On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss <danluster81@gmail.com> wrote:>A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range. > >Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode. > >https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdfI think we want the switch node to have minimum activity, so low diode impedance might be best. I tried a fb in series with the switcher output pin and it was horrible. As I discovered today, the 40 MHz ring was actually the coax to the scope. Oops. Dogged persistance is a workable substitute for extreme intelligence. This might work for startup: https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1 The laptop supply grunts as much as necessary to get the +5D (D=dirty) load up. When +5 and +24 are stable, the MAX809 waits a bit and enables the Cuk converter, which soft starts. -- I yam what I yam - Popeye
switcher ringing noise
Started by ●March 11, 2022
Reply by ●March 12, 20222022-03-12
Reply by ●March 12, 20222022-03-12
On Sunday, March 13, 2022 at 10:37:17 AM UTC+11, sea moss wrote:> A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range. > > Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode. > > https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdfBeads are nonwound components, and have appreciably lower parallel capacitance. The Toshiba link also covers wound "spike killers" which have a couple of turns of wire on a toroid, which typically means about 1pF of parallel capacitance, although their text doesn't seem to mentions this.. LTSpice lists a great many Würth Elektronik GmbH & Co. ferrite chip parts - not all of them by any means. https://www.we-online.com/catalog/en/pbs/emc_components/ferrites_for_pcb_assembly It does make it easy to play around in a simulation. -- Bill Sloman, Sydney
Reply by ●March 12, 20222022-03-12
On Sunday, March 13, 2022 at 12:17:23 PM UTC+11, jla...@highlandsniptechnology.com wrote:> On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss > <danlu...@gmail.com> wrote: > > >A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor's winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range. > > > >Have you ever tried these "amobead" parts? Might try one in series with the freewheeling diode. > > > >https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf > > I think we want the switch node to have minimum activity, so low diode impedance might be best. I tried a fb in series with the switcher output pin and it was horrible.You do have to provide an alternative (capacitative) path to ground for the high frequency current that the ferrite bead is supposed to block. Without that the ferrite bead just produces a larger voltage spike at the output of the switcher, which isn't helpful.> As I discovered today, the 40 MHz ring was actually the coax to the scope. Oops. Dogged persistance is a workable substitute for extreme intelligence.Somebody smart enough to spell "persistence" correctly might have found the problem faster.> This might work for startup: > > https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1You do need capacitors to ground before the inductors.> The laptop supply grunts as much as necessary to get the +5D (D=dirty) load up. When +5 and +24 are stable, the MAX809 waits a bit and enables the Cuk converter, which soft starts.Whatever. -- Bill Sloman, Sydney
Reply by ●March 13, 20222022-03-13
On Sat, 12 Mar 2022 13:51:33 -0800, jlarkin@highlandsniptechnology.com wrote: <snip>> >The 40 MHz ringing is in fact the coax up to the scope. If I add a DC >block and set the scope to 50 ohms, the ring goes away. What I see >then is a pretty nasty impulse with a bit of 170 MHz ring; that's what >was shocking the coax. > >https://www.dropbox.com/s/n6qcuzr752c7yqo/Z532_noise_50r.jpg?raw=1 > >It's only about 15 mV p-p. Probably the current turn-on into the catch >diode makes this noise. It's small enough that I shouldn't complain. > >Of course the DC block wrecks the low frequency response. I should >hack a giant blocking cap and a source terminator onto my board, and >run the scope hi-z again. > >Electronics is fun. You get so many puzzles to solve. Poirot had it >easy in comparison. > > >The 400 MHz ring from the LTM8078 is very real. We can see that >everywhere in our box.There's also a lower frequency concideration where the two coax pick-offs exit the board at opposite ends, then connect to the same (or connected) monitoring instrument(s). The two coax lines form a loop, that will generate antiphase readings, as common-mode shield current is seen as dif mode by the monitor. So, compare the two coax to see if the 'bump' is visible in both, inverted. RL
Reply by ●March 13, 20222022-03-13
On Sat, 12 Mar 2022 13:30:52 -0800, jlarkin@highlandsniptechnology.com wrote:>On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>jlarkin@highlandsniptechnology.com wrote: >>> On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown >>> <'''newspam'''@nonad.co.uk> wrote: >>> >>>> On 11/03/2022 21:12, John Larkin wrote: >>>>> On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> I used to love the LTM8078 dual switcher module. But it rings hard at >>>>>>> around 400 MHz at every switch transition. This is called a "Silent >>>>>>> Switcher!" >>>>>>> >>>>>>> I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576. >>>>>>> It switches at 50 KHz. And at every switching edge, it rings at about >>>>>>> 40 MHz. >>>>>>> >>>>>>> We tried all sorts of stuff on both switchers. Nothing so far has any >>>>>>> effect on the ringing frequency. >>>>>>> >>>>>>> https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0 >>>>>>> >>>>>>> The damper on the 2576 circuit reduces ring amplitude a little. >>>>>>> >>>>>>> >>>>>>> Maybe all switchers do this! >>>>>> >>>>>> In discontinuous current mode, an asynchronous switcher will produce EMI >>>>>> at the free resonance of the inductor. If you don't mind the >>>>>> efficiency hit at low current, a diode + RC snubber would probably fix it. >>>>> >>>>> The LTM is a synchronous switcher, and my 2576 is running continuous. >>>>> >>>>> Looking at the timings on by breadboard, the rings seem to start at >>>>> the big di/dt current transitions in the schottky. But nothing we can >>>>> do changes the ring frequency, so what's resonating? >>>> >>>> They will be immediately after the discontinuity aka Gibb's phenomena on >>>> a truncated Fourier expansion for a square wave. It may not be a >>>> resonance as such but a side effect of the slew rate limit of the >>>> device. It doesn't die away quickly enough to be just that though. >>>> >>>> There is a hard high frequency cutoff in gain and some ringing is pretty >>>> much what you would expect on a square wave with a truncated Fourier >>>> expansion. It may be being exaggerated in time and amplitude by some >>>> unfortunate choice of component values providing Q > 1 in addition. >>>> >>>> As Phil said some sort of snubber would be the most likely amelioration. >>>> There will be an efficiency hit though so you have to choose how quiet >>>> you need it vs what losses you can live with. >>> >>> There is an RC snubber to ground... see my schematic. The R value is >>> about optimized, and the overall effect is a very modest reduction in >>> the ringing amplitude, no visible effect on the ring frequency or Q. >>> >>> I can find only one thing that has any effect on the ringing >>> frequency: the +24 input voltage. Higher voltage results in a very >>> slight increase in ring frequency. >>> >>> It's Saturday, but I might go in and play with it for a couple more >>> hours. I need to be in that part of town anyhow. It's better commute >>> on Saturday. >>> >>> It's probably good enough, with layout improvements and secondary >>> filtering, but it's interesting and annoying. >>> >>> Next issue is soft-starting this old beast, so the system always comes >>> up. The 24v supply will be a wart type thing. We'll have a Cuk >>> converter to make +24 into -5, and that chip soft starts. My part, +24 >>> to +5, doesn't. >>> >>> I could let the Cuk start up, sense its output, and then start up my >>> LM2576... somehow. The "enable" pin is just on/off, so any soft start >>> would probably involve the fb pin. Nuisance. >> >>Another approach is to precharge the output cap before enabling the >>switcher. >> > >That would be just as bad as letting the thing just grunt. > >The laptop-type supply is rated 24v and 65 watts. If it's shorted, it >makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore >the switcher startup, on the theory that the supply can brute-force >the load up to +5, and then the switcher will start to switch. > >Laptop type supplies must be designed to pull up nasty loads.440MHz radios needed shields, preventing EMI was hard even then. I wonder how large is your coil diode system. At 400Mhz even short wires have impedance.
