Friday, March 16, 2012

How to Understand Switch Mode Power Supply (SMPS) Circuits

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. Yes the idea is to switch the 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:

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 involvment of high frequencies, eddy current losses become negligilby 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.

32 comments:

  1. please post a 12v 5amp smps ckt diagram

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

      Delete
  2. 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.

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

    ReplyDelete
    Replies
    1. Mechanical or electronic, which one are you looking for?

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

    ReplyDelete
    Replies
    1. You may try the following circuit:

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

      Delete
  5. hello,

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

    thankz

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

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

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

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

      Delete
  7. 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.

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

      Delete
  8. please post a 12v 20amp smps ckt diagram

    ReplyDelete
  9. 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.

    ReplyDelete
    Replies
    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.

      Delete
  10. 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.

    ReplyDelete
    Replies
    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.

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

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

      Delete
  12. 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

    ReplyDelete
    Replies
    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..




      Delete
    2. ok Sir
      thank you

      Delete
  13. 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

    ReplyDelete
    Replies
    1. 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.

      Delete
  14. 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 ?

    ReplyDelete
    Replies
    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.

      Delete
  15. hi swagtham

    Can I have your phone no. pls

    Regards

    ReplyDelete
    Replies
    1. 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

      Delete

Readers are requested not to include external links while commenting. For consulting a diagram, upload it on Google Drive and provide the link here.

Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.