Features of GTO
Features to GTO:
- The thyristor has reigned supreme for well over two decades in the power electronics industry and continues to do so at the very highest level of power. It, however, has always suffered from the disadvantage of being a semi-controlled device.
- Although it could be turned on by applying a gate pulse but to turn it off the main current had to be interrupted. This proved to be particularly inconvenient in DC to AC and DC to DC conversion circuits, where the main current does not naturally becomes zero.
- A bulky and expensive “commutation circuit” had to be used to ensure proper turning off of the thyristor.
- The switching speed of the device was also comparatively slow even with fast inverter grade thyristor.
- The development of the Gate Turn off thyristor (GTO) has addressed these disadvantages of a thyristor to a large extent. Although it has made a rather late entry (1973) into the thyristor family the technology has matured quickly to produce device comparable in rating (5000V, 4000Amp) with the largest available thyristor.
- Consequently it has replaced the forced commutated inverter grade thyristor in all DC to AC and DC to DC converter circuits.
- Like thyristor, the GTO is a current controlled minority carrier (i.e. bipolar) device. GTOs differ from conventional thyristor in that, they are designed to turn off when a negative current is sent through the gate, thereby causing a reversal of the gate current.
- A relatively high gate current is need to turn off the device with typical turn off gains in the range of 4-5. During conduction, on the other hand, the device behaves just like a thyristor with very low ON state voltage drop.
- Several different varieties of GTOs have been manufactured. Devices with reverse blocking capability equal to their forward voltage ratings are called “symmetric GTOs”.
- However, the most popular variety of the GTO available in the market today has no appreciable reverse voltage (20-25v) blocking capacity. These are called “Asymmetric GTOs”. Reverse conducting GTOs (RC-GTO) constitute the third family of GTOs. Here, a GTO is integrated with an anti-parallel freewheeling diode on to the same silicon wafer.
- GTO thyristors suffer from long switch off times, whereby after the forward current falls, there is a long tail time where residual current continues to flow until all remaining charge from the device is taken away. This restricts the maximum switching frequency to approx 1 kHz. It may be noted however, that the turn off time of a GTO is approximately ten times faster that of a comparable SCR.
- During turn on, the device has a maximum dI/dt rating limiting the rise of current. This is to allow the entire bulk of the device to reach turn on before full current is reached. If this rating is exceeded, the area of the device nearest the gate contacts will overheat and melt from over current.
- The rate of dI/dt is usually controlled by adding a saturable reactor (turn-on snubber), although turn-on dI/dt is a less serious constraint with GTO thyristors than it is with normal thyristors, because of the way the GTO is constructed from many small thyristor cells in parallel.
- Reset of the saturable reactor usually places a minimum off time requirement on GTO based circuits.