Converters can be made using different technologies which depend on; the type of semiconductor devices used and the power ratings of the load. Several semi conductor devices are used such as the diodes, Thyristors, Triacs, GTO (gate turn off) among others (Gureich, 2008).
Thyristor or Silicon Controlled Rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The Thyristor came from its gas tube equivalent, thyratron. Often it is a family name that includes SCRs, Triac, GTO, MCT and IGCT. A Thyristor is a controlled rectifier where the unidirectional current flow from anode to cathode is initiated by a small signal current from gate to cathode. Thyristors are classified as standard or slow-phase-control-type and fast switching, voltage fed inverter type (Ulrich et al, 1998).
Initially, when forward voltage is applied across a device, the off-state, or static dv / dt, must be limited so that it does not switch on spuriously. The dv / dt creates displacement current in the depletion layer capacitance of the middle junction, which induces emitter current in the component transistors and causes switching action (Dorf, 1997). When the device turns on; the anode current di / dt can be excessive, which can destroy the device by heavy current concentration. During conduction, the inner P-N regions remain heavily saturated with minority carriers and the middle junction remains forward biased. To recover the forward voltage capability, a reverse voltage is applied across the device to sweep out the minority carriers and the phenomena are similar to that of a diode. However, when the recovery current goes to zero, the middle junction still remains forward-biased. This junction eventually blocks with an additional delay when the minority carriers die by the recombination process. The forward voltage can then be applied successfully, but the reapplied dv / dt will be somewhat less than the static dv / dt because of the presence of minority carriers.
The volt-ampere characteristics of the device indicate that at gate current IG =0, if forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction (Littelfuse Inc, 2008). If the voltage exceeds a critical limit (break over voltage) the device switches into conduction. As the magnitude of Ig increases, the forward break over voltage is reduced and eventually at Ig, the device behaves like a diode with the entire forward blocking region removed. The device will turn on successfully if the minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse biased and the V-I curve becomes essentially similar to that of a diode rectifier. This indicates that the thyristor is composed of two diodes connected back to back. Modern Thyristors are available with very large voltage (several KV) and current ratings (several KA).
In order to turn the thyristor off, the load current must be reduced below its holding current (IH) for sufficient time to allow all the mobile charge carriers to vacate