Help with mosfets

Discussion in 'General Electronics Chat' started by Vauxpaul, Oct 30, 2015.

  1. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Can anyone help me with mosfets? Could tk12a50d be replaced with fqa13n50cf. Any help would be greatly appreciated.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It depends upon the application, which is??
     
  3. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Hello. It's a 51 inch Samsung plasma TV power supply.
     
  4. dl324

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    Mar 30, 2015
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    You'll get more help if you make it easier for someone to help you. Post a schematic and the datasheets.
     
  5. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Can you help?
     
  6. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Where do I get those?
     
  7. dl324

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    Mar 30, 2015
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    From where you bought them, manufacturer, Google,...

    They look fairly equivalent, but a schematic showing usage would allow critical parameters to be identified and scrutinized.
     
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  8. PeterCoxSmith

    Member

    Feb 23, 2015
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    similar but different.

    The package is a different size; so for a substitution in an existing design might not fit.
    The rise and fall switching times are different so different dissipation and slower means may not switch at frequency required and increase in dissipation.
    Different gate charge may mean gate drive circuit not good enough.
     
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  9. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Thanks for that. I did look at those sheets and noticed a few differences. I could just try them out.
     
  10. Roderick Young

    Member

    Feb 22, 2015
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    If it were me, I would rather get the exact part, or at least one in the same package. Heat sinking will be an issue.

    The second thing to worry about is whether the MOSFET took anything else with it when it burned out. After extracting the original part, use an ohmmeter to see whether there is any resistance between drain and gate. If there is anything but infinity, then in the process of burning, a current path was created between drain and gate, probably putting a high voltage on the gate, and destroying the driver circuit. If you have a scope, you might want to see whether the driver circuit is still putting out a waveform. Also check any diodes you see in the vicinity, and nearby resistors, even if they don't look burned.

    The third thing is whether whatever burned the MOSFET out in the first place is still lurking in the circuit.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    The first pair of numerals relate to Id, 13 is better than 12, so using a 12 instead of 13 is probably asking for it.

    The second pair of numerals is voltage x10, so both parts are 500V.

    You need to find the data sheets for both parts and verify a couple of points.

    If the replacement part has higher VGSthr; it may not switch fully on causing it to partially dissipate in linear conduction.

    In a SMPSU; you need to check the replacement is as fast as the old one, if the replacement is slower it will dissipate more during the switching transitions and maybe overheat.
     
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  12. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    Tried them in but no change. The fault must lie somewhere else. Maybe someone on here as had a similar fault with one of these TVs. Stuck on standby. Screen flashes white and led flashes.
     
  13. ian field

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    Oct 27, 2012
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    Did you test the ones you took out?

    Most often they fail short circuit all ways round. That usually takes out any fuse, possibly a resistor or two and maybe even a PCB track.

    It might simply be not starting up - there are various schemes for getting it going, the most common is a high resistance start up resistor. These can be in the habit of going high or open.
     
  14. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    I only tried the mosfets as Google and YouTube are suggesting. Have worked on other Samsung TVs that just needed capacitors replaced but all caps are good. Not too sure about the old ones. Using a cheap meter. The meter says the vs and va voltages for the screen are too high.
     
  15. ian field

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    Oct 27, 2012
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    Most likely a safety trip shutting it down then.
     
  16. recklessrog

    Member

    May 23, 2013
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    From the limited information you have provided, you have a T.V with a failed Mosfet. Have you found out WHY it has failed? Otherwise, simply replacing it may just cause the new one to fail. Look for signs of stressed capacitors, (are any leaking, or have the top bulging) high ESR capacitors are a very common cause of semiconductor failure in T.V power supplies, (or any other similar types) you say all caps are good, have you checked their E.S.R?
    You may also have other semiconductors on the board that have been overheated and will fail soon after you replace only what appears to be faulty. Careful study of the circuit diagram often reveals what else is likely to have been affected, but more often than not, a good rule of thumb is change all the electrolytics and semiconductors in that part of the PSU. This may sound like overkill, but may well save you the time, cost, and aggravation of having to do the job all over again at a later date!
     
  17. recklessrog

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    May 23, 2013
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    Be very careful connecting a 'scope to measure waveforms in the psu, as unless the T.V is run from an isolation transformer, you will either damage the scope or psu as the "earth connection" of the probe will short out the psu. Under no circumstances be tempted to just remove the mains supply earth from the scope to measure the waveforms as it will float the case and internals at line level. This practice has killed more than a few engineers in the past!
     
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  18. ian field

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    Oct 27, 2012
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    Its a trick I got away with for a long time - but it did bite me every once in a while.
     
  19. Vauxpaul

    Thread Starter New Member

    Oct 30, 2015
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    I have a meter to test capacitors. They all read 2 or 3 microfarads over what is printed on them. Don't have a scope or isolating transformer. Wouldn't know where to put a scope probe on it. Just seems a waste to dump it.
     
  20. recklessrog

    Member

    May 23, 2013
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    Electroytic capacitors used in non critical applications, ie tv power supplies, may often have a value tolerance of -50/+100% of the stated value. So if you are measuring only the value, it is possible that for example, a capacitor marked 100mfd, may have had a value of 200mfd when new. Due to ageing and deterioration, may now read only 100mfd, BUT, the equivalent series resistance may have increased significantly now rendering it unable to perform its function correctly. in switch mode power supplies, the ESR is often far more important than the exact measured capacity. Anyone who repairs things like the smp in Thompson sky satellite recievers, always do a blanket change of the electrolytics using better quality components to ensure continued reliable operation.

    I repeat my warning to everyone, DO NOT FLOAT TEST EQUIPMENT when connecting to live mains equipment!
    YOU ONLY DIE ONCE (unless you're James Bond)
     
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