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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.
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