# [EVDL] IGBTs in Parallel



## EVDL List (Jul 27, 2007)

I have to ask, after seeing messages about how difficult it is to put IGBTs
in parallel ant what rocket science takes place to do it, What exactly is
the problem? I am guessing that because of timing differences you will have
one IGBT pulling more than its share of the load at switch-on and
switch-off. It is obviously greater in an IGBT than a row of FETs. So would
the answer be to independently current limit them? Enquiring mind wants to
know.

Mark Grasser


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## EVDL List (Jul 27, 2007)

I know with FETs that the one that turns on first can hog all the gate
current. The solution for them is to use either independant gate drivers,
or a small series resistor on the gate.

IGBTs share this problem, plus I believe they have a negative temperature
coefficient which means that the one that hogs the most current will heat
up faster and will drop in effective resistance and therefor hog MORE
current, etc.

> I have to ask, after seeing messages about how difficult it is to put
> IGBTs
> in parallel ant what rocket science takes place to do it, What exactly is
> the problem? I am guessing that because of timing differences you will
> have
> one IGBT pulling more than its share of the load at switch-on and
> switch-off. It is obviously greater in an IGBT than a row of FETs. So
> would
> the answer be to independently current limit them? Enquiring mind wants to
> know.
>
> Mark Grasser
>
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>


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## EVDL List (Jul 27, 2007)

Different IGBTs will drop slightly different voltage for a given current
draw, making perfect current sharing highly unlikely if they are naively
paralleled. Carefully matching devices can help a lot.

Google is your friend. Top hit for 'parallel IGBT':
http://www.irf.com/technical-info/appnotes/an-990.pdf

Steve

> I know with FETs that the one that turns on first can hog all the gate
> current. The solution for them is to use either independant gate drivers,
> or a small series resistor on the gate.
> 
> IGBTs share this problem, plus I believe they have a negative temperature
> coefficient which means that the one that hogs the most current will heat
> up faster and will drop in effective resistance and therefor hog MORE
> current, etc.
> 
>> I have to ask, after seeing messages about how difficult it is to put IGBTs
>> in parallel ant what rocket science takes place to do it, What exactly is the
>> problem? I am guessing that because of timing differences you will have one
>> IGBT pulling more than its share of the load at switch-on and switch-off. It
>> is obviously greater in an IGBT than a row of FETs. So would the answer be to
>> independently current limit them? Enquiring mind wants to know.
>> 
>> Mark Grasser
>> 
>> 
>> _______________________________________________
>> For subscription options, see
>> http://lists.sjsu.edu/mailman/listinfo/ev
>> 
> 
> 
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

Large industrial drives use parallel IGBT modules all
the time. They derate the modules by about 10
percent, IIRC. They used to tell you to get modules
with the same batch number for parallel applications. 
Not any more. And, don't those large IGBT modules
have a bunch of small IGBTs inside in parallel? The
ones I've blown apart look like it.

Regards,

Jeff M


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