Question on Quick RC Snubber Design

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
From this document:

....Suppose the switch is an IRF740 with Io = 5 A and Eo = 160 V. For this device Coss = 170 pF and the mounting capacitance will be 40 pF. Doubling this capacitance, Cs = 420 pF. A 500V Snubber Mike capacitor would be ideal for this application and the standard values available are 390 and 470 pF. We will choose the closest standard value and set Cs = 390 pF. Rs = Eo /Io = 32 Ohms. For fs = 100 kHz, Pdiss = 1W. A 2 Watt carbon composition resistor would be ideal for Rs because it has very low self inductance. Carbon film resistors can also be used as long as those resistors which are trimmed with a spiral cut are avoided.

Now, from this other document:
(Application Guide, Snubber Capacitors Designing an RC Snubber)

...Suppose that you have designed a switchmode converter and you want to snub one of the transistor switches. The switching frequency is 50 kHz and the open-switch voltage is 160 Vdc with a maximum switch current of 5A. The resistor value must be: R <= 160/5 = 32 Ω and the capacitance value is:
Cs = 1/((Vo^2)fs) = 780 pF

From the first approach, how can I determine my mounting capacitance? Looks like the second approach does not take it into account.

Is one approach better than the other?

Mods Note:
When you provided the reference is only the link then please show the Titles of the link, now I already added the two titles of document and other document, that is to avoid when the link are lost then maybe we still can google the title to find out in some other sites, otherwise maybe the thread will be become useless.