How to calculate the junction temperature during clamping periods

AN1172

20/23

Figure 22.

Current waveform during clamping phase for 100mA / 230V RMS loads

Considering Figure 22., we can see that the load current is always inferior to a linearly

temperature elevation due to a rectangular power pulse is always superior to the one due to

a decreasing triangular power pulse of same average value (see References, 3.).

So, a pessimistic way to calculate the junction temperature is to simplify the clamping losses

on the clamping duration:

Equation C3

C.2

Transient maximum junction temperature

The clamping time is given by Equation C1, which gives way to the following one.

Equation C4

Neglecting the on-state dissipated power in front of clamping dissipated power, the junction

temperature rise at the end of the clamping phase is:

Equation C5

50

40

30

20

10

0

9

9.1

9.2

cos

= 0.9

ϕ

cos

= 0.1

ϕ

cos

= 0.7

ϕ

9.3

9.4

9.5

9.6

Time (ms)

t0

IH

tZC

()

0

ZC

H

cl

CL

t

t

I

V

2

1

E-

=

⎥

⎦

⎤

⎢

⎣

⎡

-

+

+

-

H

cl

0

ZC

i

Vac

E

V

R

1

ln

R

L

t

t

H

cl

0

ZC

j

I

V

2

1

)

t

t

(

Zth

T

⋅

-

=

∆