Electrical – Flyback converter – turn ratio

flybackpower supplyswitch-mode-power-supplytransformer

I'm building a smps using flyback topology with maximum aoutput voltage of 46V. I'm a novice, however I have read a lot about impulse power supply design. Right now I'm about to buy a transformer, and the one I want to buy has output voltage specified for 19V.

From what I understand, the transformer ratio in such a converter doesn't have to be exactly Vin/Vout, because Vout is determined by the duty cycle on primary winding.

Is that right? Is it possible to obtain 46V in this topology when Vout is only 19V?

Best Answer

Short answer: Depends on what the transformer specs and what your requirements say. But I personally don't recommend it.

Long answer: The transfer function of the converter: Vo = N . Vi . D / (1-D), where D is the duty-cycle (D = ton. fSW).

There is always an input voltage range defined for the transformer and its design should guarantee the required output voltage at minimum input voltage. And generally, it's assumed that the D is nearly 50% (0.45 - 0.47 in practice) at minimum input voltage.

For example, if the transformer is designed for 85..265 Vac (or 120..375 VDC) then this means that the output voltage will be 19 VDC and D will be nearly 0.5 at 120 VDC input in a proper designed flyback converter. Thus, if the input goes to 310 VDC (i.e. 220 Vac) then D decreases to 20%.

Now let's think about this:

The maximum value of the D should not exceed 0.48 in practice. If you want to increase the output to 46 VDC then new value of the D will be D' = D . 46 / 19 = 2.4D .

  • What's the maximum value of D'? 0.48
  • What's the nominal value? 0.48/2.4 = 0.2
  • At what input voltage D will be 0.2? 310 VDC

So, if you guarantee that the input voltage will never go below 310VDC then yes, you can use the same transformer to get 46 VDC.

Please take the assumptions above into account. So please check the specs first.

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