On 8/31/16 1:52 PM, Adrian Godwin wrote:
I'm being Devil's Advocate here because I certainly realise switchers do
generate high frequency noise. But wasn't the intention of them to make
filtering easier, with smaller filter components, exactly because they
operate at higher frequencies ?
So why do they fail ? Is it cost-cutting to make them only just good enough
for typical uses ? Shouldn't it be possible to make the best possible
supply from a switcher, if only cost and weight weren't the first
considerations ?
Sure.. you can do a bunch of LC low pass sections and knock it down, but
building a *wideband* low pass filter with good ultimate rejection is
also challenging.
In a system I'm actually sitting next to, we have a switcher followed by
a 60dB rejection lumped LC followed by the power distribution, followed
by a 60 dB rejection lumped LC followed by a linear regulator.
When you're at this kind of level, layout and mechanical arrangement is
important, because the switching noise (which is also on the input) can
couple "around" your other circuits.
The other challenge is that those filters have resistive loss - if you
want good regulation, how do you get your "sense" voltage back to the
regulator without that path being the path for the noise - so you'd need
to put LP filters on the sense line.
And of course, the L's need to be shielded.
And you put it in nesting boxes made of steel
A linear regulator is a LOT easier.
If you aren't power dissipation constrained, then it's often the easiest
way to get to where you want to be.
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