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! -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
switcher ringing noise
Started by ●March 11, 2022
Reply by ●March 11, 20222022-03-11
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. We've started putting U.FL coax jacks on all out power supply outputs, so we can figure out what's conducted and what's pickup. Helps a lot. (They don't get populated except on first articles, but they're the size of a SOT23, so nobody cares.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●March 11, 20222022-03-11
On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>:>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!Is the 10 nF 30 Ohm parallel to the diode a damping network? Use a series LC there tuned to 50 kHz to short it? That said I do not rememebr those oscillations tried a different make inductor?
Reply by ●March 11, 20222022-03-11
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?> >We've started putting U.FL coax jacks on all out power supply outputs, >so we can figure out what's conducted and what's pickup. Helps a lot. >(They don't get populated except on first articles, but they're the size >of a SOT23, so nobody cares.)That's a good idea. We should also always include some way to measure currents. We have some cute little loop-antenna scope probes. We can park one above an LTM module and get a scope trigger from its internal inductor, then signal average our noisy signals and see which noise correlates to which switcher.> >Cheers > >Phil Hobbs-- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●March 11, 20222022-03-11
On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:>On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin ><jlarkin@highland_atwork_technology.com> wrote in ><h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>: > >>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! > >Is the 10 nF 30 Ohm parallel to the diode a damping network?Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1.>Use a series LC there tuned to 50 kHz to short it?The problem isn't at 50 KHz, it's the fast ringing on both switching edges.> >That said I do not rememebr those oscillations >tried a different make inductor?This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 MHz. It's beyond the frequency ranges of the visible components. I guess we'll dump the LTM things and go with old, slow switchers, and then try to physically segregate them as much as possible, and add a lot of secondary filtering. Create clean and dirty zones on the board, draw a boundary line, and filter the power sigs that cross the line. That might work better for small 40 MHz nasties than for big 400s. But what's resonating? It doesn't seem to be the pcb itself. I thought we might have a guard-ring-SRD snap in the schottky diode, but any diode does it, and it rings on both switching edges. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●March 11, 20222022-03-11
John Larkin wrote:> On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje > <pNaonStpealmtje@yahoo.com> wrote: > >> On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >> <jlarkin@highland_atwork_technology.com> wrote in >> <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>: >> >>> 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! >> >> Is the 10 nF 30 Ohm parallel to the diode a damping network? > > Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1. > >> Use a series LC there tuned to 50 kHz to short it? > > The problem isn't at 50 KHz, it's the fast ringing on both switching > edges. > >> >> That said I do not rememebr those oscillations >> tried a different make inductor? > > This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 > MHz. It's beyond the frequency ranges of the visible components. > > I guess we'll dump the LTM things and go with old, slow switchers, and > then try to physically segregate them as much as possible, and add a > lot of secondary filtering. Create clean and dirty zones on the board, > draw a boundary line, and filter the power sigs that cross the line. > That might work better for small 40 MHz nasties than for big 400s. > > But what's resonating? It doesn't seem to be the pcb itself. > > I thought we might have a guard-ring-SRD snap in the schottky diode, > but any diode does it, and it rings on both switching edges. > >I hear you. Awhile back we did a small power supply board, in an effort to factor out the noisy stuff and put it inside a shield, so that we could concentrate on what we care about. It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24, which was then inverted by an AOZ1282 to make -16. The other rails were made using linears off those ones or off the +24 directly. (Making -16 from +24 is a bit of a strain for most integrated buck regulator chips that can go faster than 2 MHz.) It worked fine until we turned on the AOZ1282, at which point the whole board became a mass of VHF uglies. The thing was, everything was some high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected by microstrip stub resonances in the traces. We had 118 MHz ringing here, 183 MHz there, all initially very mysterious. Never did work right. We've had good success with the 150 kHz Simple Switchers, e.g. the LM2594, using powdered-iron toroids and B340A Schottky catch diodes. Our QL01 nanowatt photoreceiver has one of those within a couple of inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the switching junk is invisible on the output even using a spectrum analyzer with a 10-Hz resolution bandwidth. But even that one has issues with ground integrity--if the board doesn't make good contact with the box ground, low-level harmonics of 150 kHz start showing up. At this point we've decided we don't want to be power supply designers, so we use the 2W Murata gizmos with the embedded toroids, inside a board-level steel shield, with the whole works inside a brass or aluminum box with a laser-cut lid. (Laser cutting has recently become monstrous cheap--we pay about $2 per lid in quantity 10, with four-day turnaound.) Those U.FL connectors are super useful in distinguishing between stuff that our boards are doing and stuff that comes in over the air. The amount of tail-chasing they save is astronomical. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●March 11, 20222022-03-11
On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje >> <pNaonStpealmtje@yahoo.com> wrote: >> >>> On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >>> <jlarkin@highland_atwork_technology.com> wrote in >>> <h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>: >>> >>>> 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! >>> >>> Is the 10 nF 30 Ohm parallel to the diode a damping network? >> >> Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1. >> >>> Use a series LC there tuned to 50 kHz to short it? >> >> The problem isn't at 50 KHz, it's the fast ringing on both switching >> edges. >> >>> >>> That said I do not rememebr those oscillations >>> tried a different make inductor? >> >> This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 >> MHz. It's beyond the frequency ranges of the visible components. >> >> I guess we'll dump the LTM things and go with old, slow switchers, and >> then try to physically segregate them as much as possible, and add a >> lot of secondary filtering. Create clean and dirty zones on the board, >> draw a boundary line, and filter the power sigs that cross the line. >> That might work better for small 40 MHz nasties than for big 400s. >> >> But what's resonating? It doesn't seem to be the pcb itself. >> >> I thought we might have a guard-ring-SRD snap in the schottky diode, >> but any diode does it, and it rings on both switching edges. >> >> > >I hear you. > >Awhile back we did a small power supply board, in an effort to factor >out the noisy stuff and put it inside a shield, so that we could >concentrate on what we care about. > >It used a TI LMR23630AFDDAR (clocked at 2.15 MHz) to make +13 from +24, >which was then inverted by an AOZ1282 to make -16. The other rails were >made using linears off those ones or off the +24 directly. (Making -16 >from +24 is a bit of a strain for most integrated buck regulator chips >that can go faster than 2 MHz.) > >It worked fine until we turned on the AOZ1282, at which point the whole >board became a mass of VHF uglies. The thing was, everything was some >high harmonic of the 2.15 MHz clock synchronizing the TI chip, selected >by microstrip stub resonances in the traces. We had 118 MHz ringing >here, 183 MHz there, all initially very mysterious. Never did work right. > >We've had good success with the 150 kHz Simple Switchers, e.g. the >LM2594, using powdered-iron toroids and B340A Schottky catch diodes. >Our QL01 nanowatt photoreceiver has one of those within a couple of >inches of a very sensitive 10 megohm TIA with a 1 MHz BW, and the >switching junk is invisible on the output even using a spectrum analyzer >with a 10-Hz resolution bandwidth. But even that one has issues with >ground integrity--if the board doesn't make good contact with the box >ground, low-level harmonics of 150 kHz start showing up. > >At this point we've decided we don't want to be power supply designers, >so we use the 2W Murata gizmos with the embedded toroids, inside a >board-level steel shield, with the whole works inside a brass or >aluminum box with a laser-cut lid. (Laser cutting has recently become >monstrous cheap--we pay about $2 per lid in quantity 10, with four-day >turnaound.) > >Those U.FL connectors are super useful in distinguishing between stuff >that our boards are doing and stuff that comes in over the air. The >amount of tail-chasing they save is astronomical. > >Cheers > >Phil HobbsWe might make provision for one of those Laird shield boxes, just in case. The next challange is to soft-start the +24 to +5 switcher. Those old parts just grunt at startup. The +24 supply has to deliver 1 amp to pull up a 1 amp load. The LM2576 has an enable pin, but it's not a soft start. A time delay and huge amount of bulk capacitance on +24 is one way to do it. I have, I think, seven various goofy ideas for sorta or actually soft-starting this beast. Without a Spice model, I'll just have to try them. -- I yam what I yam - Popeye
Reply by ●March 12, 20222022-03-12
On a sunny day (Fri, 11 Mar 2022 13:29:42 -0800) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in <oren2h57dsrckacrocvdu0u9vhlm7v3dta@4ax.com>:>On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje ><pNaonStpealmtje@yahoo.com> wrote: > >>On a sunny day (Fri, 11 Mar 2022 11:39:10 -0800) it happened John Larkin >><jlarkin@highland_atwork_technology.com> wrote in >><h58n2h1ssfbd3enfcd2500eauvoi1fu8tn@4ax.com>: >> >>>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! >> >>Is the 10 nF 30 Ohm parallel to the diode a damping network? > >Yes. It reduces the 40 MHz ring amplitude a bit, but not 2:1. > >>Use a series LC there tuned to 50 kHz to short it? > >The problem isn't at 50 KHz, it's the fast ringing on both switching >edges.Yes of course I did mean that 40 MHz>> >>That said I do not rememebr those oscillations >>tried a different make inductor? > >This wouldn't normally be noticed. It's tens of mV rings at 40 or 400 >MHz. It's beyond the frequency ranges of the visible components. > >I guess we'll dump the LTM things and go with old, slow switchers, and >then try to physically segregate them as much as possible, and add a >lot of secondary filtering. Create clean and dirty zones on the board, >draw a boundary line, and filter the power sigs that cross the line. >That might work better for small 40 MHz nasties than for big 400s. > >But what's resonating? It doesn't seem to be the pcb itself.It is likely the coil !
Reply by ●March 12, 20222022-03-12
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. -- Regards, Martin Brown
Reply by ●March 12, 20222022-03-12
It is likely the coil ! Look at the coil construction, it is much like a 30 MHz or there about air coil I often use. Probably at that high MHz frequency the far away core material is ignored by them electrons. Leaves turns and circuit capacitance for tuning. I use these kind of coils, less spacing between winding and core material: http://panteltje.com/pub/LM2596_3.3_as_current_source_test_setup_2_IMG_5225.JPG I could be wrong, give it a try?
