AN437

How to design snubber circuit for turn-off improvement

13/18

Equation 11

Figure 12.

and pure inductive load (worst case))

Note:

For inductive load with rms current higher than 4 A, Snubberless TRIACs are

recommended.

In the case of a 26 W drain pump, the rms load current is 0.3 A in transient operating (worst

case). The corresponding snubber capacitor value is about 10 nF, like defined previously.

The estimated peak voltage is 613 V. The estimated peak voltage is 20% higher than the

measured value due to the RLC model approximations (refer to Section 2.2.1) and because,

in the application, the load is not purely inductive and the peak voltage is limited by the load

resistance.

E

dt

/

dV

R

R

L

2

K

OFF

S

·

+

·

=

ξ

with

f

2

I

V

L

RMS

RMS

·

·

·

=

π

Normalized peak voltage (Z = VP/E)

0

10

20

30

40

50

60

70

80

90

100

110

120

130

0

0.25 0.5 0.75 1

1.25 1.5 1.75 2

2.25 2.5 2.75 3

3.25 3.5 3.75 4

R

R

Load rms current: IRMS (A)

Snubber capacitor value (nF)

1.5

1.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

2

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 4

R

R

Load rms current: IRMS (A)