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poida

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Joined: 02/02/2017
Location: Australia
Posts: 1437

Posted: 04:07am 11 Aug 2019

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How to prove the diodes that are in parallel with the gate drive resistors?

In my case, I have a 10 Ohm resistor parallel with the diode.
The Fluke 87 diode test function applies 1mA on the probe tips
and shows the voltage across the tips.
When I try the parallel 10R/diode the Fluke says it's 0.010V
and that is for both polarities.
Not very helpful.

I did not want to desolder 16 diodes just to check if they function.

Using a bench power supply, I set it to 5V and 0.65A max current.
When I apply the 5V to the paralleled 10R/diode pair with the diode reverse biased,
I see it draw 0.5A.
When I apply the 5V but forward biased this time, we see 0.65A.
We can now see that the diode works.

With 10R resistor, and 5V across it, of course it will draw 0.5A
(I = V/R)
The diode when forward biased will draw what ever you choose to give it
and in the above test, I chose to give it 0.15A, for a second or two.
Remember both the resistor and diode are in parallel and so see the same voltage.

After the most recent inverter fail, I discovered I have 2 out of 16 diodes
that have become shorted. Both on the same 1/2 bridge, one on the high side and
one on the low side.

One step at a time.

wronger than a phone book full of wrong phone numbers

johnmc
Senior Member

Joined: 21/01/2011
Location: Australia
Posts: 282

Posted: 06:12am 11 Aug 2019

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That method sure beats unsoldering the diodes after every event.

cheers john

johnmc

mackoffgrid

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Joined: 13/03/2017
Location: Australia
Posts: 460

Posted: 06:20am 11 Aug 2019

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poida said After the most recent inverter fail, I discovered I have 2 out of 16 diodes
that have become shorted. Both on the same 1/2 bridge, one on the high side and
one on the low side.

Very interesting.

Are your driver ICs from legitimate sources?

Cheers
Andrew
Edited 2019-08-11 16:28 by mackoffgrid

poida

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Joined: 02/02/2017
Location: Australia
Posts: 1437

Posted: 06:35am 11 Aug 2019

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mackoffgrid said
Are your driver ICs from legitimate sources?

RS Components supplied the IR21844 ICs

This last fail was with the Madness power board with totem pole TIP41/42 gate drive
and the nanoverter control board. The totem pole isolates the gate drive IC outputs
from the MOSFET's gate voltages.

wronger than a phone book full of wrong phone numbers

mackoffgrid

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Joined: 13/03/2017
Location: Australia
Posts: 460

Posted: 07:08am 11 Aug 2019

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Ok, rules out bad ICs.
You're getting some sort of shoot through, cross conduction from somewhere.

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