AN1172

How to calculate the junction temperature during clamping periods

19/23

Appendix C

How to calculate the junction temperature

during clamping periods

C.1

Dissipated power evaluation

It should be kept in mind that the load inductive energy is not entirely absorbed by the die at

clamping. Another part is dissipated by the Joule effect in the load resistor or absorbed by

the mains. A way to accurately evaluate the clamping energy is then to estimate the ACS

current waveform, versus the time. The energy is then calculated by integrating this

waveform and the product of the clamping voltage along the turn-off period.

Figure 21.

Electric equivalent circuit at clamping

Let us consider a typical AC load equivalent circuit shown in Figure 21. The load current

during the clamping time is given by the following relationship.

Equation C1

It is difficult to evaluate the influence of E in a general way. Its sign depends on its phase

towards the current and the value of the holding current. But, it can be said that the back

electromotive force has little influence for most universal and synchronous motors in which

impedances are high and E is close to zero. Furthermore, for all other loads, such as

passive loads or asynchronous motors, the b.e.m.f E is null. For this reason, let us neglect

its value. Let cos

ϕ be the load power factor and V the RMS mains voltage. Since the

positive current, is given by the relationship C.2.

Equation C2

Figure 22. shows the current waveforms calculated with such an hypothesis, and assuming

that the holding current does not depend on the current rate of decrease. The junction

ACS402-5 or an ACS108-5). The ACS clamping voltage is taken equal to 700 V.

Calculations are carried out for 100 mA RMS loads with different power factors, and for a

230 V 50 Hz mains voltage.

Vac

Vcl

I

R

L

E

()

()

0

0

t

t

L

R

0

t

t

L

R

cl

e

)

t

(

i

1

e

R

Vac

E

V

)

t

(

i

-

-

-

-

⋅

+

⎟⎟⎠

⎞

⎜⎜⎝

⎛

-

-

+

=

ϕ

²

cos

1

2

V

Vac

-

⋅

-

=