A problem with MOSFETs
hyparh, Sun Jan 22 2017, 02:15PMHello 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? 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.
Re: A problem with MOSFETs
hen918, Sun Jan 22 2017, 02:44PM
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.
hen918, Sun Jan 22 2017, 02:44PM
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.
Re: A problem with MOSFETs
hyparh, Sun Jan 22 2017, 03:44PM
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.
hyparh, Sun Jan 22 2017, 03:44PM
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.
Re: A problem with MOSFETs
klugesmith, Sun Jan 22 2017, 04:13PM
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?
klugesmith, Sun Jan 22 2017, 04:13PM
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?
Re: A problem with MOSFETs
hyparh, Sun Jan 22 2017, 05:09PM
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.
hyparh, Sun Jan 22 2017, 05:09PM
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?
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.
Re: A problem with MOSFETs
johnf, Sun Jan 22 2017, 06:46PM
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
johnf, Sun Jan 22 2017, 06:46PM
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
Re: A problem with MOSFETs
hyparh, Sun Jan 22 2017, 07:51PM
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):
hyparh, Sun Jan 22 2017, 07:51PM
johnf wrote ...
Also depends if your mosfets are counterfits.
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):
Re: A problem with MOSFETs
Wolfram, Tue Jan 24 2017, 04:01PM
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.
Wolfram, Tue Jan 24 2017, 04:01PM
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.
Re: A problem with MOSFETs
Conundrum, Wed Jan 25 2017, 12:16PM
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.
Conundrum, Wed Jan 25 2017, 12:16PM
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.
Re: A problem with MOSFETs
Enceladus, Mon Jan 30 2017, 01:54PM
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.
Enceladus, Mon Jan 30 2017, 01:54PM
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.
Re: A problem with MOSFETs
Conundrum, Tue Jan 31 2017, 05:24AM
Yeah, Vgt is known to be fairly low. It isn't actually a threshold but the side effect of the parasitic diode inside the MOSFET.
I got fooled by this, had to resort to finding a known good unit and compare on Mr Peak Atlas, he say OK It did find that one of the diodes elsewhere was leaky so that got changed.
Conundrum, Tue Jan 31 2017, 05:24AM
Yeah, Vgt is known to be fairly low. It isn't actually a threshold but the side effect of the parasitic diode inside the MOSFET.
I got fooled by this, had to resort to finding a known good unit and compare on Mr Peak Atlas, he say OK
It did find that one of the diodes elsewhere was leaky so that got changed.Print this page