Until the introduction of newer power switches, the only serious contenders for high-power transportation systems and other applications were the GTO (thyristor), with its cumbersome snubbers, and the IGBT (transistor), with its inherently high losses. Rooting from the GTO is one of the newest power switches, the Gate-Commutated Thyristor (GCT). It successfully combines the best of thyristor and transistor characteristics, while fulfilling the additional requirements of manufacturabilty and high reliability. The GCT is a semiconductor based on the GTO structure, whose cathode emitter can be shut off “instantaneously,” thereby converting the device from a low conduction-drop thyristor to a low switching loss, high dv/dt bipolar transistor at turn-off. The IGCT (Integrated GCT) is the combination of the GCT device and a low inductance gate unit. This technology extends transistor switching performance to well above the MW range, with 4.5kV devices capable of turning off 4kA, and 6kV devices capable of turning off 3kA without snubbers. For more detail about GCT/IGCT technology, see the sidebar, on page 20.
Within five years of its introduction, the IGCT has penetrated virtually every power electronic application and is currently in service in equipment ranging from 200kW to 100MW with still higher ratings in design. Higher and lower voltages are under consideration, but their commercialization will depend on market demand and economics.
The vertical structure of the GCT, although derived from the GTO, has benefited from high-voltage IGBT developments and incorporates such loss-reduction techniques as the buffer layer and transparent emitter. These techniques lead to thinner wafer designs than were possible for GTOs, which further allows the monolithic integration of an antiparallel diode onto the same wafer for reverse conduction — something not optimally feasible with GTOs. The result is a switch that behaves like an IGBT at turn-off (open-base pnp transistor) but like a thyristor in conduction, thereby dissipating minimal losses.
Table 1, on page 18, lists the current range of devices rated at 4.5kV, 5.5kV, 6kV, and 6.5kV. Also, since 2001, the original 4.5kV/4kA device was relaunched in three versions for low, medium, and high frequency applications. The 3kA/6kV asymmetric device, introduced this year, is intended initially to serve induction heating applications. A newly adapted 6kV reverse blocking (symmetric) device rated for 140°C operation — a first in the history of high-voltage turn-off devices — is presented here. The feasibility of making 2.5kV and 10kV devices is currently under investigation.
Topologies and Applications
Fig. 1, on page 18, shows a typical IGCT inverter. It's characterized by the absence of turn-off snubbers (no external dv/dt limitation, “hard turn-off”) and the presence of a turn-on snubber (external di/dt limitation, “soft turn-on”). This highlights the nature of the IGCT and reveals the kind of topologies in which it can or must be used, as further illustrated in Table 2, on page 18.
Eliminating the passive components of Fig. 1 and replacing IGCTs by IGBTs gives rise to “hard-on/hard-off” topology dominating today's low-voltage power electronics.
Soft turn-off is generally avoided in power electronics today, as it imposes external dv/dt control on each of the semiconductors, adding two or three components (resistor, capacitor, and sometimes a diode) per device — up to 18 components per six devices. Hard turn-on is suitable when very fast diodes can be used. This is the preferred approach, where dc voltages are well below 2000V, eliminating the need for any passive components. Beyond this voltage, silicon diodes are slow, and their commutation rates must be reduced. Low commutation speeds mean higher values of LSNUBBER (soft-on) or slowing IGBT turn-on (hard-on). Although the total energy is typically the same, it is not dissipated in the device with the soft-on topology. Since at high voltage the turn-on losses can have a dominant share of total losses, the soft-on/hard-off topology becomes the preferred option while external di/dt control is achieved by adding four additional components per set of six devices.
The clamp circuit of Fig. 1 merits some consideration. The use of external inductance does not increase system losses. This is because the energy stored in the inductance LSNUBBER is the same as that dissipated in an IGBT at hard turn-on. Furthermore, not all the turn-on losses initially stored in LSNUBBER are dissipated in RSNUBBER. About 25% of the stored energy will subsequently serve the turn-off losses of the IGCT and its free-wheel diode (per their data sheet values). Failure to recognize this would result in double-counting this energy loss in the efficiency calculations.
The result is system turn-on losses are similar for the two topologies; however, the losses aren't dissipated in the device and are partly “recycled” in the case of soft turn-on.
Some form of fault limitation is needed at high voltage in the event of catastrophic semiconductor failure. While resistors and fuses are an option, the inductance of Fig.1 is the most practical fault limitation, because it has the merit of “already being there” for di/dt control. The press-pack construction of the IGCT, combined with this inductance, makes it resistant to explosion — even when the device's surge rating is exceeded.
The result of combining the soft-on/hard-off approach with the low-loss IGCT, allows very high power electronics to be realized cost-effectively, efficiently, and at relatively high frequencies. Today's 4.5kV IGCTs operate at up to 1 kHz, and 6.5kV devices operate up to 500 Hz. Both allow burst modes at up to 25 kHz, thermally limited to a dozen or so pulses.
These features have enabled IGCT technology to penetrate all medium-voltage applications in the past five years, including:
- Medium-voltage industrial drives.
- Marine drives.
- Traction drives.
- Induction heating inverters.
- STATCOMS (static VAR compensators).
- Wind power converters.
- Power quality (SMES, BESS, DVRs, DUPS).
- Solid-state breakers (ac and dc)
- Interies and flexible ac transmission systems.
- Pulse power.
IGCT Characteristics
It's important to recognize the features that allow the IGCT to adapt to multiple applications. IGCTs are inherently capable of operating over a temperature range of -40°C to 140°C. However, until now, none were rated above 125°C in their standard commercial specifications. Figs. 2a and 2b show the turn-off waveforms of a 4.5kV/4kA device at these two temperature extremes. The most significant difference between them is the expected increase in tail current.
Fig. 3 shows the turn-off of the same basic device as in Figs. 2a and 2b, but for three types of carrier lifetime control. Unlike IGBTs, IGCTs don't allow dv/dt control via the gate drive. You can set maximum dv/dt values for operating conditions by the anode design and choice of lifetime control (such as, by electron irradiation).
In Fig. 1, on page 18, inductance LS represents the stray inductance in the circuit composed of the phaselegs and clamp circuit CCLAMP and DCLAMP. Ideally, this should be of zero value; however, the physical size of power components and bus bar isolation requirements typically result in values of 200 nH to 600 nH. The value this inductance will have is often determined after the equipment has been built — when it's not possible to “tweak” the gate unit to adjust switching speed.
Fig. 4 shows the effect on turn-off voltage and losses when varying stray inductance. For a “normal” value of stray of 300 nH, the peak voltage at turn-off is 3kV. Increasing the stray inductance LS to 1500 nH raises this peak to 3.8kV. Thus, a fivefold increase in LS produces less than a 30% increase in peak voltage and turn-off losses. The higher inductance produces a slower fall of anode current, extracting more charge from the device, which results in a shorter tail current. Losses remain roughly proportional to the voltage. This behavior facilitates the design and construction of large systems and highlights the IGCT's robustness and user-friendliness, but should nevertheless encourage “sloppy wiring.”
Flexibility
The key to cost-effective power electronics lies in simplicity and standardization. This has been the guiding principle of IGCT technology since its inception. The 14-member family of Table 1, on page 18, uses four housings and four gate units — regardless of device voltage or type. Asymmetric and reverse-conducting devices are produced using a low-loss buffer-layer and transparent emitter technology. To preserve process standardization and allow device optimization, we realize symmetric IGCTs using an asymmetric wafer and a fast diode.
This standardization allows symmetric devices to operate at the same high temperatures as asymmetric wafers (since that's what they're made from). This is the basis for the currently launched 140°C/6kV devices.
The reverse blocking type 5SHZ15H6000 is rated at 140°C, and designed for current-source inverters. Its key parameters are:
- Direct and reverse maximum repetitive voltages: VDRM, VRRM = 6,000V, 140°C.
- Direct and reverse maximum working voltages: VDWM, VRWM = 3,600V.
- On-state voltage: VTM = 6.5V at 1,000A, 140°C.
- EOFF (IGCT turn-off energy loss) = 13.6 joules at 1,500A, 3,000V, 140°C.
- ERR (IGCT reverse recovery energy loss) = 10.7 joules at 1,500A, 1,000A/µs, 3,000V, 140°C.
- Thermal resistance junction-to-case (double-side cooled): Rth-JC = 0.0115°C/W.
The Future
Turn-off loss reductions of nearly 50% are possible with the addition of a second (anode-side) gate for n-base charge extraction — opening the door to fast, high-voltage devices. Commercialization will depend more on economics. The use of two wafers (of dissimilar function) within the same housing is established, and the use of two gates on the same device is a reality. Thus, symmetric switching devices for ac breakers or matrix converters is possible by incorporating two anti-series reverse-conducting wafers in the same presspack.
GCT/IGCT Technology
The GCT is a thyristor, turning on with a positive gate current pulse (typically 200A for a few µs) and turning off with a large negative gate current pulse of about 1µs. The IGCT is a GCT with an integrated gate drive circuit. With the IGCT, designers don't see the gate drive operation: It occurs inside the IGCT. Designers know only they must connect a power supply and fiber-optic cable to the gate unit and send the appropriate infrared signal down the fiber: light = on, dark = off. The designer does not need to understand whether the IGCT is voltage-controlled or current-controlled. GTO gate units can also have a power supply and a fiber-optic input; however, its gate drive unit isn't integrated, whereas with the IGCT it is. The IGCT is “digital” and there is no need to match the gate unit to its drive as a function of the application — the device is either on or off.
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