Broken power supply

Thread Starter

PZUFIC

Joined Jan 7, 2012
54
Hello!

I have a broken switching power supply. I accidentally push a striped wire inside the case and made a connection from FET heatsink to the case. The FET heatsink is connected to the ground of a input bridge. FET case is totally insulated from heatsink. It's a 5 and 12 V Traco power supply.

I shorted out thermistor and polyfuse, remove varistor and replace broken bridge. Then I tried it on a autotransformer. The current was low at the beginning of rising voltage, but then if it just went up and burned the resistor, which is connected between source and bridge ground. I removed the FET and tested it. My AVR tester recognized it as double diode. The method with DMM failed and also method with 5 V power supply pot and 100 Ohm resistor, there was no current going into the drain.

Does anyone have experience on that case, what can also be a common problem?

Is there any common replacement for mdf7n60b FET, because this one is kind of hard to get.
 

Hypatia's Protege

Joined Mar 1, 2015
3,226
I removed the FET and tested it. My AVR tester recognized it as double diode.
Inasmuch as the 'FET' is of the insulated gate type (i.e. a 'MOSFET' as opposed to a 'JFET') the result is indicative of failed gate 'insulation' [at least]...

Is there any common replacement for mdf7n60b FET, because this one is kind of hard to get.
It would seem you may purchase the device here:
http://www.aliexpress.com/item/Free...F7N60-SSS7N60B-7N60C-MDF7N60B/1892086767.html
http://www.aliexpress.com/item/Free...F7N60-SSS7N60B-7N60C-MDF7N60B/1892086767.html

EDIT: Please heed post #2 --- Mouser is a reliable vendor!!!:):):)

Best regards
HP
 
Last edited:
Switchmode supplies don;t really like auto-transformers. Some require at least a minimum load on the highest power supply.
I would, however use an incandescent light bulb in series with the line.

Sometimes without the minimum load the voltages keeps rising. 10-20% full load should work.

The light will limit the power and act as a current dependent resistor.

Bridge blowout is common and so is the FET
 

ian field

Joined Oct 27, 2012
6,539
Hello!

I have a broken switching power supply. I accidentally push a striped wire inside the case and made a connection from FET heatsink to the case. The FET heatsink is connected to the ground of a input bridge. FET case is totally insulated from heatsink. It's a 5 and 12 V Traco power supply.

I shorted out thermistor and polyfuse, remove varistor and replace broken bridge. Then I tried it on a autotransformer. The current was low at the beginning of rising voltage, but then if it just went up and burned the resistor, which is connected between source and bridge ground. I removed the FET and tested it. My AVR tester recognized it as double diode. The method with DMM failed and also method with 5 V power supply pot and 100 Ohm resistor, there was no current going into the drain.

Does anyone have experience on that case, what can also be a common problem?

Is there any common replacement for mdf7n60b FET, because this one is kind of hard to get.
The MOSFETS may be insulated from the heatsink, but the heatsink is often connected to the negative end of the bridge rectifier. On a 230V mains; that usually means about 320V negative half cycle peaks.

You should start by taking the MOSFETs out and test whether either are shorted, I'd also examine the PCB looking for vapourised tracks.
 
Girl fiight?
(Aleph (0) and HP?)
...Whatever she said...
;););)
'Tho no shill -- I must confess - did NTE list an 'equivalent', I would have mentioned it - albeit with the caveat that substitutes are 'iffy' at best and never preferable to proper replacements --- All of which is to say, I concur with Aleph in this instance...:)

Best regards
HP:D
 
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