Handy Guide to Power Switching Devices

The heart of any medium-voltage AC drive is the type of power electronics device that handles the rapid and controlled switching of large voltages and currents. Below is an explanation of the commonn devices used:

GTO (gate turn-off) thyristor is a power semiconductor similar to an SCR (see below), but handles lower currents and can also be turned off by a negative gate terminal signal. Its switching frequency is higher than an SCR. Low power factor, low efficiency, and need for output filtering add to its application costs.

SCR (silicon-controlled rectifier) is a one-directional, solid-state switch offering high current handling capability that retains its usage in very large MV drives. Current going to the gate terminal controls breakover voltage, the point at which conduction starts. Turn-off occurs as current is reduced below a holding value. Drawbacks include relatively slow switching speed and large size of the resulting drive. GTOs and SCRs are mature power devices.

IGBT (insulated-gate bipolar transistor) combines best features of a MOSFET (metal-oxide semiconductor field-effect transistor) input and a bipolar transistor output in a newer power-switching device. Very rapid switching results, since no junction effect exists at the input. Power consumption is small due to the insulated gate. Standard IGBTs have voltage-switching limits and need to be connected in series for MV drive usage. A still newer high-voltage device, HV-IGBT, extends operating voltages to those required by MV drives. This eases the need to gang standard devices.

IGCT (integrated gate-commutated thyristor) combines the high-switching frequency and low switching losses of IGBTs with the high voltage handling capability and low on-state (conduction) losses of GTOs. The integrated diode and gate unit lowers the parts count, resulting in increased reliability.

SGCT (symmetrical gate-commutated thyristor) is a modified version of a GTO device and similar to the IGCT. SGCTs block voltage in both directions while allowing only one-directional current to flow. However, SGCTs do not need a series diode or anti-parallel diode, as IGCTs do. This is said to result in the lowest possible component count. Presently, IGCTs and SGCTs have fewer suppliers than IGBTs.