switcher ringing noise...

[John Larkin]

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

 

[Phil Hobbs]

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

 

[Jan Panteltje]

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?

 

[John Larkin]

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

 

[John Larkin]

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

 

[Phil Hobbs]

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

 

[server]

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

 

[Jan Panteltje]

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 !

 

[Martin Brown]

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

 

[Jan Panteltje]

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?

 

[Stefan Schidl]

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

 

[server]

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

 

[legg]

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

 

[server]

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

 

[Rick C]

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

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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]

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.

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.

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.


Joe Gwinn

 

[Phil Hobbs]

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