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

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

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?

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

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

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

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 Hobbs

We 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

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 !

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

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?

On Friday, March 11, 2022 at 10:29:57 PM UTC+1, John Larkin wrote:
> But what's resonating? It doesn't seem to be the pcb itself.

It is likely that the parasitic inductance of the diode/high-side switch (L_par) resonates with the switching node capacitance (C_par). In order that your dampening network works the resistors must be about R=sqrt(L_par/C_par). My gut feeling is, that your dampening resistor is too high to work sufficiently. At typical low inductance layouts one lands in the range of 2....10R for the damping resistors these days. If the resistor is chosen too high or too low the effect on the ringing is very small.

Best regards
Stefan

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.

--

I yam what I yam - Popeye

On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin
<jlarkin@highland_atwork_technology.com> 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!

Check noise effect when scope probe/ground lead is
removed/replaced/manipulated. Above 20mhZ, it's going
to be radiated.

More ground bonds to PC ground backing near IC, on both ground
plane edges, where cut by power train.

Move your ceramic decoupling caps closer to the IC body tab.
Shuffle the polymer/ceramic positions, so both work in tandem.

Same with schottky and it's snubber. Take output gound out of
switching current loop.

SchottKy RC R too big? small? Cap on flying node - R to ground plane.

Move Noise monitors closer to filtered nodes, or filtered nodes
closer to noise monitors. Bare leads feeding sheilded coax? I arsk
yer!

RL

On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:

>On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin
><jlarkin@highland_atwork_technology.com> 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!
>
>
>
>Check noise effect when scope probe/ground lead is
>removed/replaced/manipulated. Above 20mhZ, it's going
>to be radiated.

The input and output monitors are coax. I'm monitoring the switch node
with a 10x scope probe. Removing the probe has no effect on the 40 MHz
ring on the output.

>
>More ground bonds to PC ground backing near IC, on both ground
>plane edges, where cut by power train.

The bottom is all ground. Various jumpers/plier grabs/kluges to the
ground have zero effect on the ring.

>
>Move your ceramic decoupling caps closer to the IC body tab.
>Shuffle the polymer/ceramic positions, so both work in tandem.
>
>Same with schottky and it's snubber. Take output gound out of
>switching current loop.
>
>SchottKy RC R too big? small? Cap on flying node - R to ground plane.

Different schottkies, or parallel schottkies, have no effect.

I don't think swapping the RC in the damper would affect 40 MHz.

>
>Move Noise monitors closer to filtered nodes, or filtered nodes
>closer to noise monitors. Bare leads feeding sheilded coax? I arsk
>yer!

An inch of wire flat on a ground plane won't have any effect at 40
MHz. It's only a 500 MHz scope.

The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
50r with an external DC block. The cables might be ringing. Or I can
change cable lengths and see what happens. It would be great if the
mysterious ringing is the cables, but it feels unlikely.

>
>RL

--

I yam what I yam - Popeye

On Saturday, March 12, 2022 at 10:22:01 AM UTC-5, jla...@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
> >> <pcdhSpamM...@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.

If the oscillations are in the coil self-resonance wouldn't it make sense to put the snubber across the coil? Or put one on each end of the coil? Snubbing one end of the coil to ground isn't going to stop the current or voltage of the ringing across the coil.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

Martin Brown 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.

You don't get Gibbs' ears on just any square wave--you have to use the
wrong filter. ;)

> 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.

There isn't, though. It's just a MOSFET, two poles at most.

> 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.

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

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.

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

jlarkin@highlandsniptechnology.com wrote:
> 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 Hobbs
>
> We 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.

Not all wall warts start up nicely into BFCs, though. We ship a
beautiful one with our gizmos, but 50 000 uF will make it misbehave.

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

Joe Gwinn wrote:
> 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.
>
> 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.

>> 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!

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

Joe Gwinn wrote:
> On Sat, 12 Mar 2022 13:49:10 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> Joe Gwinn wrote:
>>> 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.
>>>
>>> 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.
>
> 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.

The theoretical loss is even larger. It's very difficult to make
advanced discrete front ends without them, because single-ended
amplifier circuitry doesn't have much in the way of supply rejection.

With a switcher whose output ripple is 100 mV, 140 dB gets you down to
10 nV. That's easily visible on a spectrum analyzer, especially if you
apply it to one end of the photodiode whose other end goes to your
sub-nanovolt TIA.

If the suppression were only 85 dB, I'd need two of them in cascade.
(I've done that on occasion--four poles, two transistors.)

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

On Sat, 12 Mar 2022 08:18:34 -0800, jlarkin@highlandsniptechnology.com
wrote:

>On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:
>
>>On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin
>><jlarkin@highland_atwork_technology.com> 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!
>>
>>
>>
>>Check noise effect when scope probe/ground lead is
>>removed/replaced/manipulated. Above 20mhZ, it's going
>>to be radiated.
>
>The input and output monitors are coax. I'm monitoring the switch node
>with a 10x scope probe. Removing the probe has no effect on the 40 MHz
>ring on the output.
>
>>
>>More ground bonds to PC ground backing near IC, on both ground
>>plane edges, where cut by power train.
>
>The bottom is all ground. Various jumpers/plier grabs/kluges to the
>ground have zero effect on the ring.
>
>>
>>Move your ceramic decoupling caps closer to the IC body tab.
>>Shuffle the polymer/ceramic positions, so both work in tandem.
>>
>>Same with schottky and it's snubber. Take output gound out of
>>switching current loop.
>>
>>SchottKy RC R too big? small? Cap on flying node - R to ground plane.
>
>Different schottkies, or parallel schottkies, have no effect.
>
>I don't think swapping the RC in the damper would affect 40 MHz.
>
>>
>>Move Noise monitors closer to filtered nodes, or filtered nodes
>>closer to noise monitors. Bare leads feeding sheilded coax? I arsk
>>yer!
>
>An inch of wire flat on a ground plane won't have any effect at 40
>MHz. It's only a 500 MHz scope.
>
>The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
>50r with an external DC block. The cables might be ringing. Or I can
>change cable lengths and see what happens. It would be great if the
>mysterious ringing is the cables, but it feels unlikely.
>

The photo doesn't show much effective ground plane stitching.
Loop length from the SBD, through the board, to IC contacts is
pretty dicey, where SBD turn-off current is expected to flow.

Check a reactance chart, looking at the 40MHz line for familiar
layout and component values. You started with a 400MHz ringing
and stepped backwards to see 40MHz in earlier work. You have to
ask yourself, as well, just how much it matters in your application.
It helps if there's an actual problem that needs solving.

Probing has its own issues. Can the probe produce the coax waveform?
What do the ground plane points look like on the scope probe?
Do the coax outputs shift when the scope probe shifts or the
scope probe (plus ground probe) is removed?

Does the coax output shift as you finger certain components?
Choke bodies can be screened and grounded, if they carry a
lot of noise for re-radiation.

Traces that connect to the measurement point, without local
series impedance, can also act as a pick-up to pump current
into local non-ideal decouplers. Sense lines are the most
frequently ignored.

I scanned an old bundle of paper that sometimes saves time
wasted googling crap. It's a big file just because that's
what the scanner pumped out, so it won't be mounted for long.
Page 3 (?) is useful for checking ground plane and wide trace
L/W ratios for L.

http://ve3ute.ca/query/Trace_resistance_inductance.pdf

RL

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.

--

I yam what I yam - Popeye

On Sat, 12 Mar 2022 16:09:26 -0500, legg <legg@nospam.magma.ca> wrote:

>On Sat, 12 Mar 2022 08:18:34 -0800, jlarkin@highlandsniptechnology.com
>wrote:
>
>>On Sat, 12 Mar 2022 10:32:51 -0500, legg <legg@nospam.magma.ca> wrote:
>>
>>>On Fri, 11 Mar 2022 11:39:10 -0800, John Larkin
>>><jlarkin@highland_atwork_technology.com> 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!
>>>
>>>
>>>
>>>Check noise effect when scope probe/ground lead is
>>>removed/replaced/manipulated. Above 20mhZ, it's going
>>>to be radiated.
>>
>>The input and output monitors are coax. I'm monitoring the switch node
>>with a 10x scope probe. Removing the probe has no effect on the 40 MHz
>>ring on the output.
>>
>>>
>>>More ground bonds to PC ground backing near IC, on both ground
>>>plane edges, where cut by power train.
>>
>>The bottom is all ground. Various jumpers/plier grabs/kluges to the
>>ground have zero effect on the ring.
>>
>>>
>>>Move your ceramic decoupling caps closer to the IC body tab.
>>>Shuffle the polymer/ceramic positions, so both work in tandem.
>>>
>>>Same with schottky and it's snubber. Take output gound out of
>>>switching current loop.
>>>
>>>SchottKy RC R too big? small? Cap on flying node - R to ground plane.
>>
>>Different schottkies, or parallel schottkies, have no effect.
>>
>>I don't think swapping the RC in the damper would affect 40 MHz.
>>
>>>
>>>Move Noise monitors closer to filtered nodes, or filtered nodes
>>>closer to noise monitors. Bare leads feeding sheilded coax? I arsk
>>>yer!
>>
>>An inch of wire flat on a ground plane won't have any effect at 40
>>MHz. It's only a 500 MHz scope.
>>
>>The scope is hi-Z. It won't allow 50r and AC coupling. I might go to
>>50r with an external DC block. The cables might be ringing. Or I can
>>change cable lengths and see what happens. It would be great if the
>>mysterious ringing is the cables, but it feels unlikely.
>>
>
>The photo doesn't show much effective ground plane stitching.
>Loop length from the SBD, through the board, to IC contacts is
>pretty dicey, where SBD turn-off current is expected to flow.
>
>Check a reactance chart, looking at the 40MHz line for familiar
>layout and component values. You started with a 400MHz ringing
>and stepped backwards to see 40MHz in earlier work. You have to
>ask yourself, as well, just how much it matters in your application.
>It helps if there's an actual problem that needs solving.
>
>Probing has its own issues. Can the probe produce the coax waveform?
>What do the ground plane points look like on the scope probe?
>Do the coax outputs shift when the scope probe shifts or the
>scope probe (plus ground probe) is removed?
>
>Does the coax output shift as you finger certain components?
>Choke bodies can be screened and grounded, if they carry a
>lot of noise for re-radiation.
>
>Traces that connect to the measurement point, without local
>series impedance, can also act as a pick-up to pump current
>into local non-ideal decouplers. Sense lines are the most
>frequently ignored.
>
>I scanned an old bundle of paper that sometimes saves time
>wasted googling crap. It's a big file just because that's
>what the scanner pumped out, so it won't be mounted for long.
>Page 3 (?) is useful for checking ground plane and wide trace
>L/W ratios for L.
>
>http://ve3ute.ca/query/Trace_resistance_inductance.pdf
>
>RL
>
>
>
>

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.

--

I yam what I yam - Popeye

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.

You don't have to do it like a wildman. ;)

Something like a 78L05 with a MOSFET on its output would charge it up
quickly and then go away.

>
> 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.

I've built a fair number of POC systems powered by random old laptop bricks.

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

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