On Saturday, March 12, 2022 at 2:55:30 PM UTC-5, Joe Gwinn wrote:
> On Sat, 12 Mar 2022 13:49:10 -0500, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
> >Joe Gwinn wrote:
> >> On Fri, 11 Mar 2022 18:22:42 -0500, Phil Hobbs
> >> <pcdhSpamM...@electrooptical.net> wrote:
> >>
> >>> John Larkin wrote:
> >>>> On Fri, 11 Mar 2022 20:38:18 GMT, Jan Panteltje
> >>>> <pNaonSt...@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
> >>>>> <h58n2h1ssfbd3enfc...@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.
> >>
> >> It can be dicey to feed one switcher directly from another. The power
> >> conversion folk do know how to do this, but it requires using a spice
> >> model encompassing both switchers and the cabling and filter stuff
> >> between, as well as the loads. LTspice is what they generally use.
> >>
> >> Nor would I be surprised if the switchers were interacting with one
> >> another such that their switching frequencies adjusted (by injection
> >> locking) to be in some small-integer rational ratio to one another.
> >>
> >>
> >>> 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.
> >>
> >> If I recall, powered iron toroids have some internal damping, which
> >> will control ringing. As others have said, I'm thinking that what is
> >> bedeviling Larkin may be coil self-resonance.
> >
> >Yup. They get pretty toasty at 2 MHz, for sure.
> >
> >>> 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.)
> >>
> >> In my experience, what is mostly done these days in power supplies for
> >> low phase noise electronics is a pair of regulators before the
> >> sensitive electronics. The first regulator (a switcher) drops the
> >> voltage to almost the final output voltage (and inverts the polarity
> >> if needed). The second regulator (analog) brings the voltage down to
> >> the voltage needed by the sensitive electronics. There are low-pass
> >> and EMI filters as needed before and after the switcher, and after the
> >> analog regulator. And, the design is verified by LTspice before
> >> prototyping.
> >
> >We generally use cap multipliers right on the switcher outputs. With
> >two poles in the base circuit and one in the collector, you can get ~140
> >dB suppression in one stage at SMPS frequencies. Regulators won't get
> >into that territory.
> I don't recall people using cap multipliers. I'm sure that the power
> supply folk know of such things, so there must be a reason. I will
> ask around when I can.
I finally figured out why cap multipliers are useful compared to using an op amp and a reference. The op amp frequency response is lower than a single transistor. The cap multiplier has limitations that an op amp and voltage reference don't have, but the transistor can be faster. Both designs require a drop out voltage, so can dissipate significant heat.
> It's hard to achieve 140 dB in one stage (well, circuit board), due to
> sneak leakage paths et al, so injection locking may be able to work
> despite a 140 dB theoretical path loss. About 85 dB is more like it.
> >>> 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.
> >>
> >> I believe it. I've had the same experience with people trying to
> >> estimate the temperature of a transistor junction from six inches
> >> away. (Insert standard joke about drunk looking for car keys under
> >> the light.) The fix was to insist on a thermocouple glued to the AlN
> >> spacer between transistor casa and heat sink. Not perfect, but orders
> >> of magnitude better, cutting tail-chasing by a like ratio.
> >
> >Yup. For testing I've been known to fuse the thermocouple into a
> >heatsink using one of those big crude $150 transformer-based spot
> >welders. Dramatically better thermal contact than using epoxy!
> That would certainly do it, as would capacitor-discharge welding of TC
> wires to said heat sink. But couldn't do that without destroying the
> circuitry being debugged. What was used was silver-loaded epoxy.
When people talk about such dramatic improvements in one aspect of a design, I have to wonder how much difference it makes in performance. The overclockers use all manner of thermal paste when the thermal resistance of the few micron thick layer of paste has a lower thermal resistance than the metal of the heat sink because it is a much longer path. If you are going to optimize, why try to optimize the part that has the tiniest impact on the result?
--
Rick C.
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