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Forward converter can have max duty cycle of 50%

Discussion in 'CAD' started by Vicky, Jun 4, 2004.

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  1. Vicky

    Vicky Guest

    i would like to know why is that there is a constarint on the forward
    converter topology that the MAX duty cycle cannot be graeter than
    50%.I have read that it is due to the core saturation,but how come it
    is not applicable to other topologies.
  2. analog

    analog Guest

    There is no such absolute constraint. This is a practical limit
    that works well both with simple current mode control and when
    either no separate transformer reset winding is used or the reset
    winding has the same number of turns as the main winding. This
    limit also is very easy to produce in a control IC via a toggle
    flip-flop in the pulse stream.
    All topologies must be arranged so that there is no net dc voltage
    applied to all the inductances in the circuit, including the
    transformers. Do you have a specific circuit under consideration?
    If so, consider where the transformer core's magnetizing current
    flows during the off time of the transistor switch(es), consider
    the voltage into which it must flow, and consider how long it will
    take this magnetizing current to decay to zero (and why it must do

    If knowledge is your goal, you really should study some of the
    papers on magnetics by Lloyd Dixon from the TI/Unitrode power
    supply design seminar series instead of trying to beg someone on
    usenet to do your thinking for you. These papers are available
    on their web site (search for Lloyd Dixon). Have you seen them?
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