Rolls-Royce Corporation
University of Central Florida
23.3 Reverse-Conducting Thyristor and Asymmetrical Silicon-Controlled Rectifier
23.4 Power Bipolar Junction Transistor
23.6 Insulated-Gate Bipolar Transistor
23.7 Integrated Gate-Commutated Thyristor
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 (GTOs), power bipolar junction transistors (BJTs), power MOSFETs, insulated-gate bipolar transistors (IGBTs), and integrated gate-commutated thyristors (IGCT). 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 widely used in utility power applications such as power quality conditioning, renewable power source integration, high-voltage DC transmission (HVDC), and flexible AC transmission systems (FACTS) as well as motor drives, uninterrupted power supplies, power supplies, induction heating, and many other power conversion applications. A review of the basic characteristics of these power devices is presented in this section.
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 Figure 23.1. There are basically two classifications of thyristors: phase-controlled thyristors and fast-switching thyristors. The difference between a phase-controlled and fast-switching thyristor is the low turn-off time (on the order of a few microseconds) for the latter. The phase-controlled thyristors are slow type and are used in natural commutation (or phase-controlled) applications. Fast-switching thyristors are used in forced commutation applications such as DC–DC choppers and DC–AC inverters. The phase-controlled 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.
FIGURE 23.1 (a) Thyristor symbol and (b) volt-ampere characteristics. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, p. 5. © 1992.)
Thyristors are highly rugged devices in terms of transient currents, di/dt, and dv/dt capability. The forward voltage drop in thyristors is about 1.5–2 V, and even at a current level of several thousand amperes. Thyristors are available up to a current rating of 7000 A and a voltage rating of 10 kV, providing the highest power rating among all power semiconductor devices. A high-power thyristor is typically fabricated on an entire silicon wafer up to 150 mm in diameter, and housed in a press pack package with very low thermal resistance and low electrical parasitic impedance. When assembled in series and parallel connections, thyristors can deliver much higher voltage and current capabilities. One such example is the thyristor valves used in HVDC converters that handle a voltage level of several hundreds of kV. A thyristor’s high-voltage and current capability can be attributed to the very high concentration of excess electrons and holes in the semiconductor switch in its forward conduction state or the so-called conductivity modulation. A semiconductor device is categorized as a bipolar type when its conduction current is made of both electron and hole currents. A thyristor is a bipolar type power semiconductor device with a very high level of conductivity modulation. However due to the nature of the excess electron and hole plasma, it takes a relatively long time to turn on and off the thyristor, resulting in a high switching power loss 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.
A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol and volt-ampere characteristics are shown in Figure 23.2. 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.
FIGURE 23.2 (a) Triac symbol and (b) volt-ampere characteristics. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, p. 5. © 1992.)
The GTO is a power thyristor 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 Figure 23.3.
GTOs have the I2t 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. A GTO has a poor turn-off current gain of the order of four to five. 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 6500 V, 4000 A.
FIGURE 23.3 (a) GTO symbol and (b) turn-off characteristics. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, p. 5. © 1992.)
23.3 Reverse-Conducting Thyristor and Asymmetrical Silicon-Controlled Rectifier
Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commutation/freewheeling purposes. In reverse-conducting thyristors (RCTs), the diode is integrated with a fast-switching thyristor in a single silicon wafer or chip. Thus, 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 converter applications such as traction drives or wind power converters. The antiparallel diode limits the reverse voltage across the thyristor to 1–2 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 the 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 asymmetrical silicon-controlled rectifier (ASCR) has similar forward blocking capability to a fast-switching thyristor, but it has a much lower reverse blocking capability. It has an on-state voltage drop of about 25% less than a fast-switching 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.
23.4 Power Bipolar Junction Transistor
Power BJTs are used in applications ranging from a few to several hundred kilowatts and switching frequencies up to about 10 kHz. They may be used in utility power systems as relay gear drivers or in auxiliary power supplies. Power BJTs 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 0.5–2.5 V and increases as the current increases. Hence, the on-state losses increase more than proportionately with current. A power BJT is a bipolar type power semiconductor device but with a conductivity modulation level below that of a thyristor. Because of relatively larger switching times, the switching loss significantly increases with switching frequency for power BJTs. Power BJTs can block only forward voltages. The reverse peak voltage rating of these devices is as low as 5–10 V. Power BJTs do not have I2t withstand capability. In other words, they can absorb only very little energy before device failure. 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 configurations are commonly used. They are available in monolithic or in isolated packages. The basic Darlington configuration is shown schematically in Figure 23.4. 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 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 interdigitated emitter base geometry force more uniform current distribution and therefore considerably improve the secondary breakdown performance. Normally, a well-designed switching aid network constrains the device operation well within the SOAs.
FIGURE 23.4 Two-stage Darlington transistor with bypass diode. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, p. 6. © 1992.)
Power MOSFETs are widely used in power supplies, relay drivers, or other auxiliary circuits in utility power systems. Due to the power rating limitation, power MOSFETs are not used in the main power conversion stage of the utility power systems. 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. 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 Figure 23.5.
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 IC VCE(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.
FIGURE 23.5 Power MOSFET circuit symbol. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, p. 7. © 1992.)
An important feature of a power MOSFET is the absence of a detrimental 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 BJTs. The rugged device can withstand higher levels of diode recovery dv/dt and static dv/dt.
23.6 Insulated-Gate Bipolar Transistor
The IGBT is a switching transistor controlled by a voltage applied to its gate terminal. Device operation and structure are similar to that of a power MOSFET. The principal difference is that the IGBT relies on conductivity modulation to reduce on-state conduction losses. The IGBT has high input impedance and fast turn-on speed like a MOSFET, but exhibits an on-state voltage drop and current-carrying capability comparable to that of a bipolar transistor while switching much faster. IGBTs have a clear advantage over MOSFETs in high-voltage applications where conduction losses must be minimized. Since the initial introduction of the IGBT into market in the mid 1980s, the semiconductor industry has made great technological advancement in improving device performance and reducing fabrication cost. IGBTs are available up to a current rating of several hundred amperes and a voltage rating of 6.5 kV, and widely used in power quality conditioning, renewable power source integration, motor drives, uninterrupted power supplies, power supplies, induction heating and many other power conversion applications.
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 on-state 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-through (PT) and nonpunch-through (NPT) structures, as shown in Figure 23.6. 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 a NPT process. The devices are built on an N− float-zone (FZ) wafer substrate which serves as the N− base drift region. Experimental NPT IGBTs of up to about 6.5 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.
FIGURE 23.6 (a) Nonpunch-through IGBT, (b) punch-through IGBT, (c) IGBT equivalent circuit.
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.
Trench gate structures were introduced into the IGBT to reduce the MOS channel resistance and the so-called JFET resistance in the mid 1990s. It was found that the IGBT structure with a deep trench gate and relatively wide cell pitch considerably enhances the electron injection efficiency at the emitter side by minimizing the back injection of holes into the P-base. The large cell pitch in this type of IGBT leads to a slight penalty in the MOS channel resistance, but is more than compensated by the significant reduction of the N-base conduction resistance. Alternatively, an N-type layer can be added under the P-base to block the hole back injection current and enhance electron injection. These two carrier enhancement approaches are widely adopted in the state-of-the-art IGBTs, providing an optimum stored excess carrier profile similar to that of an ideal PiN diode and hence a reduction in forward voltage. Thin wafer punch-through IGBTs represent the latest development in IGBT technology which features a short N-base and an N-buffer layer. The difference between this new type of IGBT and the conventional PT-IGBT is that the former uses a shallow P-emitter and no carrier lifetime killing techniques very much like the conventional NPT-IGBT.
23.7 Integrated Gate-Commutated Thyristor
The IGCT is a gate-controlled turn-off switch which turns off like a transistor but conducts like a thyristor with the lowest conduction losses. The fundamental difference between a conventional GTO and the IGCT lies in the very low inductance gate driver system, inherent to the IGCT. Ultra-low inductance has been achieved through the development of a new optimized housing and integrated gate driver concept. In the IGCT, the entire anode current is commutated from cathode to gate in a very short time. Since the npn-transistor is inactive thereafter, the pnp-transistor is deprived of base current, and turns off. The IGCT, therefore, turns off in a transistor mode, thus completely eliminating the current filamentation problems inherent in conventional GTOs. Additional advantages are a dramatic reduction of storage time to less than 2 μs, and a reduction in fall time to around 1 μs. Thus, the series connection of IGCTs is facilitated, compared to GTOs, by the very low dispersion associated with these times. The key to achieving “hard” turn-off of this nature is the duration of the time interval in which it occurs. The user thus only needs to connect the device to a 28–40 V power supply and optical fiber for on/off control. Because of the topology in which it is used, the IGCT produces negligible turn-on losses. In addition, IGCT enables operation at higher frequencies than formerly obtained by other high-power semiconductor devices (Figure 23.7).
The IGCTs are available up to about 6500 V, 4000 A, and is the power switching device of choice for demanding high-power applications such as medium voltage drives (MVD), wind power converters, STATCOMs, dynamic voltage restorers (DVR), solid state breakers, DC traction line boosters, traction power compensators, and interties.
The current and future power semiconductor devices developmental direction is shown in Figure 23.8. High-temperature operation capability and low forward voltage drop operation can be obtained if silicon is replaced by silicon carbide, gallium nitride, or other wide bandgap (WBG) semiconductor materials for producing power devices. The silicon carbide has a higher bandgap 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 WBG devices are still in the very early stages of development.
FIGURE 23.7 Equivalent circuit for IGCT.
FIGURE 23.8 Current and future power semiconductor devices development direction. (From Huang, A.Q., Recent developments of power semiconductor devices, in VPEC Seminar Proceedings, Blacksburg, VA, pp. 1–9, September 1995. With permission.)
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