A switched mode power supply is an electronic power supply that uses a switching regulator to convert electric power from one form to another. The role of a power supply is to convert the mains voltage into a lower DC voltage which a load such as a personal computer and its components can run on. Although there are two types of power supply units available – linear and switched mode – the most common power supply used in computers are switched mode power supplies.
Switched mode power supplies have several advantages compared to linear power supplies. The main advantage is their greater efficiency because the switching transistor dissipates little power when it is outside of its active region. Other advantages of switched mode power supplies is that they are of a smaller size and are lighter in weight and generate less heat due to their higher efficiency. Also, due to their smaller size and lower weight, switched mode power supplies are cheaper and easier to transport.
A switched mode power supply unit consists of eight main stages – the input filtering, primary bridge rectifier, primary capacitors, primary switching transistors, transformers, secondary rectifiers, output filtering and the feedback and protection circuits. The first stage – input filtering has two main roles. The first is to prevent interferences on the mains from affecting the power supply and the second role is to prevent any interference generated by the power supply itself from returning to the grid and affecting other appliances. The primary bridge rectifier converts the AC current from the mains into a pulsed, non-continuous DC. The primary capacitors then smooth the pulsed DC current from the rectifier into a smooth constant DC for the primary switchers to work with.
The primary switching transistors or switchers then pulse the current into the transformer. It is important to note that the switchers are the most common failure point when it comes to cheap power supplies. When they are overheated or overloaded, they explode and cease functioning. The transformers then actually change the voltage. Transformers are a heap of coils that are wound around a main core. The secondary rectifiers then allow the changed current to flow one way towards the output filtering. The output filtering then smoothens out the ripple in the current generated by the switching transistors. Finally the feedback and protection circuits are used for monitoring the output voltages and adjusting the duty cycle of the switching transistors accordingly.