Monday, April 2, 2012

Induction Heater

...an old, old project which I'm finally documenting.
Instructables link

MagicDrive

This is a IGBT gate drive with desaturation detection, optocoupled input, etc. All the goodies.
Desaturation detection looks at Vce on the brick, and turns off the transistor when this is too high. I'm using two-stage turn-off - rather than clamping Vge to 0 when an short circuit is detected, it is first dropped to a point where the IGBT leaves saturation, causing the gain of the transistor to limit the current. Then it is dropped to 0. This gracefully turns off the switch without the ill side-effects associated with switching off (possibly) several thousand amps in a few microseconds, namely ringing from exciting the layout parasitics and transistor capacitance.
Soon it will be fabricated and tested, but for now, the design looks sound.
Hopefully this will prevent my QCW modulator from going boom when a fault occurs (else, a bridge failure on the coil end will propagate and destroy all of my electronics!)