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Snubber for uncoupledinductor sepic converter?

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treez

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Hello, The following article on page 17 states that the snubber across the sepic capacitor should be 220uF and 2 Ohms. How did they decide on these snubber component values?
The R=2R & C=220uF snubber has a frequency of 1/(2.pi.R.C) = 362Hz

This is more than 17 times less than the SEPIC's (L+L),C ringing frequency of 6140Hz.

Why is the snubber frequency so much less than the sepic's ringing frequency?


Article on sepic:
https://www.ti.com/lit/an/slyt411/slyt411.pdf
 

mtwieg

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The size of the damping capacitor is largely irrelevant, it just has to be large enough such that it's practically a short circuit (relative to the damping resistor) at the resonant frequency to be damped. That's why the RC of the damping network needs to be much lower. The resistor value is selected to give the desired damping factor.
 
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treez

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Thanks, so would you say R(snub) = sqrt(L/C) and C(snub) = 1/(2.pi.C(snub).F)

..where F is at least 10 times less than the resonant frequency of the sepic L's and C?
 

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That's a reasonable starting point, if you take into account the contribution of the capacitor ESR to the total resistance.
 
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Making the snubber C that large only slightly reduces the Q. Factor 1 to 2 is usually sufficient.
 
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treez

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Making the snubber C that large only slightly reduces the Q. Factor 1 to 2 is usually sufficient.

do you mean that we should make it like this..?..

R(snub) = sqrt(L/C) and C(snub) = 1/(2.pi.R(snub).F)

...where F= 1 to 2 times the resonant frequency being snubbed?

I hear you say the snubber cap doesn't need to be that big, but then why have ti.com seen fit to make the snubber cap equal to the huge 220uF?
 

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