IGBT parallel configuration

No you don't, but you may need a resistor for each gate

No you don't, but you may need a resistor for each gate

I'll bite. I understand that most IGBTs have a positive tempco for VCEsat (you'r better check your particular model) but what do the gate resistors do? Equalize the switching edges?

Yes each device still has its own gate charge.

To parallel you need to match the igbt's or use Emitter resistors if matching is unrealistic. If you match them you also wind up derating them too, and at lower currents only 1 will conduct.

Cheers

Really all that matters is that they're on the same heatsink, so junction temperature is within, ohh maybe 50C worst case. The difference in voltage drop and therefore current flow isn't all that bad. At lower currents, all the load current will go through one transistor, but since each transistor is rated for that current already, it's not a hazardous condition.

Tim

-- Deep Friar: a very philosophical monk. Website:

Thanks a lot.

Why at lower current only one conduct?

Ad low current is a function of the max reted current device or there is a treshold?

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Both questions are answered if you consider an IGBT in saturation as a voltage source, resistance and ideal diode in series.

With nearly no load, only the variations in voltage sources show up; no voltage is dropped across the resistors, so the saturation voltage equals the lowest voltage drop (notice the diodes prevent the apparent voltage sources from conducting into each other). At low currents, not much voltage is dropped across the resistor, so all the load is sunk into one transistor.

At higher currents (roughly, currents more than Vdif/Reff, where Vdif is the difference in saturation voltages and Reff is the effective collector output resistance), enough voltage is dropped across the lowest transistor's effective resistance that other transistors also start drawing current. In this range, the most conductive transistor still draws more current, equal to Vdif/Reff. Because this difference is smaller than the total current flow, sharing is much better.

Tim