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From: boB <boB@K7IQ.com>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Fri, 19 Apr 2024 11:08:36 -0700
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On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>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.


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.

boB