What is a switched mode power supply and how does it work

What is a switched-mode power supply (SMPS)

A switched-mode power supply is an electronic power supply unit. It can act as a switching regulator to convert electrical power effectively. These devices do the transferring of power from an AC or DC source to DC loads. SMPS is always regulated, to keep a constant voltage the power supply employs a feedback controller that monitors current drawn by the load. The switching cycle increases as the required output power increase. All electronic equipment needs a power supply unit, and this power supply unit will convert the input voltage that is suitable for their circuits. The switched-mode power supply is the most commonly used power supply unit. Characteristics of the current and voltage are converted while SMPS transfers power from the source to load. The regulation of voltage is done by varying the ratio of on to off time. SMPS are switching regulators of high efficiency which can step up, down and invert the input voltage. DC to DC converters and DC to AC converters belong to the SMPS category.


Buck-boost transformer

What is the function of a switched-mode power supply and why do we need SMPS

The major function of the SMPS is to convert the available electrical power to the desired power that the device circuitry can handle. The switched-mode power supply can be considered as an item that can connect to the mains, or other external supply and used to generate the source power, it can be considered as a proper power supply. So the SMPS will help the device to work at maximum efficiency without any problems. Normally standard power source is used to supply to electronic DC loads, but its voltage source doesn’t match the level required to the loads, It would definitely happen when the source voltage is not regulated, and that’s why we need SMPS.

A switched-mode power supply can be used to, step up an unregulated DC input voltage regulated DC output voltage using a boost converter circuit or step up SMPS. It can be used to step down an unregulated DC input voltage regulated DC output voltage using a buck converter circuit or step down SMPS. It can invert the DC input using the Cuk converter circuit. SMPS can produce multiple DC outputs using the flyback converter circuit. A switched-mode power supply performs the following functions like voltage conversion, rectification, filtering, regulation, and isolation.

How SMPS is different from a linear power supply

SMPS transfers power from source to the load like other power supplies, but it converts the voltage and current characteristics. In the linear regulated circuit, the excess voltage from the unregulated DC input supply drops across a series element, but in a switched-mode circuit, the unregulated portion of the voltage can be taken out by modulating the switch duty ratio.

Where are SMPS used

SMPS are mostly used in computers and in other digital systems as well as consumer electronics. We can find SMPS in

  • PC’s, minicomputers, large computers
  • Laptop and notebook computer
  • Printers and fax machines
  • Peripheral and expansion boxes
  • PDA’s both internal DC-DC converters and their AC power packs

How does an SMPS operate


In the first stage, the conversion of AC to DC occurs, by using input rectifier rectification can be done. The rectifier produces an unregulated DC voltage which is sent to a large filter capacitor. After the first stage, the inverter will convert DC, and it could be directly from the input of the rectifier. The DC will be converted to AC by the help of a power oscillator whose output transformer is very small with few windings at a frequency range between 10-100 kHz. In order to isolate the output from the input an inverter AC is used to drive the primary windings of a high-frequency transformer, and this will convert the voltage up or down to the required output level on its secondary winding and this is done with the help of the output transformer. The last stage is output rectifier and filter if a DC output is required the AC output from the transformer is rectified.

What are the types of SMPS

SMPS can be classified into four types according to the input and output waveforms they are

  • DC-DC converter
  • Forward converter
  • Flyback converter
  • Self-oscillating flyback converter

Mostly used SMPS are non-isolated ones, this type of SMPS has an inductor in place of a transformer. Buck converter, boost converter, and buck-boost converter are the non-isolated types. All these belong to the simplest class of single input, single output converters, which utilize one inductor and one active switch.

What are the applications of SMPS

  • Machine tool industry
  • Security systems
  • Personnel computers
  • Mobile phone chargers
  • Central power distribution
  • HVDC measurement
  • Vehicles
  • Consumer electronics
  • Lighting and space station

What are the advantages of SMPS

  • Higher efficiency
  • Less thermal management is required
  • Easier PFC support
  • Enhanced lifetime and reliability
  • They are less sensitive to input voltage variations
  • Generation of low and medium voltage supplies are easy

They are compact, as a result of high efficiency and low levels of heat dissipation, the switch-mode power supplies are compact. SMPS can be considered as a flexible technology because this technology can be used to provide high-efficiency voltage conversions like voltage step-up or voltage step down applications.

What are the disadvantages of SMPS

Noise is a major disadvantage, the transient spikes that occur from the switching action on switch mode power supplies are the biggest problem. If the spikes are not filtered properly then it will migrate into all areas of the circuits that the SMPS’s power. These spikes can also affect the nearby electronic component if the spikes and transients can cause electromagnetic or RF interference. External components are required such as the reservoir capacitor and filter components. The cost could be high while designing an SMPS, sometimes additional filtering will be needed and this will increase the cost.