Simple SMPS Circuit

If we need DC power supply for circuits we choose stepdown transformer based Rectifier circuit, It may give constant DC voltage under Regulator ICs but when the current fluctuations occurs at Input Power Source then the DC output supply will also gets affected.




To get rid of this drawback in traditional DC power supply method engineers, electronic designers are go with SMPS circuit. You may heard the name SMPS (Switched Mode Power Supply), It gives good constant DC output with considerably constant output current.

This page contains a simple smps circuit which is capable of producing 12 volt DC with 1 Amps current rating, and this circuit contains few easily available components, it may help you to design your own smps for your electronics projects.

SMPS Block Diagram

Before going to circuit diagram it is necessary to understand the operation of SMPS. This block diagram represents typical SMPS inner blocks.

We give high voltage AC Input and low frequency which is available on power outlet, the first stage of SMPS is Rectifier and filter hence the high AC becomes High Voltage DC from this operation we eliminate high spikes and surge, High voltage DC is controlled by High Frequency switching device (50KHz-120KHz) vary depends on design this switching block samples the high voltage DC with feed back path reference.

The flyback or tiny ferrite core  transformer step down the voltage as required depends on design, then the second stage is Rectifier and filter section, it gives Rectified constant DC output Voltage without current fluctuations.

Some portion of output is taken as feedback signal and this signal compared with reference voltage and the error (if present) is amplified depends on the error PWM pulse changes its frequency hence the switching device regulates the output. So the minimum changes in output DC regulated instantly without affecting load.

Circuit diagram

Construction & working

Main three parts of this simple SMPS circuit are TNY267 tiny switch -II family IC from power integrations. It is enhanced, energy efficient and low power offline switching device.

TNY 267 Pinout

Secondly EE20 core flyback transformer as stepdown transformer which is made up of compact ferrite EE. Middle column includes a cross-section of 4.5 x 4.5 mm, air gap 0.4 mm. Primary carries 157 turns of wire diameter 0.15 mm. Secondary incorporates (for 12V output) 14 turns of wire 0.4 mm.

Finally Optocoupler in feedback path IC2 EL817, it is a photo transistor opto coupler, when the output DC voltage goes beyond the limit then this optocoupler gives signal to TNY 267 IC, and then the PWM output pulse gets changers accordingly.

Use the components as specified and output will be 12V DC with 1 Amps current rating.

Note

1. This circuit Involves in operating  High voltage AC it is lethal, Handle with extreme care

2. Make the Flyback transformer as your requirement or get a ready made one.

3. If you are construct your own Flyback transformer, then check the operation of the device before fetch into circuit.

4. Again, All stages in SMPS circuit involved in operation of High voltage either AC or DC, Handle with extreme care     and protections.

 




18 thoughts on “Simple SMPS Circuit

  1. Can I make a 2amp 5/12volt Smps using tny 267 Ic if possible then what is transformer and circuit design.

  2. Thank you for the given information. My question is; can the current be increased more than 1Amp?. by connecting more copper wire in parallel or increase the thickness of the copper wire.

  3. very good information Smps voltage circuit analysis. small question Smps PCB board tharmistor resisrot why this resistor use in smps.

    1. to avoid heat , as it work on high frequency so the circuit will get heated,so to avoid this n keep circuit cool.

    2. Thermistor is used as a circuit protection device, this component protect from short circuit and overload conditions, by preventing current flow to the rectifier section.

  4. If an old ie transformer based supply uses a linear regulated output how does input power/voltage affect the output? If it’s designed correctly it doesn’t. The real reason smps’ are used is cost size and weight not regulation.

  5. Hi,
    Could you please advise what type of transformer I should be using to get 5v output with more than 700mA current?

    Thanks

  6. Sir i had built this above circuit instead of using tny267 i had used tny266 because it was available to me and i had seen sam circuit diagram with that on this given blog rest all things are in common.
    http://streampowers.blogspot.com/2015/08/12v-1a-smps-circuit-diagram.html?m=1

    Now here is my problem i am not getting my required current out of it. It is not able to go beyond 450ma after that i think transformer is saturating and when i measure the output voltage it shows 13.8v in no load and 13.4 in small load is it normal?

  7. Could someone tell me how the transformer was wound, primary and secondary. Is the primary shielded from the secondary? what frequency? I have a similar circuit I’m working on but 325 dc rectified from 220ac. What does it mean if I keep blowing the pin 5 on the mosfet/tnyswitch. and many times R2 in your diagram.

  8. pl. specify the input voltage does it works from 90v to 250v ac also I want to make 9V DC supply so I can use ZD1 9V/ 1Watt zener instead of 12 volts zener. Also the no of secondary turns 11 turns or 10turns.
    Pl. help in this regard.

  9. Dear Sirs… your SMPS Circuit Diagram and the PCB Board (in the Photograph) does not seem to match !
    Q1 and Q2 are not given in the circuit. What values are Q1 & Q2 and how are they connected in the circuit ? or is the pcb photograph from a different circuit ? can you please clarify ?
    Thanks in advance.

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