Electronics > Beginners

MOSFETs - Safe Operating Area

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joeyjoejoe:
Saw a nice video explaining this, so I get the concept.

However, some I notice some SOA charts do not have a DC rating.

For instance, the IRF 5305PbF doesn't.

https://www.infineon.com/dgdl/irf5305pbf.pdf?fileId=5546d462533600a4015355e370101993



What does this mean? It's not rated/certified/intended to be used for constant linear operation? Or they just haven't bothered?

(An aside : I notice International Rectifier/Infineon doesn't seem to have DC on any of the datasheets for the (very small sample set) 3 MOSFET's I chose. I went and picked 4 ON Semiconductor datasheets and they all had DC specified in the safe operating area.)

thm_w:
Its not rated to be used in that way. You can find some good discussion here: https://www.eevblog.com/forum/projects/igbt-vs-mosfet-for-dummy-load/
http://www.digikey.com.au/Web%20Export/Supplier%20Content/microsemi_278/pdf/microsemi-power-an-make-linear-mode-work.pdf?redirected=1
https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200

It could fail at less than 10x its rated current for example.
But some loads are using IRFP250, which has no DC SOA rating as well. So they have either done a lot of internal testing or discussed with the manufacturer.

joeyjoejoe:
Awesome, thanks - that first one sure is lengthy!

So, is there a way to find mosfet's that are good for linear operation, short of reading tons of data sheets? I feel like I know the answer but might as well ask :)

thm_w:
The way is to use the same parts Agilent/Rigol/BK have, and derate them in the same way (IRFP250, IRF540, etc. run at <30W each?)

Or buy purposefully linear rated FETs, some are not too expensive if you need guaranteed operation:
http://ixapps.ixys.com/family.aspx?i=5
https://www.microsemi.com/product-directory/mosfet/732-linear

joeyjoejoe:
Gotcha.

Looks like ON Semi has some nice ones for linear use.

2SJ652 (P Channel)


NTP5864N (N Channel)


Kind of nice as a hobbyist, I don't have to sweat the difference between a MOSFET that's 1.40$ versus 2$. (Or 3.25CAD$ in this case!) Just derate to make up for my lack of the ability or willingness to test.

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