Protection of Power devices and converters
Protection of Power devices and converters:
- Several protection schemes must be incorporated in a converter. It is necessary to protect both the Main Terminals and the control terminals.
- Some of these techniques are common for all devices and converters. However, differences in essential features of devices call for special protection schemes particular for those devices.
- The IGBT must be protected against latching, and similarly the GTO's turn-off drive is to be disabled if the Anode current exceeds the maximum permissible turn-off-able current specification.
Power semiconductor devices are commonly protected against:
- Voltage spike or over-voltage;
- Gate-under voltage;
- over voltage at gate;
- Excessive temperature rise;
- Electro-static discharge;
- Semiconductor devices of all types exhibit similar responses to most of the stresses, however there are marked differences.
- The SCR is the most robust device on practically all counts. That it has an I2t rating is proof that its internal thermal capacities are excellent. A HRC fuse, suitably selected, and in co-ordination with fast circuit breakers would mostly protect it. This sometimes becomes a curse when the cost of the fuse becomes exorbitant.
- All transistors, specially the BJT and the IGBT is actively protected (without any operating cost!) by sensing the Main Terminal voltage. This voltage is related to the current carried by the device. Further, the transistors permit designed gate current waveforms to minimize voltage spikes as a consequence of sharply rising Main terminal currents.
- Gate resistances have significant effect on turn-on and turn-off times of these devices - permitting optimization of switching times for the reduction of switching losses and voltage spikes.
- Protection schemes for over-voltages - the prolonged ones and those of short duration - are guided by the energy content of the surges. Metal Oxide Varistors (MOV's), capacitive dynamic voltage-clamps and crow-bar circuits are some of the strategies commonly used.
- For high dv/dt stresses, which again have similar effect on all devices, R-C or R-C-D clamps are used depending on the speed of the device. These 'snubbers' or 'switching-aid-networks', additionally minimize switching losses of the device - thus reducing its temperature rise.
- Gates of all devices are required to be protected against over-voltages (typically 20 V) especially for the voltage driven ones. This is achieved with the help of Zener clamps - the zener being also a very fast-acting device.
- Protection against issues like excessive case temperatures and ESD follow well-set practices. Forced-cooling techniques are very important for the higher rated converters and whole environments are air-cooled to lower the ambient.