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The development of semiconductor switching devices is essentially a search for the ideal switch. The effort has been made to reduce device power losses, increase switching frequencies, and simplify gate drive circuits. The evolution of the switching devices leads the pace of high-power converter development, and in the mean time the wide application of the high-power converters in industry drives the semi conductor technology toward higher power ratings with improved reliability and reduced cost.
There are two major types of high-power switching devices for use in various converters: the thyristor- and transistor-based devices. The former includes silicon controlled rectifier (SCR), gate turn-off thyristor (GTO), and gate commutated thyristor (GCT), while the latter embraces insulated gate bipolar transistor (IGBT) and injection-enhanced gate transistor (IEGT). Other devices such as power MOS FET, emitter turn-off thyristor (ETO), MOS-controlled thyristor (MCT), and static induction thyristor (SIT) have not gained significant importance in high-power applications.
Figure 1-1 shows the voltage and current ratings of major switching devices commercially available for high-power converters. Semiconductor manufacturers can offer SCRs rated at 12 kV/1.5 kA or 4.8 kV/5 kA. The GTO and GCT de vices can reach the voltage and current ratings of 6 kV and 6 kA. The ratings of IGBT devices are relatively low, but can reach as high as 6.5 kV/0.6 kA or 1.7 kV/3.6 kA.
In this section the characteristics of commonly used high-power semiconductor devices are introduced, the static and dynamic voltage equalization techniques for series connected devices are discussed, and the performance of these devices is compared.
2. HIGH-POWER SWITCHING DEVICES
High-power diodes can be generally classified into two types: (a) the general-purpose type for use in uncontrolled line-frequency rectifiers and (b) the fast recovery type used in voltage source converters as a freewheeling diode. These diodes are commercially available with two packaging techniques: press-pack and module diodes as shown in Fig. 2-1.
The device-heatsink assemblies for press-pack and module diodes are shown in Fig. 2-2. The press-pack diode features double-sided cooling with low thermal stress. For medium-voltage applications where a number of diodes may be connected in series, the diodes and their heatsinks can be assembled with just two bolts, leading to high power density and low assembly costs. This is one of the reasons for the continued popularity of press-pack semiconductors in the medium-voltage drives. The modular diode has an insulated baseplate with single-sided cooling, where a number of diodes can be mounted onto a single piece of heatsink.
2.2 Silicon-Controlled Rectifier (SCR)
The SCR is a thyristor-based device with three terminals: gate, anode, and cathode.
It can be turned on by applying a pulse of positive gate current with a short duration provided that it is forward-biased. Once the SCR is turned on, it is latched on. The device can be turned off by applying a negative anode current produced by its power circuit.
The SCR device can be used in phase-controlled rectifiers for PWM current source inverter-fed drives or load-commutated inverters for synchronous motor drives. Prior to the advent of self-extinguishable devices such as GTO and IGBT, the SCR was also used in forced commutated voltage source inverters.
The majority of high-power SCRs are of press-pack type.
The SCR modules with an insulated baseplate are more popular for low- and medium-power applications.
Fig. 2-4 shows the switching characteristics of the SCR device and typical waveforms for gate current iG, anode current iT, and anode-cathode voltage vT.
The turn-on process is initiated by applying a positive gate current iG to the SCR gate. The turn-on behavior is defined by delay time td, rise time tr and turn-on time tgt.
The turn-off process is initiated by applying a negative current to the switch at time instant t1, at which the anode current iT starts to fall. The negative current is produced by the utility voltage when the SCR is used in a rectifier or by the load voltage in a load commutated inverter. The turn-off transient is characterized by reverse recovery time trr, peak reverse recovery current Irr, reverse recovery charge Qrr, and turn-off time tq.
Table 2-1 lists the main specifications of a 12-kV/1.5-kA SCR device, where VDRM is the maximum repetitive peak off-state voltage, VRRM is the maximum repetitive peak reverse voltage, ITAVM is the maximum average on-state current, and ITRMS is the maximum rms on-state current. The turn-on time tgt is 14 _s and the turn-off time tq is 1200 _s. The rates of anode current rise diT/dt at turn-on and de vice voltage rise dvT/dt at turn-off are important parameters for converter design. To ensure a proper and reliable operation, the maximum limits for the diT/dt and dvT/dt must not be exceeded. The reverse recovery charge Qrr is normally a function of re verse recovery time trr and reverse recovery current Irr. To reduce the power loss at turn-off, the SCR with a low value of Qrr is preferred.
2.3 Gate Turn-Off (GTO) Thyristor
The gate turn-off (GTO) thyristor is a self-extinguishable device that can be turned off by a negative gate current. The GTOs are normally of press-pack design as shown in Fig. 2-5, and the modular design is not commercially available. Several manufacturers offer GTOs up to a rated voltage of 6 kV with a rated current of 6 kA. The GTO can be fabricated with symmetrical or asymmetrical structures. The symmetric GTO has reverse voltage-blocking capability, making it suitable for cur rent source converters. Its maximum repetitive peak off-state voltage VDRM is approximately equal to its maximum repetitive peak reverse voltage VRRM. The asymmetric GTO is generally used in voltage source converters where the reverse voltage-blocking capability is not required. The value of VRRM is typically around 20 V, much lower than VDRM. The switching characteristics of the GTO thyristor are shown in Fig. 2-6, where iT and vT are the anode current and anode-cathode voltage, respectively. The GTO turn-on behavior is measured by delay time td and rise time tr. The turn-off transient is characterized by storage time ts, fall time tf , and tail time ttail . Some manufacturers provide only turn-on time tgt (tgt = td + tr) and turn-off time tgq (tgq =ts + tf) in their datasheets. The GTO is turned on by a pulse of positive gate current of a few hundred milliamps. Its turn-off process is initiated by a negative gate current. To ensure a reliable turn-off, the rate of change of the negative gate current diG2/dt must meet with the specification set by the device manufacturer.
Table 2-2 gives the main specifications of a 4500-V/4000-A asymmetrical GTO device, where VDRM, VRRM, ITAVM, and ITRMS have the same definitions as those for the SCR device. It is worth noting that the current rating of a 4000-A GTO is defined by ITGQM, which is the maximum repetitive controllable on-state current, not by the average current ITAVM. The turn-on delay time td and rise time tr are 2.5 _s and 5.0 _s while the storage time ts and fall time tf at turn-off are 25 _s and 3 _s, respectively. The maximum rates of rise of the anode current, gate current, and device voltage are also given in the table.
The GTO thyristor features high on-state current density and high blocking volt age. However, the GTO device has a number of drawbacks, including (a) bulky and expensive turn-off snubber circuits due to low dvT/dt, (b) high switching and snubber losses, and (c) complex gate driver. It also needs a turn-on snubber to limit diT/dt.
2.4 Gate-Commutated Thyristor (GCT)
The gate-commutated thyristor (GCT), also known as integrated gate-commutated thyristor (IGCT), is developed from the GTO structure. Over the past few years, the industry has seen the GTO thyristor being replaced by the GCT device.
The GCT has become the device of choice for medium voltage drives due to its features such as snubberless operation and low switching loss.
The key GCT technologies include significant improvements in silicon wafer, gate driver, and device packaging. The GCT wafer is much thinner than the GTO wafer, leading to a reduction in on-state power loss. As shown, a special gate driver with ring-gate packaging provides an extremely low gate inductance (typically < 5 nH) that allows the GCT to operate without snubber circuits. The rate of gate current change at turn-off is normally greater than 3000 A/_s instead of around 40 A/_s for the GTO device. Since the gate driver is an integral part of the GCT, the user only needs to provide the gate driver with a 20- to 30-V dc power supply and connect the driver to the system controller through two fiber-optic cables for on/off control and device fault diagnostics.
Several manufacturers offer GCT devices with ratings up to 6 kV/6 kA. 10-kV GCTs are technically possible, and the development of this technology depends on the market needs. The GCT devices can be classified into asymmetrical, reverse-conducting and symmetrical types. The asymmetric GCT is generally used in voltage source converters where the reverse voltage-blocking capability is not required. The reverse-conducting GCT integrates the freewheeling diode into one package, resulting in a reduced assembly cost. The symmetric GCT is normally for use in current source converters.
Figure 2-8 shows the typical switching characteristics of the GCT device, where the delay time td, rise time tr, storage time ts and fall time tf are defined in the same way as those for the GTO. Note that some semiconductor manufacturers may define the switching times differently or use different symbols. The waveform for the gate current iG is given as well, where the rate of gate current change diG2/dt at turn-off is substantially higher than that for the GTO.
GCT Device Classification
Asymmetrical GCT: Reverse-conducting; GCT; Symmetrical; GCT N (reverse blocking)
For use in voltage source converters with antiparallel diodes.
For use in voltage source converters.
For use in current source converters.
Table 2-4 gives the main specifications of a 6000-V/6000-A asymmetrical GCT, where the maximum repetitive controllable on-state current ITQRM is 6000 A. The turn-on and turn-off times are much faster than those for the GTO. In particular, the storage time ts is only 3 _s in comparison with 25 _s for the 4000-A GTO device in Table 2-2. The maximum dvT/dt can be as high as 3000 V/_s.
The maximum rate of gate current change, diG2/dt, can be as high as 10,000 A/_s, which helps to reduce the switching time at turn-off. The on-state voltage at IT = 6000 A is only 4 V in comparison with 4.4 V at IT = 4000 A for the GTO device.
The GCT device normally requires a turn-on snubber since the diT /dt capability of the device is only around 1000 A/_s. Figure 2-9a shows a typical turn-on snubber circuit for voltage source converters . The snubber inductor Ls limits the rate of anode current rise at the moment when one of the six GCTs is gated on. The energy trapped in the inductor is partially dissipated on the snubber resistor Rs. All six GCTs in the converter can share one snubber circuit. In current source converters, the snubber circuit takes a different form as shown in Fig. 2-9b, where a di/dt limiting inductor of a few microhenries is required in each of the converter legs, but no other passive components are needed.
2.5 Insulated Gate Bipolar Transistor (IGBT)
The insulated gate bipolar transistor (IGBT) is a voltage-controlled device. It can be switched on with a +15 V gate voltage and turned off when the gate voltage is zero.
In practice, a negative gate voltage of a few volts is applied during the device off period to increase its noise immunity. The IGBT does not require any gate current when it is fully turned on or off. However, it does need a peak gate current of a few amperes during switching transients due to the gate-emitter capacitance.
The majority of high-power IGBTs are of modular design. Press-pack IGBTs are also available on the market for assembly cost reduction and efficient cooling, but the selection of such devices is limited.
The typical switching characteristics of the IGBT device are where the turn-on delay time td_on, rise time tr, turn-off delay time td_off , and fall time tf are defined. The waveforms for gate driver output voltage vG, gate-emitter voltage vGE, and collector current iC are also given. The voltage vGE is equal to vG after the IGBT is fully turned on or off. These two voltages, however, are not the same during switching transients due to the gate-emitter capacitance. The gate resistor RG is normally required to adjust the device switching speed and to limit the transient gate current.
Table 2-5 gives the main specifications of a 3.3-kV/1.2-kA IGBT, where VCE is the rated collector-emitter voltage, IC is the rated dc collector current and i_cm is the maximum repetitive peak collector current. The IGBT has superior switching characteristics. It can be turned on within 1 _s and turned off within 2 _s.
The IGBT device features simple gate driver, snubber-less operation, high switching speed, and modular design with insulated baseplate. More importantly, the IGBT can operate in the active region. Its collector current can be controlled by the gate voltage, providing an effective means for reliable short-circuit protection and active control of dv/dt and overvoltage at turn-off.
The construction of a medium-voltage converter with series connected IGBT modules should consider a number of issues such as efficient cooling arrangements, optimal dc bus-bar design, and stray capacitance of baseplates to ground. In contrast, press-pack IGBTs allow direct series connection, where the mounting and cooling techniques developed for press-pack thyristors can be utilized.
2.6 Other Switching Devices
There are a number of other semiconductor devices, including power MOSFET, emitter turn-off thyristor (ETO) , MOS-controlled thyristor (MCT), and static induction thyristor (SIT). However, they have not gained significant importance in high-power applications. The injection enhanced gate transistor (IEGT) seems to be a promising new switching device for high-power converters.
3. OPERATION OF SERIES-CONNECTED DEVICES
In medium voltage drives, switching devices are normally connected in series. It is not necessary to parallel the devices since the current capacity of a single de vice is usually sufficient. For instance, in a 6.6-kV 10-MW drive the rated motor current is only 880A in comparison with the current rating of a 6000A GCT or 3600A IGBT.
Since the series-connected devices and their gate drivers may not have exactly the same static and dynamic characteristics, they may not equally share the total voltage in the blocking mode or during switching transients. The main task for the series-connected switches is to ensure equal voltage-sharing under both static and dynamic conditions.
3.1 Main Causes of Voltage Unbalance
The static voltage unbalance is mainly caused by the difference in the off-state leak age current Ilk of series-connected switches. Furthermore, the leakage current is a function of device junction temperature and operating voltage. The causes of the dynamic voltage unbalance can be divided into two groups: (a) unbalance due to the difference in device switching behavior and (b) unbalance caused by the difference in gate signal delays between the system controller and the switches. Table 3-1 summarizes the main causes of unequal voltage distribution, where _ represents the discrepancies between series-connected devices.
---- Main Causes of Unequal Voltage Distribution Between Series-Connected Devices
Type Causes of Voltage Unbalance
Static voltage _Ilk: Device off-state leakage current unbalance _Tj : Junction temperature Dynamic voltage Device _td_on:
Turn-on delay time unbalance _td_off : Turn-off delay time (IGBT)
_ts: Storage time (GCT)
_Qrr: Reverse recovery charge
_Tj : Junction temperature Gate driver _tGD_on:
Gate driver turn-on delay time
_tGD_off : Gate driver turn-off delay time
_Lwire: Wiring inductance between the gate driver output and the device gate
3.2 Voltage Equalization for GCTs
(1) Static Voltage Equalization. --- shows a commonly used method for static voltage equalization, where each switch is protected by a parallel resistor Rp. Its resistance can be determined by an empirical equation where _VT is the desired maximum voltage discrepancy between the series switch es and _Ilk is the allowable tolerance for the off-state leakage current. Equation (3-1) is valid for both asymmetrical and symmetrical GCTs, and the value of Rp is normally between 20 k_ and 100 k_.
(2) Dynamic Voltage Equalization. For the dynamic voltage equalization, three modes of GCT operation need to be considered:
_ Turn-on transient
_ Turn-off transient by gate commutation
_ Turn-off transient by natural commutation (for symmetrical GCT only)
The first two operating modes are for asymmetrical and reverse-conducting GCTs used in voltage source converters, while all three operating modes are applicable to symmetrical GCTs in current source converters.
To ensure equal dynamic voltage sharing, the following techniques can be employed:
_ Use devices of one production lot to minimize _tdon, _ts and _Qrr.
_ Match the device switching characteristics to minimize _tdon, _ts and _Qrr.
_ Make the device cooling condition identical to minimize _Tj
_ Design symmetrical gate drivers to minimize _tGD_on and _tGD_off
_ Place the gate drivers symmetrically to minimize _Lwire.
The implementation of the above-mentioned techniques can help to reduce the de vice voltage unbalance during switching transients, but does not guarantee a satisfactory result. The series connected switches are often protected by RC snubber circuits shown in Fig. 3-1b.
The snubber capacitor Cs should be sized to minimize the effect of the delay time inconsistency on the GCT voltage equalization. Since the turn-on delay time td_on is normally much shorter than the storage time ts at turn-off, the requirements for turn off dominate. The value of Cs can be found from an empirical equation where _t_delay is the maximum tolerance in the total turn-off delay time including ts and the delay time caused by gate drivers, IT max is the maximum anode current to be commutated, and _VT max is the maximum allowed voltage deviation between the series switches.
The GCTs used in the current source converter may be turned off by natural commutation, where the device is commutated by a negative anode current produced by its power circuit. The commutation process is similar to that of an SCR device. The dominant factor affecting the dynamic voltage unbalance in this case is the discrepancy in the GCT reverse recovery charge _Qrr. This adds another criteri on for choosing the capacitance value:
The value of Cs is normally in the range of 0.1 to 1 _F for the GCT devices, much lower than that for the GTOs. The snubber resistance Rs should (a) be sized such that it should be small enough to allow fast charging and discharging of the snubber capacitor to accommodate the short pulse-widths of the PWM operation and (b) be large enough to limit the discharging current that flows through the GCT at turn-on.
A good compromise should be made.
3.3 Voltage Equalization for IGBTs
The static and dynamic voltage equalization techniques for the GCT devices can be equally applied to the IGBTs. In addition, an active overvoltage clamping scheme can be implemented to limit the collector-emitter voltage during switching transients. This scheme is invalid for the GCTs due to the latching mechanism of the thyristor structure.
Fig. 3-2 illustrates the principle of an active overvoltage clamping scheme. The collector-emitter voltage vCE of each IGBT is detected and compared with a reference voltage V* max that is the maximum allowed voltage for the device. The difference _v is sent to a comparator. If the detected vCE is lower than V* max at turnoff, the output of the comparator is zero and the operation of the device is not affected. At the moment that vCE tends to exceed V* max, |_v| is added to the gate signal vG, forcing vCE to decrease. Through the feedback control in the IGBT active region, vCE will be clamped to the value set by V*max during switching transients, effectively protecting the device from overvoltage. However, this is achieved at the expense of an increase in the device switching loss.
The section focuses on commonly used high-power semiconductor devices including SCRs, GTOs, GCTs, and IGBTs. Their switching characteristics are introduced and main specifications are discussed. Since these devices are often connected in series for high-power medium-voltage applications, the static and dynamic voltage equalization techniques are elaborated. To summarize, a qualitative comparison for the GTO, GCT, and IGBT devices is given as follows.
Item GTO GCT IGBT Maximum voltage and current High, High Low ratings Packaging Press pack Press pack Module or press pack Switching speed Slow Moderate Fast Turn-on (di/dt) snubber Required Required Not required Turn-off (dv/dt) snubber Required Not required Not required Active overvoltage clamping No No Yes Active di/dt and dv/dt control No; No Yes Active short-circuit protection No; No Yes On-state loss Low
Low High Switching loss High Medium Low Behavior after destruction Short-circuited
Open-circuited Gate driver Complex, Complex, Simple, separate integrated compact Gate driver power consumption High Medium Low