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Snubber Network

Discussion in 'Electronic Design' started by [email protected], Apr 4, 2007.

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  1. Guest

    I've been reading and decided
    it would be interesting to lay down on my power supply board with a
    view to investigating when i had the time. However i'm left a little
    confused as to where to place the RC network. The PSpice model in
    figure 7 seems to suggest across the diode to GND effectively and the
    text talks about the equivalent of a full wave bridge rectifier having
    both diodes in series (which of course makes sense), which then places
    the RC across both diodes to GND. However other texts seem to suggest
    the RC should go across each diode? Can anyone give me any clues on
    this one?

  2. Paul Mathews

    Paul Mathews Guest

    RC across each diode is best. The theory is interesting and sometimes
    useful, but here is an easy procedure that usually works:
    1) observe ringing with oscilloscope and measure ring frequency f0
    2) add capacitance C across the switching element (diode in this case)
    until f = f0/2. i.e., ring freq is half of f0. This capacitance will
    be in the range of the stray and parasitic capacitances of the circuit
    and the diode, between 10 pF and 1000 pF usually.
    3) Calculate R = 1/(2*pi*f) as a rough estimate of required damping
    resistance, and choose a miniature trimpot adjustable above and below
    R. Adjust pot for critical damping.

    Take care to use very short leads for everything, including scope
    ground. In switching power supplies, resistor dissipation may be an
    issue. You can estimate power dissipation from:

    P = F * C * V^2 / 2 where F is the switching freq and V is the
    peak to peak voltage change across the switching element for each
    switch cycle. If P is too large for practical reasons, compromise by
    reducing C and readjusting R for best results. You can still achieve
    critical damping, but the amplitude of the overshoot pulse will be
    Paul Mathews
  3. Guest

    Excellent, thanks for your reply its exactly what i needed.

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