SMPS Full Form – Switched Mode Power Supply

SMPS Full Form – Switched Mode Power Supply

What is the full form of SMPS?

Full form of the SMPS is (Switched Mode Power Supply) or Switching Mode Power Supply But the Right Full Form of the SMPS is (Switched Mode Power Supply)

SMPS Full Form - Switched Mode Power Supply
SMPS Full Form – Switched Mode Power Supply

Working principles of SMPS

Switch Mode Power Supply two major components involved in power supply. One of them is the Switch Mode Power Supply (SMPS) and the other is Uninterrupted Power Supply (UPS). The SMPS provides DC voltage the components of the computer like processor, hard disk drive, floppy disk drive and so on. UPS, on the hand ensures that the power supply provided to the different components is uninterrupted and consisted.

In this Blog, you will learn the different types of power supplies, the different power terminologies power connectors and also learn about the methods involved in testing the power supply. At your place of Work, you would be required to test the different power conditions under which the different PC components work, You would also be able to identify the right and required power units for each component so that they work.

Full Form SMPS

Another important aspect that you would need to take care of is consistency in power supply and the need maintain this consistency. For this, you must have a solid understanding of UPS, its types, and its specification there might be situations where you might have to decide the type and specifications of a UPS to solve particular power related problem. This would require good analytical skills where you identify the problem are and good problem-solving skills where you identify corrective steps to solve the problem on hand. These skill would to a greater extent help in troubleshooting issues related to power supplies.

After completing this Blog you will be able

  • Discuss about the different types of Power Supplies
  • Explain about the different component of Switch Mode Power Supply
  • Discuss about the different output DC voltages
  • Describe the different Power terminologies
  • Identify the different Connectors present in ATX Power Connector
  • Explain about the need for Line Conditioners and Surge suppressors
  • Discuss the process of testing ATX power supply


The Power supply unit used in a computer is called Switch Mode Power Supply (SMPS). The SMPS is used to DC voltage to the components present in the motherboard. It is used to produce different DC voltages to drive the various components like the processor, the hard disk, floppy drive, the processor fan and the components present in the system unit of the computer. The input given to the SMPS is Alternating Current, which is converted into Direct Current for operating with different Integrated Components present in the system.

Classification of Power Supplies

The power supplies can be classified into two types, these are Linear Power Supply and Switch Mode Power Apply These two types of power supplies are explained as follows.

1-Linear Power Supply

The linear power supplies dissipate nine power during the conversion process. As the process is efficient and requires a large heat sink to cool the system. The amount of power that is wasted as dissipation depends on the input voltage applied and the lead actuations. They show poor efficiency and hence am got suitable for computers, the example of a linear power supply is the buttery adapter.

2-Switch Mode Power Supply

The switch mode power supply uses a transistor switch and a closed feedback loop to generate DC The output from the SMPS is a regulated voltage irrespective of the variations in the input voltage.

Different component of Switch Mode Power Supply

  • Primary Rectifier
  • Primary Filter
  • Solid state Switch
  • Transformer
  • Secondary Rectifier
  • Secondary Filler
  • Sensing Switching Circuit

Primary Rectifier

The 230V AC input voltage is sent to the SMPS unit. The output of the SMPS is the regulated DC voltage of low range. The rectifier is a circuit made of electronic components called diodes to convert the bidirectional Alternating Current to Unidirectional DC needed for the computer.

Primary Filter

Filter is an electronic circuit designed to allow only particular range of frequencies disregarding the rest. A her circuit is then used to remove the undesired signals like spikes.


Transformer is used to boost up the voltage to the required level. The voltage can be increased or decreased using the transformer the process can be explained as follows. AC voltage is given input to the SMPS. It converted into DC using the primary rectifiers The Rectified DC from the output of the primary rectifier is sent to filters to remove the spikes or the undesired signals if any present in the rectified output,

Transformer in SMPS is used to step down the voltage. The transformer operates only in AC and hence the rectified DC current is converted into pulsating DC using the switching transistor. The pulsating DC applied to the transformer and the high voltage is converted into the required low voltage with high frequency.

The output of the transformer is sent to the secondary rectifier to convert the AC to DC. The rectified DC output is then sent to the secondary filter circuit to remove the spikes and then sent as output through the different power connectors.

Output DC Voltages

The output DC voltages of SMPS are shown in the figure below. The different output DC voltages are

  • -12V DC
  • -12V DC
  • +5V DC
  • -5V DC
  • +3.3V DC

-12V DC:-

+12V used to drive the disk drive motors. It is also used by components like fans and other cooling devices.

-12V DC

The RS-232C interface of the serial port follows the negative logic. This voltage is provided to the motherboard to power up the serial port. This voltage is rarely used but is provided to the motherboard for downward compatibility.

+5V DC

All the integrated components of the motherboard operate with +5V and hence the motherboard is provided with +5V power supply. On the new systems the CPU has now designed to operate with a lower voltage of +3.3V.

-5V DC

The motherboard is provided with SV for backward compatibility. The older Floppy disk drives connected the ISA slots, the DRAM memory chips in the original PC needed 5V. This voltage level is now obsolete and is not used by components of recent days.

+3.3V DC

+3.3V voltage level is the modern voltage level introduced by the ATX form factor, It is not present in the older. From factors +5V was the lowest voltage level in the aider computers, The +5V provided power to the CPU Memory and all the other integrated components on the motherboard, As the technology improved, the second Pentium chips are designed to operate at reduced voltage of 3,3V This would reduce the power consumption as the speed of operation of the chip is increased. The motherboard manufacturers had to design gal voltage regulators o on board to reduce the 5V to 3.3V.

As a result, lot of heat was generated by the voltage regulators and this method of voltage regulation on the motherboard was not efficient. Hence the supply unit was itself designed to provide the +3.3V separately for the CPU the +3.3V power supply used by the CPU in addition to the AGP cards, system memory and other circuits.

+5 V Power OK (Power Good Signal)

When the system is started for the first time, the power supply unit takes some time until the power is sufficient drive all the components present in the motherboard, After a stabilized output is obtained power can be give the processor and the other components present in the motherboard. To prevent the computer from opera a power good signal is generated to indicate that the power obtained is proper and c given to other devices on the motherboard. If the power good signal is not obtained, then the CPU restarts prematurely immediately until a stabilized power is generated for driving the devices present in the motherboard.

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