How computer power supplies work – KitGuru Guide

1. Introduction2. The two types of power supplies - Linear power supplies3. The two types of power supplies - Switching power supplies4. The main components of a computer switching power supply - AC input and transient filter5. The main components of a computer switching power supply - AC Rectifier and PFC (Primary side)6. The main components of a computer switching power supply - Main transformer and primary/secondary side heatsinks.7. The main components of a computer switching power supply - The cables and connectors8. View All Pages

The AC receptacle is the socket where you connect the power cable at, often accompanied by a simple on/off switch. The switch does nothing more than isolating the power supply from the power grid by interrupting the live wire.

On older power supplies you could find an AC voltage selection switch which had to be set to comply with the utility grid voltage (110V or 220V) depending on where you live at. This is no longer necessary because newer units automatically sense the grid voltage through the active PFC circuit. Since the installation of an active PFC circuit became obligatory by law in many of the world's countries, it is highly unlikely you will find a passive PFC unit (or a unit without a PFC circuit at all) being sold today.

The first thing the power supply has to do is make sure that the AC power is as clean as possible. That includes both the AC power the power supply is receiving but also the AC line itself; switching power supplies generate harmonics which must be filtered before entering the AC power grid. This is done by implementing a transient filter circuit.

The transient filter usually consists of capacitors and induction coils and sometimes includes a metal-oxide varistor (commonly known as a MOV) and a simple glass fuse for extra protection. We say “usually” and “sometimes” because there are no restrictions concerning what manufacturers can and cannot use. High quality units try to meet and exceed the ATX specification guidelines. Very poor quality units will probably have no transient filter at all, so they will flood your household power grid with electrical noise and can be seriously damaged even by a slight power surge.

The most common transient filter components are:

• Y type capacitors : Used for line bypass, connected between Live or Neutral to Ground. They are used in applications where capacitor damage may lead to electric shock and they shunt current to Ground.

• X type capacitors : X type capacitors are being used across the lines, connecting Live to Neutral. They are used where damage to the capacitor cannot induce electric shock and also operate as fire retardants.

• Induction common mode (filtering) coils : Common mode choke coils are used to suppress common mode noise (EMI/RFI filtering). This type of coil is produced by winding the signal or supply wires one ferrite core. Since magnetic flux flows inside the ferrite core, common mode choke coils work as an inductor against common mode current. Accordingly, using a common mode choke coil provides larger impedance against common mode current and is more effective for common mode noise suppression than using several normal inductors.

• Metal Oxide Varistor (MOV) : Metal oxide varistors (MOVs) are semiconductors that protect electronic components and systems from transient voltages. Their design allows current to flow in only one direction. MOVs have very high resistance at low voltage and very low resistance at high voltage, which is where their name comes from (Variable Resistor). Thus applying low to moderate voltage has little to no effect, however a high voltage triggers an “avalanche effect” and causes the diode to break down. In short, these very little components are your typical, reliable surge suppressor. MOVs tend to degrade if they are repeatedly exposed to transients.

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