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Power Supply with transformer 300V

Discussion in 'Electronic Repair' started by Dotinho, Mar 8, 2007.

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  1. Dotinho

    Dotinho Guest

    Hello, i have i power supply of Siemens in one olde machine, and i
    have to repair it, but i don't know how.

    the essencial, the 1 step is transform 220 VAC to 315VDC and then a
    MOSFET will pulse the 300VDC at 50KHz to a transformer that out 5V at
    secudary, the problem is that tranformer giver out 8V and don't
    work..


    Anyone can tell me if i change the frequency,. the output volt will
    decrease?


    Thanks.
     
  2. Arfa Daily

    Arfa Daily Guest

    It sounds as though you are talking about a switch mode power supply here.
    Although the switching frequency has some effect on the final output due to
    the fact that the transformer is designed to run at a particular frequency,
    the basic control for this type of supply is achieved by altering the mark
    space ratio of the drive to the MOSFET.

    However, I'm not convinced that this is actually what you are talking about.
    Are you saying that the supply has a *DC* output of 8v instead of a correct
    5v ? If so, then it is impossible to offer valid advice on what the problem
    might be, without knowing the *actual* regulation scheme that the supply
    uses. For instance, all voltage setting and regulation might be achieved by
    a control loop that operates on the mark space ratio of the MOSFET drive, in
    a scheme known as pulse width modulation, or PWM. On the other hand, there
    might be a basic fixed drive ratio that results in some arbitrary voltage a
    few volts above the required 5v - possibly the 8v that you are seeing. This
    might then be followed by either a linear 5v regulator, or even in some
    cases, a further switching regulator. Still further, the supply could use a
    combination of any or all of these schemes.

    If you are not familiar with the principles of switchers, you might struggle
    to get to the bottom of any problem that there might be. These items are
    notorious for being very fickle. Also, very importantly, I would warn
    against working on the supply without it being at least connected via a
    proper bench isolation transformer. These supplies are VERY dangerous to the
    point of being potentially lethal, if you do not understand fully their
    principles, and the places to avoid touching.

    Arfa
     
  3. Another point is that if you're testing it without a proper load, outputs
    could be higher or lower than spec'd.

    There is general info on SMPS repair at the sites below.

    http://www.repairfaq.org/sam/smpsfaq.htm

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  4. Arfa Daily

    Arfa Daily Guest

    Thanks Sam. A valid point I forgot to mention.

    Arfa
     
  5. Dotinho

    Dotinho Guest

    Its okay sam, but the PWM have some protections, and if output voltage
    was more than 6.5V it disable every thing, with or without load..

    the tranformer are malfunction, and i can't get another becouse don't
    produce anymore, this power supply is very old, and i have to repair
    it, decrease the output tension, and i dont know how..

    there is no regulation, just exist a small feedbak for output
    protecion, if voltage was superior to 6.5V it disable everything.
    thats all..

    The shematic ir very simple, it rectifier 220VAC from nets, to 315VDV
    wih 4 diods and 2 big capacitors, than the tranformer is conected in
    serie with MOSFET, pulsing that 315VDC to tranformer, and the there is
    3 output, one at 5VDC with 18A and another 24V with 0.5A...

    The problem is the tranformer, the mosfet is directly connected to PWM
    with one wire, there is no regulation!!

    How can i reduce the power changing the shema??

    thanks
     
  6. Arfa Daily

    Arfa Daily Guest

    I would suggest that you are missing something somewhere. OK, so the MOSFET
    is connected to the PWM with one wire, but I would be 99% sure that this is
    where your trouble lies. Something somewhere is generating that PWM, and it
    is likely doing it with the wrong mark space ratio. I would be very
    surprised if a switcher is producing a 5v rail specced to 18 amps, without
    some kind of basic feedback. Usually, a 5v rail in older kit, with that sort
    of current capability, is supplying a bunch of 'straight' or 'LS' 74 series
    logic. That stuff is critical of its supply voltage to around +/- 0.2v.

    I can't actually think of a failure mode for the transformer, which would
    cause it to produce excess output. In fact, SMPS transformers are incredibly
    reliable across the board. Since they've become really popular over the last
    25 - 30 years, I can only think of one item of equipment that had the
    transformer fail regularly, and that was an early DB sat receiver. In that
    case, the primary used to fail with shorted turns, which led to catastrophic
    failure of the MOSFET and input fuse.

    How is the PWM generated in the first place, and are you absolutely certain
    that there is not an optoisolator hiding somewhere ? How does the
    overvoltage shutdown circuit work ? Are you sure that it does not also serve
    as the basic control loop, and only has the shutdown function as a last
    resort if control is lost ?

    Arfa
     
  7. Dotinho

    Dotinho Guest

    Arfa Daily escreveu:

    ya ya, i know that. perhaps i should send the shmatic? is that okay?

    dotinho
     
  8. Arfa Daily

    Arfa Daily Guest

    No problem. Send it.

    Arfa
     
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