AN1379

ACS: an overvoltage protected AC device

Doc ID 8317 Rev 5

3/10

Then, the switch turns on in breakover mode [2] [3]. Such an event is particularly stressful

on the semiconductor especially so if the current and its rate of increase are both high. The

worst case occurs when ACSs are driving low resistance, non inductive loads (only a few

tens of µH as a series parasitic inductance).

For example, Figure 3 and Figure 4 have been recorded with a thermal active door lock

system at a low temperature controlled by an ACS108-5TA device. The 2 kV surge is super-

imposed on the 230 V, 50 Hz mains and synchronized with its peak value, as shown in

Figure 3. Figure 4 shows the device turn on in this mode. As the load was previously off, its

resistance is cold and equals 150 ohm. In this case, the current rises at a rate of 100 A/µs

and reaches 15 A. Such transient surges would damage Triacs, but not ACSs, as they are

designed to turn on in breakover mode. The varistor is then no longer needed in parallel

across ACSs, unlike Triacs. The difference between ACS and Triac + varistor is that, with the

ACS, the load is switched on during a half or one mains cycle. This can be accepted as such

events happen a few times in the system's life.

Figure 3.

2 kV surge on the mains

Figure 4.

ACS breakover

VAC (500 V/div)

Iout (5 A/div)

VAC (200 V/div)

Iout (5 A/div)