Base/Gate drive circuit
Base/ Gate Drive Circuit:
Fig: Simple gate-drive and protection circuit for a stand-alone IGBT and a SCR
- All discrete controlled devices, regenerative or otherwise have three terminals. Two of these are the Main Terminals. One of the Main Terminals and the third form the Control Terminal.
- The amplification factors of all the devices (barring the now practically obsolete BJT) are quite high, though turn-on gain is not equal to turn-off gain.
- The drive circuit is required to satisfy the control terminal characteristics to efficiently turn-on each of the devices of the converter, turn them off, if possible, again optimally and also to protect the device against faults, mostly over-currents.
- Being driven by a common controller, the drives must also be isolated from each other as the potentials of the Main Terminal which doubles as a Control terminal are different at various locations of the converter.
- Gate-turn-off-able devices require precise gate drive waveform for optimal switching. This necessitates a wave-shaping amplifier. This amplifier is located after the isolation stage.
- Thus separate isolated power supplies are also required for each Power device in the converter (the ones having a common Control Terminal - say the Emitter in an IGBT - may require a few less).
- There are functionally two types of isolators: the pulse transformer which can transmit after isolation, in a multi-device converter, both the un-shaped signal and power and optical isolators which transmit only the signal.
- The former is sufficient for a SCR without isolated power supplies at the secondary. The latter is a must for practically all other devices.