Power semiconductor devices and converter hardware issues

The advances of high voltage/current semiconductor technology directly affect the power electronics converter technology and its progress. The 'perfect' high-power semiconductor is yet to be fully developed and become commercially available.

However, new semiconductors have changed the way that power switches are protected, controlled and used and an understanding of the device characteristics is needed before a system is developed successfully.

Technological progress in the power electronics area over the last twenty years or so has been achieved due to the advances in power semiconductor devices. In this chapter, these devices are presented and current developments are discussed.

Power semiconductor devices

The various semiconductor devices can be classified into three categories with respect to the way they can be controlled:

1. Uncontrolled. The diode belongs to this category. lts on or off state is controlled by the power circuit.
2. Semi-controlled. The thyristor or silicon controlled rectifier (SCR) is controlled by a gate signal to turn-on. However, once it is on, the controllability of the device is lost and the power circuit controls when the device will turn-off.
3. Fully-controlled. Over the last twenty years a number of fully controlled power semiconductors have been developed. This category includes the main kind of transistors such as the bipolar junction transistor (BJT) and the metal oxide semiconductor field effect transistor (MOSFET). New hybrid devices such as the insulated gate bipolar transistor (IGBT), the gate turn-off thyristor (GTO), the mos-controlled thyristor (MCT), and many others have recently been introduced.

Diode

Details in diodes are discussed here- Power Semiconductor Devices-Diode

Thyristor

Details in Thyristor are discussed here-THYRISTORS

Light-triggered thyristor (LTT)

The thyristor represents a mature technology and is already the most widely used device especially in high and very-high power applications for decades. However, there are a number of developments happening in order to further improve the performance characteristics of the device.

In the early 1970s the electrically triggered thyristor (ETT) was developed. How- ever, when such devices are used in series in large numbers to develop a high-voltage valve, the electrical triggering and the required insulation were complex making the hardware equipment expensive. In the late 1970s, a light-sensitive gating method was developed and the associated amplifying layers were built integrally into the power thyristor to facilitate the light-triggering concept (EPRI, 1978). The main reasons of using LTT technology are as follows:

1. Light signals are not affected by ElectroMagnetic Interference (EMl).
2. The optical fibre provides one of the best available electrical isolation and transmits the light directly into the gate of the device.

The blocking voltage of the initial devices was relatively low (Temple, 1980,. Temple, 1981). Since then continually new devices were manufactured that were able to block higher voltages (Tada et al., 1981; Katoh et al., 1997). Another important aspect was the protection of the device against dv/dt and di/dt (Przybysz et al., 1987). This resulted in the development of the self-protected LTT (Cibulka et al., 1990,. Aliwell et al., 1994).

Today, research and development aims mainly at reducing the complexity of the device itself while improving its reliability. Each valve in high-power applications is built with a number of thyristors and work in recent years has resulted in an increase of the blocking voltage level so that the number of thyristors required to build the same valve from the blocking voltage point of view can be reduced. It is widely accepted by manufacturers that the highest blocking voltage that results in optimized device as far as cost, power losses, reliability and fabrication process are concerned is about 8-kV (Ruff et al., 1999; Asano et al., 1998).

Fig. 5.5 Photograph of a 5-inch light-triggered thyristor with integrated breakover diode. (Courtesy of Siemens ant EUPEC.)

Today, manufacturers try to integrate the drive and protection circuitry within the device. Specifically, the light and overvoltage triggering functions have been integrated with the device. When compared with the ETT, the component count of the drive circuit is reduced substantially (Lips et al., 1997). Furthermore, the overvoltage protection is also integrated into the device which further reduces the complexity of the circuitry arrangement required to ensure safe operating conditions and risk of failure reduction. An improved 8-kV LTT with the overvoltage protection developed by Siemens and EUPEC has become available (Schulze et al., 1996; Schulze et al., 1997; Ruff et al., 1999). An 8-kV LTT with integrated diode (Niedernostheide et al., 2000) is shown in Figure 5.5.

Many LTT devices have been successfully used in Japan (Asano et al., 1998). The recently developed 8-kV LTT by Siemens, AG was tested as commercial product at Bonneville Power Administration's (BPAs), Celilo Converter Station at The Dalles, Oregon, USA in 1997. Celilo is the northern end of BPAS 3.1-MW HVDC line from the Columbia River system to Southern California.

Current R&D work aims at developing fully self-protected devices with break over diode (BOD), forward recovery protection (FRP) and dv/dt protection as a high- power LTT (Ruff et al., 1999).

next Desired characteristics of fully-controlled power semiconductors