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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi guys, what do you think of this circuit? The idea was to drive IGBT bricks without dedicated dead time generator and avoid shoot-through. The transformer action of the GDT should eliminate the possibility of both gates being positive at any given time (low leakage inductance is essential), and the larger charging resistor further delays the turn-on, so when the other transistor starts turning on, the gate of the first one is surely negative. The advantage would be that you can still set both the turn-on and turn-off resistances.
Edit: I know it is similar to many circuits people use, but the idea here was to put the turn-off resistor on the primary.
Registered Member #93
Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64
I'm pretty sure this approach has been suggested in the past in one of our forums, and the idea was generally regarded as sound. I just can't to find the reference to the discussion at the moment. If I do, I'll post it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It is standard practice when driving MOSFETs or IGBTs from a transformer. I prefer a dedicated deadtime generator though. It's easier to adjust on the fly for research.
Unlike MOSFETs, IGBTs have a finite turn-off time. The MOSFET part of the device turns off as soon the gate voltage falls below the threshold, but the bipolar part suffers from charge storage and takes a while to recover, just like a saturated BJT or a diode.
The amount of charge storage and resulting turn-off delay depends strongly on the amount of current that was flowing. ZCS can almost eliminate it: by the time the switching event comes around, the current has fallen to zero and the bipolar part of the device is already off.
FWIW, my DRSSTC seems quite happy with about 50ns added deadtime and no diodes across the gate resistors.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve, I try to avoid generating the dead time because this slows down the turn-off on the gate (the voltage differential is only 15V not 30V), overshoot can become a problem (partially turning the other device ON when it still should be OFF) and also IGBTs like to have negative voltage on the gate during the off-state, not just zero. Of course circuits for this can be made - eg. using two GDTs to drive the transistors diagonally, but such configuration is not foolproof because both transistor in one branch can be turned on in case of control circuit malfunction.
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Simple anti-shoot-through circuit for IGBT bricks
I prefer a dedicated deadtime generator though. It's easier to adjust on the fly for research.