Protection of Power Devices and Converters
Power electronic devices operate from the supply of utility power. And the utility power may contain disturbances. Therefore, the protection of power devices and converters is exposed to its disturbances.
And also, these devices and converters are associated with the transient and the fault at the load side. So, several protection schemes must be integrated into a converter to protect the power electronics switches (devices) and converter.
There are many techniques developed for the Protection of power devices and converters. Some of the protection schemes are common for all devices and converters. Differences in essential features of devices need special protection schemes, particular for those devices.
If the anode current exceeds the maximum permissible turn-off-able current specification, the IGBT must be protected against latching, and similarly, the GTO‘s turn-off drives are disabled. Power semiconductor devices commonly protected against:
- Voltage spike or over-voltage
- Gate-under voltage
- Overvoltage at gate
- Excessive temperature rise
- Electro-static discharge
Most semiconductor devices exhibit similar responses to most of the stresses. But there marked differences. The SCR is the most robust device on practically all counts compared to other power electronics switches.
An HRC fuse, suitably selected and coordinated with fast circuit breakers, would mostly protect it.
When the cost of the fuse becomes prohibited, sometimes this becomes a curse. By sensing the main terminal voltage, all transistors (especially the BJT and the IGBT) is actively protected without any operating cost. This voltage interconnected to the current passes through the device.
Further, the transistors permit designed gate current waveforms to minimize voltage spikes (sharply rising Main terminal currents). To reduce switching losses and voltage spikes, gate resistances have a significant effect on the turn-on and turn-off times of these devices.
Protection schemes for over-voltages prolonged ones and those of short duration. The energy content of the surges guides these. To avoid over-voltage, typical strategies are used like capacitive dynamic voltage-clamps, Metal Oxide Varactors (MOV’s), and crowbar circuits.
High dv/dt stresses have a similar effect on all devices. Depending on the device’s speed, R-C or R-C-D clamps are used. To minimize switching losses of the device, these ‘snubbers’ or ‘switching-aid-networks’ circuits are used additionally. And these also reduce its temperature rise.
Against over-voltages (typically + 20 V), gates of all devices are required to protect. Zener clamps help to solve this problem. The Zener is also a very fast-acting device.
Protection against issues like excessive case temperatures. For the higher-rated converters, forced-cooling techniques are fundamental. And whole environments air-cooled to lower the ambient.
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