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27th February 2008, 08:03 AM  #1 
diyAudio Member
Join Date: Apr 2007

Calculation for Snubber Value
Please find attached file and help me to calculate the value of capacitor & Resistor for RC Snubber. For the frequencies of 20KHz & 50KHz. Please also provided a simple formula.

27th February 2008, 11:34 PM  #2 
diyAudio Member
Join Date: Apr 2006
Location: Minnesota

The values depend on the leakage inductance (Le) of the transformer. Off the top of my head select C such that the resonant frequency of the LC circuit is 10 times that of the switching frequency. Fres=(1/2*pi)*1/(sqrt(L*C)). Select R=sqrt(L/C); it might be 2*sqrt(l/c)

28th February 2008, 10:37 PM  #3 
diyAudio Member

Look here:
http://www.ridleyengineering.com/snubber.htm
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28th February 2008, 10:42 PM  #4 
diyAudio Member

Hi
But you are not making push pull offline, do you?? 
28th February 2008, 10:46 PM  #5 
diyAudio Member

Great link indeed
A more elaborate method includes finding out the characteristic RLC values of the circuit (no snubber), and then using simulation software to find out a RLC network capable of providing critical damping (single cycle with no oscillation) with minimum losses. Series inductance in snubbers is actually useful as long as it's not too far from the optimum value.
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1st March 2008, 10:32 AM  #6 
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Join Date: Mar 2008

what ridley doesnt bother to explain is whats going on and how to figure it out when you DONT know either L or C.
this can also be found in an old unitrode app note, but this is how I snub ringing circuits:  measure ringing period T1, using nice low capacitance scope probe.  T1 = 2*pi*sqrt(Lo*Co) where Lo, Co are the elements responsible for the ringing.  add some capacitor C1 to the circuit  eg across the primary winding if we are snubbing that, or across a diode etc.  this capacitor will increase the resonant period, T2 = 2*pi*sqrt(Lo*(Co+C1)) (if T2 doesnt really change this is because either C1 << Co, or you stuck the cap in the wrong place) the ratio of these two periods T2/T1 = sqrt((Co+C1)/Co) so (T2/T1)^2  1 = C1/Co thus you now know Co, and from T1 can calculate Lo then calculate Zo = sqrt(Lo/Co). this is the value to use for the snubber resistor Rs The snubber capacitor Cs needs to be 2x or 3x Co. Using Cs < 2Co decreases the amount of damping; using Cs > 3Co wont increase the damping, but does increase Fsmps*Cs*Vpk^2 losses  it is clear from the above maths that if you keep adding C1 until T2 = 2*T1 then C1 = 3*Co measure the peak snubber cap voltage Vpk. average losses are Fsmps*Cs*Vpk^2 measure the peak snubber resistor voltage, and calculate Ppeak = Vpeak^2/Rs. make sure your snubber resistor can handle this note also that if you know your scope probe capacitance, you can correct for it when calculating snubber components 
3rd March 2008, 12:09 AM  #7 
diyAudio Member

That looks like good stuff, Terry.
Check out this Maxim appnote, which also has a similar procedure to determine R and C for a snubber, using scope measurements: http://www.maximic.com/appnotes.cfm...te_number/3835 And here is the rest of my snubberdesign appnote/article collection: http://www.cornelldubilier.com/design.pdf http://www.hagtech.com/pdf/snubber.pdf http://www.ridleyengineering.com/snubber.htm http://archive.chipcenter.com/circui.../c1100rp58.htm
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