Hi,

Triacs commute at the zero cross. However due to current lag the voltage zero cross and the current zero cross do not coincide. So the other half part of the triac can come into conduction thus phase angle control is not achieved. If you use a 1:1:1 pulse transformer then the firing signal is fed to the pulse transformer. You can AND a logic drive with a 10 KHz PWM of 10%-25% duty to derive the final driving signal for the PulseTr. The two isolated windings then serve the purpose of firing to Anti-Parallel connected SCRs. So you need 3 drive signals only and not six.