What is a Switched-Mode Power Supply (SMPS)?

You may have heard about SMPS power supplies for your computer. It’s the same topic we’ll be discussing. SMPS can also power a variety of other devices, such as CCTV cameras, motors, and anything else that requires a DC supply.

In this blog, we are going to discuss the basics of SMPS, Its Working, Types, Advantages, Disadvantages & Applications in detail.

What is a Switch mode power supply?

A switched-mode power supply (SMPS) is an electronic circuit that converts power utilizing switching devices that are turned on and off at high frequencies, as well as storage components like inductors or capacitors to provide power while the switching device is not conducting.

Switching power supplies have high efficiency and are used in a wide range of electronic equipment, including computers and other sensitive equipment that requires a consistent and efficient power supply.

A switched-mode power supply is also known as a switching-mode power supply. Taking a high-voltage AC input and converting it to a low-voltage DC output, has become the workhorse of efficient power conversion.

It uses a semiconductor switch type MOSFET to regulate the output voltage by switching on and off the supply voltage at a specific switching frequency. The output voltage will change as the switching frequency is changed. The supplies are smaller in size, have higher efficiency of up to 90%, and are commonly utilized in computers and other sensitive electronic equipment.

Need for SMPS

The task of power conversion was previously performed using a linear method. Heavy and large transformers with analog “linear” control circuits are often required for linear power supplies. Transformers generate a significant quantity of waste heat, which must be dissipated, due to average conversion efficiency of less than 65 percent. This led to the design of SMPS.

Switched-mode power supplies are small, lightweight, and have a high power efficiency of more than 85%. From the design perspective, switched-mode power supplies are also incredibly adaptable, allowing designers to find the best solution for whatever power requirements their end product may have.

The main advantages of SMPS over conventional linear power supply are as under :

  • SMPS is less sensitive to input voltage variations
  • For the same power rating, SMPS is of smaller size, lighter in weight, and possesses higher efficiency because of its high-frequency operation.

The disadvantages of SMPS are as under :

  • SMPS has a higher output ripple and its regulation in worse
  • SMPS is a source of both electromagnetic and radio interference due to the high-frequency switching
  • Control of radio frequency noise requires the use of filters on both input and output of SMPS

How Does It work?

When we offer an SMPS AC input, it converts it to DC right away. Rectification is the term for the process of converting AC to DC. When we offer DC voltage, it will be skipped. The rectifier is designed as a voltage doubler in PC power supplies by adding a switch that can be operated manually or automatically. The rectifier’s unregulated DC voltage is now sent to a filter capacitor. Short pulses of electricity are extracted from the mains supply around the AC voltage peaks.

The high-frequency energy in these pulses lowers the power factor. In today’s SMPS, a unique power factor correction circuit is used to make the input current follow the sinusoidal shape of the AC input voltage, thereby correcting the power factor. Active power factor correction power supplies are typically auto-ranging, with input voltages ranging from 100 VAC to 250 VAC and no input voltage selector switch.

Because the DC passes through the rectifier unmodified, SMPS designed for AC input can be powered by a DC supply. SMPS made for AC input can run from a DC supply, as the DC will pass through the rectifier unchanged.

SMPS working flow
  • Inverter stage

Also known as a chopper.

This step transforms DC to AC by passing it through a power oscillator, whose output transformer is small and has few windings, at a frequency of tens or hundreds of kilohertz, regardless of whether it came from the input or the rectifier stage described above. To make it inaudible to humans, the frequency is normally above 20 kHz. MOSFET amplifiers are switched in numerous stages (to get high gain). MOSFETs are transistors having a low on-resistance and a large current-handling capability.

  • Voltage converter and output rectifier

Inverted AC must be used to drive the primary winding of a high-frequency transformer if the output must be isolated from the input, which is frequently the case with main power supplies. On its secondary winding, this changes the voltage up or down to the required output level. This is done by the output transformer in the block diagram.

If a DC output is required, the transformer’s AC output is rectified. Ordinary silicon diodes are usually employed for output voltages above 10 volts or so. Schottky diodes are often employed as rectifier elements at lower voltages because they offer faster recovery times (enabling low-loss operation at higher frequencies) and a lower voltage drop when conducting than silicon diodes. MOSFETs can be utilised as synchronous rectifiers for even lower output voltages; they have smaller conducting state voltage drops than Schottky diodes.

A filter made up of inductors and capacitors must smooth the rectified output. Components with lower capacitance and inductance are required for higher switching frequencies. Instead of inductors and transformers, other forms of SMPSs use a capacitor–diode voltage multiplier. Typically, these are employed to generate high voltages at modest currents (Cockcroft-Walton generator). A charge pump is a low-voltage variation.

  • Regulation

The output voltage is monitored by a feedback circuit, which compares it to a reference value. The controller may have an isolation device (such as an optocoupler) to isolate it from the DC output, depending on design and safety considerations. These optocouplers are used in switching supplies in computers, TVs, and VCRs to carefully control the output voltage.

There is no feedback circuit in an open-loop regulator. Instead, they feed a steady voltage to the transformer or inductor’s input and trust that the output will be accurate. The impedance of the transformer or coil is compensated for in regulated devices. Monopolar designs additionally account for the core’s magnetic hysteresis.

Types of SMPS

The Switched Mode Power Supply (SMPS) circuit is used to generate a regulated DC output voltage from an unregulated DC or AC input voltage. There are four types of SMPS, for example:

  • DC to DC Converter
  • AC to DC Converter
  • Flyback Converter
  • Forward Converter
  • DC-DC Converter

It turns on and filters high voltage DC when AC power is applied. It is sent to the main side of the step-down transformer at a very quick rate of change. This transformer is merely a fraction of the size of a 50 Hz counterpart, eliminating the size and weight issues. The filtered and rectified o/p on the transformer’s minor side. After that, it must be sent to the power supply’s o/p. To control the output voltage, a sample of this output must be given back to the button.

  • Forward Converter

The choke in a forward converter conducts current both when the transistor is leading and when it is not. The current is transmitted through the diode during the transistor’s OFF time. As a result, current flowed into the load throughout both periods. During the ON time, the choke stores energy and also allows some energy into the o/p load.

Flyback Converter

During the ON time of the switch, the magnetic field of the inductor supplies the energy in this converter. The energy collapses in the output voltage circuit when the button is in the open condition. The output voltage is controlled by the duty cycle.

  • Self-Oscillating Flyback Converter

This is the most basic converter, based on the flyback principle. The flow of current through the transformer primary switches ramps up linearly with the angle equal to Vin/Lp during the switching transistor’s conduction duration. The induced voltage in the secondary winding and feedback windings reverse bias the fastest recovery rectifier, keeping the conducting transistor ON. The primary current inclines to increase extremely fast when it reaches a peak value of ‘Ip’, where the core activates to saturate. We are unable to support them using the feedback winding’s fixed-base drive. As a result, the switching turns on in order to get out of saturation.

Things to take care of while using an SMPS:

  • Don’t open the SMPS
  • Make sure the connection is secure. The cable should not come into contact with the SMPS’s body.
  • Make sure the screws and connections are tight.
  • If you don’t connect the proper load according to the requirements, it could burn.
  • When adjusting the voltage adjuster, use a multimeter to check the changed voltage.
  • Do not expose it to direct sunlight or a humid environment for an extended period of time.
  • If you don’t want your cables to burn out, make sure they’re the right thickness.

Advantages of SMPS

  • The efficiency is high as compared to LPS, with more than 80 to 90%
  • The device is used in SMPS is compact and very small in size
  • The manufacturing cost is reduced
  • Provide isolation between multiple outputs
  • Low power wastage
  • Less heat generation
  • High power density
  • Flexible technology
  • Lower weight
  • Providing the required number of voltage
  • Strong anti-interference
  • Reduced the harmonic feedback into the supply mains
  • Wide ac input voltage

Disadvantages of SMPS :

  • Greater circuit complexity
  • Expert design is required
  • High-frequency electrical noise
  • It can be used as step down regulator 
  • The noise present due to high-frequency switching
  • It produces electromagnetic interference
  • Harmonic distortion
  • Only one output voltage
  • Expensive compared to LPS

Application of Switched Mode power supply (SMPS)

  • It is used in servers, power stations, and personal computers.
  • It is used in vehicles for charging batteries
  • It is used in factories and industries for power.
  • It is used in the railway system and security systems.
  • It is also used on mobile and also as lighting.


Hope this blog helps you to understand the basics of SMPS Its construction, working, configurations, advantages and disadvantages, and applications. We, MATHAELECTRONICS will come back with more informative blogs.

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