# How to calculate secondary RMS current of each winding in multiplre output flyback converter?

As I am designing multiple output flyback converter and I have confusion in selection of wire gauge of the secondary winding. There are 4 secondary windings and one bias winding. Mode of operation is CCM. V_out1 48 V I_out1 1.5 A

V_out2 48 V I_out2 0.15A

V_out3 25 V I_out3 0.7 A

V_out4 25 V I_out4 0.3 A

V_bias 16 V I_bias 0.05 A

280 V dc input voltage. D_max=0.47, Lprimary=3.1mH, Fsw=65KHz

• You might get more help if you were to respect the guidelines of this site and upvote answers that were useful and formally accept answers that were both appropriate and useful. This is how this site works. It's give and take. You have been informed of this previously on a couple of occasions. – Andy aka Jul 17 '18 at 8:44
• Sir, I am not being rude but really am not getting satisfactory answers. That's why I didn't upvote an answer. I will upvote answer as soon as I get a solution to my question. – vck Jul 17 '18 at 8:54
• My answer here explained why you were seeing a different waveform on light loads compared to full loads. How do you justify not formally marking that answer as accepted? – Andy aka Jul 17 '18 at 9:13
• Sir, At that time I corrected that problem by making changes in a feedback loop. Please help me on this question if you know the answer. – vck Jul 17 '18 at 9:27
• Your question was correctly answered irrespective of what changes you made. Trying to squirm out of things is not doing you any favours. – Andy aka Jul 17 '18 at 9:29

Wire gauge is determined by the required output current. Estimate the wire length from the core size and number of turns, the calculate the resistance. Remember that the resistance of copper is a function of temperature, so you can use the wire resistance at your expected transformer temperature. The wire resistance is in series with your output, and you can perform the R*I^2 calculation to determine the power you will be generating which will be both a transformer loss and a heat source for your design. Remember that you will be supplying pulsed current, so your voltage drop will be greatest immediately after switching off the primary, and the voltage must be high enough immediately after switching.

You will have to do some math. When your power supply is fully loaded, what is the duty cycle? It depends on whether you are in discontinuous mode or continuous mode and the duty cycle, but the peak current on the output winding will be much higher than the average current. It should be relatively easy to draw a theoretical waveform, and then calculate the result of adding the expected winding resistance.

• Sir, I will try it by this method. Can you suggest any book or online link for reference. – vck Jul 17 '18 at 17:15
• ti.com/lit/ml/slup127/slup127.pdf – John Birckhead Jul 17 '18 at 19:33