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Thermal Runway in MOSFETs

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atripathi

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Do MOSFETs also exhibit thermal runway phenomena, similar to BJTs when used in parallel for current sharing?
 

I hope not!
Mosfets have a negative thermal/gain relationship as opposed to the positive one exhibited by bi-polar power semiconductors such that thermal runaway for Mosfets is not normally an issue.
This characteristic of Mosfets is used widely in Audio Amplifier designs such that output circuit protection is very often omitted.

Gain matching when paralleling Mosfets is important, as if one within an array has greater gain then it will 'unbalance' the current distribution.

hope this answers your question, my answer may be a bit over simplified though
Mik
 
What about current generation MOSFETs? Has any one faced issues like thermal runaway due to gate parasitic induced oscillations?
 

atripathi said:
What about current generation MOSFETs? Has any one faced issues like thermal runaway due to gate parasitic induced oscillations?
Click to expand...
I have never seen anything like this. I've never heard of a MOSFET having a negative temperature coefficient, so paralleling should always work. As for gate oscillations - a proper gate driver design should be able to limit these to a very safe level.
 

The drain current temperature coefficient for a MOSFET depends on its inversion coefficient. In weak inversion, the drain current increases with temperature due to negative dependance of Vt on temperature, while in strong inversion the drain current reduces due to mobility degradation taking precedence over Vt decrease. The point is that even if the MOSFET was originally in weak inversion, it cannot increase forever with temperature as at some point it will be in strong inversion which will limit furthur increase. Current imbalance if it exists can only be ignorably small in paralleled MOSFETs. There is no secondary breakdown in MOSFETs to limit the SOA.
 

There is certainly a point where you put so much heat into
the device that the parasitic BJT can light up from leakage
and degraded base resistance. But that will be outside the
manufacturer's ratings and your responsibility to make sure
it doesn't happen.
 

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