Re: gEDA-user: Switching regulator question

[John Doty <jpd@xxxxxxxxxxxxx>]

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/
jpd@xxxxxxxxx




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