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MOSFET's; Magical Parameters
#1
Hi, all. Haven't been on here since before the fancy new interface. Glad to see this is still going on.

So I have an application for a MOSFET source follower. The goal is simply a transparent buffer. I've gone through spec sheets for this before, and the dozens of different parameters just makes my eyes glaze over.

Merlin mentions using power MOSFETs with a lower-than-usual "reverse transfer capacitance" in his Hifi book. So I know I'm looking for low capacitance, nominal voltage higher than my supply rail, maybe concerned about power/current handling. Apart from these things, are any of the myriad other specs of any concern to me? Any magical parameters to look for? Or, for all practical purposes, is one N-channel power MOSFET as good as another?

Thanks.
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#2
Check the SOA graph, make sure there's a DC rating (and that your operating point falls within it). This indicates the FET is suitable for linear operation, and not just switching. If it doesn't have a DC curve, it's safest to assume that FET is not suitable for linear operation.
Many *may* be suitable with sufficient de-rating, because the DC SOA is rarely important enough to the target market (switching) to be characterised, specified and therefore guaranteed by the manufacturer, so sometimes they simply don't bother.
So rather than it being explicitly unsuited, we just don't know.
Without that curve on the graph, the only way to know is to test and qualify yourself, and at a DIY level we usually don't want to have to blow up a whole lot of parts to find out what their limits are, so it's easier and cheaper to make a safe bet and choose one that's specified to work correctly.
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#3
an IFR740 or IFR820 should work for you. http://www.geofex.com/Article_Folders/mo...tfolly.htm
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#4
(05-31-2020, 10:22 AM)Greg Robinson Wrote: Check the SOA graph, make sure there's a DC rating (and that your operating point falls within it). This indicates the FET is suitable for linear operation, and not just switching. 

Greg, thanks. That totally makes sense. Who the target market is for these devices wasn't even in my mind.

Stack, I read R.G. Keen's stuff once upon a time - good information. I have a bunch of IFR820's I use for power supplies, but I'm looking to play with devices other than the usual suspects. Thanks.
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