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Old 20th February 2012, 01:08 AM   #1
dgta is offline dgta  United States
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Default MOSFETs and SOA

Looking through some data sheets for a MOSFET to use as pass element in a HV regulator, I come across this:
http://skory.gylcomp.hu/alkatresz/IRF840.pdf

with the following statement:
"SOA is Power Dissipation Limited"

What exactly does that mean? I can read that in at least 2 different ways. I'm not looking for guesses, I'm looking for someone who actually knows what they meant.

BTW Siliconix and IR don't have that statement on their data sheets, only Fairchild.

Thanks
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Old 20th February 2012, 01:35 AM   #2
jcx is offline jcx  United States
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it means they don't have an internal thermal runaway mechanism from local hot spotting

while Mosfets don't have the same Second Breakdown problem as BJT some of the higher power switching Mosfet devices with super low Ron are subject to SOA limiting - if the manufacturer didn't think to test Linear operation then the silence on this spec is no guarantee of good behavior

I would search the Cordell BJT vs Mosfet thread for more details

But I think generally Lateral Mosfets, and higher Ron devices – like the high voltage Fairchild part don’t have the problem
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Old 20th February 2012, 01:46 AM   #3
CBS240 is offline CBS240  United States
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It means that the mosfet will destruct due to exceeding the thermal limits of the die, de-rated of course. Mosfets like the IRF840 do not suffer from secondary breakdown as BJT's do. With higher Vce a BJT cannot handle the same power as with a lower Vce. I'm not in the mood to explain why (local hot-spotting), but it is due to the physics behind how each device works. Compare the SOAR graphs of a BJT to that of a Mosfet and you will see the difference.

A mosfet that is poorly heatsinked may not be able to handle the same power as the same one that is well sinked. Seems obvious eh? I have found that by mounting a mosfet directly to a piece of brass without an insulator, I can use two TO-220 devices in class AB and reliably get >60Wrms output from them. TO-220 BJT's would self destruct rather quickly if abused this way. I have seen mosfets with TO-220 and TO-3P packages that use the exact same die, but the TO-3P is rated with much higher SOA.
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Old 20th February 2012, 02:51 AM   #4
dgta is offline dgta  United States
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Ummm... OK, I thought no hotspots/secondary breakdown was pretty elementary, intrinsic and not a "feature".

Yes CBS240, it does seem obvious.
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Old 20th February 2012, 05:21 AM   #5
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Actually there is a hot spotting secondary breakdown effect in MOSFETs too (spirito effect). Some generations are more prone to this than others, old planar deices are rather robust in that matter.
Anyway, the SOA charts in the HV area are often not accurate for not taking that into account.
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