Phase-fired controller
Phase firing is a triggering method often used in controlling SCR, thyristor, thyratron and other such gated diode like devices.
A common role for these devices is for their cathode to anode junction to be connected directly to some form of modulated supply, such as the national AC mains.
By conducting 100% of each modulation cycle, 100% of the energy within that cycle will be delivered to the load connected to the output of the device.
Provided the modulation during each cycle is predictable or repetitive, as it is on the national AC mains, phase firing allows the device to be switched on at a specific phase angle within that cycle so as to deliver a known percentage of the cycle to the load.
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 delivered to the load. In essence, a duty cycle or pulse width modulation mode of operation.
The output of such regulation is equal to or less than it's input. A phase fired controller, like a buck topology switched-mode power supply, is only about to deliver a maximum voltage or current equal to whatever is available to it at it's input.
To obtain an output lower than it's input, a phase fired control simply switches off for a given phase angle of the input's modulation cycle.
As a phase fired controller is unable to boost the voltage or current beyond that present at it's input, it is common for designers to purposely design the maximum output requirements to be lower than the minimum possible input; derating the supply. When operating within a derated specification, the controller will initially switch off for some phase angle of input's modulation on every cycle so as to maintain a design specified output. When first turned on, the supply may be off for some percetage of the modulation cycle even when running under normal conditions. This way, any deviation in the load towards a higher demand or variations on the input level towards a lower level can be accounted for by calling on the spare phase angle set aside during deration.
Derating of mains powered, phase fired supplies is important as they are often used to control resistive loads, such as heating elements. Over time, the resistance of heating elements often increases. To account for this, a phase fired control must be able to provide some degree of voltage boost 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, opening the supply up progressively towards delivering 100% of the input modulation cycle as the elements age.
A duty cycle or pulse width modulated controller is usually fed with DC. Whilst such an input may also contain a noise signal, the energy delivering component of DC is constant. This means that it is not important when a controller switches the supply on as it will be garanteed a consistent level of energy delivering potential from it's input; DC has no phase. Thus, the only factor the controller needs concerned with modulating is the time period the supply is on for.
For a phase fired supply, it's input will be modulated. The modulation in each cycle represents a changing ability of the input to delivery energy and so it is important that the controller can synchronise when it's virtual phasing with the real phase of it's input. This is important because the phase of it's input may not stay constant and may drift back and forth. In order for the controller to deliver an accurate amount of energy to it's load, it will need to sense the phase of it's input and then align it's self for it's prediction and the real world load delivering potential of the input to be in synchronisation with each other.
Taking the UK AC mains for example, a controller connected to this input may detect when it's input voltage falls to 0V and use this as a trigger for the beginning of a new modulation cycle. As the frequency, voltage and waveform of the input is acceptably constant at 50Hz, 230V and sinusoidal in the UK, the controller can predict when it should and shouldn't be conducting so as to produce a given output. The supply is effectively resynchronising it's self every time the modulation at it's input begins a new cycle. More advanced controllers would also be able to evaluate their input's frequency, voltage and waveform to better regulate the load.
Given that a number of phase fire capable devices can also handle high power demands 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. Previously, extremely expensive and heavy multi-tapped transformers where 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.
In modern equipment of a high power nature, 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 realistically needs it. This is because removal of the mains transformer means that the load is in direct galvanic contact with the input. The input is often the national AC mains, which is it's self referenced to the Earth for ease of transmission. 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 415V, high current capable inputs and having the entirity of any metallic housing or framework present Earthed, this is a serious risk that must be assessed with care.
Many gated diode devices have relatively slow slew rates, but this is only of serious concern in high speed circuits, such as switch mode power supplies, where the switching frequency may be hundred of thousands, or even millions, of cycles per second.
More expensive phase fired controlled may also use [[Distributed Buffer - Gate Turn-off Thyristor (DB-GTO) ] | [gate turn off]], or [[Distributed Buffer - Gate Turn-off Thyristor (DB-GTO) ] | [gate controlled switching]], semiconductor devices that can not only be triggered into conduction at a specific phase angle but also back into their blocking state before the input modulation cycle restarts.