Connect with us

Reasonable to try to sub power MOSFET's?

Discussion in 'Electronic Repair' started by Mr. Land, Jan 22, 2008.

Scroll to continue with content
  1. Mr. Land

    Mr. Land Guest

    Hi, I'm working on a home theater receiver that has a blown MOSFET
    (2SK3599-01MR) in one of the switching power supplies. I can't seem
    to find a replacement from any of the sources I'm aware of.

    I was able to find a datasheet on it, and I'm wondering if it's even a
    reasonable idea to try to use that to substitute some other MOSFET for
    it by comparing datasheets.

    If this isn't a completely stupid idea, what parameter(s) would be key
    for making such a substitution?

    Thanks.
     
  2. JeffM

    JeffM Guest

  3. Arfa Daily

    Arfa Daily Guest

    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
     
  4. JW

    JW Guest

  5. Mr. Land

    Mr. Land Guest

  6. Mr. Land

    Mr. Land Guest

    Thanks for that advice. I found the device listed on a manufacturer's
    site
    (http://www.fujisemiconductor.com/html/QckSelGuides/mosfet.htm) 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.
     
  7. Arfa Daily

    Arfa Daily Guest

    Thanks for that advice. I found the device listed on a manufacturer's
    site
    (http://www.fujisemiconductor.com/html/QckSelGuides/mosfet.htm) 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
     
  8. Guest

    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
     
  9. Mr. Land

    Mr. Land Guest

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

    Mr. Land Guest

    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.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-