AC switch control with an inductive load

AN441

6/16

Doc ID 3579 Rev 3

2.2

Gate control for full-wave operation

2.2.1

Gate current pulse width

the current waveform at zero point, holding current also has to be taken into consideration

Equation 4

To reduce the pulse width duration, a more sensitive Triac could also be used.

2.2.2

Gate current synchronization with mains voltage

In case of inductive load, the current is lagging regarding to the mains voltage. This is why,

in case of control by gate current pulse, the gate pulse has to be applied with a delay after

each line zero voltage point which approximately equals the current-voltage phase shift (as

shown on Figure 6)

Figure 6.

Triac pulse width synchronization

In case the zero line voltage event is not sensed by the Triac control circuit or no timer is

available to manage triggering times, one solution is to apply a DC gate current. In this case

the average current consumption for the power supply will be higher. But today more and

more switching mode power supplies (SMPS) are used offering a higher output current

capability.

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