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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Fri, 19 Apr 2024 08:22:44 -0700
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On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>wrote:
>>
>>>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.
>>>>
>>>>On a multi-channel power supply, there may be some small advantage to
>>>>have a separate spread per channel. That would be easy.
>>>
>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>using multiple channels in some signal path.
>>
>>When my engineers get too fussy about stuff like that, I remind them
>>"it's just a power supply."
>
>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.
>
>Telling the power-supply folk that it's just a power supply is a good
>way to get buried in details.
>

My intent was to keep it simple and get it done.

>
>>>Depending on details, the problem could manifest itself as peaks or
>>>ripples in the time domain, your beloved homeland.
>>>
>>>Joe Gwinn
>>>
>>
>>TI has a couple of interesting appnotes
>>
>><https://www.ti.com/lit/pdf/slyt809>
>>
>><https://www.ti.com/lit/SLVAF18>
>>
>>
>>Their little TPS54302 type parts have radical looking PWM, but the
>>final DC is super clean. Nice trick.
>>
>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>
>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>
>>We may get some EMI from switching rise/fall ringing too, in the
>>hundred-MHz ballpark. It would help to de-phase that too.
>
>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.
>
>Joe Gwinn

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.

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.

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.

A pseudorandom pattern will average to 0.5, which affects the average
switcher frequency, but we can deal with that.

I suppose I could draw a diagram.

We'd like the fine-grain spectra to not overlap, across all 8
channels. Fun.

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.