Rectifier question

[Rich M]

This must be a stupid question, but hopefully asking it will make me a little less stupid, even though it will appear otherwise!

Anyway, on B+ supplies with CT secondaries, why do they use several (2-3) rectifiers in series. One could handle the reverse voltage and I imagine the combo doesn't help with storage time. Is it for reliability (in case one fails)? Transient protection? If one swaps in a SiC schottky's to eliminate storage time, is this redundancy necessary?

[number9]

This must be a stupid question, but hopefully asking it will make me a little less stupid, even though it will appear otherwise!

Anyway, on B+ supplies with CT secondaries, why do they use several (2-3) rectifiers in series. One could handle the reverse voltage and I imagine the combo doesn't help with storage time. Is it for reliability (in case one fails)? Transient protection? If one swaps in a SiC schottky's to eliminate storage time, is this redundancy necessary?

No question is stupid... I forget the real reason they did it, but from an electrical
stand point of view, one could argue a few things. It gives you a slightly larger voltage
drop (say 0.7V X 3 instead of one), it can also protect against bigger reverse transients,
and can actually help with forward transitnets due to the tiny delay in turn on time from
one diode to the next. I have heard the argument that it helps with ripple, again due to the
delay in the diode turn on time... I would have to break out my scope or some paper to
prove that one to myself though.

I have seen amps both ways, one with a single large diode on each leg, one with multiple 1n4007's, one with a single 1n4007 in each leg. I typically use tube rectification, so I suppose in reality I have not given it too much thought, but perhaps next time I am in front of one I will really check it out now since you have raised my curiosity more.

[VaughnC]

This must be a stupid question, but hopefully asking it will make me a little less stupid, even though it will appear otherwise!

Anyway, on B+ supplies with CT secondaries, why do they use several (2-3) rectifiers in series. One could handle the reverse voltage and I imagine the combo doesn't help with storage time. Is it for reliability (in case one fails)? Transient protection? If one swaps in a SiC schottky's to eliminate storage time, is this redundancy necessary?

It depends on the voltage of the power supply and the peak reverse voltage rating of the diodes. When a diode is alternately shut off in a full wave rectifier, it actually sees 2.8 times the power supply voltage in the reverse direction. So, diodes are connected in series so not to exceed and individual diodes PRV rating.

[Rich M]

Thanks for the replies. Good point, VaugnC, but wouldn't it be 2.8 times the RMS voltage of the xformer secondary ( x 2 x sqrt(2) ) or roughly 2x B+ (neglecting sag) ?