I am designing a SEPIC supply around an LT3757. The MOSFET will have ~6.5A pulses with a strange ramping pattern (see image), but the RMS current is only 3.38A and the average current is 2.60A. Assuming 50% over-rating, which rating should I be looking for in a suitable FET? Also, does this apply for resistors, inductors and capacitors, or are there other specifications to look out for?
Best Answer
In short you need to choose the part to survive the peak current.
Longer Answer:
I'd work your way through the SEPIC topology's design calculations again, your current ripple is really high, the difference between average and peak current should be much smaller.
The datasheet says that ΔIsw should be 20-40% of Isw(average max). Or to use the datasheet's terminology χ should be between 0.2 and 0.4.
In your case (estimating by looking at the simulation output graph):
Isw(average max) = ~2.75A
ΔIsw = ~5.5A
χ = ~2.0
Try rerunning your simulation with larger L1/L2 inductors, this should reduce the ripple currents.
EDIT: try a larger cap for C3 also (try 10uf or a second 4.7uf in parrellel), the ESR / ESL of this part is also critical, check what the values of these are for your simulation model.