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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Fri, 19 Apr 2024 10:30:45 -0400
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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.


>>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