Broken power supply

Discussion in 'General Electronics Chat' started by PZUFIC, Sep 25, 2015.

  1. PZUFIC

    Thread Starter New Member

    Jan 7, 2012
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    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.
     
  2. #12

    Expert

    Nov 30, 2010
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  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    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]...

    It would seem you may purchase the device here:
    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: Sep 25, 2015
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  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    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
     
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  5. PZUFIC

    Thread Starter New Member

    Jan 7, 2012
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  6. Hypatia's Protege

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  7. PZUFIC

    Thread Starter New Member

    Jan 7, 2012
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  8. Hypatia's Protege

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    Putatively, yes -- That said; you are advised to compare the maximum and application critical specifications of any replacement with those of the OEM part...

    Best regards
    HP
     
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  9. Aleph(0)

    Member

    Mar 14, 2015
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    Say what HP? No New Tone equivalent?:p

    I say mosfet at link will work cause absolute maximum ratings ok and on resistance is lower and gate charge and parasitic capacitance favorable for switching service
     
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  10. #12

    Expert

    Nov 30, 2010
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    Obsolete? The whole page? Twenty-five different 7N60 transistors in stock by the hundreds? Some in stock by the thousands...You call that Not Available?
     
    Last edited: Sep 25, 2015
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  11. Gdrumm

    Distinguished Member

    Aug 29, 2008
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    Girl fiight?
    (Aleph (0) and HP?)
    ...Whatever she said...
     
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  12. ian field

    Distinguished Member

    Oct 27, 2012
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    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.
     
  13. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    ;););)
    '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
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
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    Depends on the OP's CoO.
    In Canada for example, DigiKey is just about the best bet.
    The trouble with NTE they are upwards of 4x the cost usually for a sub device.
    Max.
     
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