The most common type of varistor is a metal-oxide varistor (MOV). This is the rectangular box with the diagonal line through it on the input in Figure 1. The input filtering block will also typically include a voltage dependent resistor, or varistor to prevent high voltage spikes on the electrical power grid from damaging the power supply.
Capacitors do not pass DC, and can be used in series (as DC blocking ‘high pass filter’ elements), or parallel (to shunt high frequencies to ground preventing them from getting through to the converter). As with other parts of an AC to DC converter, reactive elements like capacitors and inductors perform the important role of frequency – selective suppression. The filter passes through 50/60Hz mains frequency, and attenuates higher frequency noise and harmonics that might be present. It also prevents noise that may be on the mains power supply getting into subsequent circuits. Apart from this difference, they can utilize the same essential blocks.įigure 1: Functional Block Diagram of a Forward Converter AC/DC Power SupplyĪn input filter is important as it prevents noise produced in the power supply switching elements from getting back onto the mains power supply. Although not discuss in detail, a flyback converter differs from a forward converter in that its operation depends upon energy stored in the airgap of the transformer in the circuit. The converter depicted in figure 1 is called a ‘forward converter”, which is a higher efficiency than a slightly simpler architecture a ‘flyback converter’. This circuit can take many forms but always comprises of the same essential elements, and may have one or more stages of conversion. Filters are used to smooth out the energy stored, resulting in creation of a DC source for other circuits. This process separates the power associated with the positive and negative potentials. In the U.S., the typical AC main line is 120V.Ĭonverters steer an alternating current, as its voltage also alternates, into reactive impedance elements, such as inductors (L) and capacitors (C), where it is stored and integrated. As a reminder, AC mains in homes range from 110V to 250V, depending on which part of the world you live it. Depending on the location, high voltage can range from 4kV (kilo-volts) up to 765kV. AC power that is sent at high voltage over long distances and then converted down to a lower voltage is a more efficient and safer source of power in homes. AC is the dominant method of transporting power because it offers several advantages over DC, including lower distribution costs and simple way of converting between voltage levels thanks to the invention of the transformer. This is to ensure it is safe, does not interfere with other people or contribute noise to the AC main power lines.Įlectric power is transported on wires either as a direct current (DC) flowing in one direction at a non-oscillating constant voltage, or as an alternating current (AC) flowing backwards and forwards due to an oscillating voltage. More than this, it must have been tested and certified to do so – an expensive process. When you plug in anything to a mains plug socket, it must comply with legal certification standards in the country it will be used in. If possible, use a preapproved off-the-shelf plug pack. Without exception, this subsystem should be designed and implemented by a qualified expert. When the AC source is a mains power socket, great care must be taken to ensure an implementation is safe to use. Understanding the key concepts associated with conversion and the practical alternatives available is a good start towards a successful design. Circuits often require an integrated AC power source as the optimum strategy to reduce size, cost or due to application specific needs.