Phase-fired controller

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 angle 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 use 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 the next zero crossing.

A phase-fired controller, like a buck-topology switched-mode power supply, is only able to deliver an output voltage not exceeding 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.

To achieve a "boost"-like effect, the PFC designs must be derated such that the maximum present at the input is higher than the nominal output requirements. When the supply is first turned on or operating under nominal conditions, the controller will continually be delivering less than 100% of its input. When a boost is required, the controller delivers a percentage closer to 100% of the maximum input available.

Derating of mains-powered phase-fired controllers is important as they are often used to control resistive loads, such as heating elements. Over time, the resistance of heating elements can increase. To account for this, a phase-fired control must be able to provide some degree of extra voltage to draw the same heating current through the element. The only way of achieving this is to purposely design the supply to require less than 100% of the input's modulation cycle when the elements are first put in place, progressively opening the supply up towards delivering 100% of the input modulation cycle as the elements age.