Re: spread-spectrum model

On Sun, 21 Apr 2024 12:32:01 -0700, boB <boB@K7IQ.com> wrote:

On Fri, 19 Apr 2024 17:33:48 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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On Fri, 19 Apr 2024 13:43:59 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:
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On Fri, 19 Apr 2024 11:08:36 -0700, boB <boB@K7IQ.com> wrote:
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On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:
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On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
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.
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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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In my world, we have multiple parallel components (like array
sections) in the signal path powered by independent power supplies
that are required to have independent noise, to prevent correlated
gain when these parallel paths are summed, say in a radar beamformer.
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Telling the power-supply folk that it's just a power supply is a good
way to get buried in details.
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My intent was to keep it simple and get it done.
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Depending on details, the problem could manifest itself as peaks or
ripples in the time domain, your beloved homeland.
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Joe Gwinn
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TI has a couple of interesting appnotes
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<https://www.ti.com/lit/pdf/slyt809>
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<https://www.ti.com/lit/SLVAF18>
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Their little TPS54302 type parts have radical looking PWM, but the
final DC is super clean. Nice trick.
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<https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
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<https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
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We may get some EMI from switching rise/fall ringing too, in the
hundred-MHz ballpark. It would help to de-phase that too.

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TI stuff is widely used in radar, but in the most capable radars the
dithering is provided by bespoke radar firmware, and not left to the
converter chip.  But those chips do work well.
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Joe Gwinn

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We've decided to use home-made half bridges in the 8-channel
programmable power module. The TI and ADI switching regs are just too
smart. We'll use the reg chips when we just want a fixed power supply.
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I was thinking that we could use a DDS architecture to generate the
PWM into the switching half-bridges. We could wobble the frequency
setting to spread the spectrum.
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Maybe replace some of the LSBs of the frequency-set register with a
pseudorandom pattern, a different one for each power supply channel.
Change those LSBs at some rate, 20 KHz or something, to fool an
EMI-test spectrum analyzer.
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A pseudorandom pattern will average to 0.5, which affects the average
switcher frequency, but we can deal with that.
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I suppose I could draw a diagram.
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We'd like the fine-grain spectra to not overlap, across all 8
channels. Fun.
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Given eight unipolar half-bridges, we'll allow users to use a pair as
a full bridge to drive one bipolar load, or three to drive a 3-phase
load like a torque motor. In those cases, I think we can still allow
each phase to have its own independent spread-spectrum thing. The
motors won't care.

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The STM32F4  that I use in my inverter/charger  has a SS clock option.
I have not enabled that yet but intend to eventually for lower peaks
from the PWM outputs driving all the power circuitry.
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boB

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That's cool. I'm designing a bunch of plugin modules that would all
get a 50 MHz clock from the backplane. Since my PWM frequencies will
be fairly low, we should be able to fuzz up the spectra in the FPGA on
each board.
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We could code a generic ss PWM block and use it everywhere.
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<https://www.dropbox.com/scl/fi/2ypg6qhnalmixv6kx44if/Spread_Spectrum_Apr_19.jpg?rlkey=d3hiwl4mj57erk82629fyouse&raw=1>

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This dithers the frequency, which is a valid approach.  But dithering
the phase before going to the DAC core to generate the waveform is
also widely used.  And one can do both at the same time, particularly
with different sequences, so the peaks are spread out in 2D.
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Joe Gwinn

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Pseudo-random frequency is probably best as long as the average
frequency is the specified and wanted center frequency.  So the PRN
source may need to be massaged to do that ?
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What I see most often in processors and an old SMPS chip I seem to
remember all use triangle wave modulation to the clock frequency.
This way, it averages to the center frequency.

Yes, triangle wave modulation is common, but if center frequency is
important, they modulate phase, and maybe amplitude.

I remember hearing about a 12V switchmode power supply for ham radio
where you could adjust the switching frequency up or down slightly so
that the EMI could be tuned away from the operator's radio frequency
he was operating on so it would not interfere.  I guess if you can't
make the EMI go away by design, that might be the next best thing.

It's far easier to move the frequency than to suppress the EMI
everywhere.  Ham operator budgets are not large.

I have yet to find a 12V SMPS supply that I could not hear on my HF
radios so I just use a linear PS for that at home.

That's the cheaper approach.

I don't know if this would be good enough, but a standard dodge is to
have a switcher go from AC line in (120 or 240 Vac) to 48 Vdc, and
then use linear regulators fed from that 48-V bus for all DC loads.

Joe Gwinn