DocID025296 Rev 1

3/20

AN4363

Current rating selection

20

1

Current rating selection

1.1

Load current and operating temperature

The maximum current an AC switch (we call AC switch Triac, ACS or ACST in this

document) can handle is limited only by the maximum allowed operating temperature,

except for specific applications where the turn-off capability of the AC switch can also

become a limiting factor (refer to Section 1.2: Turn-off limitation for specific applications).

But, for most applications, where the current waveform is sinusoidal, the current of the AC

switch will depend only on the operating temperature and the device power losses.

For an AC switch, as the switching times (around 100 ns) are very low compared to the

switching period (50 Hz or 60 Hz for most cases), the device power losses are considered

as only linked to the conduction losses.

Then, the junction temperature will be (refer to STMicroelectronics Application note AN533

for further information):

Equation 1

or:

Equation 2

With respectively:

Equation 3

AC switch ON-state voltage drop.

For each device, the datasheet gives the maximum allowed current calculated so that the

associated power losses (with Equation 2) will keep the junction temperature below the

maximum operating temperature (with Equation 1). This maximum temperature is usually

125 °C or 150 °C for high temperature devices.

Figure 1 and Figure 2 give the example of such datasheet curves for respectively an

ACS108-6SA and a T1635T-8FP. As the ACS108-6SA package is a TO92, there is no way

to add a heatsink to the device to evacuate its power losses. That is the reason why the

curve is given versus the operating ambient temperature.

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