Secondary rectifier ratings in forward topology

[LongRunner]

Is there really no difference in terms of current limit, between both diodes fully conducting at a 50% duty cycle in a single schottky package and one diode freewheeling at a 33% duty cycle and the other diode conducting at a 66% duty cycle?

Well, the datasheets do say that it's the instantaneous current that affects the forward voltage drop - so in theory at least, the heating ought to be the same whether you have each diode in the part conducting half of the time, one for one third and the other for two thirds, one for one quarter and the other for three quarters, etc.

I ask because I noticed that even at 230V, judging by c_hegge's reviews, the HP-D3057F3H isn't really efficient at all (it hovers between ~68% and ~75% efficiency at low and high loads respectively, possibly because the fan spins very slow at low loads, probably much slower than the H305P-01 fan controller has it spinning, but that should only be at low loads). In fact, it seems on par with the H305P-01 in terms of efficiency, and that linear regulates the +3.3V rail, which squanders lots of power.

If you look at the load tables for the HP-D3057F3H, you will see that, for the time it could handle the overload, its efficiency actually rose marginally, unlike the H305P-01 where it dropped to 69%. The HP-D3057F3H also maintained tight regulation of the main rails under the overload, suggesting that it could actually manage ≈450W continuously if the fan speed was further increased (though obviously things would get even louder then).

So if the HP-D3057F3H can sustain a 50% overload without the voltages falling, it must operate at a rather low duty cycle in normal use, don't you think???

I get that the H305P-01 has a 6A bridge rectifier on a heatsink and the HP-D3057F3H has an 8A bridge without one...

The bridge drops maybe 2V from the entire mains supply so it shouldn't have any appreciable effect on efficiency.

To reach the theoretical maximum duty cycle, not only would you have to load the PSU to at least the rating, but you would also have to lower the mains voltage (for non-APFC units) quite substantially.

[c_hegge]

Well, the datasheets do say that it's the instantaneous current that affects the forward voltage drop - so in theory at least, the heating ought to be the same whether you have each diode in the part conducting half of the time, one for one third and the other for two thirds, one for one quarter and the other for three quarters, etc.

That is correct. You can work out hopw much heat will be dissipated by a rectifier by using the formula H = I x Vf (where H = Heat output, I = Current Draw, and Vf = Forward voltage drop).

For example, The popular STPS3045CW rectifier has a rated Vf of 0.57V, so if we were to pull the full 30A from it, the heat output would be 17.1W (30 x 0.57)

[Wester547]

Well, the datasheets do say that it's the instantaneous current that affects the forward voltage drop - so in theory at least, the heating ought to be the same whether you have each diode in the part conducting half of the time, one for one third and the other for two thirds, one for one quarter and the other for three quarters, etc.

That is correct. You can work out hopw much heat will be dissipated by a rectifier by using the formula H = I x Vf (where H = Heat output, I = Current Draw, and Vf = Forward voltage drop).

For example, The popular STPS3045CW rectifier has a rated Vf of 0.57V, so if we were to pull the full 30A from it, the heat output would be 17.1W (30 x 0.57)

The datasheet says the forward voltage is 0.57V @ 15A and 0.72V @ 30A for STPS3045CW (@ 125*C). That works out to 21.6W of heat output whilst conducting 30A in total.

To reach the theoretical maximum duty cycle, not only would you have to load the PSU to at least the rating, but you would also have to lower the mains voltage (for non-APFC units) quite substantially.

So then most PSUs operate at lower duty cycles at lower loads and higher duty cycles at higher loads (to simplify what's being said)? Would a higher duty cycle at lower loads be how some half bridge PSUs that seem to be built really cheaply achieve great efficiency (80%+) at low loads?