Choosing a diode for flyback SMPS. Average current or peak current?

[debininja]

As you guys already know, well before I did, FBT secondary side peak currents are much higher than average output.
For an example: an 1Amp average output current FBT might output around ~4.5Amps peak, which then slopes down to 0 (in 45% dtc. DCM).

Question is: do I choose the average current value as my limiting factor or the instantaneous (peak)?
Take for example, the SB240 (click here for datasheet). Also, would this diode be a good choice for a 5V 1A FBT SMPS?

[fourtytwo42]

You need to consider both, if your peak current exceeds the diodes repetitive peak current rating it will fail. The average power involves the diodes dissipation and is usually quoted for a half sine, not very representative of an SMPS because the peak to average ratio is totally different but if possible calculate your average power and choose a diode capable of that. In all cases ensure your chosen diode is well able to handle the situation, not borderline or only just. Many converters have higher currents during startup charging output capacitors etc and this also needs to be taken into account.

[T3sl4co1l]

If you use the basic Vf + Rs model, there is a linear power term (from Vf) and a quadratic term (from Rs).  That is, power dissipated is: Vf * I + Rs * I^2.

For a waveform, the power from Vf comes from average current, while that from Rs comes from RMS current.

So it's definitely more than average, but not quite RMS, and certainly not peak.

It gets more complicated if you include a more nuanced model of Vf (a power series, or the exponential proper), and temperature dependence.  (Note that, as the diode is allowed to heat up, it becomes more efficient in the forward direction!  And also less efficient in reverse -- watch out for that leakage at high temperatures.)  You'd be better off SPICEing that, though.

Why power?  Until extremely high currents (above the surge rating), the limiting factor is heat.  So the diode rating also depends on the heatsinking: a B340 with minimal pads is good for about as much as a (frame-lead) SOD-123F with a smaller die, but more heatsinking.  (PMEGxxxx parts come to mind.)

Small dies are also better for high frequency operation.  This isn't a problem at your frequencies, though.

Tim