Rajashekara, K, Bhat, A.K.s., Bose, B K " Power Electronics The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Rajashekara, K., Bhat, A.K.S., Bose, B.K. “Power Electronics” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
30 Power electronics 30.1 Power Semiconductor Devices Thyristor and Triac. Gate Turn-Off Thyristor(GTO).Reverse- Conducting Thyristor(RCT)and Asymmetrical Silicon-Controlled Rectifier (ASCR). Power Transistor. Power MOSFET Insulated-Gate Bipolar Transistor(IGBT).MOS Controlled Thyristor(MCT) Kaushik rajashekara 30.2 Power Conversion elphi Energy e Engine AC-DC Converters. Cycloconverters. DC-to-AC Management Systems Converters DC-DC Converters Ashoka K.s. bhat 30.3 Power Supplies DC Power Supplies. AC Power Supplies.Special Power Supplies 30.4 Converter Control of Machines Bimal K. bose Converter Control of DC machines Converter Control of ac University of Tennessee Machines 30.1 Power Semiconductor devices Kaushik rajashekara The modern age of power electronics began with the introduction of thyristors in the late 1950s. Now there several types of power devices available for high-power and high-frequency applications. The most notable ower devices are gate turn-off thyristors, power Darlington transistors, power MOSFETS, and insulated -ga bipolar transistors(IGBTs). Power semiconductor devices are the most important functional elements in all power conversion applications. The power devices are mainly used as switches to convert power from one form to another. They are used in motor control systems, uninterrupted power supplies, high-voltage dc transmission, power supplies, induction heating, and in many other power conversion applications. A review of the basic characteristics of these power devices is presented in this section Thyristor and Triac The thyristor, also called a silicon-controlled rectifier(SCR), is basically a four-layer three-junction pnpn device It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device. The turn-off is achieved by applying a reverse voltage across the anode and cathode The thyristor symbol and its volt-ampere haracteristics are shown in Fig. 30. 1. There are basically two classifications of thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter-grade thyristor is the low turn-off time(on the order of a few microseconds) for the latter. The converter-grade thyristors are slow type and are used in natural commutation(or phase-controlled)applications. Inverter-grade thyristors are used in forced commutation applications such as dc-dc choppers and dc-ac inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an external commutation circuit. This requires additional commutating components, thus resulting in additional losses in the inverter. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 30 Power Electronics 30.1 Power Semiconductor Devices Thyristor and Triac • Gate Turn-Off Thyristor (GTO) • ReverseConducting Thyristor (RCT) and Asymmetrical Silicon- Controlled Rectifier (ASCR) • Power Transistor • Power MOSFET • Insulated-Gate Bipolar Transistor (IGBT) • MOS Controlled Thyristor (MCT) 30.2 Power Conversion AC-DC Converters • Cycloconverters • DC-to-AC Converters • DC-DC Converters 30.3 Power Supplies DC Power Supplies • AC Power Supplies • Special Power Supplies 30.4 Converter Control of Machines Converter Control of DC Machines • Converter Control of AC Machines 30.1 Power Semiconductor Devices Kaushik Rajashekara The modern age of power electronics began with the introduction of thyristors in the late 1950s. Now there are several types of power devices available for high-power and high-frequency applications. The most notable power devices are gate turn-off thyristors, power Darlington transistors, power MOSFETs, and insulated-gate bipolar transistors (IGBTs). Power semiconductor devices are the most important functional elements in all power conversion applications. The power devices are mainly used as switches to convert power from one form to another. They are used in motor control systems, uninterrupted power supplies, high-voltage dc transmission, power supplies, induction heating, and in many other power conversion applications. A review of the basic characteristics of these power devices is presented in this section. Thyristor and Triac The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpn device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device. The turn-off is achieved by applying a reverse voltage across the anode and cathode. The thyristor symbol and its volt-ampere characteristics are shown in Fig. 30.1. There are basically two classifications of thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter-grade thyristor is the low turn-off time (on the order of a few microseconds) for the latter. The converter-grade thyristors are slow type and are used in natural commutation (or phase-controlled) applications. Inverter-grade thyristors are used in forced commutation applications such as dc-dc choppers and dc-ac inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an external commutation circuit. This requires additional commutating components, thus resulting in additional losses in the inverter. Kaushik Rajashekara Delphi Energy & Engine Management Systems Ashoka K. S. Bhat University of Victoria Bimal K. Bose University of Tennessee
Forward FIGURE 30.1 (a) Thyristor symbol and(b) volt-ampere characteristics. Source: B K. Bose, Modern Power Electronics Evaluation, Technology and Applications, P 5.0 1992 IEEE) Thyristors are highly rugged devices in terms of transient currents, dildt, and dw/dt capability. The fc voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 1000 A, it exceeds 3 V. While the forward voltage determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device junction temperature at high operating frequencies. Because of this, the maximum switching frequencies possible using thyristors are limited in comparison with other power devices considered in this section Thyristors have I2t withstand capability and can be protected by fuses. The nonrepetitive surge current capability for thyristors is about 10 times their rated root mean square(rms)current. They must be protected by snubber networks for dv/dt and di/dt effects. If the specified dv/dt is exceeded, thyristors may start conducting without applying a gate pulse In dc-to-ac conversion applications it is necessary to use an antiparallel diode of similar rating across each main thyristor. Thyristors are available up to 6000 V, 3500A A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol and volt-ampere characteristics are shown in Fig. 30. 2. Because of the integration, the triac has poor reapplied dv/dt, curren tivity at turn-on, and longer turn-off time. Triacs are mainly used in phase control applications such as in ac regulators for lighting and fan control and in solid-state ac relays Gate Turn-Off Thyristor (GTO The Gto is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for high frequency operation than thyristors. The GTO symbol and turn-off characteristics are shown in Fig. 30.3 GTOs have the Pt withstand capability and hence can be protected by semiconductor fuses. For reliable operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber circuit. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Thyristors are highly rugged devices in terms of transient currents, di/dt, and dv/dt capability. The forward voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 1000 A, it seldom exceeds 3 V. While the forward voltage determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device junction temperature at high operating frequencies. Because of this, the maximum switching frequencies possible using thyristors are limited in comparison with other power devices considered in this section. Thyristors have I 2t withstand capability and can be protected by fuses. The nonrepetitive surge current capability for thyristors is about 10 times their rated root mean square (rms) current. They must be protected by snubber networks for dv/dt and di/dt effects. If the specified dv/dt is exceeded, thyristors may start conducting without applying a gate pulse. In dc-to-ac conversion applications it is necessary to use an antiparallel diode of similar rating across each main thyristor. Thyristors are available up to 6000 V, 3500 A. A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol and volt-ampere characteristics are shown in Fig. 30.2. Because of the integration, the triac has poor reapplied dv/dt, poor gate current sensitivity at turn-on, and longer turn-off time. Triacs are mainly used in phase control applications such as in ac regulators for lighting and fan control and in solid-state ac relays. Gate Turn-Off Thyristor (GTO) The GTO is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for highfrequency operation than thyristors. The GTO symbol and turn-off characteristics are shown in Fig. 30.3. GTOs have the I2 t withstand capability and hence can be protected by semiconductor fuses. For reliable operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber circuit. FIGURE 30.1 (a) Thyristor symbol and (b) volt-ampere characteristics. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 5. © 1992 IEEE.)
G MI-mode FIGURE 30.2 (a)Triac symbol and(b) volt-ampere characteristics. Source: B K. Bose, Modern Power Electronics: Evalu ation, Technology and Applications, p 5. e 1992 IEEE. Anode A IGURE 30.3 (a)GTO symbol and(b)turn-off characteristics. Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 5.@ 1992 IEEE) A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current GTO may require as high as 500 A of reverse gate current. Also, a gto has the tendency to latch at temperatures above 125". GTOs are available up to about 4500 V, 2500 A. Reverse-Conducting Thyristor(RCT)and Asymmetrical Silicon-Controlled Rectifier(ASCR) Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commutation/free wheeling purposes. In RCTs, the diode is integrated with a fast switching thyristor in a single silicon chip. Thus, c 2000 by CRC Press LLC
© 2000 by CRC Press LLC A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current GTO may require as high as 500 A of reverse gate current. Also, a GTO has the tendency to latch at temperatures above 125°C. GTOs are available up to about 4500 V, 2500 A. Reverse-Conducting Thyristor (RCT) and Asymmetrical Silicon-Controlled Rectifier (ASCR) Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commutation/freewheeling purposes. In RCTs, the diode is integrated with a fast switching thyristor in a single silicon chip. Thus, FIGURE 30.2 (a) Triac symbol and (b) volt-ampere characteristics. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 5. © 1992 IEEE.) FIGURE 30.3 (a) GTO symbol and (b) turn-off characteristics. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 5. © 1992 IEEE.)
ne number of power devices could be reduced. This integration brings forth a substantial improvement of the static and dynamic characteristics as well as its overall circuit performance The RCTs are designed mainly for specific applications such as traction drives. The antiparallel diode lin the reverse voltage across the thyristor to 1 to 2 V. Also, because of the reverse recovery behavior of the diodes, the thyristor may see very high reapplied dv/dt when the diode recovers from its reverse voltage. This necessitates se of large RC snubber networks to suppress voltage transients. As the range of application of thyristors and diodes extends into higher frequencies, their reverse recovery charge becomes increasingly important. Higl recovery charge results in high power dissipation during switching. The ASCR has a similar forward blocking capability as an inverter-grade thyristor, but it has a limited reverse blocking(about 20-30 V)capability. It has an on-state voltage drop of about 25% less than an inverter-grade thyristor of a similar rating. The ASCR features a fast turn-off time; thus it can work at a higher frequency n an SCR. Since the turn-off time is down by a factor of nearly 2, the size of the commutating components can be halved. Because of this, the switching losses will also be low Gate-assisted turn-off techniques are used to even further reduce the turn-off time of an ASCR. The appli cation of a negative voltage to the gate during turn -off helps to evacuate stored charge in the device and aids the recovery mechanisms. This will in effect reduce the turn-off time by a factor of up to 2 over the conventional Power transist Power transistors are used in applications ranging from a few to several hundred kilowatts and switchin frequencies up to about 10 kHz. Power transistors used in power conversion applications are generally npn type. The power transistor is turned on by supplying sufficient base current, and this base drive has to be maintained throughout its conduction off by remo ng voltage slightly negative(within -VBE max). The saturation voltage of the device is normally 0.5 to 2.5V and increases as the current increases. Hence the on-state losses increase more than proportionately with current. The transistor off-state losses are much lower than the on-state losses because the leakage current of the device is of the order of a few milliamperes. Because of relatively larger switching times, the switching loss significantly increases with switching frequency Power transistors can block only forward voltages. The reverse peak voltage rating of these devices is as low as 5 to 10 V. Power transistors do not have Ft withstand capability. In other words, they can absorb only very little energy before breakdown. Therefore, they cannot be protected by semiconductor fuses, and thus an electronic pro- tection method has to be used To eliminate high base current requirements, Darlington con figurations are commonly used. They are available in monolithic or in isolated packages. The basic Darlington configuration is shown schematically in Fig. 30.4. The Darlington configuration resents a specific advantage in that it can considerably Increase the current switched by the transistor for a given base drive. The 1 Transat for the Darlington is generally more than that of a single transistor of similar rating with corresponding increase in on state power loss. During switching, the reverse-biased collector junction may show hot spot breakdown effects that are specified by reverse-bias safe operating area(RBSOA)and forward bias safe operating area(FBSOA). Modern devices with highly inter- digited emitter base geometry force more uniform current dis- FIGURE 30.4 A two-stage Darlington transis- tribution and therefore considerably improve second breakdown tor with bypass diode.(Source:BKBose,Moo effects. Normally, a well-designed switching aid network con- ern Power Electronics: Evaluation, Technology strains the device operation well within the soas and Applications, P 6.0 1992 IEEE. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC the number of power devices could be reduced. This integration brings forth a substantial improvement of the static and dynamic characteristics as well as its overall circuit performance. The RCTs are designed mainly for specific applications such as traction drives. The antiparallel diode limits the reverse voltage across the thyristor to 1 to 2 V. Also, because of the reverse recovery behavior of the diodes, the thyristor may see very high reapplied dv/dt when the diode recovers from its reverse voltage. This necessitates use of large RC snubber networks to suppress voltage transients. As the range of application of thyristors and diodes extends into higher frequencies, their reverse recovery charge becomes increasingly important. High reverse recovery charge results in high power dissipation during switching. The ASCR has a similar forward blocking capability as an inverter-grade thyristor, but it has a limited reverse blocking (about 20–30 V) capability. It has an on-state voltage drop of about 25% less than an inverter-grade thyristor of a similar rating. The ASCR features a fast turn-off time; thus it can work at a higher frequency than an SCR. Since the turn-off time is down by a factor of nearly 2, the size of the commutating components can be halved. Because of this, the switching losses will also be low. Gate-assisted turn-off techniques are used to even further reduce the turn-off time of an ASCR. The application of a negative voltage to the gate during turn-off helps to evacuate stored charge in the device and aids the recovery mechanisms. This will in effect reduce the turn-off time by a factor of up to 2 over the conventional device. Power Transistor Power transistors are used in applications ranging from a few to several hundred kilowatts and switching frequencies up to about 10 kHz. Power transistors used in power conversion applications are generally npn type. The power transistor is turned on by supplying sufficient base current, and this base drive has to be maintained throughout its conduction period. It is turned off by removing the base drive and making the base voltage slightly negative (within –VBE(max)). The saturation voltage of the device is normally 0.5 to 2.5 V and increases as the current increases. Hence the on-state losses increase more than proportionately with current. The transistor off-state losses are much lower than the on-state losses because the leakage current of the device is of the order of a few milliamperes. Because of relatively larger switching times, the switching loss significantly increases with switching frequency. Power transistors can block only forward voltages. The reverse peak voltage rating of these devices is as low as 5 to 10 V. Power transistors do not have I2 t withstand capability. In other words, they can absorb only very little energy before breakdown. Therefore, they cannot be protected by semiconductor fuses, and thus an electronic protection method has to be used. To eliminate high base current requirements, Darlington con- figurations are commonly used. They are available in monolithic or in isolated packages. The basic Darlington configuration is shown schematically in Fig. 30.4. The Darlington configuration presents a specific advantage in that it can considerably increase the current switched by the transistor for a given base drive. The VCE(sat) for the Darlington is generally more than that of a single transistor of similar rating with corresponding increase in onstate power loss. During switching, the reverse-biased collector junction may show hot spot breakdown effects that are specified by reverse-bias safe operating area (RBSOA) and forward bias safe operating area (FBSOA). Modern devices with highly interdigited emitter base geometry force more uniform current distribution and therefore considerably improve second breakdown effects. Normally, a well-designed switching aid network constrains the device operation well within the SOAs. FIGURE 30.4 A two-stage Darlington transistor with bypass diode. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 6. © 1992 IEEE.)