Electronics > Beginners
Why am I having so Much Trouble Choosing a Mosfet?
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mbless:

--- Quote from: cellularmitosis on June 08, 2018, 10:41:33 pm ---This MOSFET appears to have a very unusual safe operating area.  It appears it isn't rated for DC above 0.2V.

--- End quote ---

That's not how SOA works; it's the VDS when the mosfet is on. Given RDS ~= 2mOhm, VDS ought to be 0.5mV at 250mA! That's easily inside the DC SOA.

Again, this is why we are telling OP to measure VDS. OP also doesn't say if the MOSFET is PWM'd or not. The gate charge is rather large, so fast PWM with a large gate resistor could result in a lot of time in the linear region.
neppo1345:
I checked gate voltage at 3.4V.

Voltage drop across the transistor is checking at 0.5mV

Transistor is an infineon and came from DigiKey (as reputable of a source as I know, so I hope its not fake).

And hold on...the Vds used in the SOA is the voltage drop across the transistor? Holy crap, I had no idea I was reading those charts wrong. Why the heck do all the datasheets just straight up call it drain to source voltage?

I'll solder up another board...maybe I damaged this mosfet while handling it prior to assembly.

I really appreciate the help, looks like I may have had some major misunderstandings.
mbless:

--- Quote from: neppo1345 on June 09, 2018, 03:54:49 pm ---And hold on...the Vds used in the SOA is the voltage drop across the transistor? Holy crap, I had no idea I was reading those charts wrong. Why the heck do all the datasheets just straight up call it drain to source voltage?

--- End quote ---

It's all the same thing. Vds is the voltage difference between the drain and source which is the voltage drop across the transistor (in the current path). The SOA uses the actual operating conditions: off and on. When off Vds = 5V and Id = 0A. When on Vds = 0.5mV and Id = 250mA. You can't combine the off Vds and the on Id together.
sleemanj:

--- Quote from: neppo1345 on June 09, 2018, 03:54:49 pm ---I checked gate voltage at 3.4V.

Voltage drop across the transistor is checking at 0.5mV


--- End quote ---

At 250mA load through it?  Sounds about right then at 2mR Rds (0.0005V / 0.25A).  Dissipation at that should be miniscule.

Unless you meant 0.5V instead of 0.5mV - in which case I'd suggest you checked the pinout.



Teledog:
2n7000   cheap like borscht
Maybe put a 1-10k resistor on the gate &/or a 10k from gate to source if there's any floating logic issues


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