AN308 is ST's application note, which introduces how to use TRIAC to control inductive loads, including the introduction and analysis of different control methods, and finally proposes a simple and reliable control circuit.
The document discusses various triggering methods for TRIACs used with inductive loads. It evaluates circuits synchronized across the TRIAC voltage versus those synchronized by the mains voltage, highlighting issues like asymmetrical operation and load dependency.
A specific circuit suitable for industrial applications is proposed, utilizing a pulse train synchronization method to ensure perfect symmetry of current regardless of the phase shift or variable cos phi values.
It can result in unacceptable asymmetrical operation, which is dangerous for highly inductive loads.
It maintains the triggering signal during each half wave using a pulse train to retrigger the TRIAC.