switcher ringing noise...

[Joe Gwinn]

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.

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.

Joe Gwinn

 

[Phil Hobbs]

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

 

[legg]

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

 

[server]

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

 

[server]

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

 

[Phil Hobbs]

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

 

[sea moss]

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

 

[Rick C]

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.

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

 

[server]

On Sat, 12 Mar 2022 17:17:21 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

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.

There\'s still the load. Disconnecting the load and charging the cap
won\'t help much. As soon as the load is connected, the output cap will
collapse, the switcher will go to 100% duty cycle, the input current
will equal the load current, and there we are.

Besides that, the 7805 and the mosfet and the timing would be a
nuisance.

If the 24 supply can provide 1.5 amps, the max 5v load current, it
will just work. I think. The switcher *is* the linear regulator during
startup!

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.

Right. My concerns are probably silly, given that things like this
usually work.



--

I yam what I yam - Popeye

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Sat, 12 Mar 2022 17:17:21 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

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.

There\'s still the load. Disconnecting the load and charging the cap
won\'t help much. As soon as the load is connected, the output cap will
collapse, the switcher will go to 100% duty cycle, the input current
will equal the load current, and there we are.

Don\'t think so. Simple switchers do everything cycle-by-cycle, so if it
doesn\'t collapse during normal operation, it shouldn\'t do it on the
first cycle either.

Besides that, the 7805 and the mosfet and the timing would be a
nuisance.

Sure. It\'s just a possible alternative.

If the 24 supply can provide 1.5 amps, the max 5v load current, it
will just work. I think. The switcher *is* the linear regulator during
startup!

Sort of.

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

 

[server]

On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss
<danluster81@gmail.com> wrote:

A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor\'s winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

Have you ever tried these \"amobead\" parts? Might try one in series with the freewheeling diode.

https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

I think we want the switch node to have minimum activity, so low diode
impedance might be best. I tried a fb in series with the switcher
output pin and it was horrible. As I discovered today, the 40 MHz ring
was actually the coax to the scope. Oops. Dogged persistance is a
workable substitute for extreme intelligence.

This might work for startup:

https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1

The laptop supply grunts as much as necessary to get the +5D (D=dirty)
load up. When +5 and +24 are stable, the MAX809 waits a bit and
enables the Cuk converter, which soft starts.



--

I yam what I yam - Popeye

 

[Anthony William Sloman]

On Sunday, March 13, 2022 at 10:37:17 AM UTC+11, sea moss wrote:

A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor\'s winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

Have you ever tried these \"amobead\" parts? Might try one in series with the freewheeling diode.

https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

Beads are nonwound components, and have appreciably lower parallel capacitance. The Toshiba link also covers wound \"spike killers\" which have a couple of turns of wire on a toroid, which typically means about 1pF of parallel capacitance, although their text doesn\'t seem to mentions this..

LTSpice lists a great many Würth Elektronik GmbH & Co. ferrite chip parts - not all of them by any means.

https://www.we-online.com/catalog/en/pbs/emc_components/ferrites_for_pcb_assembly

It does make it easy to play around in a simulation.

--
Bill Sloman, Sydney

 

[Anthony William Sloman]

On Sunday, March 13, 2022 at 12:17:23 PM UTC+11, jla...@highlandsniptechnology.com wrote:

On Sat, 12 Mar 2022 15:37:09 -0800 (PST), sea moss
danlu...@gmail.com> wrote:

A ferrite bead in series with the buck inductor might be worth trying. If the SMPS spike is using the inductor\'s winding capacitance as a conduction path, then the ferrite bead should definitely make a difference in that 10-100MHz range.

Have you ever tried these \"amobead\" parts? Might try one in series with the freewheeling diode.

https://www.toshiba-tmat.co.jp/pdf/en/product/3-1_am_parts_absse.pdf

I think we want the switch node to have minimum activity, so low diode impedance might be best. I tried a fb in series with the switcher output pin and it was horrible.

You do have to provide an alternative (capacitative) path to ground for the high frequency current that the ferrite bead is supposed to block. Without that the ferrite bead just produces a larger voltage spike at the output of the switcher, which isn\'t helpful.

> As I discovered today, the 40 MHz ring was actually the coax to the scope.. Oops. Dogged persistance is a workable substitute for extreme intelligence.

Somebody smart enough to spell \"persistence\" correctly might have found the problem faster.

This might work for startup:

https://www.dropbox.com/s/bhfr9yod5261t8e/T501_Sw_4.jpg?raw=1

You do need capacitors to ground before the inductors.

> The laptop supply grunts as much as necessary to get the +5D (D=dirty) load up. When +5 and +24 are stable, the MAX809 waits a bit and enables the Cuk converter, which soft starts.

Whatever.

--
Bill Sloman, Sydney

 

[legg]

On Sat, 12 Mar 2022 13:51:33 -0800, jlarkin@highlandsniptechnology.com
wrote:

<snip>

The 40 MHz ringing is in fact the coax up to the scope. If I add a DC
block and set the scope to 50 ohms, the ring goes away. What I see
then is a pretty nasty impulse with a bit of 170 MHz ring; that\'s what
was shocking the coax.

https://www.dropbox.com/s/n6qcuzr752c7yqo/Z532_noise_50r.jpg?raw=1

It\'s only about 15 mV p-p. Probably the current turn-on into the catch
diode makes this noise. It\'s small enough that I shouldn\'t complain.

Of course the DC block wrecks the low frequency response. I should
hack a giant blocking cap and a source terminator onto my board, and
run the scope hi-z again.

Electronics is fun. You get so many puzzles to solve. Poirot had it
easy in comparison.


The 400 MHz ring from the LTM8078 is very real. We can see that
everywhere in our box.

There\'s also a lower frequency concideration where the two coax
pick-offs exit the board at opposite ends, then connect to the
same (or connected) monitoring instrument(s). The two coax lines
form a loop, that will generate antiphase readings, as common-mode
shield current is seen as dif mode by the monitor.

So, compare the two coax to see if the \'bump\' is visible in both,
inverted.

RL

 

[LM]

On Sat, 12 Mar 2022 13:30:52 -0800, jlarkin@highlandsniptechnology.com
wrote:

On Sat, 12 Mar 2022 13:05:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Sat, 12 Mar 2022 09:05:02 +0000, Martin Brown
\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 11/03/2022 21:12, John Larkin wrote:
On Fri, 11 Mar 2022 15:35:08 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
I used to love the LTM8078 dual switcher module. But it rings hard at
around 400 MHz at every switch transition. This is called a \"Silent
Switcher!\"

I breadboarded a 24-to-5 volt switcher with an ancient bipolar LM2576.
It switches at 50 KHz. And at every switching edge, it rings at about
40 MHz.

We tried all sorts of stuff on both switchers. Nothing so far has any
effect on the ringing frequency.

https://www.dropbox.com/sh/ly0hfcysz13pi89/AAAiXJd3dHAQyg_Ga-OxFJb2a?dl=0

The damper on the 2576 circuit reduces ring amplitude a little.


Maybe all switchers do this!

In discontinuous current mode, an asynchronous switcher will produce EMI
at the free resonance of the inductor. If you don\'t mind the
efficiency hit at low current, a diode + RC snubber would probably fix it.

The LTM is a synchronous switcher, and my 2576 is running continuous.

Looking at the timings on by breadboard, the rings seem to start at
the big di/dt current transitions in the schottky. But nothing we can
do changes the ring frequency, so what\'s resonating?

They will be immediately after the discontinuity aka Gibb\'s phenomena on
a truncated Fourier expansion for a square wave. It may not be a
resonance as such but a side effect of the slew rate limit of the
device. It doesn\'t die away quickly enough to be just that though.

There is a hard high frequency cutoff in gain and some ringing is pretty
much what you would expect on a square wave with a truncated Fourier
expansion. It may be being exaggerated in time and amplitude by some
unfortunate choice of component values providing Q > 1 in addition.

As Phil said some sort of snubber would be the most likely amelioration.
There will be an efficiency hit though so you have to choose how quiet
you need it vs what losses you can live with.

There is an RC snubber to ground... see my schematic. The R value is
about optimized, and the overall effect is a very modest reduction in
the ringing amplitude, no visible effect on the ring frequency or Q.

I can find only one thing that has any effect on the ringing
frequency: the +24 input voltage. Higher voltage results in a very
slight increase in ring frequency.

It\'s Saturday, but I might go in and play with it for a couple more
hours. I need to be in that part of town anyhow. It\'s better commute
on Saturday.

It\'s probably good enough, with layout improvements and secondary
filtering, but it\'s interesting and annoying.

Next issue is soft-starting this old beast, so the system always comes
up. The 24v supply will be a wart type thing. We\'ll have a Cuk
converter to make +24 into -5, and that chip soft starts. My part, +24
to +5, doesn\'t.

I could let the Cuk start up, sense its output, and then start up my
LM2576... somehow. The \"enable\" pin is just on/off, so any soft start
would probably involve the fb pin. Nuisance.

Another approach is to precharge the output cap before enabling the
switcher.


That would be just as bad as letting the thing just grunt.

The laptop-type supply is rated 24v and 65 watts. If it\'s shorted, it
makes a 100 ms 9 amp pulse about once a second. So maybe I can ignore
the switcher startup, on the theory that the supply can brute-force
the load up to +5, and then the switcher will start to switch.

Laptop type supplies must be designed to pull up nasty loads.

440MHz radios needed shields, preventing EMI was hard even then.
I wonder how large is your coil diode system. At 400Mhz even short
wires have impedance.