gEDA-user: Switching regulator question

[DJ Delorie]

> 24VAC RMS means 34V peak,

HA!  24VAC means 38V peak (i.e. after the bulk caps) on my board.  Had
to change modules because I designed for 34V.

> My question (one among many) is how much do I need to filter the
> ripple coming out of the full-wave rectifier?

Input ripple is telegraphed to the output as part of its reponse curve
- how fast can the switcher respond to a change of input voltage.  The
switchers I got (Acon modules) want 2Vpp ripple max.

> It seems that the lower the average input voltage, the higher the
> average efficiency of the regulator is going to be,

Hmmm... most I've seen have a "sweet spot" that I'm guessing has to do
with the duty cycle of the switcher.  But note that the efficiency
depends on the quality of the catch diode on output; get the lowest Vf
schottky you can, and look for a switcher that has a FET helper for
it.

> so I would ideally aim to have the largest input ripple possible,
> which coincidentally allows me to choose a smaller, cheaper input
> capacitor.

What I did was put in four smaller caps in a row, which reduces the
ripple current through each.  That helps keep cost down too.

http://www.delorie.com/house/furnace/pcb2/board-full.html