Path: Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Fri, 19 Apr 2024 15:24:28 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: spread-spectrum model Date: Fri, 19 Apr 2024 08:22:44 -0700 Organization: Highland Tech Reply-To: xx@yy.com Message-ID: References: <7qk22jlrrc9949ccrkdk058b4dinnnt75f@4ax.com> X-Newsreader: Forte Agent 3.1/32.783 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 96 X-Trace: sv3-pfyhrz+DCeHonpyyPIvg95YBSITAoPRph7kx9HAsieTXLqLAcVB04rnKJAYHJjyHJ1vz8RYhHMt50Gl!hQmGYxhh/JBGXUhKMKcJBuA9LJCsz1H9JwrqqEguplNgnA5sutKkZwVxN9QfoLXzcT/mCeQlg6Pf!PW7RLg== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 4914 On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn wrote: >On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin > wrote: > >>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn >>wrote: >> >>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin >>> 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 >> >> >> >> >> >> >>Their little TPS54302 type parts have radical looking PWM, but the >>final DC is super clean. Nice trick. >> >> >> >> >> >>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.