Phase control

Phase-fired control (PFC), also called phase cutting or "phase angle control", is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.

Phase-fired control (PFC) is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous.

PFC differs from Pulse-width modulation (PWM) in that it addresses supplies that output a modulated waveform, such as the sinusoidal AC waveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration.

Phase-fired controllers take their name from the fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a controller that can synchronise itself with the modulation present at the input.

Most phase-fired controllers use thyristors or other solid state switching devices as their control elements. Thyristor-based controllers may utilise gate turn-off (GTO) thyristors, allowing the controller to not only decide when to switch the output on but when to turn it off, rather than having to wait for the waveform to return to zero.

A phase-fired controller, like a buck topology switched-mode power supply, is only able to deliver an output maximum equal to that which is present at its input, minus any losses occurring in the control elements themselves. Provided the modulation during each cycle is predictable or repetitive, as it is on the national grid's AC mains, to obtain an output lower than its input, a phase-fired control simply switches off for a given phase angle of the input's modulation cycle. By triggering the device into conduction at a phase angle greater than 0 degrees, a point after the modulation cycle starts, a fraction of the total energy within each cycle is present at the output.

...
Wikipedia