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 18:08:37 +0000 From: boB Newsgroups: sci.electronics.design Subject: Re: spread-spectrum model Date: Fri, 19 Apr 2024 11:08:36 -0700 Message-ID: <7mc52j9ub78dqtdo1a6oj2lfhtgktvt9is@4ax.com> References: <7qk22jlrrc9949ccrkdk058b4dinnnt75f@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 105 X-Trace: sv3-fZSp7F+4DbSVTb37xwjfeG/6fpPTnIuxCAxfxeSDFKoo4vGpMqDSMh3WDzRutbYDLLUo0uwpDr2/LBp!EmX1vG7wBQHRooeRM8inXHpGu1+PlpJPnkolXjSEpgXGjrL2JzN3J3aTqwVjS3wSmMUCUNosdWUQ!IalIlnU= 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: 5276 On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin wrote: >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. 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