AN437

How to design snubber circuit for turn-off improvement

11/18

Using Equation 6, the snubber circuit divider ratio M is equal to:

M = 0.77 with R = 190

2.

Damping factor definition

The snubber capacitor depends on the undamped natural resonance (

factor (

ξ). Consider the following modified forms of Equation 4 and Equation 3:

Equation 4 modified

Where K is the normalized voltage rise slope (refer to the lower graphic in Figure 8).

Equation 3 modified

From the two previous equations, the ratio between the normalized voltage rise slope (K)

and the damping factor (

ξ) is given by Equation 10. Figure 10, derived from the lower graph

in Figure 8 gives the variation of this ratio with

ξ:

Equation 10

For the drain pump controlled (L = 2.4 H and R = 190

Ω at 50 Hz) and by using Equation 5,

the final voltage value E is:

maximum allowed (dV/dt)c, see Figure 2).

Thus according to Equation 10, the ratio between the normalized voltage rise slope and the

damping factor is equal to 38. Figure 10 gives then the damping factor value (

ξ = 0.026).

Figure 10.

damping factor (

ξ) according to the damping factor (ξ)

L

1

C

2

0

S

·

=

ω

with

K

E

dt

/

dV

OFF

0

·

=

ω

2

2

S

S

)

R

R

(

L

4

C

ξ

·

+

·

=

E

dt

/

dV

R

R

L

2

K

OFF

S

·

+

·

=

ξ

0.1

1

10

100

0.01

0.1

1

10

K /

ξ

M = 1

M = 0.75

M = 0.5

M = 0.25

M = 0

ξ = 0.026

38

M decreases

M = 1

M = 0.75

M = 0.5

M = 0.25

M = 0

M decreases

Damping factor:

ξ