# RC Snubber for synchronous MOSFET (SMPS)

I am trying to understand the theory of RC snubber for synchronous buck converters. Some statements seem to be contradictory to me in the below mentioned application notes:

1. “The snubber capacitance is designed to block lower frequencies (such as the switching frequency) but appears as a short at the ringing frequency. “ http://www.ti.com/lit/an/slpa010/slpa010.pdf - Page 12

2. “It is important to notice that only the magnitude of the ringing was affected by the boot-resistor and gate resistor methods. The snubber method also changed the ringing frequency and damped out the ringing waveform” http://www.ti.com/lit/an/slyt465/slyt465.pdf - Page 6

My doubts:

1. How does the snubber capacitor acts as a short circuit during the ringing frequency ? After all it is the one which decides the new ringing frequency.

2. Why is Cp (parasitic capacitance) used in the characteristic impedance calculation? The equivalent capacitance should have been considered, which is Cp + Csnubber.

How does the snubber capacitor acts as a short circuit during the ringing frequency ? After all it is the one which decides the new ringing frequency.

Texas Instruments are somewhat polarizing what happens - it doesn't act as a complete short but it does reduce the ringing frequency AND it also reduces the ringing amplitude. The amplitude reduces because the Q factor of the resonating components is reduced by the snubber resistor.

Why is Cp (parasitic capacitance) used in the characteristic impedance calculation? The equivalent capacitance should have been considered, which is Cp + Csnubber.

I'm unsure which document you found this statement - maybe you can point it out?

• ti.com/lit/an/slpa010/slpa010.pdf – Divya K.S Jun 25 at 10:07
• You might need to take the tour to find out why people give up their free time to help - I'm saying this because I notice that you haven't formally accepted any answer to your previous 9 questions. I know you have done so on earlier questions but, let's not be mean here. – Andy aka Jun 25 at 10:13
• I can't find the term "characteristic impedance" in that document. Please be clear about what you ask. – Andy aka Jun 25 at 10:16
• Iam so sorry Andy. I completely appreciate the help you and other guys offering. Its just that I took this lightly and didnt realise the bare minimum etiquettes here. Once again thanks for telling me this. – Divya K.S Jun 25 at 10:17
• Reading between the lines I would say that because the snubber capacitor is used "apply" the snubber resistor to reduce the Q-factor of resonance, the natural impedance ($\sqrt{L/C}$) is largely unaffected by whatever value Csnubber is. That's me polarizing things because there are subtleties to this around any resonant point. – Andy aka Jun 25 at 10:25