Simple 12V, 1A SMPS

AshvinI Vishvakarma and Atanu Dasgupta


Flyback transformer

A flyback circuit is simply a pair of coupled inductors. If a current is passed through one inductor, it will store energy E = ½ (L.I2), where ‘L’ stands for inductance in henry, and ‘I’ for current in amperes. This energy can later be taken out of the second inductor, which is coupled to the former at a different volt-current ratio. The flyback’s energy storage and extraction mechanism is interesting. The key point is the polarity of the winding; the secondary is out of phase with the primary, as is evident in Fig. 2 (the dots indicate polarity).

Fig. 2: Circuit of the 1A, 12V SMPS
Fig. 2: Circuit of the 1A, 12V SMPS
Fig. 3: A single-side PCB for the 1A, 12V SMPS
Fig. 3: A single-side PCB for the 1A, 12V SMPS
Fig. 4: Component layout for the PCB
Fig. 4: Component layout for the PCB

Download PCB and Component Layout PDFs: Click here

21B_PartsWhen the MOSFET of IC1 is ‘closed,’ the current flows through L1. Point A on L1 goes positive and by transformer action, and considering the polarity of dots, point C on L2 goes negative. This reverse biases diode D4, and no current flows in the secondary winding. Similarly, when the MOSFET is ‘open,’ the current flow through L1 is interrupted and, by Lenz’s Law, a voltage of polarity opposite to the applied voltage appears on L1 and L2. Thus, point A on L1 goes negative and point C on L2 goes positive. This situation forward biases diode D4. The energy stored in the core causes the current to flow through winding L2. This charges capacitor C2 and also powers the load. The charge on C2 is used in the next half of the cycle to keep the current through the load somewhat constant. The cycle repeats endlessly. The MOSFET is switched on/off continuously at a frequency of around 120 kHz to


  1. Dear Sir,
    i have try this circuit at home but i didn’t get its out put. i check its AC &DC voltage across snubber circuit (Diode across primary side of transformer)& capacitor , it shown 696 V & 295V respectively.

    i have design its ferrite core E20 transformer my self using you tube videos , but i didnt get its secondary side voltage . here i am using schottky diode SB560 insist of SB 160.
    Kindly guide what is a fault in the circuit.

    vipin pandey
    [email protected]

  2. I have tried this circuit. But facing one problem. Output current is getting only less than 300mA.
    Getting only less than 30KHz at TP2. Kindly help.

  3. How come, they have not given any specific number of turns? I am novice in Inductors. Can you guys please help?

    • Reply from author Ashvini Vishvakarma: EE20 has no replacements. It is widely available, and very cheap.
      You can find it it in SMPS of every DTH set top box.

  4. Good day Sir,
    Please, I need your help. I have tried building the circuit using your schematic diagram and it work, but the output I’m getting is 3V at the secondary side and it’s the highest I could get even when I increase the number of turns.
    please I need 12V output, what must I do to be able to get this?
    Thank you.


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