Re: spread-spectrum model

On Sun, 21 Apr 2024 08:50:23 -0400, legg <legg@nospam.magma.ca> wrote:

On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:
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On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:
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On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:
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On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>

I'm designing a switching power supply module and could reduce EMI by
going spread-spectrum on the switching frequency. The simple one below
reduces things by 20 dB. Probe the SS node and FFT.
>
The ss inside switching reg chips is no doubt more sophisticated. In
an FPGA, we could do some sort of pseudo-random thing.
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On a multi-channel power supply, there may be some small advantage to
have a separate spread per channel. That would be easy.

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I'd check for cross-correlation as well, so no ganging up in systems
using multiple channels in some signal path.

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When my engineers get too fussy about stuff like that, I remind them
"it's just a power supply."

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Noise at the local level is best correlated, as it is more
predictable - you avoid low-frequency beat frequencies in the
local regulators - which can and will show up in a detector's
BW and in the regulators' outputs.

>
But...but... it's just a power supply!
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Presumably uncorrelated spread-spectrum will make wideband noise at an
output, not a beat.
>

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A master clock, phase shifted for various local users, can be dithered
for the system (box), which is the actual, final radiator.

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Our box has a 50 MHz clock that is bussed to all the plugin modules,
and it can be locked to other boxes or to a 10 MHz reference, so we
can't usefully dither that. I guess each module could have its own
VCO, but that would mess up synchronizing modules, and complicate
things. Spread-spectrum sounds easier.
>

>
Your engineers can get REAL fussy, if the system's non-compliant
way past the development's due date.

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Eventually, some giant customer may want CE stickers, so we'll do the
easier things now, to improve our chances of passing an EMI test. A
bit of VHDL in the FPGAs would be easy.

>
Unsynchronized power supplies on the same board can
influence each other, unpredictably with load, to produce
audible harmonics.
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Ignore the effects at your peril.
>
RL

Given a common 50 MHz clock and a sensible architecture, we plan to
synchronize modules. Like, for instance, paralleling 3-phase AC
sources to get more power, or running two at phase offsets.

But I'm not concerned with acoustics, given that the modules are in a
rackmount chassis with big fans. And a hiss would be better than a
whine, which is the heart of the spead-spectrum concept. Ears are
spectrum analyzers too.