Reasonable to try to sub power MOSFET's?

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In my experience, if you enjoy Disney's firework displays, go right ahead and sub for the original ... !!

Seriously though, I've never had any luck going down the substitution route for power MOSFETs in switchers. It seems like no matter how close - or even apparently better - you get a match on the quoted parameters of the original, then if it doesn't try to take the retinas out of the backs of your eyes at first switch on, it will try to melt the supply to your house two days later ...

Others on here might have found differently ?

Arfa

Nice! Thanks for posting that.

Thanks, I will have a look at that.

e n

Thanks for that advice. I found the device listed on a manufacturer's site

and it's listed with "related" devices with a small set of comparative characteristics.

I'm (only) guessing that I(D) is maximum drain current, R(DS)on is the maximum drain-to-source resistance when the device is fully on, V(DSS) is the maximum drain-to-source voltage, and Q(G) Typ would be expected capacitance at the gate.

I was hoping that, since these specs are the only ones on this page, and the page seems to be intended to help you choose a device for a design, that if I could find one of the other numbers that came in close, I'd be OK.

But it sounds like I'm just asking for trouble. Thanks again.

Thanks for that advice. I found the device listed on a manufacturer's site

and it's listed with "related" devices with a small set of comparative characteristics.

I'm (only) guessing that I(D) is maximum drain current, R(DS)on is the maximum drain-to-source resistance when the device is fully on, V(DSS) is the maximum drain-to-source voltage, and Q(G) Typ would be expected capacitance at the gate.

I was hoping that, since these specs are the only ones on this page, and the page seems to be intended to help you choose a device for a design, that if I could find one of the other numbers that came in close, I'd be OK.

But it sounds like I'm just asking for trouble. Thanks again.

Well, you might be ok. You seem to have the abreviations for the various parameters down pat, so that's a good start. Before carrying out any repair on it, you might want to check that the current sensing resistor in the source has not also gone open. Check the gate coupling resistor for open. Check the main input filter cap for poor ESR / low value. Check the main primary side rectifier diodes for short circuit.If it uses a control chip, check the startup resistor(s) for value, and the supply decoupling cap. If the gate coupling resistor is blown, or even just open, suspect that the chip is also damaged. If you can source one, replace the feedback opto as well. Check the secondary side for short circuit rectifiers (but don't get fooled by the fact that they are 'fast' types that read very low in one direction anyway). Shorted secondary side reccies can place such a strain on the primary side circuitry, that it results in a blown FET.

If you have access to a variac, you can check for chopper drive on most supplies, by leaving out the FET initially, and 'scoping the output pin of the control chip, using primary side rectifier "-" (main filter cap "-") as a reference, but be careful of your 'scope grounding, and the fact that you are very close to rectified line voltage when doing this. You can often get the chip basically 'going' with no more than 30% line input voltage for US

110v or 15% for EU 230v, which makes the checking a lot safer.

If you can source the correct FET, and it's not too expensive, save the postage and order two of them. You can be sure that the first one will go bang again ...

Arfa

Arfa

Arfa is dead on about that resistor. The source resistor is about the same thing as an emitter resistor to a bipolar. There is also usually a zener from gate to source, it may be shorted and if it is the drive circuitry might be fried. In fact if the source resistor is open the drive circuitry is usually fried.

That's why we change fuses LAST.

Now as for a replacement. This is a switching FET. You must have the same or better gain in case it is a self oscillator, switching times must be faster or at least equal, not knowing what frequency it is supposed to run at. That is after all the maximum ratings have been met or exceeded.

Do not change case styles, especially to a metal tabbed one. It is not only a pain in the ass but might be illegal. If you are doing this in your basement it is unsafe. This is a hot side component. If it is all plastic and you get the insulator kit, and put in a non all plastric device, there could be an insidious shock hazard. You certainly blow the UL or CSA or whatever rating, and if this is done at an ISO certified company it is so close to illegal you can smell it. It would certainly be grounds for dismissal. So if it is plastic, stick with plastic.

Just remember, after all the Id, Vd all that max, even if the replacement device meets that criteria there is still transconductance and switching time. If you have higher gain and faster switching time you should be alright. Of course use the right type of transistor. Some are optimized for switching, you don't want the ones that are optimized to be used in the linear mode, such as an audio output.

There is a safety factor, mainly keep the same case style. Other than that you can always upgrade a semiconductor device (in the US) because of a mandate in 1976 that safety of a product cannot depend on the failure of a semiconductor device.

There were TVs with absolutely no high voltage shutdown circuits at all, the idea was, well the horizontal output will fail before it gets that high. Well some didn't.

Just recently I had a dog case where the other tech replaced a damper diode. This application requires a 1200 V doide. He put in a 400 V diode, and the thing lasted for like a month. He must have read the numbers wrong or something, and then by chance got a diode that was able to take it. When it came back he replaced the diode again and it failed immediately. So did another.

I finally had a look, saw where it was and said wait a minute, you can't.......I went to the computer and found out for sure, it was the wrong diode.

But you can't count on it failing, that is the rule.

Check that continuity from the source to the negative end of the main filter, then make sure it reads open circuit, or close, from source to gate, positive probe on the gate. Follow the foil from the gate and there will be a resistor, make sure that is not open, and check where it comes from for a shorted condition between it and hot ground or the Vcc or Vdd of the chip. It a transistor check if it is shorted.

Also with as much unplugged as you can unplug, check all the rectifiers on the seconday side for shorts. Many times the current limiting works fine when one of the output voltages is overloaded, but I have seen more than one design where if one of those rectifiers is shorted it does not work, letting it fry another chopper transistor. Happened more than once, so it is worth checking.

At any rate, have fun with it. Good luck.

Also I would check anything that drives an optocoupler. Alot of times that fails and leads to overvoltage, frying it. Run your eyes down the foils from the pins, if there are any electrolytic caps connected, check them.

After all of this, you might not have a fireworks display. I would suggest firing it up on a variac, but there are alot of designs that simply will not start that way, so forget it.

JURB

Thanks for the great info - I learned a lot. Cheers.

I'm not sure if anyone will catch this reply, but I just wanted to thank all you folks again for all the info and guidance on this and other questions I've posted recently.

Thanks to you, I now have a working 7.1 home theater receiver (now to find a remote.)

I am grateful.