Re: gEDA-user: Switching regulator question

["Tom Zych" <freethinker@xxxxxxxxx>]

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.)
>
> 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?

Well, actually, smaller and cheaper are always better, as long as
the thing still works. What is engineering but an endless quest for
ways to make things work better, for less money, and especially in
electronics, in a smaller package?

I don't know too much about design (yet), but I have played around
with this sort of design so I have some idea about it.

Think of the waveform coming out of the rectifier. It's abs(sin(t)),
so it looks like a bouncing ball: zero, curves up to 34V, falls back
down to zero, bounces back up, etc. Now, the waveform going into the
regulator will be like a bouncing ball with a parachute: from zero
to 34V, starts to fall but the capacitor makes it glide down instead
of falling. Then the next wave comes along and tops it up again.

Consider some extremes. No capacitor: bouncing ball. Minimum
capacitor that will keep the voltage above 8V: falls pretty quickly
from 34V to 8V before the next wave pushes it back up. A huge
capacitor: it stays pretty close to 34V all the time. It depends on
how much current you're drawing too, of course.

Now, with a linear regulator, the more voltage it has to drop from
input to output, the more power it's going to waste. So yes, with
the minimal capacitor you're better off.

However, that's for a *linear* regulator. I'm not familiar with
switching regulators, but I would think this would be much less of
an issue, because as I understand it, whereas a linear regulator is
like someone watching the input voltage and turning a variable
resistor (which wastes power), a switching regulator is like someone
watching the input voltage and turning a switch on and off. It won't
waste much power in either position.

Ok, now that I'm done hand-waving, everyone who actually understands
this stuff may now proceed to tell me how many ways I'm off base :)

Cheers,
-- 
Tom Zych / freethinker@xxxxxxxxx
Hamlet: Ha, ha! What say thou now, Polonius?
Polonius: Thou wert ever a pain in the arras. (Dies)




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