Inductive loads on/off control

AN1172

6/23

2.1.2

Maximum switching frequency

As far as thermal management involving clamping phases is concerned, a maximum load

commutation frequency must be defined to avoid excessive device heating. Figure 5. gives

the maximum supplementary temperature rise due to recurrent clampings, versus the ACS

switching period (see Appendix C). This value is given for a 230 V - 50 Hz mains voltage

(110 V mains is less stressing), for the worst case of load (power factor = 0.1, peak load

this case, the energy absorbed by the die equals 25 mJ.

The chosen package is the TO92 one (ACS108-5SA device) because it presents the highest

Rth value, among ACS packages on offer (DIL20, TO92, SOT223, DIL8).

It can be seen that this temperature elevation can be neglected (< 4° C) as long as the

control frequency is less than one Hertz. Such a value is suitable for most appliance

applications where loads are at most controlled once per second. For that reason, in ACS

data-sheets, the maximum allowed current is given for a 1 Hertz maximum frequency 0.1

minimum load power factor. Turn-off dissipated power is then reviewed for a wide range of

application needs.

This enables us to conclude that no varistor is needed across ACSs to clamp the loads

inductive energy at turn-off, even with electromagnets which are the highest inductive loads

in Appliances.

Figure 5.

Supplementary temperature elevation due to repetitive clampings (@

clamping energy = 25 mJ, package: TO92)

2.2

Pumps and Fans ON / OFF control

2.2.1

Application requirements for (dI/dt)c and (dV/dt)c

There is a higher risk that a triac or an ACS will fail to turn-off when both the load current

rate of decrease and the reapplied voltage rate across the device are steep (see

References, 3.). This risk increases as the junction temperature increases. The maximum

current decreasing rate that ACS can switch off, called (dI/dt)c is defined for a maximum

Appliances

operation field

ACS switching period (T) (s)

∆ T

(° C)

rep

0

0

0.5

1.0

1.5

2.0

10

20

30

40