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Switch mode power supplies

Discussion in 'Hobby Electronics' started by Glenn, Aug 24, 2006.

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  1. Glenn

    Glenn Guest

    I have a dead colour TV that I'm finally having a go at fixing - no
    expert on these things though.

    The chopper transistor on the switch mode supply has shorted out.
    Replaced it and same again (very quickly fails), so there is a problem
    somewhere. I have a couple of questions:

    1. Anyone know a good online description, in detail, of the operation
    of these supplies. For example, the feedback from the output side of
    the supply that apparently controls the chopper transistor is through a
    single four pin IC that I can't identify. I suspect it may be an
    optoisolator but not certain. It carries markings OA . P621 . 4 G.,
    The manufacturer logo looks like a "T", but don't recognise it (its not
    an SGS Thomson logo).

    2. I can think of at least two possible generic causes of the failure:
    a) Failure somewhere in the actual TV circuit that is causing
    excessive current drain and overloading the power supply, eg horizontal
    output transistor (but this looks OK).
    b) Failure in the switchmode supply itself such that the chopper
    transistor is forced permanently on, or is being driven at a much
    higher duty cycle than it can handle. Are there any other
    possibilities, and testing strategies?

  2. Alf Katz

    Alf Katz Guest

    ----- Original Message -----
    From: "Glenn" <>
    Newsgroups: aus.electronics
    Sent: Thursday, August 24, 2006 8:57 PM
    Subject: Switch mode power supplies

    Probably a Toshiba TLP621 opto isolator.
    Usually when the transistor goes short something else is gone, too. Cause
    and effect are usually hard to separate. What else is gone depends on the
    topology of the supply. Is there a shottky diode that may have upgraded
    itself from semiconductor to full conductor, perhaps?

  3. Glenn

    Glenn Guest

    Thanks for the tips. The supply is all discrete components, no IC
    (except for the suspected optocoupler, which is providing a feedback
    signal to the primary side. I've checked the two electros on the
    primary side, and the main smoothing capacitor, but they look OK. Also
    done a look around and resolder for dry joints. I'm attempting to
    check every component on the primary side - so far, all are showing up
    fine (except for the chopper).

    Unfortunately, I have no circuit diagram so the job is a bit more of a

  4. "ian field" wrote

    Possibly the 2 most common causes of SMPSU failure are dry joints and
    electrolytics gone high ESR, its no good just measuring the capacitance as
    this will often read acceptable within limits on a faulty cap.

    ***** Ian,your good advice is certainly wasted if we are to believe the OP:-

    "I've checked the two electros on the primary side, and the main smoothing
    capacitor, but they look OK".

    You understand,they must be in good working order because they LOOK OK!

    Brian Goldsmith.
  5. Ross Herbert

    Ross Herbert Guest

    I was asked last Friday if I could fix a single output 24V, 60W SMPS
    for a Zebra-Eltron P310C plastic ID card printer. The local support
    company wanted a mere $AU450 for a new one shipped to my door.

    There appeared to be drastic failure of a number of components (turned
    out that I had to replace 13 componets in all) with burnt resistors,
    shattered output rectifier, shorted and cracked transorb, dried out
    electro caps, leaky 1N4148, dead 3842 PWM chip, and of course the
    power FET (STW13NB60A a TO247 device). There was nothing wrong with
    the opto-coupler or the TL431 feedback regulator but the resistor
    which determines the current through the opto diode was burnt so that
    the value was unrecognisable. A bit of intuitive guesstimation showed
    that 1.8K was about right and this proved to be so under load testing.

    I couldn't see the need for the original specified 13A 600V FET for a
    nominal 60W PSU because other similar supplies I had worked on used no
    more than an 8A device. Anyway RS Components only had a couple of near
    equivalents for the STW13NB60A at between $AU43 and $AU52 each - far
    too much. I substituted a IRFIB6N60A, a TO220 device rated at 5.5A
    600V. Under load testing I had no trouble getting 3.2A at 24 volts
    (better than 72W) and the FET heatsink only got moderately warm. The
    substitute FET was much cheaper at around $AU8.
  6. Ross Herbert

    Ross Herbert Guest

    I also had no circuit diagram for the Zebra-Eltron PSU I mentioned in
    my other post. Luckily it was fairly easy to work out the schematic so
    I do now have a circuit.

    I find the easiest way to do the shematic is to divide the schematic
    into 3 functional blocks and draw each out on a separate piece of
    paper. The first block I do, for example, is the secondary low voltage
    side. Nxxt I do the primary high voltage side and lastly the mains
    input rectifier part. Many PSU's are hard to work out because of the
    number of heatsinks and or the desire to achieve highest packing
    density and the only way to work out the schematic for these is to
    remove the components which prevent you from reading values etc. An
    arduous task but if you plan to work on the same type again the work
    involved is invaluable.
  7. Glenn

    Glenn Guest

    Thanks for all the posts and tips. I need to be more careful how I put
    things. "look OK" is my way of saying "tests OK", although that is a
    basic test since I don't have an ESR meter. Anyway, I have replaced
    them just in case. On more digging, I've found a couple of suspect low
    power transistors that look like they are in the circuit driving the
    base of the chopper. I'll let you know if I have success.

    The set doesn't owe me anything and would otherwise be carted off to
    the tip. So I don't have much too lose except a few hours of mucking

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