Snubber Network

Apr 04, 2007 2 Replies
W

I've been reading

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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?



Thanks Andrew



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P

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 greater. Paul Mathews

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W

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

Andrew

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