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.

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60 comments

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June 18, 2012 at 10:36 AM delete

please post a 12v 5amp smps ckt diagram

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July 15, 2012 at 7:30 PM delete

Dear

Does you have the circuit diagram and working of horn being used in two wheeler,scooters and bikes.
Contact me at :
jasdeepsinggh@gmail.com
provide me your mail id also so that we can discuss.

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July 15, 2012 at 7:32 PM delete

does you have horn curcuit being used in two wheelers like scooter and bike.

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July 17, 2012 at 3:19 PM delete

Mechanical or electronic, which one are you looking for?

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October 22, 2012 at 5:49 PM delete

Please provide me simple SMPS circuit with list items with specifications for 3V DC.

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October 22, 2012 at 7:34 PM delete

You may try the following circuit:

http://homemadecircuitsandschematics.blogspot.in/2012/05/cheapest-smps-circuit-using-mje13005.html

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March 28, 2013 at 12:35 PM delete

hello,

how do i know that maximum current supplied by smps supply that i made??

thankz

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March 29, 2013 at 8:52 AM delete

connect the output to an ammeter set at 10 amp range, it will provide you with the maximum current rating of the circuit.

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April 7, 2013 at 10:53 AM delete

sir,
could you plz tell me more about "self oscillating type of configuration of transistor" in smps....

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April 8, 2013 at 8:00 AM delete

You may refer to this circuit, it employs a self-oscillating topology.

http://homemadecircuitsandschematics.blogspot.in/2012/05/cheapest-smps-circuit-using-mje13005.html

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May 10, 2013 at 3:03 AM delete

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.

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May 10, 2013 at 10:21 AM delete

try googling VFD circuit, or variable frequency drive circuit, you would find some interesting info...

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June 3, 2013 at 4:31 PM delete

please post a 12v 20amp smps ckt diagram

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Anonymous
June 24, 2013 at 3:23 AM delete

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.

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June 24, 2013 at 11:47 AM delete

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.

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Anonymous
June 24, 2013 at 3:33 PM delete

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.

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June 24, 2013 at 9:13 PM delete

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.

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June 28, 2013 at 9:30 AM delete

dir sir,
I want to know the kind of input and output of high frenquency transformer.

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June 28, 2013 at 3:21 PM delete

transformer data needs complex calculations, difficult to produce it here.

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Anonymous
August 12, 2013 at 8:48 PM delete

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

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August 13, 2013 at 10:33 AM delete

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..




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Anonymous
August 13, 2013 at 8:25 PM delete

ok Sir
thank you

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raj
October 12, 2013 at 1:33 AM delete

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

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October 12, 2013 at 3:59 PM delete

The coil is the main component which needs proper dimensioning, and is most difficult to design, it's better to buy one ready-made sample and copy it as it is.

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Anonymous
October 24, 2013 at 4:42 PM delete

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 ?

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October 24, 2013 at 7:47 PM delete

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

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December 7, 2013 at 9:10 AM delete

hi swagtham

Can I have your phone no. pls

Regards

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December 7, 2013 at 12:27 PM delete

Hi Hemant,

I am sorry a telephonic conservation won't be possible, you can contact me via the email IDs given in the "contact" link below

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March 28, 2014 at 10:53 AM delete

I want 12 volt 5 amp built circuit for AC to DC in regular quantity. deepakfromagra@hotmail.com

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November 14, 2014 at 9:54 AM delete

I wanted to make a 48 volt SMPS. Kindly share schematic for 48 volts supply.

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November 14, 2014 at 9:55 AM delete

Can you please share 48 volt SMPS circuit.

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November 14, 2014 at 5:23 PM delete

you can built the following circuit:

http://homemadecircuitsandschematics.blogspot.in/2012/03/how-to-make-simple-12-v-1-amp-switch.html

double the number of turns in the secondary and adjust R6 until 48V is achieved at the output

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January 28, 2015 at 3:42 PM delete

how much amps is possible in viper22a sir

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Anonymous
May 19, 2015 at 4:49 PM delete

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?

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May 19, 2015 at 6:30 PM delete

yes that's correct, PWMs or the duty cycle can be used for adjusting current, not voltage.

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July 27, 2015 at 12:55 PM delete

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 :)

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July 28, 2015 at 8:27 AM delete

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:

http://www.homemade-circuits.com/2015/07/turns-and-voltage-ratios-total-voltage.html

http://www.homemade-circuits.com/2015/04/how-to-make-variable-smps-driver-circuit.html

http://www.homemade-circuits.com/2015/04/how-to-make-variable-current-smps.html

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July 28, 2015 at 4:26 PM delete

Thanks Swagatam. What happens when input ac voltage varies ? For example can it be capable to provide the rated output when AC voltage varies from 110V to 220V ?

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July 29, 2015 at 10:12 AM delete

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)

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August 7, 2015 at 6:14 PM delete

Hey please send me the circuit for 5v DC 1Amp from 230v AC. kmhatre14@gmail.com

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August 8, 2015 at 3:25 PM delete

you can refer to the following article:

http://www.homemade-circuits.com/2014/02/220v-smps-cell-phone-charger-circuit.html

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September 16, 2015 at 6:48 AM delete

sir how to do an smps wich is controlling by mosfet as a switch pls reply simbu389@gmail.com

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September 16, 2015 at 2:19 PM delete

you can try the following circuit:

http://www.homemade-circuits.com/2014/03/12v-5-amp-transformerless-battery.html

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September 30, 2015 at 1:22 PM delete

Can you please tell me if a UPS WITH 12 volts 1 amp OUTPUT. Is available.I require this for the modem .since the modem takes a few minutes for configuration each time the current goes off

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September 30, 2015 at 8:54 PM delete

you may try the circuits explained here:

http://www.homemade-circuits.com/2013/03/simple-dc-ups-circuit-for-modemrouter.html

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November 5, 2015 at 12:54 PM delete

hello sir,which device can replace transformer in regulated power supply???

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November 5, 2015 at 12:55 PM delete

hello sir, which device can be used as a replacement of transformer in regulated power supply

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November 6, 2015 at 11:11 AM delete

In SMPS the ferrite transformer cannot be replaced, it's the heart of the system.

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November 12, 2015 at 7:24 PM delete

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 ?

Thanks
Krishna

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November 12, 2015 at 7:25 PM delete

Is it safe to remove c7 if I want to connect the ground on the dc side to mains ac ?

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November 12, 2015 at 8:23 PM delete

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...

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November 12, 2015 at 8:24 PM delete

...you can try the following circuit if isolation is not a concern for your need:

http://www.homemade-circuits.com/2014/02/simple-1-watt-to-12-watt-smps-led.html

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November 12, 2015 at 8:43 PM delete

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 ?
Thanks
Krishna

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November 13, 2015 at 11:01 AM delete

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

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November 13, 2015 at 2:13 PM delete

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 http://www.ebay.in/itm/DC-DC-3A-Buck-Converter-Step-Down-Module-Power-Supply-DIY-4-5-28V-to-0-8-20V- /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.


Thanks
Krishna

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November 14, 2015 at 12:20 PM delete

Dear Krishnakumar,

you can take the help of the following article for further info:

http://www.homemade-circuits.com/2015/10/calculating-inductor-value-in-smps.html

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