Switch Mode Power Supply (SMPS)

Switch Mode Power Supply (SMPS)

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Every electrical or electronic device needs power for work and the computer is not exception to it. Computers, too, have a particularly designed power supply component known as Switch Mode Power Supply (SMPS). SMPS converts raw input power to controlled voltage and current for the operation of various components of the computer. SMPS uses switches for high efficiency. The primary function of SMPS is to convert the alternating current (AC) power available in homes into direct current (DC) required for a computer system. In desktop computers, a metal box found in the corner of the CPU case supplies power to various components in the CPU box. The power supply converts 115-230 volt AC into DC that is required for computer components to work.



The terms such as voltage, AC, DC, etc., are closely associated with the power supply or SMPS.

Voltage is an electric potential difference between two points and is measured in volts.

AC stands for alternating current. It is an electric current whose magnitude and direction vary cyclically.

DC stands for direct current. It is considered as the constant flow of electrons in a single direction from low to high potential. Computers use DC power. Because of the advantage of alternating current over direct current in transforming and transmission, electric power distribution today is nearly everywhere alternating current.

The SMPS or power supply of a computer comes in different form factor styles. The form factor refers to the physical dimensions of a component. The form factor of the power supply must match with the form factor of the computer case into which the SMPS is inserted. There are various industry standard form factors available. Some of the commonly used form factors with their characteristics are given below:

• LPX
• ATX
• SFX

The LPX form factor style power supply has exactly the same motherboard and disk drive connectors as the previous standard power supply form factors. LPX form factor power supply differs in its reduced size that allows building much smaller and consumer oriented PCs. Due to their small size, they can be put into almost every type of computer cases.

The ATX form factor is developed by Intel in 1995. ATX is similar to LPX in physical dimensions. The difference between the two is that the power pass through-outlet for the monitor has been removed from ATX. Another difference is that in ATX, the cooling fan is mounted along the inner side of the power supply. With this kind of arrangement, the fan draws air in from the back of the chassis and blows it inside across the motherboard.

The SFX form factor style power supply is 100 mm wide, 125 mm deep, and 63.5 mm in height. It includes a 60 mm power supply fan for cooling. The main SFX motherboard connector is in the same shape and size as the ATX connector. The one difference here is that the SFX power supply specification does not support the -5V compatibility voltage and, therefore, should not be used with motherboards that have ISA slots.

Each SMPS or power supply on a computer contains a connector that connects to the motherboard, which supplies power to the system processor, memory, and all add-on cards connected to slots such as ISA, PCI, AGP, etc.

Industry standard PC, XT, AT, and LPX motherboards use the same type of main power supply connectors. They use two main power connectors: P8 and P9. Each of the P8 and P9 connectors has 6 pins that connect power supply to the motherboard.



ATX main power connector is used to connect to the power connectors on the ATX, SFX, and in all ATX based motherboards. ATX main power connector is a 20-pin connector with a square hole for pin #1 and round holes for the other 19 pins.



ATX auxiliary power connector is a 6-pin connector. Intel designed this to fulfill the additional power requirement of motherboards, e.g., a motherboard that requires 18A of +3.3v or 24A of +5v power. A higher level of power is required when power supply system is using 250 watt to 300 watt supplies.

ATX12V power connector provides localized power to the CPU, while the AUX connector provides additional power mainly to the AGP card.

The computer components and circuits such as motherboard, adapter cards, and disk drive logic board use +3.3v or +5v, and the disk drive motors and cooling fans require +12v power. The components that require voltage level other than this are powered through onboard voltage regulators. These voltage regulators are built into the motherboard.

Power Good Signal: The power supply not only converts AC to DC but also ensures that the system does not run unless the power supplied is proper. Power supply conducts an internal check before the computer starts. If the check is successful, it sends a signal called Power Good to the motherboard. The Power Good signal is present in the system. When the AC voltage dips and the power supply is unable to provide outputs, the Power Good signal is withdrawn and the system is reset. The Power Good signal is a +5v active high signal, which is supplied to the motherboard after the internal checks have been successfully performed. The processor timer chip present on the motherboard that controls the reset line receives the Power Good signal. After receiving the signal, it releases the reset line, and the CPU starts executing the code present at FFFF:0000 memory address (ROM BIOS).

Power supply transmits power not only to the motherboard but also to the floppy disk drive, hard disk drive, and CD/DVD drive and other devices. A four-wire connector is attached at the back of each drive. The four wires provide +5v, +12v power, along with two grounds, to the various drives that use them. The connectors are available in two different sizes. The large size connector known as Molex is used on most internal drives, including hard disk, CD/DVD, Zip drives, and the older 5.25" floppy disk drives. The smaller size power connector, called as Berg, is used for the newer style of 3.5" floppies.



Serial-ATA (SATA) power connector is used to provide power to SATA hard disk drives.



The number of power connectors on the power supply varies with the type of power supply. If the power supply is large, the connectors are more in number so that more devices can be attached to it.

Power supplies have many other specifications and features to protect computers. A good power supply should have very low current leakage, i.e., less than 500 microamps, to ground.

While adding add-on cards or other components on a computer, a technician should make sure that the power supply is not overloaded. Most of the time the power supply is overloaded by filling up the expansion slots and adding more hard disk or CD-ROM drives. Today's processors may also have high current requirement for the +5v or +3.3v supplies. Therefore, when selecting an SMPS for a computer, technicians should have future system upgrades in mind.

In case of a defect or problem with power supply, it is not recommended that an inexperienced user open a power supply for repairing. The power supply has dangerously high voltage. Even after unplugging, power supplies can retain dangerous voltage and must be discharged (like a monitor) before servicing it. These internal repairs are not recommended unless the technician is specially trained to repair such components.

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