IGCTs improve megawatt range power thyristors

Continuous progress in the development and production of GTO thyristor and IGBT technologies has taken place in the last few years, and the performance of both devices has improved steadily as a result. Now a series of linked innovations by ABB has created a platform for the design and production of a power silicon switch which extends beyond the IGBT and GTO thyristor performance envelope.

Known as the IGCT (Integrated Gate Commutated Thyristor), it can switch more quickly and has lower losses than either the GTO thyristor or IGBT. More importantly, it has characteristics which enable electrical power system engineers to shrink the size and cost of medium voltage systems while boosting their efficiency and reliability.

A better power switch

In the 30 years since their introduction, power silicon switches have increased steadily in complexity and capability. The first silicon-controlled rectifiers could switch power off only at the end of an AC cycle. Progress came with the GTO thyristor, which can switch at any point in the cycle. The introduction of IGBTs brought faster switching, but at present their switching losses are acceptable only at low voltage levels.

GTO thyristors consist of thousands of individual switching elements fabricated on a silicon wafer. Losses occur in all four conditions of operation (on, off, switching on, switching off). At a medium voltage, GTOs exhibit very low on-state losses and reasonable turn-off losses. However, due to switching being non-homogeneous, external snubber circuits take up more than half the volume of the final equipment and account for much of the design complexity, costs and losses.

Conversely, IGBTs have comparatively higher conduction losses but switch homogeneously, i.e. they do not need a snubber. However, they are not yet available for direct operation at all medium voltage levels. To overcome this handicap, designers must connect low voltage IGBTs in series, dramatically increasing the complexity, increasing losses and reducing system reliability. A converter designed for operation at 4.16 kV, for example, requires four series-connected 1.8 kV IGBTs per phase.

GTO thyristors can be produced economically for applications at most medium voltage levels. It is anticipated that 3.3 kV and 4.5 kV IGBTs will become available and simplify the design of medium voltage power circuits, but it is known already today that they will have high losses. To overcome these losses and the resulting heat build-up, the IGBTs will have to have a larger silicon area, and this will increase costs.

The ideal power switch would switch like and IGBT and conduct like a GTO thyristor, and it would have the low fabrication costs and high yields of the GTO thyristors. ABB claims that this is exactly what the IGCT achieves (see Table 1).

In IGCT technology, a combination of design innovations permits the thousands of individual power switching structures in a modified GTO thyristor to switch fast and simultaneously. What is more, the low on- and off-state losses inherent in thyristor designs are retained.

The first two key innovations is the buffer layer design, which has allowed the on-state and switching losses to be reduced by a factor of 2 to 2.5 and makes the optimum doping profile of a GTO and a diode virtually identical.

Previously, integrating a diode with a GTO had resulted in severe degradation of the diode's performance. Although the idea of the buffer layer is almost as old as the GTO itself, it has never been used before for the following reason: to reduce the switching losses, the charge stored in the device in the conducting phase has to be removed quickly at turn-off. In a conventionally designed GTO this function is performed by anode shorts, which provide a path for the electrons to flow out.

The combination of anode shorts and a buffer layer, however, leads to extremely high trigger and holding currents. To solve this problem the anode shorts have been omitted. Instead, the anode is made 'transparent', i.e. permeable, to the electrons, with the result that the trigger currents are reduced by almost one order of magnitude compared with a conventional GTO without buffer.

The second design innovation addresses the gate control. GTOs and thyristors are four-layer (npnp) devices. As such, they only have two stable points in their characteristics – 'on' and 'off'. Every state in between is unstable and results in current filamentation. The inherent instability is worsened by processing imperfections. This has led to the widely accepted myth that a GTO cannot be operated without a snubber.

Essentially, the GTO has to be reduced to a stable pnp device (i.e., a transistor) for the few critical microseconds during turn-off. To stop the cathode (n) from taking part in the process, the bias of the cathode n-p junction has to be reversed before voltage starts to build up at the main junction. This calls for commutation of the full load current from the cathode (n) to the gate (p) within about one microsecond.

Thanks to a new low inductive housing design, 4000 A/s can be achieved with a low cost 20 V gate unit. Current filamentation is totally suppressed and the turn-off waveforms and safe operating area are identical to those of a transistor (e.g., an IGBT). Also, GTOs can now switch instantly, without jitter, so that series connection is no longer a challenge.

Fusion of designs

The IGCT technology is the result of intense collaboration between device designers at ABB Semiconductors and power circuit designers at ABB Industrial Systems. In fact, it was the co-development of the power silicon, the packaging and the additional circuitry needed to make the power switch suitable for industrial applications that made the IGCT's unique combination of characteristics possible in the first place.

IGCT technology brings together the power handling device (GCT) and the device control circuitry (freewheeling diode and gate driver) in an integrated package. By offering four levels of component packaging and integration it permits simultaneous improvement in four interrelated areas: low switching and conduction losses at medium voltage; simplified circuitry for operating the power semiconductor; reduced power system costs; and enhanced reliability and availability.

Advantages of IGCT technology

Low switching losses: An advantage of low-loss switching is that it allows equipment designers to choose the switching rate that best matches the end-application requirements. Previous power devices limited operation to 250 Hz at full rated current. IGCT technology can operate at up to four times that speed. In motor drives systems, for example, a designer might choose faster switching to obtain better system efficiency. Alternatively, the efficiency of inverter systems is improved and their losses reduced by selecting a slower switching rate for an IGCT.

Reduction of associated circuitry: The unique device-level characteristics of the GCT permit snubberless operation, with important benefits for design engineers. Inverter designs with snubbers are large and complex, while snubberless inverters are compact and have far fewer components. Reliability is better as a result.

Equipment designs based on IGCT technology are also simplified by the integration of the freewheeling diodes in the GCT structure. This is possible as the reduction in thickness of the GCT silicon (the same reduction that makes low-loss switching possible) also permits an efficient diode to be fabricated on the same wafer.

Lower cost power systems: The cost of power control equipment can be reduced by 30 per cent or more through the use of IGCT technology. Several factors account for this.

GCTs can be fabricated using existing GTO production processes. Since these processes are well understood and available equipment can be used for them, GCT costs are in line with those of GTO thyristors. Compared with IGBTs, GCTs are less sensitive to process variations in terms of how these affect the turn-off performance. Consequently, the yield is higher and costs are lower. In addition, GCT design simulation is simpler, reducing costs and speeding up system development.

GCT technology simplifies power circuits to the extent that the number of components is reduced by as much as 50 per cent. This is due to the integration of the diodes in the GCT and the reduction in cabling and interconnection made possible by the generally high level of integration. A further reduction of costs is possible due to the higher operating frequency, which allows some components to become smaller. In addition, the gate drive circuit power is substantially reduced, allowing less costly components to be employed.

IGCT-based equipment can be designed with higher efficiencies than when other technologies are used. Losses in the power circuits and in the associated circuitry are lower, translating into more compact cooling equipment for a further reduction in costs.

Reliability and availability

In a larger context, the cost of power equipment is small in comparison with the cost of downtime of the process in which it operates. Equipment availability is therefore paramount. IGCT technology was conceived specifically for use at medium voltage levels and ensures optimum equipment reliability through:

homogeneous switching

robust, thyristor-like packaging technology (no wire bonds)

simplified gate control units

reduced number of parts.

Further, in the unlikely event of a failure, the modular nature of IGCT technology makes replacement of power components quick and simple. This reduces spare parts costs and keeps processes on-line.

Applications of IGCT technology

At the core of the IGCT's performance advantage is its ability to turn off in 2 microseconds and conduct like a thyristor. IGCT technology therefore permits simple inverter designs with half the losses of alternative technologies.

Thanks to IGCTs, previously impractical circuit topologies rated up to 100 MW and requiring series connection of many devices can now be realised. Medium voltage equipment based on this technology exhibits very high reliability.

For the first time, a power silicon technology has been matched to medium voltage, high-power applications. This enables equipment designers working with IGCTs to build less costly, more reliable and more compact power control systems, including:

railway power supply frequency changers

utility interties

static var compensators for power factor control

power flow controllers for utilities

medium voltage drives with line voltages of up to 6.9 kV rms

pump and fan drives for the chemical, oil and power sectors

marine drives/all-electric ships

transformerless traction supplies

locomotive drives

induction heating resonant inverters

static breakers.

IGCT technology is available now

ABB is a leading supplier of high-power switching devices to both ABB Group companies and to external firms. This leadership is exemplified by the innovations that have produced IGCT technology. IGCT-based equipment benefits from lower costs and high reliability even at the highest power levels.

Designers of medium voltage equipment can now choose from three power silicon switch technologies: GTO thyristors, IGBTs and IGCTs. IGCT technology will be the technology of choice for all applications in which the priorities are compactness, high efficiency, fast development and proven reliability.
Tables

Table 1.
IGCT technology combines the advantages
of GTO thyristors and IGBTs