# SMPS power calculation

I am planning to design smps power supply that using flyback topology. It will have 12V@5A output values. But it is not clear for me how to estimate input power. It will supply from 220 AC input port. I am trying to find it from my transformer inductance which is 75uH. It is logical to find from V=Lxdi/dt ? My switching frequency is 100kHZ and will work %45 duty cycle. Therefore ramp signal rises to peak current in 4.5us. I have some ideas like this. Can u help me how to find input power and from there eff?

• Calculate all the losses you can imagine (such as MOSFET and rectifier conduction and switching losses; transformer winding and core losses, etc.) and make your first estimate. Build your prototype, realize that you're wayyyy off, then compute even more losses. Unless you're very experienced in this sort of thing, your initial estimates are likely going to be wildly optimistic. – Adam Lawrence Apr 18 '18 at 19:26
• There are a lot of loss sources (e.g. switching losses, transformer losses which is the sum of core and copper losses, diode dissipation, passives, dummy load etc), so why dont you estimate? I'm sure that flyback converters have at least 80% efficiency. – Rohat Kılıç Apr 18 '18 at 19:32
• thank you. If I assume that my eff is 80%, based on this I can find currrent that flow through mosfet. And I can begin to compute power losses on other components. – Ali Mutlu Apr 18 '18 at 19:35
• 75uH seems too low a value for your transformer (unless you meant secondary?). There are plenty of good online design web pages, you should try there and compare with your attempts. – a concerned citizen Apr 18 '18 at 19:55
• This might help a bit and take note that 75 uH primary inductance is too low for 100 kHz operation given the power levels you are talking about. AC powered efficiency is usually about 75% and 80% for flybacks fed directly from a DC bus. – Andy aka Apr 19 '18 at 7:53