efficiency of flyback converter

How do i practically calculate input power for efficiency of flyback converter

The input power is simply the sum of the output power plus all the losses: rectifier losses, transformer losses, transistor losses, auxiliary (bias, gate drive, etc.) losses and losses in the capacitors (yes, their ESR dissipates power).

Our friend VVV has explained it correctly you can use following formula

Vout X Iout Power output
----------------- = ------------------ = Effiency
Vin X Iin Power input

Effiency figure will give you the idea of loses in any kind of AC-DC or DC-DC converter.

While all of those losses are a part of the overall calculation the biggest contributor to any loss could be the flyback transformer.

Before anyone says otherwise know this; the biggest contributor to any power supply with a low efficiency rating is always the magnetic. If designed and built properly you almost be guaranteed nearly 90% efficiency.

E

Sorry, nxtch,

You are incorrect. Usually, we allow about 3-4% of the total losses to transformer and usually the loss is split equally between the core and the windings. This loss is either temperature-rise-limted or absolute-loss-limited. For more info see Lloyd Dixon's paper "Transformer and inductor design for optimum circuit performance" on the TI website (SEM1500). There are other papers on the TI webiste, too. Search for transformer or inductor design.

The biggest loss usually comes from the rectifiers. And that is easy to see why. consider a power supply that delivers 10 at 5V. Assume it uses a Schottky rectifier with an on voltage drop of 0.5V. Right away you see that the diode conduction losses are 0.5V*10A=5W or about 10% of the total output power. Add to that the switching losses and you can easily approach 15-20% of the total output power, especially for lower output voltges, such as 3.3V, for example.

This is the reason why designers came up with synchronous rectifiers, to reduce the rectifier losses, since they are the major contributor to low efficiency at relatively high currents.