OK we all know the purpose of diodes are as uncontrolled switches, blocking current flow in one direction...

Now on a diode's datasheet it states the blocking voltage, recovery time and if you are lucky the switching time.

IF you have a 1200V (or any other voltage) diode, how much can it support in forward cunduction.

What I mean is When a diode is forward biased the voltage across it is between 0.3V (for schokky) to 3V (for power diodes), but it take time for the diode to start to conduct.

Can a 1200V diode support 900V in the forward direction befor conduction starts?

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Hi Styx,
A diode starts conducting pretty fast so the voltage across it hardly overshoots.
Its main problem is with its reverse recovery time where it remains conducting when switched from forward conducting to reverse blocking and it remains conducting for a fairly long time.

Attached Images

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yupyup,

the problem i have at work is in an IGBT testing rig with the test IGBT and DIODE setup in a chopper arranement with a 200uH load.

when switching at 450V 800A, the Vce peak is 940V in 700ns.
Thus for a link of 450V that means that IF the diode wasn't turn on (it does just pointing something oit) there is 490V across the diode in hte forward direction.


Now the diode does conduct, but it wont be instantanious.
The reason I am asking is I have taken out 2 IGBT-DIODE modules and each time it was the diode that had failed in over-voltage (and then the IGBT is seriosly damage, gate capacitance reduced from 190nF to 70nF)

We cannot see how the diode could have died?

thanks

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details