Power electronics digital notes b. Tech III year

Capacitors selection: 
Snubber capacitors are subjected to high peak and RMS currents and high 
dv/dt
. An example is turn-on 
and turn-off current spikes in a typical RCD snubber capacitor. The pulse will have high peak and RMS 
amplitudes. The snubber capacitor has to meet two requirements. First, the energy stored in the snubber 
capacitor must be greater 
than the energy in the circuit’s inductance. Secondly, the time constant of 
snubber circuits should me small compared to shortest on time expected, usually 10% of the on time. By 
allowing the resistor to be effective in the ringing frequency this capacitor is used to minimize the 

---

39 | 
P a g e
dissipation at switching frequency. The best design is selecting the impedance of the capacitor is same 
that of resistor at the ringing frequency. 
Resistors selection: 
It is important that R in the RC snubber, have low self inductance. Inductance in R will increase the peak 
voltage and it will tend to defeat the purpose of the snubber. Low inductance will also be desirable for R 
in snubber but it is not critical since the effect of a small amount of inductance is to slightly increase the 
reset time of C and it will reduce the peak current in switch at turn-on. The normal choice of R is usually 
the carbon composition or metal film. The resistor power dissipation must be independent of the 
resistance R because it dissipates the energy stored in the snubber capacitor in each transition of voltage 
in the capacitor. If we select the resistor as that the characteristic impedance, the ringing is well damped. 
When comparing the Quick design to optimum design, the required snubber resis
tor’s power capability 
will be reduced. Usually the “Quick” design is completely adequate for final design. Going to the 
“Optimum” approach is only if power efficiency and size constraints dictate the need for optimum 
design.