTs can be paralleled because of their positive temperature coefficient property MOS-Controlled Thyristor (MCT The MCT is a new type of power semiconductor device that combines the capabilities of thyristor voltage and current with MOS gated turn-on and turn-off. It is a high power, high frequency, low conduction drop and ugged device, which is more likely to be used in the future for medium and high power applications. A cross sectional structure of a p-type MCT with its circuit schematic is shown in Fig. 30.7. The MCT has a thyristor type structure with three junctions and PNPn layers between the anode and cathode In a practical MCT, about 00,000 cells similar to the one shown are paralleled to achieve the desired current rating. MCT is turned on by a negative voltage pulse at the gate with respect to the anode, and is turned off by a positive voltage pulse The MCT was announced by the General Electric R D Center on November 30, 1988. Harris Semiconductor Corporation has developed two generations of p-MCTs Gen-1 p-MCTs are available at 65 A/1000 V and 75A/600 V with peak controllable current of 120 A Gen-2 p-MCTs are being developed at similar current and voltage ratings, with much improved turn-on capability and switching speed. The reason for developing p-MCT is the fact that the current density that can be turned off is 2 or 3 times higher than that of an n-MCT; but n-MCTs are the ones needed for many practical applications. Harris Semiconductor Corporation is in the process of developing n-MCTS, which are expected to be commercially available during the next one to two years C 2000 by CRC Press LLC
© 2000 by CRC Press LLC that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum junction temperature of the device under all conditions for guaranteed reliable operation. The onstate voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied. In general, IGBTs can be classified as punchthrough (PT) and nonpunch-through (NPT) structures, as shown in Fig. 30.6. In the PT IGBT, an N+ buffer layer is normally introduced between the P+ substrate and the N– epitaxial layer, so that the whole N– drift region is depleted when the device is blocking the off-state voltage, and the electrical field shape inside the N– drift region is close to a rectangular shape. Because a shorter N– region can be used in the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V. High voltage IGBTs are realized through nonpunch-through process. The devices are built on a N– wafer substrate which serves as the N– base drift region. Experimental NPT IGBTs of up to about 4 KV have been reported in the literature. NPT IGBTs are more robust than PT IGBTs particularly under short circuit conditions. But NPT IGBTs have a higher forward voltage drop than the PT IGBTs. The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of parallel-connected IGBTs are (1) on-state current unbalance, caused by VCE(sat) distribution and main circuit wiring resistance distribution, and (2) current unbalance at turn-on and turn-off, caused by the switching time difference of the parallel connected devices and circuit wiring inductance distribution. The NPT IGBTs can be paralleled because of their positive temperature coefficient property. MOS-Controlled Thyristor (MCT) The MCT is a new type of power semiconductor device that combines the capabilities of thyristor voltage and current with MOS gated turn-on and turn-off. It is a high power, high frequency, low conduction drop and a rugged device, which is more likely to be used in the future for medium and high power applications. A cross sectional structure of a p-type MCT with its circuit schematic is shown in Fig. 30.7. The MCT has a thyristor type structure with three junctions and PNPN layers between the anode and cathode. In a practical MCT, about 100,000 cells similar to the one shown are paralleled to achieve the desired current rating. MCT is turned on by a negative voltage pulse at the gate with respect to the anode, and is turned off by a positive voltage pulse. The MCT was announced by the General Electric R & D Center on November 30, 1988.Harris Semiconductor Corporation has developed two generations of p-MCTs. Gen-1 p-MCTs are available at 65 A/1000 V and 75A/600 V with peak controllable current of 120 A. Gen-2 p-MCTs are being developed at similar current and voltage ratings, with much improved turn-on capability and switching speed. The reason for developing p-MCT is the fact that the current density that can be turned off is 2 or 3 times higher than that of an n-MCT; but n-MCTs are the ones needed for many practical applications. Harris Semiconductor Corporation is in the process of developing n-MCTs, which are expected to be commercially available during the next one to two years. FIGURE 30.6 Nonpunch-through IGBT, (b) Punchthrough IGBT, (c) IGBT equivalent circuit
TYPICAL CELL CIRCUIT SCHEMATIC CROSS SECTION FOR P-MCT MCT OFF-FET IGBT OFF-FET CHANNEL ON-FET LOWER BASE WDE SE LOWER BASE BUFFER LOWER EMITER CK METAL ○ CATHOOE FIGURE 30.8 Current and future pwer semiconductor devices development direction(Source: A Q. Huang, FIGURE 30.7 ( Source: Harris Semiconductor, User Recent Developments of Power Semiconductor Devices Guide of mos controlled Thyristor, With permission.) VPEC Seminar Proceedings, Pp. 1-9. with permission. The advantage of an MCT over-IGBT is its low forward voltage drop. N-type MCTs will be expected to have a similar forward voltage drop, but with an improved reverse bias safe operating area and switching speed MCTs have relatively low switching times and storage time. The MCT is capable of high current densities and blocking voltages in both directions. Since the power gain of an MCT is extremely high, it could be driven directly from logic gates. An MCT has high dildt(of the order of 2500 A/us)and high dv/dr(of the order of 20,000 V/us) capability. The MCT, because of its superior characteristics, shows a tremendous possibility for applications such as motor drives, uninterrupted power supplies, static VAR compensators, and high pe active power line conditioners The current and future power semiconductor devices developmental direction is shown in Fig. 30.8. High temperature operation capability and low forward voltage drop operation can be obtained if silicon is by silicon carbide material for producing power devices. The silicon carbide has a higher band gap tha Hence higher breakdown voltage devices could be developed. Silicon carbide devices have excellent characteristics and stable blocking voltages at higher temperatures. But the silicon carbide devices are still in the very early stages of development. Defining Terms di/dt limit: Maximum allowed rate of change of current through a device. If this limit is exceeded, the devie may not be guaranteed to work reliabl dv/dt: Rate of change of voltage withstand capability without spurious turn-on of the device Forward voltage: The voltage across the device when the anode is positive with respect to the cathode. t: Represents available thermal energy resulting from current flow. reverse voltage: The voltage across the device when the anode is negative with respect to the cathode Related Topic 5.1 Diodes and Rectifie References B K. Bose, Modern Power Electronics: Evaluation, Technology and Applications, New York: IEEE Press, 1992. Harris Semiconductor, USser's Guide of MOS Controlled Thyrist c 2000 by CRC Press LLC
© 2000 by CRC Press LLC The advantage of an MCT over-IGBT is its low forward voltage drop. N-type MCTs will be expected to have a similar forward voltage drop, but with an improved reverse bias safe operating area and switching speed. MCTs have relatively low switching times and storage time. The MCT is capable of high current densities and blocking voltages in both directions. Since the power gain of an MCT is extremely high, it could be driven directly from logic gates.An MCT has high di/dt (of the order of 2500 A/ms) and high dv/dt (of the order of 20,000 V/ms) capability. The MCT, because of its superior characteristics, shows a tremendous possibility for applications such as motor drives, uninterrupted power supplies, static VAR compensators, and high power active power line conditioners. The current and future power semiconductor devices developmental direction is shown in Fig. 30.8. High temperature operation capability and low forward voltage drop operation can be obtained if silicon is replaced by silicon carbide material for producing power devices. The silicon carbide has a higher band gap than silicon. Hence higher breakdown voltage devices could be developed. Silicon carbide devices have excellent switching characteristics and stable blocking voltages at higher temperatures. But the silicon carbide devices are still in the very early stages of development. Defining Terms di/dt limit: Maximum allowed rate of change of current through a device. If this limit is exceeded, the device may not be guaranteed to work reliably. dv/dt: Rate of change of voltage withstand capability without spurious turn-on of the device. Forward voltage: The voltage across the device when the anode is positive with respect to the cathode. I2t: Represents available thermal energy resulting from current flow. Reverse voltage: The voltage across the device when the anode is negative with respect to the cathode. Related Topic 5.1 Diodes and Rectifiers References B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, New York: IEEE Press, 1992. Harris Semiconductor, User’s Guide of MOS Controlled Thyristor. FIGURE 30.8 Current and future pwer semiconductor devices development direction (Source: A.Q. Huang, Recent Developments of Power Semiconductor Devices, VPEC Seminar Proceedings, pp. 1–9. With permission.) FIGURE 30.7 (Source: Harris Semiconductor, User’s Guide of MOS Controlled Thyristor, With permission.)
A.Q. Huang, Recent Developments of Power Semiconductor Devices, VPEC Seminar Proceedings, Pp. 1-9, Sep- N. Mohan and T. Undeland, Power Electronics: Converters, Applications, and Design, New York: John Wiley J. Wojslawowicz, Ruggedized transistors emerging as power MOSFET standard-bearers, Power Technics Mag- azine, pp 29-32, January 1988. Further Information B M. Bird and K G. King, An Introduction to Power Electronics, New York: Wiley-Interscience, 1984 R Sittig and P. Roggwiller, Semiconductor Devices for Power Conditioning, New York: Plenum, 1982. V.A.K. Temple, " Advances in MOS controlled thyristor technology and capability, Power Conversion, pp 544-554,Oct.1989 B W. Williams, Power Electronics, Devices, Drivers and Applications, New York: John Wiley, 1987. 30.2 Power Conversion Kaushik rajashekara Power conversion deals with the process of converting electric power from one form to another. The power ctronic apparatuses performing the power conversion are called power converters Because they contain no moving parts, they are often referred to as static power converters. The power conversion is achieved using ower semiconductor devices, which are used as switches. The power devices used are SCRs(silicon controlled rectifiers,or thyristors), triacs, power transistors, power MOSFETs, insulated gate bipolar transistors(IGBTs) and MCTs(MOS-controlled thyristors). The power converters are generally classified as 1. ac-dc converters(phase-controlled converters) 2. direct ac-ac converters(cycloconverters) 3. dc-ac converters(inverters) 4. dc-dc converters(choppers, buck and boost converters) AC-DC Converters The basic function of a phase-controlled converter is to convert an alternating voltage of variable amplitude and frequency to a variable dc voltage. The power devices used for this application are generally SCRs. The average value of the output voltage is controlled by varying the conduction time of the SCRs. The turn-on of the SCR is achieved by providing a gate pulse when it is forward-biased. The turn-off is achieved by the commutation of current from one device to another at the instant the incoming ac voltage has a higher instantaneous potential than that of the outgoing wave. Thus there is a natural tendency for current to be commutated from the outgoing to the incoming SCR, without the aid of any external commutation circuitry. This commutation process is often referred to as natural commutation. A single-phase half-wave converter is shown in Fig. 30.9. When the SCR is turned on at an angle a, full supply voltage(neglecting the SCR drop) is applied to the load. For a purely resistive load, during the positive half cycle, the output voltage waveform follows the input ac voltage waveform. During the negative half cycle the SCR is turned off. In the case of inductive load, the energy stored in the inductance causes the current to flow in the load circuit even after the reversal of the supply voltage, as shown in Fig. 30.9(b). If there is no off the SCR as soon as the input voltage polarity reverses, as shown in Fig. 30.9(c). When the SCR is of o freewheeling diode D, the load current is discontinuous. A freewheeling diode is connected across the load t the load current will freewheel through the diode. The power flows from the input to the load only when the SCR is conducting. If there is no freewheeling diode, during the negative portion of the supply voltage, SCR returns the energy stored in the load inductance to the supply. The freewheeling diode improves the input c 2000 by CRC Press LLC
© 2000 by CRC Press LLC A.Q. Huang, Recent Developments of Power Semiconductor Devices, VPEC Seminar Proceedings, pp. 1–9, September 1995. N. Mohan and T. Undeland, Power Electronics: Converters, Applications, and Design, New York: John Wiley & Sons, 1995. J. Wojslawowicz, “Ruggedized transistors emerging as power MOSFET standard-bearers,” Power Technics Magazine, pp. 29–32, January 1988. Further Information B.M. Bird and K.G. King, An Introduction to Power Electronics, New York: Wiley-Interscience, 1984. R. Sittig and P. Roggwiller, Semiconductor Devices for Power Conditioning, New York: Plenum, 1982. V.A.K. Temple, “Advances in MOS controlled thyristor technology and capability,” Power Conversion, pp. 544–554, Oct. 1989. B.W. Williams, Power Electronics, Devices, Drivers and Applications, New York: John Wiley, 1987. 30.2 Power Conversion Kaushik Rajashekara Power conversion deals with the process of converting electric power from one form to another. The power electronic apparatuses performing the power conversion are called power converters. Because they contain no moving parts, they are often referred to as static power converters. The power conversion is achieved using power semiconductor devices, which are used as switches. The power devices used are SCRs (silicon controlled rectifiers, or thyristors), triacs, power transistors, power MOSFETs, insulated gate bipolar transistors (IGBTs), and MCTs (MOS-controlled thyristors). The power converters are generally classified as: 1. ac-dc converters (phase-controlled converters) 2. direct ac-ac converters (cycloconverters) 3. dc-ac converters (inverters) 4. dc-dc converters (choppers, buck and boost converters) AC-DC Converters The basic function of a phase-controlled converter is to convert an alternating voltage of variable amplitude and frequency to a variable dc voltage. The power devices used for this application are generally SCRs. The average value of the output voltage is controlled by varying the conduction time of the SCRs. The turn-on of the SCR is achieved by providing a gate pulse when it is forward-biased. The turn-off is achieved by the commutation of current from one device to another at the instant the incoming ac voltage has a higher instantaneous potential than that of the outgoing wave. Thus there is a natural tendency for current to be commutated from the outgoing to the incoming SCR, without the aid of any external commutation circuitry. This commutation process is often referred to as natural commutation. A single-phase half-wave converter is shown in Fig. 30.9. When the SCR is turned on at an angle a, full supply voltage (neglecting the SCR drop) is applied to the load. For a purely resistive load, during the positive half cycle, the output voltage waveform follows the input ac voltage waveform. During the negative half cycle, the SCR is turned off. In the case of inductive load, the energy stored in the inductance causes the current to flow in the load circuit even after the reversal of the supply voltage, as shown in Fig. 30.9(b). If there is no freewheeling diode DF , the load current is discontinuous. A freewheeling diode is connected across the load to turn off the SCR as soon as the input voltage polarity reverses, as shown in Fig. 30.9(c). When the SCR is off, the load current will freewheel through the diode. The power flows from the input to the load only when the SCR is conducting. If there is no freewheeling diode, during the negative portion of the supply voltage, SCR returns the energy stored in the load inductance to the supply. The freewheeling diode improves the input power factor
Fa L FIGURE 30.9 Single-Phase half-wave converter with freewheeling diode (a)Circuit diagram;(b)waveform for inductive load with no freewheeling diode; (c)waveform with freewheeling diode The controlled full-wave dc output may be obtained by using either a center tap transformer(Fig. 30.10)or by bridge configuration(Fig. 30. 11). The bridge configuration is often used when a transformer is undesirable and the magnitude of the supply voltage properly meets the load voltage requirements. The average output voltage of a single-phase full-wave converter for continuous current conduction is given where Em is the peak value of the input voltage and a is the firing angle. The output voltage of a single-phase bridge circuit is the same as that shown in Fig. 30.10. Various configurations of the single-phase bridge circuit can be obtained if, instead of four SCRs, two diodes and two SCRs are used, with or without freewheeling diodes. A three-phase full-wave converter consisting of six thyristor switches is shown in Fig. 30.12(a). This is the most commonly used three-phase bridge configuration. Thyristors TT, and Ts are turned on during the positive half cycle of the voltages of the phases to which they are connected, and thyristors T2, Ta, and ts are urned on during the negative half cycle of the phase voltages. The reference for the angle in each cycle is at the crossing points of the phase voltages. The ideal output voltage, output current, and input current waveforms are shown in Fig. 30.12(b). The output dc voltage is controlled by varying the firing angle a. The average output voltage under continuous current conduction operation is given by E cOS aL where Em is the peak value of the phase voltage. At a=90, the output voltage is zero. For 0< a<90%, v, is positive and power flows from ac supply to the load. For 90< a< 180%,v, is negative and the converter operates in the inversion mode. If the load is a dc motor, the power can be transferred from the motor to the ac supply, a process known as re
© 2000 by CRC Press LLC The controlled full-wave dc output may be obtained by using either a center tap transformer (Fig. 30.10) or by bridge configuration (Fig. 30.11). The bridge configuration is often used when a transformer is undesirable and the magnitude of the supply voltage properly meets the load voltage requirements. The average output voltage of a single-phase full-wave converter for continuous current conduction is given by where Em is the peak value of the input voltage and a is the firing angle. The output voltage of a single-phase bridge circuit is the same as that shown in Fig. 30.10. Various configurations of the single-phase bridge circuit can be obtained if, instead of four SCRs, two diodes and two SCRs are used,with or without freewheeling diodes. A three-phase full-wave converter consisting of six thyristor switches is shown in Fig. 30.12(a). This is the most commonly used three-phase bridge configuration. Thyristors T1, T3, and T5 are turned on during the positive half cycle of the voltages of the phases to which they are connected, and thyristors T2, T4, and T6 are turned on during the negative half cycle of the phase voltages. The reference for the angle in each cycle is at the crossing points of the phase voltages. The ideal output voltage, output current, and input current waveforms are shown in Fig. 30.12(b). The output dc voltage is controlled by varying the firing angle a. The average output voltage under continuous current conduction operation is given by where Em is the peak value of the phase voltage. At a = 90°, the output voltage is zero. For 0 < a < 90°, vo is positive and power flows from ac supply to the load. For 90° < a < 180°, vo is negative and the converter operates in the inversion mode. If the load is a dc motor, the power can be transferred from the motor to the ac supply, a process known as regeneration. FIGURE 30.9 Single-phase half-wave converter with freewheeling diode. (a) Circuit diagram; (b) waveform for inductive load with no freewheeling diode; (c) waveform with freewheeling diode. v E d m a p = 2 cos a v E o m = 3 3 p cos a