Troubleshooting after discovering a faulty mosfet.

Discussion in 'General Electronics Chat' started by gsnowak0w0w, Apr 29, 2016.

  1. gsnowak0w0w

    Thread Starter New Member

    Apr 29, 2016
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    I acquired an lcd tv for free that didn't have a picture. But while the tv is on in a dark room you can see that the tv is clearly on.

    After my first visual inspection on the power supply unit I came across a round fuse with a black mark on both sides of it and I checked It and it was open. So I moved on to the rectifier diodes nearby and pulled a leg on all four of them. They all checked out good. The next thing I pulled was a pair of MosFets that apon testing them via transistor tester were bad. I checked the surrounding resistors tied to the gate and they read fine.

    I did a lot of reading and I'm unsure why they went bad. Did one just go bad taking the other? I bought two more but am nervous install them. (New fuse already installed.)

    Should I troubleshoot further in some ones opinion? Or when should I stop troubleshooting?
     
  2. Roderick Young

    Member

    Feb 22, 2015
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    168
    I would check every single component nearby, with special attention to anything connected to the gate or drain of the transistors. For example, you may see a resistor and capacitor in series, where the resistor is marked 82 ohms, but reads 500 ohms. Or maybe the capacitor is marked 2200 pF, but has gone open circuit. That could be a bad snubber. My limited experience is that when I replace a bad MOSFET without finding what the problem was in the first place, the circuit will work for a few seconds or minutes, then the transistors will blow again.

    Also FWIW, http://pididu.com/wordpress/blog/repair-a-24-volt-electric-bike-charger/
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,026
    I've had somewhat better (and far more limited) experiences. Whenever I've replaced a blown MOSFET, I was lucky that it was indeed the problem and the device came back to life. I had one take the gate resistor with it, though.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    In flyback PSUs they usually take out the current sensing resistor in the source circuit, LCD TVs usually have half bridge PSUs which don't always have a source resistor.

    Most manufacturers sail close to the wind on MOSFET voltage rating - UK mains would put about 320V on a reservoir capacitor, most have a PFC front end which boosts that to just under 450V - yet the MOSFETs are rarely rated more than 500V.

    The TS could get lucky and find there's no other damage and replacing both MOSFETs as a pair does the trick. With the benefit of a mains isolating transformer, it would be worth checking the drive waveform - obviously it would be at maximum M/S ratio trying to get some voltage at the regulation sensing circuit, but good clean square waves would eliminate some possible causes.

    In these circumstances; I'd certainly check whether the PSU input circuit is MOV protected - and add one if it isn't.

    On the plus side; a salvaged part is certain to be an acceptable voltage rating, on the downside; they take a hit every time they clamp a spike, so some of its life is probably used up.

    The tricky part of ordering a new one is deciding what standoff voltage to select.
     
  5. gsnowak0w0w

    Thread Starter New Member

    Apr 29, 2016
    3
    0
    Alright. Thanks for the good info. I double checked all the gate resistors and other components nearby.

    Unfortunately it didn't fix my issue. The tv turns on for a second then shuts back off.

    If anyone has more suggestions for me to try I plan on doing more troubleshooting. All the power coming off the power supply is good. The only thing I couldn't check is the transformer output to the lcd inverter board. I don't have a multimeter to test 2100 volts.

    Thanks if you have suggestions.
     
  6. gsnowak0w0w

    Thread Starter New Member

    Apr 29, 2016
    3
    0
    Alright. Thanks for the good info. I double checked all the gate resistors and other components nearby.

    Unfortunately it didn't fix my issue. The tv turns on for a second then shuts back off.

    If anyone has more suggestions for me to try I plan on doing more troubleshooting. All the power coming off the power supply is good. The only thing I couldn't check is the transformer output to the lcd inverter board. I don't have a multimeter to test 2100 volts.

    Thanks if you have suggestions.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    I'd check the secondary side rectifiers.

    You have to be careful though, any Shottky barrier rectifiers show some leakage as normal. When they fail - there usually isn't any doubt.

    Regular fast silicon rectifiers shouldn't have any detectable leakage that you can measure with usual bench instruments. Any leakage is very thermal, so leakage causes heating - and more leakage.
     
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