Simple 12V, 1A SMPS

AshvinI Vishvakarma and Atanu Dasgupta

77369
 

The design data for the transformer is as follows:
1. Duty cycle = 0.45 (max. duty cycle for DCM flyback = 0.5; less 10% safety margin)
2. Core saturation magnetisation Bsat = 0.24T
3. Core area of EE20 = 25mm2

Winding details computed for the SMPS are shown in Table I.

The feedback circuit

Regulated output needs feedback to control the pulse width modulation (PWM) of the MOSFET. TNY266 has a fabulous control feature; it stops the switching cycle as soon as any current is taken out of pin 4 of the device. If SMPS output exceeds the zener break-down voltage then ZD1 conducts. This lights the opto-LED and the opto-transistor grounds pin 4 of IC1, resulting in immediate stoppage of the switching cycle. Also, when the primary is conducting, diode D4 on the secondary side is reverse biased. At this time, if the voltage across D4 exceeds its reverse breakdown voltage, the SMPS will fail. Here we have used an SB160 Schottky diode with breakdown voltage= 60V.

Connector CON2 provides 12V regulated DC supply.

Construction and testing

A general core-selection ‘rule of thumb’ for SMPS below 50 watts is 2-3 mm2 core area per watt. For a primary input power of 16W, a core with a core area of 32-48 mm2 is needed. EE20 core will work well for this design.

Transformer wire

Any wire that can carry the required current can be used. To know how much current a given wire can carry, SMPS designers use a number called current density [J] for this calculation. Empirically, a good starting point is J = 5 amps/mm2. The primary carries 0.3 amps, so it can be wound using SWG 38. The secondary carries 5 amps, so it can be wound with SWG 26. A good practice would be to wind the secondary using two parallel strands of SWG 28. This reduces the skin effect. The key problem in flyback transformers is leakage inductance, which is caused by poor coupling between the primary and secondary windings. So wind them tight, with full overlap.

The air gap

Flybacks made from power ferrites must have an air gap. The energy stored in a flyback primary is E = ½ L.I2. Peak primary current depends inversely on primary inductance. An air gap increases the energy storage capacity of a flyback transformer. It is calculated as follows:

Air gap = (µ0 x N2 x Ac)/Lprimary
where:
µ0 = permeability of free space, 4π x 10-7
Ac = Core area (m2)
N = Primary turns
Lprimary = (Vprimary.pk × Ton.primary)/ Iprimary.pk

The primary inductance computes to ~3mH, so the air gap is approximately 0.2mm. However, this value is not critical, as was experienced by the authors during their experiments. Any air gap in the neighbourhood of the calculated value works well. A thin sheet of plastic or paper works fine.

A single-side PCB for the simple 1A, 12V SMPS is shown in Fig. 3 and its component layout is shown in Fig. 4. Assemble the circuit on the recommended PCB to minimize assembly errors. Use IC base for IC1.

To test if the circuit is functioning properly, first check the regulated output at TP1 with respect to TP0. The voltage should be stable with or without a load.


Ashivini Vishvakarma is associated with IBM as a telecom systems architect and a radio amateur. Atanu Dasgupta was a powergrid executive in telecom division and is also a radio amateur.

16 COMMENTS

  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.

    regards,
    vipin pandey
    7208446650
    [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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