How Switch Mode Power Supply (SMPS) Work

SMPS is the acronym of the word Switch Mode Power Supply. The name clearly suggests that the concept has something or entirely to do with pulses or switching of the employed devices. Let’s learn how SMPS adapters work for converting mains voltage to a lower DC voltage.

Yes in SMPS adapters the idea is to switch the mains input voltage into the primary winding of a transformer so that a lower value DC voltage may be obtained at the secondary winding of the transformer.

However the question is, the same can be done with an ordinary transformer, so what is the need of such complicated configuration when the functioning can be simply implemented though ordinary transformers?

Well, the concept was developed precisely for eliminating the use of heavy and bulky transformers with much efficient versions of SMPS power supply circuits.

Though the principle of operation is quite the similar, the results are hugely different.

Our mains voltage is also a pulsating voltage or an AC which is normally fed into the ordinary transformer for the required conversions, but we cannot make the transformer smaller in size even with current as low as 500 mA.

The reason behind this is the very low frequency involved with our AC mains inputs.
At 50 Hz or 60 Hz, the value is tremendously low for implementing them into high DC currents outputs using smaller transformers.

This is because as the frequency decrease, the eddy current losses with the transformer magnetization increases, which results in huge lose of current through heat and subsequently the whole process becomes very inefficient.

To compensate the above loss, relatively larger transformer cores are involved with relevant degree of wire thickness, making the entire unit heavy and cumbersome.

A switch mode power supply circuit tackles this issue very cleverly.

If lower frequency increases eddy current losses, means an increase in the frequency would do just the opposite.

Meaning if the frequency is increased, the transformer could be made much smaller yet would provide higher current at their outputs.

That’s exactly what we do with an SMPS circuit. Let’s understand the functioning with the following points:

How SMPS adapters work

In a switch mode power supply circuit diagram, the input AC is first rectified and filtered to produce relevant magnitude of DC.

The above DC is applied to an oscillator configuration comprising a high voltage transistor or a mosfet, rigged to a well dimensioned small ferrite transformer primary winding.

The circuit becomes a self oscillating type of configuration which starts oscillating at some predetermined frequency set by other passive components like capacitors and resistors.

The frequency is usually above 50 Khz.

This frequency induces an equivalent voltage and current at the secondary winding of the transformer, determined by the number of turns and the SWG of the wire.

Due the involvement of high frequencies, eddy current losses become negligibly small and high current DC output becomes derivable through smaller ferrite cored transformers and relatively thinner wire winding.

However the secondary voltage will also be at the primary frequency, therefore it is once again rectified and filtered using a fast recovery diode and a high value capacitor.

The result at the output is a perfectly filtered low DC, which can be used effectively for operating any electronic circuit.

In modern versions of SMPS, hi-end ICs are employed instead of transistors at the input.
The ICs are equipped with a built in high voltage mosfet for sustaining high frequency oscillations and many other protection features.

These ICs have adequate built in protection circuitry like avalanche protection, over heat protection and output over voltage protection and also a burst mode feature.

Avalanche protection ensures that the IC does not get damaged during power switch ON current in rush.

The over heat protection ensures that the IC is automatically shuts off if the transformer is not wound correctly and draws more current from the IC making it dangerously hot.

The burst mode is an interesting feature included with the modern SMPS units.

Here, the output DC id fed back to a sensing input of the IC. If due to some reason, normally due to wrong secondary winding or selection of resistors the output voltage rises above a certain predetermined value, the IC shuts off the input switching and skips the switching into intermittent bursts.

This helps to control the voltage at the output and also the current at the output.

The feature also ensures that if the the output voltage is adjusted to some high point and the output is not loaded, the IC switches to burst mode making sure that the unit is operated intermittently until the output gets adequately loaded, this saves power of the unit when in standby conditions or when the output is not operative.

The feedback from the output section to the IC is implemented via an opto-coupler so that the output remains well aloof from the input high voltage mains AC, avoiding dangerous shocks.

68 Replies to “How Switch Mode Power Supply (SMPS) Work”

  1. I need to convert 60Hz to 50 Hz @ 2500-4000 watts of power. I am thinking go to DC with a full bridge rectifier, smoothing caps, maybe a choke, then go back to a fixed freq of 50 HZ SMPS. I suppose a sine wave is not absolutely needed, & square would work, but a sine wave would be preferable. 10-13 amps @ 240 volts should work fine depending on the efficiency of the circuit. Any circuits you know of already on the web?? THANKS!!!
    P.S. 2000 watts out would (SHOULD) be more than sufficient. I just need to slow down some AC motors that were made for the European market.

  2. hi sir.i am Girish. i have few questions.

    1) i have heard that smps convert AC into DC then into AC then finally into DC. so there is no problem with output regardless of input wave form.
    I had a amplifier which runs on 12vdc when i connect the amplifier to 12v smps with SQUARE WAVE INVERTER (made by me) there is heavy hum noise in the amplifier such that no music can be heard, when it runs on AC mains no problem. is that true smps give output regardless of input wave form.

    2) please answer to this . When the smps convert DC into AC(at this stage) what is the wave form square wave or sine wave.

    1. Yes that's correct, a 12V SMPS will convert 220V AC to 330V DC then to 12V AC then finally to 12V DC, so square wave inverter with correct RMS will not affect the load performance from an smps.

  3. HI sir . if i use a modified square wave inverter, with the smps ,then load performance will be ok?

    please, please answer to this, what is the wave form of the smps at the stage of converting DC to AC
    i,e 330VDC to 12VAC. is it square wave or any thing else? – GIRISH.

    1. The input waveform is irrelevant for an smps circuit because right at the initial stage the 220V is converted to DC, after that it's converted to flat square wave by an internal oscillator and then stepped down to 12V AC and finally tis 12V is rectified and filtered to make 12V Dc.

      The input RMS is crucial, if it's more then the DC stage capacitor will burst.

  4. Hi Sir
    this is swetha

    the Circuit diagram for Personal computer is simply an SMPS circuit or any other circuit?
    pls provide me the equivalent circuit of personal computer.
    it is for analysing the harmonics in the PC

    1. Hi Shweta,

      It is a regular SMPS having multiple voltage outputs.

      The ferrite transformer involved with PC smps have many taps and winding which are very complicated, so i am sorry it would be difficult for me to figure out and present the date and the configurations..

  5. i am beginner…and i am planning to start manufactoring mobile chargers.can u please suggest me some way to learn these circults and which circults i should prefer

  6. The switching transformers used in smps are available in different types such as EE,EEL,EER, etc. What are the significances of these types ?
    Also vertical and horizontal type of transformer are available, what is the functional difference between them, apart from there mechanical structures ?

    1. Not much difference as far as efficiency is concerned, they just facilitate different PCB designs pinout arrangement, and other dimension considerations according to me.

  7. So if I have understood correctly the final voltage is not determined by the switching frequency and duty cycle but solely by the number of turns on the secondary of the transformer?
    Therefore if I reduce the turns on the secondary I will reduce the voltage accordingly?

  8. Thank you very for the valuable tutorial. Can I get some ideas for designing various voltage and current rated switched mode power supplies? Are there any easy understanding steps ? What should be modified in the design to change the o/p to 5V-2A of ratings. Please let me understand the way for choosing the transformer for different ratings of o/p. Thank you 🙂

    1. the primary side of an SMPS is a complex section and cannot be modified or tweaked easily, and is not recommended.

      the transformer secondary can be adjusted some for getting a desired selected output.

      You may refer to the article for more info:

    2. Thanks Lomas, an SMPS will continue to produce a constant voltage at the output as per its rating, regardless of the input mains input (within 100V and 285V)

  9. Hi,
    I see that most smps mobile chargers have a 2.2nf or so capacitor connecting the dc ground on primary and the dc ground on the secondary side. I couldn't understand the use of this capacitor. Why is this cap(c7) needed ?

    I want to measure the primary ac voltage without isolation and hence want to connect the ground of the dc to the ground on the primary side. Is it safe to do this ?


    1. Hi, if the capacitors are referenced to ground then most probably these are for filtering high frequency content or noise in the circuit.

      It's absolutely not safe to use the primary side of any SMPS for a DC application…

    2. Dear Swagatam,
      Assuming that there is going to be no human contact on the dc side, is it safe to remove c7 in this circuit OR replace c7 with a short.

      With c7 in place, is this an example of a 100% isolated dc smps power supply ?

    3. Dear Krishnakumar,

      C7 is not required according to me….I can't figure out the exact intention behind introducing this capacitor, because it might affect the 100% isolation criterion of the SMPS

    4. Dear Swagatham,
      Thanks for your quick response. I have been opening up mobile chargers from ranging 60/- to 500/- and observed that all of them have this capacitor. The cheap ones use self oscillatory circuit based on Transistor and the expensive ones use IGBT based drives with forced oscillations. I will try out my instrumentation circuit by removing this capacitor and keep you updated.

      For the next phase I am planning for a transformer less power supply followed by a cheap 5v regulator like /131634145521?hash=item1ea601a8f1:g:D~gAAOSwA4dWJ1LQ

      I plan to use a zener to clip the input voltage to ~ 15v @ ~200mA. The 5v regulated output should be able to reach upto 500ma. Let me know yout thoughts on such a circuit.


  10. Hi Swagatam da,
    Thanks a lot for such a simple yet comprehensive Idea. I am very much confident now to go for my first self-made PSU. Just one thing, can you please suggest me a place where I can order the Ferite E Core Transformer, because till now I used to go to Radio Equipments Co. @ Chandni Chowk (Kolkata) for my electronic needs, but it seems they are not able to provide customized transformers as in this case.
    Is it recommended to self-wind the coils at home with prescribed gauzed wires?

    1. Thank you Rajarshi, I am glad I could help….

      …. yes, the transformer cannot be procured ready made from anywhere, only the transformer spares needs to be procured and then assembled at home by following the given winding data

  11. Hi Swagatam,If you could help me by sending the circuit of +ve & -ve 38 volt 10 amp SMPS Circuit for my mosfet amplifier.please mention the specifications of transformer also.

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