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158 Power semiconductor devices and converter hardware issues

cathode. There is a need for two conditions to be present simultaneously to turn on
the thyristor:
1. forward voltage from the anode (A) to the cathode (K)
2. positive gate current to be applied for a short time.
When the two conditions are met, the thyristor can be triggered or as also referred the
thyristor can be fired into its on state. It then behaves almost as a short circuit with a
low voltage drop across (typically few Volts depending upon the type of the thyristor
and its power ratings). Once the thyristor enters its on state, the controllability of the
device through the gate circuit is lost and the thyristor behaves as an ordinary diode.
This happens even if the gate current is removed. The thyristor cannot be turned off
through the gate. Only if, due to the operation of the power circuit, the current from
the anode to the cathode tries to change direction (becomes negative or else flows from
the cathode to the anode), the thyristor turns off and the anode to the cathode current
becomes eventually zero. This happens under certain conditions and the circuit design
must ensure that while the anode to the cathode current is negative and finally
becomes zero, a negative voltage must be present across the thyristor to ensure that
it turns off completely. Manufacturers' data sheets specify times and requirements for
the thyristor to turn off. When the voltage across the thyristor is negative (reverse
bias) a very small current (leakage current) flows from the cathode to the anode.

5.2.3 Light-triggered thyristor (LT T)

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:
. Light signals are not affected by ElectroMagnetic Interference (EMI).
. The optical fibre provides one of the best available electrical isolation and trans-
mits 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