Calculation example

AN316

14/17

DocID3567 Rev 4

Figure 11. Transistor protection

Experience shows that this hypothesis is confirmed in most cases with a Transil. Therefore

the Transil should be initially selected according to its thermal characteristics.

An approximate value can be obtained by supposing that the Transil absorbs the energy

contained in the inductance. This hypothesis is close to reality when the ratio:

is significant.

Equation 11:

5.2.2

First choice

We choose ST product type SMCJ70A.

Equation 12

T

Repetitive f = 50Hz

Tamb Max = 50°C

R = 45W

L = 0.35H

12V ± 20%

V

V

V

BR

W

W

I

L

9

.

0

)

(

50

45

4

.

2

12

35

.

0

2

1

2

1

2

2

=

×

⎥

⎦

⎤

⎢

⎣

×

=

×

=

P

AV

ƒ

××

×

C

W

C

V

°

×

°

=

=

-

/

10

5

.

10

/

75

58

.

85

4

T

=

α

R

th

V

BRmax