Offline flyback without C(in)?

[eem2am]

We are designing a 41W offline, isolated flyback converter for Vout = 36V +/-5% and 41W.

There is going to be 30mF of capacitance on the secondary due to the fact that there must be some supply of power in the event of a mains power cut.

Given that there is so much capacitance on the secondary, I assume I am right in assuming that we might as well have virtually no capacitance on the primary, since the ripple at 100Hz is going to be virtually non-existent on the secondary?

 

[Militoy]

Most offline PFC converters step-up voltage - and have virtually no holdup capacitance on the front end - just the X and Y caps in the EMI filter, and maybe a small film or ceramic cap after the bridge to polish off HF noise. This is to take advantage of the HV rail voltage after the secondary, and the benefit of higher voltage to energy storage in the cap (E=CV^2). In your case, you might want to consider giving up some input power factor, in exchange for longer holdup time - and put the bulk of your capacitance in the front end, where it will do the most good. In a step-down flyback converter, you have a whole lot of duty-cycle bandwidth to work with. You should take full advantage of the square of that higher voltage at the front side of your transformer. At only 41W, the lower power factor will typically not be as much an issue as holdup time. You are going to have to trade a lot of space, if you are stuck trying to trade capacitance volume for squared voltage.