# Flyback converter - turn ratio

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?

• Simulate it! Flyback are the hardest to design since every single parameter on both the primary and secondary is determined by your transformer. To get any type of efficiency and/or cost due to semiconductor rating, you’ll end up having to wind your own and learn tons of stuff in the process. Mar 1, 2018 at 10:40
• @winny Alright, what simulators are suitable for such simulation? Mar 1, 2018 at 12:08
• LTspice. There are other too. Mar 1, 2018 at 12:21

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%.

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.

• The formula you have given for Vo/Vin is in CCM mode and chances are that a typical flyback will operate in DCM and have a totally different formula based on output load. May 15, 2018 at 17:16
• It is much more likely that a typical flyback converter will only enter CCM during start up and that it will be in DCM for most of its normal operation especially very light loads. You need to look at what you are saying. May 15, 2018 at 22:05

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.

In a flyback convertor you apply a DC voltage to the primary and this causes a ramp-up in current to some value determined by the "on" time, the applied voltage and the primary inductance. The current rises linearly to a peak then you disconnect the applied DC voltage and the magnetic energy stored is released into the secondary.

That energy is dictated by $\frac{LI^2}{2}$ and if you perform this cycle F times per second you get a power converter because power is energy per cycle transferred multiplied by the number of cycles per second (F).

So, the output voltage is as a consequence of the load resistance and the power transferred.

Right now I'm about to buy a transformer, and the one I want to buy has output voltage specified for 19V.

This doesn't sound right to me. You should be considering a flyback transformer and not a regular AC transformer.

• Well, the transformer I'd like to buy is specified for flyback 130kHz (so it's not an ordinary one) and for 19V, whatever it means (??? there is no datasheet). Mar 1, 2018 at 12:07
• No data sheet = no guarantees = waste of time. I'm serious about this @EmKa. Only buy from reputable dealers that provide a data sheet that appears to properly describe performance. This applies from humble resistors through semi conductors all the way to complex circuit functions. Do you have a link to it so I can check? Mar 1, 2018 at 12:15
• I've finally found the datasheet, but it is in Polish. However you should be able to figure something out: dokumentacja.feryster.pl/… Mar 1, 2018 at 12:22
• I think the 19 volts is when using a TOP249 controller (as specified on the data sheet). I suspect the transformer will be fine based on what you've revealed; The 19 V/3,6 A bit will refer to the final rectified voltage when using the controller specified. If you want to find the controller go looking at the power integrations website. Mar 1, 2018 at 12:42
• Yeah, I want to use UC3845 with slightly more frequency (more like 300kHz) in order to increase maximum current Mar 1, 2018 at 12:49