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Old 19th October 2011, 04:43 PM   #1
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Default Testing MOSFET's: suspected failure.

Hi again,

Ok, forgive me if this has been asked many times before, but I searched the forum and couldn't find the exact answer.

Whilst debugging a buzzy sub-amp (150W) I desoldered the power supply caps, and checked then, 40-55mohm ESR. Seem fine. But while testing the MOSFET's, I noticed some dodgy readings on my multimeter.

Theres two upper N-channels (BUZ900) and two lower P-channels (BUZ905), all are TO3 mounted on a massive heatsink, as expected. Now, I'm not familiar with audio MOSFET's, only power ones (I've designed several SMPS with power MOSFETS), but is a diode drop reading of 0.07V one way, and 0.17V the other normal?

I did not desolder these from the board, but because the cases (source) were only electrically connected to the circuit via mountings, I removed these, and double checked the source of each MOSFET was isolated. Only using a multimeter for diode testing, and (and resistance for kicks) heres the results:

BUZ905s:
Drain to source diode drop: 0.41
Source to drain diode drop: 0.47 (can't remember which way round these were, but they are similar).
Both similar in specs.

BUZ900s:
For First one: D-S: 0.056V (126ohm), S-D: 0.130V (190ohm)
Second: D-S: 0.074V (128ohm), S-D: 0.172V (170ohm)

I was expecting roughly 0.5-0.7V from source to drain for the internal body diode, but both are well below this. And the BUZ900 (N-channel) are WAY below. I have neer seen this before, but as I said, I have only ever worked with power MOSFET's. When these go, theres a very low Vdrop from drain to source either direction, and very low resistance. But these are half way there....

I just thought I would check that these are indeed 'effed' before I buy replacements, or if perhaps my crude testing is flawed (charged gate?). The datasheets on these devices seem sparse on info with regard to internal diode drop. Ultimately, I'm out of my depth.

Any help would be greatly appreciated! And if someone has some BUZ900's laying around, any chance of a quick test? I seriously doubt this is 'normal', but I have doubts...

BC

Last edited by Buriedcode; 19th October 2011 at 04:45 PM.
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Old 20th October 2011, 11:59 AM   #2
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Ok, after much headscratching I realised in circuit, the gates of these MOSFETs were floating (duh), so I soldered a small wire to each gate, and connected them all to the main speaker out - which is directly connected to the sources of all devices.

Now things are looking clearer, the P-channels show 0.48V across the body diode (source to drain). But the N-channels still show 0.085V. However, when measuring diode vdrop and resistance fom drain to source on these N-channels, the diode drop is unmeasurable (high, good thing) and the resistance is around 4.7K. All of these tests were done double checking each gate was shorted to the common source.

So the P-chans haev been eliminated, sitll not what I expected but well within spec. The N-chans seems fine in the 'on' direction. As in, when switched off, no significant current can flow. But the body diode's Vf is lower than most Schottky diodes I've measured - the datasheet provides nothing in terms of the body diode specs (why should they, for audo, its used purely for protection).

Again, if anyone can quickly measure the Vdrop from Source to Drain on an N-channel Audio mosfet, I would be grateful. I do not trust myself here - I guess powering up the amp with no load couldn't hurt, as the low-side MOSFET's look good. Then Measuring the voltage on the speaker output to see if its being pulled high by these dodgy transistors.

Was this the wrong place to post a technical question about testing MOSFET's ?
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Old 20th October 2011, 04:02 PM   #3
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Location: Jeffersonville, Indiana USA
Right place, wrong people. The experts have semi curve tracers and expect you to own one too. Us amateur repair people with a DVM & a scope don't know. My favorite power amp repair accessory is a pair of 10 ohm 450W resistors with 12 ga wires soldered to them. I hook up the resistors & turn on, can determine something about state of O.T.'s with a meter without damaging speakers. I got my resistors at the surplus house. Actually 4 each 5 ohms 225 w each. You have to get creative with surplus. The 5 ohms are gone but somebody has 2 ohm 50 watt ones that could be useful. I put my resistors on steel brackets hung in the air, so I don't set the coffee table on fire if the O.T's are shorted. You can see smoke from the oil of your fingers sometimes before you can read the DVM.
Fairchild FET datasheets are specific about the internal zener etc. That is as far as I have gotten checking. I blew one once, it read shorted across drain-source at all times. I short the gate to source on N channel mosfet before I do anything with a meter, usually.
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Last edited by indianajo; 20th October 2011 at 04:11 PM.
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Old 20th October 2011, 08:07 PM   #4
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Hey Joe,

Where you going with that gun of....couldn't resist, sorry..
Thanks for the reply! Good call on the power resistors for a non-reactive dummy load... I have some 30W 10 ohms somewhere, bolted to something.... in the garage (under years of crap).

I think I'm going to risk it, and power it up, first with a lower voltage (+/- 30v) fom a smaller transformer to limit current - this is without a load attached. Then if it passes some crude measurements, the full +/- 65V power supply on the original amp. Still with no speaker attached. For now, thats all I can really do - trust my meter, and the LTspice simulation I ran of this circuit.

If I post back tomorrow, then I haven't had 30lb's of subwoofer driver fired into my face. But maybe with more questions!

Cheers
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