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A problem with MOSFETs

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hyparh

Sun Jan 22 2017, 02:15PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

Hello everyone!
Recently I've purchased 10 new IRFP250, but all of them seem to be in a constant "off" state. What do I mean - I set my multimeter to diode check, put negative lead to Source, charge the Gate by touching it with the positive lead, and then when I touch Drain with the positive it shows around 900 ohms instead of a closed circuit. And this is for all of the 10 brand new mosfets

My DMM is with new battery and measures everything else properly.
Why this could be happening?ï»¿ Is there any simple schematic (with a small light bulb and battery for instance) to test them?
I tried one of them in a flyback driver and it didn't work properly.

hen918

Sun Jan 22 2017, 02:44PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I am a little confused. A diode check measures the forward voltage of a diode, in volts or sometimes millivolts. Putting the positive lead onto the source and the negative onto the drain should give you a reading somewhere around one volt. With the gate shorted to the source, the positive lead onto the drain and negative on the source, should not give a reading (it would be OL, like a resistance check, because the voltage would be above the multimeter's threshold.

hyparh

Sun Jan 22 2017, 03:44PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

I'm sorry, it was the continuity mode I used for measure (the painted diode there was confusing me with "diode check", or maybe they're both at once on some multimeters?).
So with briefly charging the Gate and then measuring with positive to D and negative to S, it should be like closed circuit with the "beep" signal, right? But in my case they all show some values between 800 and 1000.

klugesmith

Sun Jan 22 2017, 04:13PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dou you get more satisfying results with different MOSFETs, using the same method and meter? What gate voltage will marginally turn "on" an IRFP250?

What's the open-circuit output voltage of your meter in continuity mode, which probably displays ohms? Is it enough to get a finite reading of the D-S diode when probes are reversed?

What's the open-circuit output voltage of your meter in diode mode, which probably displays volts?

hyparh

Sun Jan 22 2017, 05:09PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

klugesmith wrote ...

Dou you get more satisfying results with different MOSFETs, using the same method and meter? What gate voltage will marginally turn "on" an IRFP250?

Yes, via this method and the same meter the previous two IRFP250 were showing 0 (or very near 0 with the beep signal presenting) and were working just fine until I blew them up with a faulty circuit

Now I am realy confused with the new ones which are the very same type, but showing different things when measured.

My DMM is VC890D and has "continuity" and "diode" checks at the same division. I measured 3.2V between probes in this mode.

johnf

Sun Jan 22 2017, 06:46PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Also depends if your mosfets are counterfits.
this has happened before.

If your gate voltage is not clamped to protect it ie Transil diode or Zener diode --it is very likely that you have punctured the gate insulating layer

hyparh

Sun Jan 22 2017, 07:51PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

johnf wrote ...

Also depends if your mosfets are counterfits.

That crossed my mind too. Well, I guess I'll have to try another ones from another store

Though I've used IRFP250s from the same store before without problems, but this was more than year ago, so who knows...

Here are three of them (unused, as they were in the package):

1485114244 1054 FT178767 20170122 213026

1485114244 1054 FT178767 20170122 213050

Wolfram

Tue Jan 24 2017, 04:01PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

The test is not very reliable, as the compliance voltage of the diode check function on most multimeters is close to the gate threshold voltage of many power MOSFETs, including the IRFP250. Test them with a higher gate drive voltage to get a more conclusive result.

Conundrum

Wed Jan 25 2017, 12:16PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Hi, if you are interested I have a used Peak Atlas (pcb only) here which does work but the battery discharges in a couple of days. If you added a switch to it this would be a nice test instrument. PM me.

Enceladus

Mon Jan 30 2017, 01:54PM

Registered Member #61428 Joined: Sat Jan 14 2017, 12:39PM
Location:
Posts: 50

Have you tried measuring while charging the gates to perhaps 10V with a separate supply? Most datasheets recommend driving Vgs to at least 10V for an R(ds) ON measurment test.

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