Phase fired controllers

Phase control (PFC), also called phase cutting, is a method of pulse width modulation (PWM) 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.

Overview

Phase fired control is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this in much the same way that a pulse width modulated (PWM) supply would pulse on and off 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 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 that fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a PWM 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 pulse the output on but also when to turn it off, rather than having to wait for the waveform to pass within the element's Zero Cross Point.

Output reduction by bucking

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.

'Boosting' by derating

To achieve a 'boost' like effect, the PFC designs must be derated such that 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.

Applications

Previously, extremely expensive and heavy multi-tapped transformers were used as the supplies for such elements, with the corresponding winding tap being connected to the element to produce the desired temperature. This limited the temperature resolution to the number of tap combinations available. They often find their way into controllers designed for equipment such as electric ovens and furnaces. These applications are basic, consisting of what amounts to a resistor across a power supply's output.

In modern, usually high power, equipment, the transformer is replaced with phase fired controllers connecting the load directly to the mains, resulting in a substantially cheaper and lighter system. However, the method is usually limited to use in equipment that would be unrealistic without it. This is because removal of the mains transformer means that the load is in direct galvanic contact with the input. For industrial ovens and furnaces the input is often the national grid AC, which is itself galvanically referenced to the Earth. With the controller's output referenced to the Earth, a user need only be in contact with the Earth and one of the output terminals to risk receiving an electrical shock. With many high power pieces of equipment running from three phase 415 V, high current capable inputs and having the entirety of any metallic housing or framework present Earthed (grounded), this is a serious risk that must be assessed with care.

References