SMPSU goes BUZZZZZ

Discussion in 'General Electronics Chat' started by iamspook, Sep 5, 2008.

  1. iamspook

    Thread Starter Member

    Aug 6, 2008
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    0
    I repaired a blown DVD's SMPSU by inspection of the components
    (No Schematic). The following problems found:

    1) The 240v input terminated on a dry cracked joint - presumably the root
    cause of the failure.

    2) A power MOS FET was short circuit everywhere. This took out the BR
    and a 15v zener and a fast recovery diode.

    3) A C3112 NPN high voltage switching transistor popped. It looks like it is a single transistor oscillator to drive the MOS FET (primary side).

    The nearest but uncomfortable equiv I could find is a BC547CG.

    This main diff is the C3112 has an amazing high DC gain. Somewhere
    between 600 and 3600 while the BC547CG has a respectable typical 600
    and nowhere near the max of 3600.

    It all works, but audibly buzzes the transformer.
    (If I put a plastic ruler on the
    transformer, It travels up the ruler.) I'd guess it to be 700 Hz or so.

    Questions:

    Is the lower HFE likely to be causing this?
    Is there a better substitute or a reliable supplier for the C3112?

    Could I use two BC547CGs in a darlington pair arrangement to get
    the gain without losing any significant characteristics? I guess I need
    to know if there are any particular disadvantages of using a darlington
    pair under these circumstances. I should add that I know that the
    DP will have a lower speed than an equivalent single transistor,
    and possibly temperature instability which can be solved with a
    resistor but this affect the turn-on voltage. But will this matter?

    Or would a Sziklai pair be more beneficial?

    Thanks!
     
    Last edited: Sep 5, 2008
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    The 2sc3112 is a darlington transistor.
    You should try to find one with about the same specs.

    Greetings,
    Bertus
     
  3. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    The MPSA18 is closer to what you are looking for.

    The 2SC3112 is a "super-beta" NPN not a Darlington. There are 2 grades A:600-1800 and B:1200-3600. If it is just driving the FET then the beta is not the reason for the buzz. You may have a damaged transformer or there is some current noise getting coupled to it or there is just plain undesired DC current running through it.
     
  4. iamspook

    Thread Starter Member

    Aug 6, 2008
    27
    0
    Thanks for the advice. I changed the transistor for a trusty robust little 2n2222 and turned it on. No buzzing. Go figure! I'll have to soak-test it
    to see if it is reliable. It should be ok for breakdown voltage and power and Ic and so on. Beta is about 100 on this one. (I measured it) I guess the
    high beta requirement was a furfy. Maybe the BC547CG was not stable in the circuit.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    One thing to double check is the pinouts of the transistors... it is easy to overlook.
    The P2n222a is CBE and the PN222A is EBC and the 2SC3112 is ECB
     
  6. iamspook

    Thread Starter Member

    Aug 6, 2008
    27
    0
    You are right. I had to insulate and cross one of the legs to make it fit. It all works perfectly.
     
  7. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    good to hear you got it working!
     
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