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smps problem

Discussion in 'Hobby Electronics' started by ksoner, Sep 18, 2006.

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  1. ksoner

    ksoner Guest


    hope the questions is relevant with the group:

    I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
    The topology is flyback and I decide the output(27V or 13V) with two
    jumpers. the problem is that when I switch from 13.8 to 27.6V, VCC also
    increases ()and I dont want VCC to increase above 30V because the FPS
    of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
    to limit the current of zener on VCC circuitry. but those dissipate too
    much power and zener live on the limits. Is there a nice method(fast
    and cheap) to limit the voltage on VCC rather than using a
    zener+resistor. thanks in advance
  2. Bushy Pete

    Bushy Pete Guest

    If I understand your circuit from your description, you are disconnecting
    one jumper and inserting it on a second jumper. Your problem is occurring
    while the jumper is disconnected.

    Change your system to one jumper and with the jumper removed you get one
    voltage and the with the jumper connected you get the other.

    Use two resistors that will give you the alternate resistance in parallel,
    or the other resistance with just the other one. I'm sure you can cope with
    resistors in parallel calculations!

    This way, your jumpers are never both disconnected and you don't have to
    worry about unconnected voltages.....

    And you will be able to get rid of the "Brother-in-Law Diode"

    Hope this helps,

    p.s. A "Brother-in-Law Diode" is one that is in circuit for no other reason
    than the designer's Brother-in-Law owns the diode factory!
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