On Jun 16, 2007, at 5:43 PM, Randall Nortman wrote: > 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.)
If I was expecting 36, I'd design for at least 50. The AC mains aren't well regulated, and large transients aren't uncommon. > > 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). Depends on how much ripple on the output you can tolerate and how fast the control loop is. The regulator acts as a lowpass filter, but it will pass some ripple to the output. Know your requirements. Know the specs of the stuff you're using. > 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 higher ripple voltage on the capacitor, which may break it. > > But it can't be that easy. Smaller and cheaper is never better, so > what am I missing? Smallness and cheapness beats bulk and squander. Not that in my business we're good at avoiding either... John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [EMAIL PROTECTED] _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user