Basic Components of a Power Supply Unit

A power supply is an electronic circuit that converts an ac voltage to dc voltage. It is basically consisting of the following elements: transformer, rectifier, filter and regulator circuits. Power supply units (PSU) are used in computers, amateur radio transmitters and receivers, and all other electronic equipment that use dc voltage as an input. Uninterruptible power supply is a must for computers which holds volatile data from time to time. This prevents corruption of data due to power failure and low voltage.

The transformer is a static device that transfers electrical energy from the primary winding to the secondary winding without affecting the frequency. It is used to step-up or step-down the ac voltage level and isolates the remainder of the electronic system from the ac power.

The primary winding of the transformer is connected to an ac voltage source that produces alternating current while the secondary is connected to a load. The primary and secondary windings are not physically connected to each other but due to electromagnetic induction following Faraday's law, there is an induced voltage in the secondary winding. There are three main functions of transformers namely: stepping the voltage up, stepping the voltage down and providing isolation between the primary and secondary circuits.

The rectifier is a device used to change the ac power into pulsating dc. The basic rectifier is the diode. This diode is a unidirectional device that operates as rectifier in the forward direction. The three basic rectifier circuits using diodes are the half-wave, full-wave center-tapped and full-wave bridge type.

The filter of the power supply is used to keep the ripple component from appearing in the output. It is designed to convert pulsating DC from rectifier circuits into a suitably smooth dc level. The two basic types of power supply filters are the capacitance filter (C-filter) and RC-filter. The C-filter is the simplest and most economical filter available. On the other hand, RC-filter is used to reduce the amount of ripple voltage across a capacitor filter. Its primary function is to pass most of the dc component while attenuating the ac component of the signal.

Ripple and Ripple factor

Ripple is the unwanted ac component of the signal after rectification. It is unwanted because it can destroy or damage the load. This is the main reason why filters are installed in power supply - to prevent high ripples. The job of the filter is to smoothen the signal and suppress the ac component or variations. Ripple factor is the ratio of the root mean square of the ripple voltage to the value of dc component at the output voltage. It is sometimes expressed in percentage or in peak-to-peak value. The ripple factor determines the effectivity of a filter being used in the circuit.

A voltage regulator is designed to provide a very steady or well regulated dc output. It is always ideal to have a steady output voltage so that the load will operate properly. The output level is maintained regardless of the variation of the input voltage. The commonly used transistor voltage regulators are the series voltage regulator and the shunt voltage regulator.

Series voltage regulator

The series element controls the amount of the unregulated input voltage that goes to the output as a regulated output. The regulated output voltage is sampled by a circuit that provides a feedback to the comparator circuit and is compared to a reference voltage.

Shunt voltage regulator

The shunt voltage regulator provides regulation by shunting current away from the load to regulate the output voltage.

A regulator Integrated Circuit (IC) unit contains the circuitry - the reference source, comparator, amplifier, control device, and the overload protector - inside a single IC. There are also adjustable voltage regulators which allow the user to set the desired output level. Other IC regulators have fixed output values. It is said that IC regulators are superior compared to transistor voltage regulators when it comes to linearity of the output voltage.