TPS6420x snubber C calculation

Also, in the datasheet you reference, the capacitor is selected by the following method:

"The first step in the snubber design is to measure the oscillation frequency of the sine wave.

Then, a capacitor has to be connected in parallel to the Schottky diode which causes the frequency to drop to

half of its original value."

So there is no equation needed, only measurement and then testing.  I can tell you that most designs will require something in teh range of 1000 - 10000 pF.  I usualy start with 2200 pF and work up or down from there.

The methodology goes like this...

Add capacitance in parallel until the frequency drops in half.  When this occurs you now have quadrupled the total capacitance in the circuit.  Recall that the resonant frequency is 1/sqrt(L x (C+C_ext)).  As a resutl, the amount of external capacitance that you've added is three times the parasitic capacitance. 

Since you now know the natural resonant frequency and the parasitic capacitance, you can calculate the parasitic inductance.  Once this is known you can calculate the characteristic impedance of the circuit:  Z_o=sqrt(L/C_total).  For a Q=1, set the damping resistor equal to this value since Q=R/Z_o in this resonant circuit.  For more damping, reduce the resistance further. 

Use the smallest damping capacitor possible since high capacitance values will ultimately result in excessive power dissipation in the damping resistor.  Also, try to use non-inductive resistors that can withstand the heat and any possible surges.  No wire-wounds! 

If this is an off-line switcher requiring safety agency approval, be aware that the agencies will short out each snubber capacitor to simulate a single-point failure and look to see if any devices or the surrounding PWB temperatures exceed their ratings and/or become safety hazards.  You can get around this by placing two snubber capacitors in series.

Hope this helps!

Ray Mayer