Weird/interesting FET problem - Page 1
Forum section: electronics / projects

author: hammil
author_rank: Regular Contributor
timestamp: March 13, 2013, 02:44:21 pm
content: So I have this p-channel FET I got from Farnell (an IRLIB9343 to be specific) but I can't get it to work as I would have thoughtI can't seem to confirm the pinout of the p-channel MOSFET in TO-220, so I just assumed it was the same as the n-channel, Gate Drain Source when viewed from the front.I had an adjustable negative supply connected to the gate pin - from the output of an inverting amplifier, and had it at 0 initially. I then connected the source pin to ground... and the amplifier started to oscillate dramatically. Like, saturating either way. And it was conducting about 15mA through the gate pin to the source, ground.However.. just shorting the amplifier's output did nothing. Which makes sense, since there was no voltage difference between them.Now.. unless I've got the pinout wrong, which from some passive measurements I can't really see how it could be the case, the gate pin should be very high impedance and not conduct at all. I find it difficult to see how I could have broken it, since the maximum possible voltage I could have put across any of the pins is 15V, and the most current I could put through it was about 2A, which is well below the maximum rating in the datasheet.Is this a defective part, perhaps? Either way, I'd be interested to know how it managed to cause the amplifier to oscillate like that.

author: Icarus
author_rank: Regular Contributor
timestamp: March 13, 2013, 08:39:16 pm
content: Source should go positive for this type of mosfet

author: muvideo
author_rank: Frequent Contributor
timestamp: March 13, 2013, 09:01:40 pm
content: Hello, as for the oscillation, you should post the schematic, but in general thegate of a mosfet is capacitive, and a capacitive load on the amplifier output caneasily make it unstable.As for the part, it's the first time I see a datasheet without pinout, dont knowif it's funny or depressing, but surely it will be the usual GDS as you said.Checking a mosfet with a dmm is an easy task, many words for a very easytest sequence (for a P mosfet):- put the dmm in diode test mode- put the negative meter's lead on the source and with the positive lead touchthe gate, you should not see any conduction.- hold the negative lead on the source and put the positive on the drain, hereyou measure the S-D junction, it's like a normal diode, you will read 0.5-0.6V.- swap meter leads: positive on the source and negative on the drain, noconduction- hold the positive lead on the source and touch the gate with the negative oneno conduction here, but doing this you have charged the Gate to the test voltageof the meter, if the threshold voltage of the mosfet (check Vth) is low enough it willstay in conduction for few seconds- quickly move the negative lead from the gate to the drain, you should see someconduction or even a dead short for few seconds.Sometime the mosfet contains a protective zener that connects the gate to thesource, but usually it's in the datasheet part drawing, and this seem not the casewith yor part.If your mosfet is gone, probably you will find a perforated gate, maybe it was subjectto excessive voltage gate to source, also static electricity can be a culprit, for exampletouching the part when it's connected to a power supply not grounded.Fabio.Edit: for an N-channel mosfet the test is the same but with lead polarity swapped.As for the current you measured, how did you was measuring it? If the ampliferwas in autooscillation it's normal that you have measured AC current flowingtrough gate and source, since as I said before the gate to source (and also to drain)behaves like a capacitor.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 13, 2013, 11:50:05 pm
content: Quote from: AcHmed99 on March 13, 2013, 10:34:16 pmOps, i missed it, my apologies to IRFFabio.