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# Transformer on switching mode power supply

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#### Toru Fujinami

##### Newbie level 3
Hi,
I have a question regarding transformer turn ratio on DC-DC switching mode power supply (smps).

I am testing smps that converts (12 to 24)V to 3.6V. The circuit is designed by other person (without documentation...) so I do not know why he chose the way he did.

The system has a feedback circuit which regulates the output by controlling the switch to toggle. A transformer with turn ratio of 1:0.63 is used.

For the lowest input, the output to input voltage ratio is 3.6/12 = 0.3. Therefore, My turn ratio is half of the voltage ratio. But due to feedback I am still getting 3.6V.

Is the turn ratio I have a reasonable and healthy for the circuit? Would that affect the efficiency of the system? Is it better to use 1:0.3 turn ratio instead?

There are different operation principles of switched mode power supplies with transformer, particularly forward and flyback converter.

To describe it a bit simplified, a forward converter works similar to a low frequency transformer power supply, the output to input voltage ratio is primarly defined by the transformer winding ratio, unless you implement an additional storage inductor in the output rectifier. The input power is direcetly transferred from the primary to the secondary, the transformer has a high primary inductance and doesn't store considerable energy.

In a flyback converter, the energy is stored in the transformer inductance in one half cycle and released to the secondary side in the other half cycle, at a possibly different voltage level. The transformer windings ratio does not necessarily correspond to the voltage ratio.

It's however not clear why the present converter has been designed with the said windings ratio. There's no obvious advantage at first sight.

so is it forward or flyback......ccm flyback you have vo/vi = ND/(1-D)

and in forward you have vo/vi = ND

where N = ns/np.

usually you select your wanted duty cycle at your lowest vin, then you go from there and calc the turns ratio which gives you that.
If much more current in sec than pri then you often want low duty cycle.

You know some controllers have limited duty cycle so you cannot do more than that.

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