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Power electronic control in electrical systems 163
It should be noted that although the GTO can be turned on like a thyristor,
with a low positive gate current pulse, a large negative pulse is required to turn it
off. These are relatively slow devices when compared with other fully controlled
semiconductors. The maximum switching frequency attainable is in the order of
1kHz. The voltage and current ratings of the commercially available GTOs are
comparable to the thyristors approaching 6.5 kV, 4.5 kA and are expected to
increase to cover completely the area occupied by thyristors as shown in Figure 5.12.
5.2.6 Metal-oxide-semiconductor field effect transistor
The metal-oxide semiconductor field effect transistor (MOSFET) is a transistor
device capable of switching fast with low switching losses. It cannot handle high
power and is mostly suited for low-power applications. These include switch-mode
power supplies (SMPS) and low voltage adjustable speed motor drives used in
copier machines, facsimiles and computers to name a few. In fact for very low
Fig. 5.8 Power MOSFET: (a) circuit symbol for an n-channel; (b) circuit symbol for a p-channel; (c) basic
structure of an n-channel device; and (d) voltage signal control of a typical n-channel device.