gEDA-user: Switching regulator question

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

Anybody here knowledgeable in the field of switching power supply
design?  I'm designing my first one, to knock 24VAC down to 5VDC/0.5A,
so it's a fairly small supply, but I don't think I'd want to do that
much of a drop with a linear regulator.  I'm settling on the TPS5420
integrated switcher (controller with on-chip MOSFET) from TI, which
has a switching frequency of 500kHz.  (Mostly chosen because of its
large input range -- 24VAC RMS means 34V peak, plus I have to assume
that it might be as much as 10% above nominal, then subtract the diode
drops from the rectifier, so I'm designing for 36V peak.)

My question (one among many) is how much do I need to filter the
ripple coming out of the full-wave rectifier?  Given a switching
frequency of 500kHz, I would think that 120Hz ripple on the input
would not bother the thing, even if it's large ripple, so long as the
voltage never drops below the minimum required to still enable a 5V
output (i.e., about 8V).  It seems that the lower the average input
voltage, the higher the average efficiency of the regulator is going
to be, so I would ideally aim to have the largest input ripple
possible, which coincidentally allows me to choose a smaller, cheaper
input capacitor.  That also means lower peak currents through the
rectifier and a better power factor.

But it can't be that easy.  Smaller and cheaper is never better, so
what am I missing?


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