How to design snubber circuit for turn-off improvement

AN437

12/18

3.

Snubber capacitance and overvoltage calculations

The snubber capacitance is given by the modified form of Equation 3:

The peak voltage is given in the upper graphic of Figure 8. A component with a 600 V

capability will be suitable.

4.

RC snubber circuit validation

The turn-off and turn-on behaviors must be checked experimentally to validate the designed

RC snubber circuit.

For turn-off commutation, the measured slope of the voltage rise is 1.7 V/µs (Figure 10) and

is very close to the theoretical slope (2 V/µs). The measured peak voltage (520 V) is lower

than the calculated value (607 V) due to the RLC model approximations (refer to

Section 2.2.1).

Figure 11.

Z0103 TRIAC turn-off on inductive load with snubber circuit

(C = 10 nF and R = 620

Ω)

The second method of RC snubber circuit design allows a quicker snubber capacitor choice.

The capacitor is directly chosen from the load rms current (refer to Figure 12). Pure

inductive loads are considered and the slope of the reapplied voltage is fixed to 2 V/µs.

For a given rms load current and according to the snubber resistance used (47

Ω or 620 Ω),

factor (

ξ) is defined (refer to Equation 11). Then, as in the first snubber design method, the

damping factor is given by Figure 10, the capacitor value by Equation 3 modified and the

peak voltage by the upper graph of Figure 8.

2

2

S

S(nF)

)

R

R

(

L

4

C

x

·

·10

9

+

·

=

9.9 nF with

= 0.026

»x

V

I

V

V

I

V

dV/dt

dI/dt

OFF

V