Electronics > Beginners
Why am I having so Much Trouble Choosing a Mosfet?
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neppo1345:
Long time lurker, first time posting. Bear with me

Trying to figure out circuit that will let me turn on/off a 5v 250mA load using a microcontroller with 3.3v logic. I'd like everything to be fairly compact, SO-8 size mosfet or smaller...but still solderable by hand.

After some research I determined the easiest way to do this is probably using an N Channel Mosfet/low side switching. Ideally it would be high-side switching (to keep a common ground reference between the load and other components) with a P-channel, but then I would need to pick out a mosfet AND a transistor...and I was having enough trouble picking out a mosfet by itself....so I gave up and compromised on the low side.

So.. did a bunch or research, realized I needed to keep an eye on thermal requirements no matter what the 'max' current was advertised on the datasheet.

Looked at the IRF6201...2.10 mOhm @ VGS = 2.5V so that's what... (.250A^2)*(.002Ohm) = .125mW IRF6201 datasheet shows Pd @70c of 1.6W. Built it up, it works...but the mosfet gets screaming hot.

So I go back to the datasheet to figure out WTF I did wrong and stumble upon Figure 8 showing 'Maximum Safe Operating Area' and find that my current/voltage combination is way out of the safe range for DC.

Now that I know the datasheets have these figured I'm trying to find something based on other datasheets and their 'safe operating area' figures and for the life of me can't find a small mosfet that will fit my bill.

Am I going about this the right way and just falling into a trap of impossible requirements (I didn't think 250mA at 5v sounded too crazy)? Am I just asking too much for an SO-8 package?

If not can someone nudge me in the right direction to help me figure out what I'm doing wrong?

Thanks,

-Tim

2N3055:
Measure voltage drop on the mosfet...
capt bullshot:
From your description, the MOSFET cannot get warm at all, since you intend to use it as a switch, and you wouldn't exceed SOA since you have either 5V or 0.25A (but not at the same time) across the MOSFET.

Something must be wrong with your circuit.
mbless:

--- Quote from: 2N3055 on June 08, 2018, 07:16:16 pm ---Measure voltage drop on the mosfet...

--- End quote ---

Also what do you measure for the gate voltage? Is there a pull-down resistor on the gate?
Dukov Ahzrukhal:
Could it be that 3.3v is not enough to fully turn on the MOSFET? You could try disconnecting it from the microcontroller and applying 5v to the gate to see if it still runs too hot.
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