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how to improve voltage reguration.

Discussion in 'Electronic Basics' started by KM, Oct 19, 2004.

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  1. KM

    KM Guest

    I have use a LC3803 Linear regurator to design a flyback regurator. A
    tronsformer is wind with single primary and double secondary coil (one
    for 3volts and one 7volts) the FB was return from 3 volt output. It is
    found that the output of 7 volts when unloaded is floating high to 13v
    when only 3volts output are loaded. It is found that the overshoot is
    causing the raise. What is the possible solution to this?
  2. Are the 3 and 7 volt outputs both positive?
    Do the 3 and 7 volt outputs share a common return?
    What are the rated full load currents for each output?
  3. Terry Given

    Terry Given Guest

    basically your problem is due to the lack of coupling between the
    regulated and unregulated outputs. Firstly, ensure tight coupling in
    your transformer. Secondly, "stack" the windings - say you have 3T and
    7T each for the 3V and 7V supplies. rather than having a 3T and a 7T
    winding, use a 3T winding & a 4T winding - in other words, tap a single

    Thirdly, the lack of load on the 7V supply is also a problem. Depending
    on the 7V load, a simple solution is a series R such that the RC time
    constant is longer than the "overshoot" (which is probably only 1us or
    so). Or slap a minimum load on the 7V supply.

    Fourthly, you could change your feedback circuit - instead of a voltage
    divider connected to the 3V rail only, attach another resistor from the
    divider to the 7V rail. select the resistors such that when 3V = 3 and
    7V = 7 the voltage divider sits at the correct voltage (whatever that
    happens to be). This method basically averages out the errors - instead
    of one well-regulated output and one badly regulated output, you get two
    moderately well regulated outputs.

  4. Rich Grise

    Rich Grise Guest

    What's a linear regurgitator?

    (no offense intended, KM - I'm making a joke about "Engrish" at your
    expense. :) )
  5. KM

    KM Guest

    No, there are individual return seperately
    the rated full load are 2.2A for 3 Volts and 1.2 A for 7volts respectively.
  6. N. Thornton

    N. Thornton Guest

    Sounds like a good description: feed the current in, then when you
    open the switching tr, the inductor regurgitates the energy.

  7. That rules out sharing currents in the 3 volt winding.
    I think I would try winding part of the 7 volt winding bifilar with
    the 3 volt winding and putting the rest on top of that. And you will
    probably need to add some minimum load to the 7 volt output so that it
    is never completely unloaded.
  8. Clarence

    Clarence Guest

    How about a "Snubber" on the unloaded winding to control overshoot? I can not
    imagine a supply design without some form of rise-time load for a lightly
    loaded output. Ripple control is most important at low currents.
  9. Isn't that what a small resistive load across the filter cap amounts
  10. Terry Given

    Terry Given Guest

    depends what you mean by small, but yeah. likewise for a resistor in
    series with the "low" power winding (in this case that wont work as 1.2A
    aint small)

  11. Clarence

    Clarence Guest

    No. It is a value of a resistor which is a load only when the rise time (Dv/Dt)
    of the overshoot is fast enough to pass through a Cap in series with the
    resistor. An Ac terminator is one way to look at it.

    I usually use something like a .1mfd in series with a 100 ohm resistor. Only
    conducts the ac component. It was very common when a Vibrator was the AC
  12. Rich Grise

    Rich Grise Guest

    Would it be way persnickety to do 2 turns of the 7, then 3 bifilar, then
    the other 2 of the 7? i.e., not only embed it sideways, but lengthwise,
    so to speak. (transformer design is black magic, AFAIK.)

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