DS3279 Rev 5

7/17

ACS120

AC line switch basic application

17

2.2

High inductive load switch-off: turn-off overvoltage clamping

With high inductive and low RMS current loads the rate of decrease of the current is very

low. An overvoltage can occur when the gate current is removed and the OUT current is

lower than IH.

As shown in Figure 15 and Figure 16, at the end of the last conduction half-cycle, the load

current decreases (1). The load current reaches the holding current level IH (2), and the

ACS turns off (3). The water valve, as an inductive load (up to 15 H), reacts as a current

generator and an overvoltage is created, which is clamped by the ACS (4). The current

flows through the ACS avalanche and decreases linearly to zero. During this time, the

voltage across the switch is limited to the clamping voltage VCL. The energy stored in the

inductance of the load is dissipated in the clamping section that is designed for this purpose.

When the energy has been dissipated, the ACS voltage falls back to the mains voltage

value (230 V rms, 50 Hz) (5).

Note:

Same working principle described in item 2.2 is valid for both current directions.

Figure 15. Turn-off operation of the ACS120

switch with an electro-valve

Figure 16. ACS120 switch static characteristic

1

2

3

4

5

I

H

V

PEAK =V CL

100μs/div

I

(5 mA/div)

OUT

V

(200 V/div)

OUT

I

H

V

CL

V

I

1

2

3

4

5

I

H

V

CL

V

T

I

T

1

2

3

4

5