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Old 19th Oct 2021, 11:06 am   #1
McMurdo
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Default MOSFET Identity?

Can anyone recognise this SMD part please? Some sort of mosfet I suppose, I would guess ST5NF20 but nothing comes up in google. It's part of a dc-dc converter and has gone short. It's in a DPAK or TO-252 package.
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Old 19th Oct 2021, 11:26 am   #2
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Default Re: mosfet I.D.

https://www.worldwayelec.com/pro/stm.../5nf20/3084346

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Old 19th Oct 2021, 11:28 am   #3
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Default Re: mosfet I.D.

Datasheet https://datasheetz.com/data/Discrete...atasheetz.html

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Old 19th Oct 2021, 1:07 pm   #4
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Default Re: MOSFET Identity?

mm that says its a TO247 package!
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Old 19th Oct 2021, 1:29 pm   #5
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Default Re: MOSFET Identity?

https://www.st.com/content/st_com/en...roducts-page=1
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Old 19th Oct 2021, 3:48 pm   #6
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Default Re: MOSFET Identity?

That datasheet says the device marking code is '25NF20', rather than the 5NF20 that mine says. Is it the same thing?
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Old 19th Oct 2021, 4:11 pm   #7
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Default Re: MOSFET Identity?

Quote:
Originally Posted by McMurdo View Post
mm that says its a TO247 package!
It also shows a D2PAK, which is what you have - the surface mount version.
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Old 19th Oct 2021, 4:24 pm   #8
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Default Re: MOSFET Identity?

Mine's the DPAK rather than D2PAK, which is similar to a TO220 with the tab cut off.
But when I look at the datasheet for the DPAK version of the 25A unit, it still says the code is 25NF20, rather than my 5NF20.

Am I right in thinking I have a 5A,200V mosfet rather than the 25NF20 which is a 25A, 200V version? Or did they just drop the leading '2' at some point?
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Old 19th Oct 2021, 11:56 pm   #9
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Default Re: MOSFET Identity?

What I find odd are:

There are no actual datasheets anywhere on the net for '5NF20' with any of the standard 'STP, STC etc lettering in front of the number.

It is not listed on the STmicro website, either as a current or obsolete device.

A 2015 datasheet for 25NF20 shows that the devices are marked as 25NF20

I did come across some 5NF20's from China which are in TSSOP8 packages.
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Old 20th Oct 2021, 1:05 am   #10
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Default Re: MOSFET Identity?

I think you might have a 5A version.

You would have to find out which prefix letter ST uses. The letter P (as in STP5NF20) designates a TO-220 case while TO-263 is indicated by a B but I don't have a clue for the TO-252 case, let alone the TSSOP8.
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Old 20th Oct 2021, 8:52 am   #11
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Default Re: MOSFET Identity?

I would assume STD5NF20 but google shows nothing, as per my original post.

I have tried a couple of educated guesses using a 9A 200V and 9A 400V fets, one of them works but overheats after approx 15 minutes and the other just causes a squealing noise and excessive current draw (the converter should draw 70mA at 65V).
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Old 20th Oct 2021, 9:52 am   #12
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Default Re: MOSFET Identity?

MOSFETs are pretty interchangeable in my experience, but I suspect you knew that already! The main things I'd confirm are drain-source voltage and current and gate threshold voltage. Can you cope the gate drive waveform and find out how big it is? Then you can check it's enough for the MOSFET you've substituted.

However, the symptoms you describe give me the feeling that there's something else wrong - a shorted diode or capacitor on the secondary or output, or (gulp) shorted turns in an inductor or transformer.

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Old 22nd Oct 2021, 6:50 pm   #13
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Default Re: MOSFET Identity?

I decided to put the fet back in that worked the best, and then scope what was happening. All looked good for a while then the waveform started jittering badly, frequency wise (amplitude ok). During the jittering, the current draw went up.
While I was staring at the amps on the power supply, a wisp of smoke came from somewhere in the vicinity of the transformer, so I switched off quick.

I had a hunch it might have been arcing under one of the capacitors as I'd had that before on something similar, so I removed them, all to no avail, I removed the little transformer (all of the size of a sugar lump), gave it a good look over with a glass and a good sniff, but nothing looked untoward.

In the end time was marching on so I've ordered a new board, around the same price as a long weekend in a swanky hotel...
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