DC power filter

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We sometimes have a "quiet" power pour on a pc board, like +3.3Q.
Ideally is has its own LDO from +5 or something.

But sometimes we just add a lowpass filter from, say, +3.3V to +3.3Q,
and that tends to be a ferrite bead and a biggish cap. That's not very
scientific, and the bead inductance can resonate with downstream
capacitance if the load current wiggles.

Here's one experiment:

https://www.dropbox.com/sh/bt5zefqg2v9dfd4/AABb2HjhkbNtFvI2EXVNr7EQa?dl=0

The bead is
  
VISHAY              ILHB1206ER601V
MOUSER              70-ILHB1206ER601V

rated 600r, 2.5 amps. Measures 3uH and 70 mohms.

VISHAY              ILHB1206ER601V
MOUSER              70-ILHB1206ER601V

Q is OK, but low frequency filtering looks mediocre. The 56u polymer
cap catches fast spikes pretty well, although we'll also have a lot of
ceramic bypass caps everywhere too. Power pours themselves filter the
really fast stuff.

Supply ripple changes gate and FPGA prop delay, which is jitter in our
world.


Re: DC power filter
On Mon. 29 Jun.-20 3:31 p.m., John Larkin wrote:
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I was wondering the intent behind your test config for this RLC filter.  
1. What does the R=1 simulate? Doesn't that unload the LC resonance of  
~10kHz

2. Why drive with 0.1 ? voltage source ~ 1kHZ square wave then capture  
with 1M? DSO?  Wouldn't it better to use actual ESR of regulator for DC  
source and use a current pulse pump to simulate dynamic CMOS loads.

3. Would it be better to use an AC coupled 50? termination in the DSO to  
look for ripple above the SMPS switch rate or LC resonant frequency  
since this is where load regulated noise comes from in an LDO?

Tony Stewart
Test Engineer EE'75

Re: DC power filter
On Mon, 29 Jun 2020 17:13:31 -0400, Tony Stewart

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R=1 kills the resonant Q a little. It's not simulated.


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That shouldn't matter too much. My big concern was resonance, and this
combination looks pretty good. The regulators and loads could vary a
lot.

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I'm mostly concerned with mid-frequency ripple, so the scope
termination doesn't matter.  

Regulators generally have mediocre high-frequency loops, so we rely on
output capacitance to keep things stiff at all but low frequencies.  

A more compulsive test would combine a specific regulator (switch or
linear) with a filter, and test the load transient response at various
base loads. That could become a project.

I wonder if just a regulator and its output caps ever parallel
resonate. Probably so, maybe often.

I've done some load transient testing with LM317 types with ceramic
output caps, and they do ring pretty good after a load step. I posted
that a while ago, with a fix that seems to work.


Re: DC power filter
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Well, easy enough.  Solve the network and add ESR until the poles become  
real.

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Yeh, at what frequency?  Ferrite beads have a diffusive characteristic,  
i.e., Z ~ sqrt(F) over a substantial range.  By Kramers-Kronig, that's  
equivalent to saying X_L = R, or Q = 1, over the same range.

Hence, it might measure "3uH" at 1MHz, "1uH" at 10MHz, etc.

Real parts vary above and below that curve, of course, and so the Q follows  
the slope.  Q generally stays low, so it doesn't matter much.  It's not like  
you can make them resonate any sharper than the Q.

Q generally peaks at low frequencies, above the L/R time constant set by DCR  
+ asymptotic L, and below the core loss curve.

And, at what bias current?  Ferrite beads saturate typically by 1/20th or so  
of their DC current rating.  Laird is one of the few that provides bias  
curves; I don't bother with anyone else.


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Well, yeah.  The FB looks like R || L.  The R adds a zero, giving a shitty  
asymptote.

Easy solution: make a nice, clean CLCLC, and put damping on either end to  
keep it well-behaved under load conditions.  At each end, an R+C provides  
parallel termination resistance, and a series L (optionally L || R) isolates  
that termination from the source and load.  So it overall looks like:
L(R+C)CLCLC(R+C)L

With good layout and component choice, and shielding to exclude radiating  
fields, this many stages should be good enough for practically unlimited  
attenuation (>100dB) at high frequencies, and can always be extended with  
larger values and more stages for lower frequencies.

I made a particularly nice CDN, LISN, bias tee, whatever you want to call  
it, with this scheme.  The crossover frequency varies a bit with load  
impedance (say in the 10-30MHz range); the Q is never very high, for any  
point on the Smith chart.
https://www.seventransistorlabs.com/Images/LISN_Built.jpg
EUT side has a coupling cap to 50 ohms; R+C in the middle of two chokes; DC  
side stays nice and quiet.


Mind the strays of larger components, which will introduce zeroes in the  
transfer function.  No, this does not happen randomly or unpredictably,  
they're quite predictable.  Just need suitable models.  The real problem is  
a lot of models are either overly simple (e.g. RLC parallel or series), hard  
to use, or don't model modes beyond the first SRF.  Consequently, you have  
to either assume, measure, or guard-band those modes.

Coilcraft provide extensive models, but they use nonphysical elements which  
makes simulations difficult or impossible.  Luckily, I'd found a converter  
which makes them work in any analysis:
https://www.seventransistorlabs.com/Calc/Coilcraft1.html

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: DC power filter
On Mon, 29 Jun 2020 16:34:24 -0500, "Tim Williams"

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I did this experimentally because I don't have a good model of the
bead. And I'd prefer to not add resistance in series with my big cap;
its ESR is nice, around 20 mOhms. To kill Q, I prefer to add a
resistor across the inductor. The bead Q isn't high, which is good.

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On my AADE meter, which runs it in the very roughly 500 KHz range, I
think. Pretty low. Inductance is a fuzzy concept.

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That's why we buy them.  

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Beads seem to have pretty good Qs at low frequencies. They behave like
inductors in the roughly 1 uH range. LT Spice has sims of the Wurth
parts, but we don't use them. The correlation of Z-at-100-MHz and
inductance is weak.

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I'm not counting on that.

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I'm expecting 100 mA, maybe 200. This 1206 bead is rated for 2.5 amps.

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It cuts the ringing about in half. Hardly worth it, but the cost of
the resistor is maybe a penny.  

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The problem is that the supply current can depend on things out of my
control, namely the customer's timing pattern. If I have any supply
resonance, somebody will find it. And I want to keep it simple. There
are something like 16 supply rails on this board.

The real concern is: does a bead and a cap have low frequency
resonances? Sometimes they do. This combo ain't bad.

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