I'm designing a switching power supply module and could reduce EMI by
going spread-spectrum on the switching frequency. The simple one below
reduces things by 20 dB. Probe the SS node and FFT.

The ss inside switching reg chips is no doubt more sophisticated. In
an FPGA, we could do some sort of pseudo-random thing.

On a multi-channel power supply, there may be some small advantage to
have a separate spread per channel. That would be easy.

Version 4
SHEET 1 880 680
WIRE 144 80 80 80
WIRE 240 80 144 80
WIRE 80 112 80 80
WIRE 432 112 384 112
WIRE 464 112 432 112
WIRE 80 224 80 192
FLAG 80 224 0
FLAG 144 80 MOD
FLAG 432 112 SS
SYMBOL voltage 80 96 R0
WINDOW 0 43 80 Left 2
WINDOW 3 12 111 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(0 1 0 1m 1u 1u 1m)
SYMBOL SpecialFunctions\\modulate 240 80 R0
WINDOW 0 48 -48 Left 2
SYMATTR InstName A1
SYMATTR SpiceLine mark=220K space=280K
TEXT 462 54 Left 2 !.tran 2m
TEXT 400 144 Left 2 ;Basic spread-spectrum
TEXT 408 176 Left 2 ;for P943 8-ch supply
TEXT 432 208 Left 2 ;JL Apr 18 2024

On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>I'm designing a switching power supply module and could reduce EMI by
>going spread-spectrum on the switching frequency. The simple one below
>reduces things by 20 dB. Probe the SS node and FFT.
>
>The ss inside switching reg chips is no doubt more sophisticated. In
>an FPGA, we could do some sort of pseudo-random thing.
>
>On a multi-channel power supply, there may be some small advantage to
>have a separate spread per channel. That would be easy.

I'd check for cross-correlation as well, so no ganging up in systems
using multiple channels in some signal path.

Depending on details, the problem could manifest itself as peaks or
ripples in the time domain, your beloved homeland.

Joe Gwinn

>Version 4
>SHEET 1 880 680
>WIRE 144 80 80 80
>WIRE 240 80 144 80
>WIRE 80 112 80 80
>WIRE 432 112 384 112
>WIRE 464 112 432 112
>WIRE 80 224 80 192
>FLAG 80 224 0
>FLAG 144 80 MOD
>FLAG 432 112 SS
>SYMBOL voltage 80 96 R0
>WINDOW 0 43 80 Left 2
>WINDOW 3 12 111 Left 2
>WINDOW 123 0 0 Left 0
>WINDOW 39 0 0 Left 0
>SYMATTR InstName V1
>SYMATTR Value PULSE(0 1 0 1m 1u 1u 1m)
>SYMBOL SpecialFunctions\\modulate 240 80 R0
>WINDOW 0 48 -48 Left 2
>SYMATTR InstName A1
>SYMATTR SpiceLine mark=220K space=280K
>TEXT 462 54 Left 2 !.tran 2m
>TEXT 400 144 Left 2 ;Basic spread-spectrum
>TEXT 408 176 Left 2 ;for P943 8-ch supply
>TEXT 432 208 Left 2 ;JL Apr 18 2024

On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>I'm designing a switching power supply module and could reduce EMI by
>>going spread-spectrum on the switching frequency. The simple one below
>>reduces things by 20 dB. Probe the SS node and FFT.
>>
>>The ss inside switching reg chips is no doubt more sophisticated. In
>>an FPGA, we could do some sort of pseudo-random thing.
>>
>>On a multi-channel power supply, there may be some small advantage to
>>have a separate spread per channel. That would be easy.
>
>I'd check for cross-correlation as well, so no ganging up in systems
>using multiple channels in some signal path.

When my engineers get too fussy about stuff like that, I remind them
"it's just a power supply."

>
>Depending on details, the problem could manifest itself as peaks or
>ripples in the time domain, your beloved homeland.
>
>Joe Gwinn
>

TI has a couple of intersting appnotes

https://www.ti.com/lit/pdf/slyt809

https://www.ti.com/lit/SLVAF18

Their little TPS54302 type parts have radical looking PWM, but the
final DC is super clean. Nice trick.

https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1

https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1

We may get some EMI from switching rise/fall ringing too, in the
hundred-MHz ballpark. It would help to de-phase that too.

On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>I'm designing a switching power supply module and could reduce EMI by
>>>going spread-spectrum on the switching frequency. The simple one below
>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>
>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>an FPGA, we could do some sort of pseudo-random thing.
>>>
>>>On a multi-channel power supply, there may be some small advantage to
>>>have a separate spread per channel. That would be easy.
>>
>>I'd check for cross-correlation as well, so no ganging up in systems
>>using multiple channels in some signal path.
>
>When my engineers get too fussy about stuff like that, I remind them
>"it's just a power supply."

In my world, we have multiple parallel components (like array
sections) in the signal path powered by independent power supplies
that are required to have independent noise, to prevent correlated
gain when these parallel paths are summed, say in a radar beamformer.

Telling the power-supply folk that it's just a power supply is a good
way to get buried in details.

>>Depending on details, the problem could manifest itself as peaks or
>>ripples in the time domain, your beloved homeland.
>>
>>Joe Gwinn
>>
>
>TI has a couple of interesting appnotes
>
><https://www.ti.com/lit/pdf/slyt809>
>
><https://www.ti.com/lit/SLVAF18>
>
>
>Their little TPS54302 type parts have radical looking PWM, but the
>final DC is super clean. Nice trick.
>
><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>
><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>
>We may get some EMI from switching rise/fall ringing too, in the
>hundred-MHz ballpark. It would help to de-phase that too.

TI stuff is widely used in radar, but in the most capable radars the
dithering is provided by bespoke radar firmware, and not left to the
converter chip. But those chips do work well.

Joe Gwinn

On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>wrote:
>>
>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>
>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>
>>>>On a multi-channel power supply, there may be some small advantage to
>>>>have a separate spread per channel. That would be easy.
>>>
>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>using multiple channels in some signal path.
>>
>>When my engineers get too fussy about stuff like that, I remind them
>>"it's just a power supply."
>
>In my world, we have multiple parallel components (like array
>sections) in the signal path powered by independent power supplies
>that are required to have independent noise, to prevent correlated
>gain when these parallel paths are summed, say in a radar beamformer.
>
>Telling the power-supply folk that it's just a power supply is a good
>way to get buried in details.
>

My intent was to keep it simple and get it done.

>
>>>Depending on details, the problem could manifest itself as peaks or
>>>ripples in the time domain, your beloved homeland.
>>>
>>>Joe Gwinn
>>>
>>
>>TI has a couple of interesting appnotes
>>
>><https://www.ti.com/lit/pdf/slyt809>
>>
>><https://www.ti.com/lit/SLVAF18>
>>
>>
>>Their little TPS54302 type parts have radical looking PWM, but the
>>final DC is super clean. Nice trick.
>>
>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>
>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>
>>We may get some EMI from switching rise/fall ringing too, in the
>>hundred-MHz ballpark. It would help to de-phase that too.
>
>TI stuff is widely used in radar, but in the most capable radars the
>dithering is provided by bespoke radar firmware, and not left to the
>converter chip. But those chips do work well.
>
>Joe Gwinn

We've decided to use home-made half bridges in the 8-channel
programmable power module. The TI and ADI switching regs are just too
smart. We'll use the reg chips when we just want a fixed power supply.

I was thinking that we could use a DDS architecture to generate the
PWM into the switching half-bridges. We could wobble the frequency
setting to spread the spectrum.

Maybe replace some of the LSBs of the frequency-set register with a
pseudorandom pattern, a different one for each power supply channel.
Change those LSBs at some rate, 20 KHz or something, to fool an
EMI-test spectrum analyzer.

A pseudorandom pattern will average to 0.5, which affects the average
switcher frequency, but we can deal with that.

I suppose I could draw a diagram.

We'd like the fine-grain spectra to not overlap, across all 8
channels. Fun.

Given eight unipolar half-bridges, we'll allow users to use a pair as
a full bridge to drive one bipolar load, or three to drive a 3-phase
load like a torque motor. In those cases, I think we can still allow
each phase to have its own independent spread-spectrum thing. The
motors won't care.

On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>wrote:
>>>
>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>
>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>
>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>
>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>have a separate spread per channel. That would be easy.
>>>>
>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>using multiple channels in some signal path.
>>>
>>>When my engineers get too fussy about stuff like that, I remind them
>>>"it's just a power supply."
>>
>>In my world, we have multiple parallel components (like array
>>sections) in the signal path powered by independent power supplies
>>that are required to have independent noise, to prevent correlated
>>gain when these parallel paths are summed, say in a radar beamformer.
>>
>>Telling the power-supply folk that it's just a power supply is a good
>>way to get buried in details.
>>
>
>My intent was to keep it simple and get it done.
>
>>
>>>>Depending on details, the problem could manifest itself as peaks or
>>>>ripples in the time domain, your beloved homeland.
>>>>
>>>>Joe Gwinn
>>>>
>>>
>>>TI has a couple of interesting appnotes
>>>
>>><https://www.ti.com/lit/pdf/slyt809>
>>>
>>><https://www.ti.com/lit/SLVAF18>
>>>
>>>
>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>final DC is super clean. Nice trick.
>>>
>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>
>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>
>>>We may get some EMI from switching rise/fall ringing too, in the
>>>hundred-MHz ballpark. It would help to de-phase that too.
>>
>>TI stuff is widely used in radar, but in the most capable radars the
>>dithering is provided by bespoke radar firmware, and not left to the
>>converter chip. But those chips do work well.
>>
>>Joe Gwinn
>
>We've decided to use home-made half bridges in the 8-channel
>programmable power module. The TI and ADI switching regs are just too
>smart. We'll use the reg chips when we just want a fixed power supply.
>
>I was thinking that we could use a DDS architecture to generate the
>PWM into the switching half-bridges. We could wobble the frequency
>setting to spread the spectrum.
>
>Maybe replace some of the LSBs of the frequency-set register with a
>pseudorandom pattern, a different one for each power supply channel.
>Change those LSBs at some rate, 20 KHz or something, to fool an
>EMI-test spectrum analyzer.
>
>A pseudorandom pattern will average to 0.5, which affects the average
>switcher frequency, but we can deal with that.
>
>I suppose I could draw a diagram.
>
>We'd like the fine-grain spectra to not overlap, across all 8
>channels. Fun.
>
>Given eight unipolar half-bridges, we'll allow users to use a pair as
>a full bridge to drive one bipolar load, or three to drive a 3-phase
>load like a torque motor. In those cases, I think we can still allow
>each phase to have its own independent spread-spectrum thing. The
>motors won't care.

The STM32F4 that I use in my inverter/charger has a SS clock option.
I have not enabled that yet but intend to eventually for lower peaks
from the PWM outputs driving all the power circuitry.

boB

On Fri, 19 Apr 2024 11:08:36 -0700, boB <boB@K7IQ.com> wrote:

>On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>>wrote:
>>
>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>
>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>
>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>
>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>have a separate spread per channel. That would be easy.
>>>>>
>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>using multiple channels in some signal path.
>>>>
>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>"it's just a power supply."
>>>
>>>In my world, we have multiple parallel components (like array
>>>sections) in the signal path powered by independent power supplies
>>>that are required to have independent noise, to prevent correlated
>>>gain when these parallel paths are summed, say in a radar beamformer.
>>>
>>>Telling the power-supply folk that it's just a power supply is a good
>>>way to get buried in details.
>>>
>>
>>My intent was to keep it simple and get it done.
>>
>>>
>>>>>Depending on details, the problem could manifest itself as peaks or
>>>>>ripples in the time domain, your beloved homeland.
>>>>>
>>>>>Joe Gwinn
>>>>>
>>>>
>>>>TI has a couple of interesting appnotes
>>>>
>>>><https://www.ti.com/lit/pdf/slyt809>
>>>>
>>>><https://www.ti.com/lit/SLVAF18>
>>>>
>>>>
>>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>>final DC is super clean. Nice trick.
>>>>
>>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>>
>>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>>
>>>>We may get some EMI from switching rise/fall ringing too, in the
>>>>hundred-MHz ballpark. It would help to de-phase that too.
>>>
>>>TI stuff is widely used in radar, but in the most capable radars the
>>>dithering is provided by bespoke radar firmware, and not left to the
>>>converter chip. But those chips do work well.
>>>
>>>Joe Gwinn
>>
>>We've decided to use home-made half bridges in the 8-channel
>>programmable power module. The TI and ADI switching regs are just too
>>smart. We'll use the reg chips when we just want a fixed power supply.
>>
>>I was thinking that we could use a DDS architecture to generate the
>>PWM into the switching half-bridges. We could wobble the frequency
>>setting to spread the spectrum.
>>
>>Maybe replace some of the LSBs of the frequency-set register with a
>>pseudorandom pattern, a different one for each power supply channel.
>>Change those LSBs at some rate, 20 KHz or something, to fool an
>>EMI-test spectrum analyzer.
>>
>>A pseudorandom pattern will average to 0.5, which affects the average
>>switcher frequency, but we can deal with that.
>>
>>I suppose I could draw a diagram.
>>
>>We'd like the fine-grain spectra to not overlap, across all 8
>>channels. Fun.
>>
>>Given eight unipolar half-bridges, we'll allow users to use a pair as
>>a full bridge to drive one bipolar load, or three to drive a 3-phase
>>load like a torque motor. In those cases, I think we can still allow
>>each phase to have its own independent spread-spectrum thing. The
>>motors won't care.
>
>
>The STM32F4 that I use in my inverter/charger has a SS clock option.
>I have not enabled that yet but intend to eventually for lower peaks
>from the PWM outputs driving all the power circuitry.
>
>boB

That's cool. I'm designing a bunch of plugin modules that would all
get a 50 MHz clock from the backplane. Since my PWM frequencies will
be fairly low, we should be able to fuzz up the spectra in the FPGA on
each board.

We could code a generic ss PWM block and use it everywhere.

https://www.dropbox.com/scl/fi/2ypg6qhnalmixv6kx44if/Spread_Spectrum_Apr_19.jpg?rlkey=d3hiwl4mj57erk82629fyouse&raw=1

On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>wrote:
>>>
>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>
>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>
>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>
>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>have a separate spread per channel. That would be easy.
>>>>
>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>using multiple channels in some signal path.
>>>
>>>When my engineers get too fussy about stuff like that, I remind them
>>>"it's just a power supply."
>>
>>In my world, we have multiple parallel components (like array
>>sections) in the signal path powered by independent power supplies
>>that are required to have independent noise, to prevent correlated
>>gain when these parallel paths are summed, say in a radar beamformer.
>>
>>Telling the power-supply folk that it's just a power supply is a good
>>way to get buried in details.
>>
>
>My intent was to keep it simple and get it done.
>
>>
>>>>Depending on details, the problem could manifest itself as peaks or
>>>>ripples in the time domain, your beloved homeland.
>>>>
>>>>Joe Gwinn
>>>>
>>>
>>>TI has a couple of interesting appnotes
>>>
>>><https://www.ti.com/lit/pdf/slyt809>
>>>
>>><https://www.ti.com/lit/SLVAF18>
>>>
>>>
>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>final DC is super clean. Nice trick.
>>>
>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>
>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>
>>>We may get some EMI from switching rise/fall ringing too, in the
>>>hundred-MHz ballpark. It would help to de-phase that too.
>>
>>TI stuff is widely used in radar, but in the most capable radars the
>>dithering is provided by bespoke radar firmware, and not left to the
>>converter chip. But those chips do work well.
>>
>>Joe Gwinn
>
>We've decided to use home-made half bridges in the 8-channel
>programmable power module. The TI and ADI switching regs are just too
>smart. We'll use the reg chips when we just want a fixed power supply.

The radar folk prefer TI over ADI for such things because TI does the
digital parts better.

>I was thinking that we could use a DDS architecture to generate the
>PWM into the switching half-bridges. We could wobble the frequency
>setting to spread the spectrum.

This is a common approach in radar.

>Maybe replace some of the LSBs of the frequency-set register with a
>pseudorandom pattern, a different one for each power supply channel.
>Change those LSBs at some rate, 20 KHz or something, to fool an
>EMI-test spectrum analyzer.

Yep.

Another reason to dither the lsbs is to allow correlation processing
to pull things up from below despite ADC quantization, where things
get very granular and thus non-linear.

>A pseudorandom pattern will average to 0.5, which affects the average
>switcher frequency, but we can deal with that.
>
>I suppose I could draw a diagram.
>
>We'd like the fine-grain spectra to not overlap, across all 8
>channels. Fun.

The max EMI regulations specify something like a one-second averaging
window, so some alignments are probably OK. A sufficiently long
pseudo random sequence will make coincidences rates small enough to
not matter, even if the sequences are not actually orthogonal.

>Given eight unipolar half-bridges, we'll allow users to use a pair as
>a full bridge to drive one bipolar load, or three to drive a 3-phase
>load like a torque motor. In those cases, I think we can still allow
>each phase to have its own independent spread-spectrum thing. The
>motors won't care.

Small motors (and transformers) won't care. But one thing I learned
after being buried by the power folk is that at the megawatt level,
harmonics and overlaps must be handled, or the motor or transformer
will fail prematurely due to corona induced within the windings, there
being multiple ways to cause this.

..<https://en.wikipedia.org/wiki/Partial_discharge>

Joe Gwinn

On Fri, 19 Apr 2024 13:43:59 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Fri, 19 Apr 2024 11:08:36 -0700, boB <boB@K7IQ.com> wrote:
>
>>On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>wrote:
>>>
>>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>wrote:
>>>>>
>>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>>
>>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>>
>>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>>
>>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>>have a separate spread per channel. That would be easy.
>>>>>>
>>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>>using multiple channels in some signal path.
>>>>>
>>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>>"it's just a power supply."
>>>>
>>>>In my world, we have multiple parallel components (like array
>>>>sections) in the signal path powered by independent power supplies
>>>>that are required to have independent noise, to prevent correlated
>>>>gain when these parallel paths are summed, say in a radar beamformer.
>>>>
>>>>Telling the power-supply folk that it's just a power supply is a good
>>>>way to get buried in details.
>>>>
>>>
>>>My intent was to keep it simple and get it done.
>>>
>>>>
>>>>>>Depending on details, the problem could manifest itself as peaks or
>>>>>>ripples in the time domain, your beloved homeland.
>>>>>>
>>>>>>Joe Gwinn
>>>>>>
>>>>>
>>>>>TI has a couple of interesting appnotes
>>>>>
>>>>><https://www.ti.com/lit/pdf/slyt809>
>>>>>
>>>>><https://www.ti.com/lit/SLVAF18>
>>>>>
>>>>>
>>>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>>>final DC is super clean. Nice trick.
>>>>>
>>>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>>>
>>>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>>>
>>>>>We may get some EMI from switching rise/fall ringing too, in the
>>>>>hundred-MHz ballpark. It would help to de-phase that too.
>>>>
>>>>TI stuff is widely used in radar, but in the most capable radars the
>>>>dithering is provided by bespoke radar firmware, and not left to the
>>>>converter chip. But those chips do work well.
>>>>
>>>>Joe Gwinn
>>>
>>>We've decided to use home-made half bridges in the 8-channel
>>>programmable power module. The TI and ADI switching regs are just too
>>>smart. We'll use the reg chips when we just want a fixed power supply.
>>>
>>>I was thinking that we could use a DDS architecture to generate the
>>>PWM into the switching half-bridges. We could wobble the frequency
>>>setting to spread the spectrum.
>>>
>>>Maybe replace some of the LSBs of the frequency-set register with a
>>>pseudorandom pattern, a different one for each power supply channel.
>>>Change those LSBs at some rate, 20 KHz or something, to fool an
>>>EMI-test spectrum analyzer.
>>>
>>>A pseudorandom pattern will average to 0.5, which affects the average
>>>switcher frequency, but we can deal with that.
>>>
>>>I suppose I could draw a diagram.
>>>
>>>We'd like the fine-grain spectra to not overlap, across all 8
>>>channels. Fun.
>>>
>>>Given eight unipolar half-bridges, we'll allow users to use a pair as
>>>a full bridge to drive one bipolar load, or three to drive a 3-phase
>>>load like a torque motor. In those cases, I think we can still allow
>>>each phase to have its own independent spread-spectrum thing. The
>>>motors won't care.
>>
>>
>>The STM32F4 that I use in my inverter/charger has a SS clock option.
>>I have not enabled that yet but intend to eventually for lower peaks
>>from the PWM outputs driving all the power circuitry.
>>
>>boB
>
>That's cool. I'm designing a bunch of plugin modules that would all
>get a 50 MHz clock from the backplane. Since my PWM frequencies will
>be fairly low, we should be able to fuzz up the spectra in the FPGA on
>each board.
>
>We could code a generic ss PWM block and use it everywhere.
>
><https://www.dropbox.com/scl/fi/2ypg6qhnalmixv6kx44if/Spread_Spectrum_Apr_19.jpg?rlkey=d3hiwl4mj57erk82629fyouse&raw=1>

This dithers the frequency, which is a valid approach. But dithering
the phase before going to the DAC core to generate the waveform is
also widely used. And one can do both at the same time, particularly
with different sequences, so the peaks are spread out in 2D.

Joe Gwinn

On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>I'm designing a switching power supply module and could reduce EMI by
>>>going spread-spectrum on the switching frequency. The simple one below
>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>
>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>an FPGA, we could do some sort of pseudo-random thing.
>>>
>>>On a multi-channel power supply, there may be some small advantage to
>>>have a separate spread per channel. That would be easy.
>>
>>I'd check for cross-correlation as well, so no ganging up in systems
>>using multiple channels in some signal path.
>
>When my engineers get too fussy about stuff like that, I remind them
>"it's just a power supply."

Noise at the local level is best correlated, as it is more
predictable - you avoid low-frequency beat frequencies in the
local regulators - which can and will show up in a detector's
BW and in the regulators' outputs.

A master clock, phase shifted for various local users, can be dithered
for the system (box), which is the actual, final radiator.

Your engineers can get REAL fussy, if the system's non-compliant
way past the development's due date.

RL

On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:

>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>wrote:
>>
>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>
>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>
>>>>On a multi-channel power supply, there may be some small advantage to
>>>>have a separate spread per channel. That would be easy.
>>>
>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>using multiple channels in some signal path.
>>
>>When my engineers get too fussy about stuff like that, I remind them
>>"it's just a power supply."
>
>Noise at the local level is best correlated, as it is more
>predictable - you avoid low-frequency beat frequencies in the
>local regulators - which can and will show up in a detector's
>BW and in the regulators' outputs.

But...but... it's just a power supply!

Presumably uncorrelated spread-spectrum will make wideband noise at an
output, not a beat.

>
>A master clock, phase shifted for various local users, can be dithered
>for the system (box), which is the actual, final radiator.

Our box has a 50 MHz clock that is bussed to all the plugin modules,
and it can be locked to other boxes or to a 10 MHz reference, so we
can't usefully dither that. I guess each module could have its own
VCO, but that would mess up synchronizing modules, and complicate
things. Spread-spectrum sounds easier.

>
>Your engineers can get REAL fussy, if the system's non-compliant
>way past the development's due date.

Eventually, some giant customer may want CE stickers, so we'll do the
easier things now, to improve our chances of passing an EMI test. A
bit of VHDL in the FPGAs would be easy.

On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin
<jjSNIPlarkin@highNONOlandtechnology.com> wrote:

>On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:
>
>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>wrote:
>>>
>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>
>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>
>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>
>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>have a separate spread per channel. That would be easy.
>>>>
>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>using multiple channels in some signal path.
>>>
>>>When my engineers get too fussy about stuff like that, I remind them
>>>"it's just a power supply."
>>
>>Noise at the local level is best correlated, as it is more
>>predictable - you avoid low-frequency beat frequencies in the
>>local regulators - which can and will show up in a detector's
>>BW and in the regulators' outputs.
>
>But...but... it's just a power supply!
>
>Presumably uncorrelated spread-spectrum will make wideband noise at an
>output, not a beat.
>
>>
>>A master clock, phase shifted for various local users, can be dithered
>>for the system (box), which is the actual, final radiator.
>
>Our box has a 50 MHz clock that is bussed to all the plugin modules,
>and it can be locked to other boxes or to a 10 MHz reference, so we
>can't usefully dither that. I guess each module could have its own
>VCO, but that would mess up synchronizing modules, and complicate
>things. Spread-spectrum sounds easier.
>
>>
>>Your engineers can get REAL fussy, if the system's non-compliant
>>way past the development's due date.
>
>Eventually, some giant customer may want CE stickers, so we'll do the
>easier things now, to improve our chances of passing an EMI test. A
>bit of VHDL in the FPGAs would be easy.

Unsynchronized power supplies on the same board can
influence each other, unpredictably with load, to produce
audible harmonics.

Ignore the effects at your peril.

RL

On Sun, 21 Apr 2024 08:50:23 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:
>>
>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>
>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>
>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>
>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>have a separate spread per channel. That would be easy.
>>>>>
>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>using multiple channels in some signal path.
>>>>
>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>"it's just a power supply."
>>>
>>>Noise at the local level is best correlated, as it is more
>>>predictable - you avoid low-frequency beat frequencies in the
>>>local regulators - which can and will show up in a detector's
>>>BW and in the regulators' outputs.
>>
>>But...but... it's just a power supply!
>>
>>Presumably uncorrelated spread-spectrum will make wideband noise at an
>>output, not a beat.
>>
>>>
>>>A master clock, phase shifted for various local users, can be dithered
>>>for the system (box), which is the actual, final radiator.
>>
>>Our box has a 50 MHz clock that is bussed to all the plugin modules,
>>and it can be locked to other boxes or to a 10 MHz reference, so we
>>can't usefully dither that. I guess each module could have its own
>>VCO, but that would mess up synchronizing modules, and complicate
>>things. Spread-spectrum sounds easier.
>>
>>>
>>>Your engineers can get REAL fussy, if the system's non-compliant
>>>way past the development's due date.
>>
>>Eventually, some giant customer may want CE stickers, so we'll do the
>>easier things now, to improve our chances of passing an EMI test. A
>>bit of VHDL in the FPGAs would be easy.
>
>Unsynchronized power supplies on the same board can
>influence each other, unpredictably with load, to produce
>audible harmonics.
>
>Ignore the effects at your peril.
>
>RL

Given a common 50 MHz clock and a sensible architecture, we plan to
synchronize modules. Like, for instance, paralleling 3-phase AC
sources to get more power, or running two at phase offsets.

But I'm not concerned with acoustics, given that the modules are in a
rackmount chassis with big fans. And a hiss would be better than a
whine, which is the heart of the spead-spectrum concept. Ears are
spectrum analyzers too.

On Sun, 21 Apr 2024 08:50:23 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:
>>
>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>
>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>
>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>
>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>have a separate spread per channel. That would be easy.
>>>>>
>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>using multiple channels in some signal path.
>>>>
>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>"it's just a power supply."
>>>
>>>Noise at the local level is best correlated, as it is more
>>>predictable - you avoid low-frequency beat frequencies in the
>>>local regulators - which can and will show up in a detector's
>>>BW and in the regulators' outputs.
>>
>>But...but... it's just a power supply!
>>
>>Presumably uncorrelated spread-spectrum will make wideband noise at an
>>output, not a beat.
>>
>>>
>>>A master clock, phase shifted for various local users, can be dithered
>>>for the system (box), which is the actual, final radiator.
>>
>>Our box has a 50 MHz clock that is bussed to all the plugin modules,
>>and it can be locked to other boxes or to a 10 MHz reference, so we
>>can't usefully dither that. I guess each module could have its own
>>VCO, but that would mess up synchronizing modules, and complicate
>>things. Spread-spectrum sounds easier.
>>
>>>
>>>Your engineers can get REAL fussy, if the system's non-compliant
>>>way past the development's due date.
>>
>>Eventually, some giant customer may want CE stickers, so we'll do the
>>easier things now, to improve our chances of passing an EMI test. A
>>bit of VHDL in the FPGAs would be easy.
>
>Unsynchronized power supplies on the same board can
>influence each other, unpredictably with load, to produce
>audible harmonics.
>
>Ignore the effects at your peril.

Yes. Fix the shielding and grounding story until these effects no
longer matter. This is done in radar, as discussed upthread.

Joe Gwinn

On Fri, 19 Apr 2024 17:33:48 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Fri, 19 Apr 2024 13:43:59 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Fri, 19 Apr 2024 11:08:36 -0700, boB <boB@K7IQ.com> wrote:
>>
>>>On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>
>>>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>>wrote:
>>>>>>
>>>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>>>
>>>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>>>
>>>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>>>
>>>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>>>have a separate spread per channel. That would be easy.
>>>>>>>
>>>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>>>using multiple channels in some signal path.
>>>>>>
>>>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>>>"it's just a power supply."
>>>>>
>>>>>In my world, we have multiple parallel components (like array
>>>>>sections) in the signal path powered by independent power supplies
>>>>>that are required to have independent noise, to prevent correlated
>>>>>gain when these parallel paths are summed, say in a radar beamformer.
>>>>>
>>>>>Telling the power-supply folk that it's just a power supply is a good
>>>>>way to get buried in details.
>>>>>
>>>>
>>>>My intent was to keep it simple and get it done.
>>>>
>>>>>
>>>>>>>Depending on details, the problem could manifest itself as peaks or
>>>>>>>ripples in the time domain, your beloved homeland.
>>>>>>>
>>>>>>>Joe Gwinn
>>>>>>>
>>>>>>
>>>>>>TI has a couple of interesting appnotes
>>>>>>
>>>>>><https://www.ti.com/lit/pdf/slyt809>
>>>>>>
>>>>>><https://www.ti.com/lit/SLVAF18>
>>>>>>
>>>>>>
>>>>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>>>>final DC is super clean. Nice trick.
>>>>>>
>>>>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>>>>
>>>>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>>>>
>>>>>>We may get some EMI from switching rise/fall ringing too, in the
>>>>>>hundred-MHz ballpark. It would help to de-phase that too.
>>>>>
>>>>>TI stuff is widely used in radar, but in the most capable radars the
>>>>>dithering is provided by bespoke radar firmware, and not left to the
>>>>>converter chip. But those chips do work well.
>>>>>
>>>>>Joe Gwinn
>>>>
>>>>We've decided to use home-made half bridges in the 8-channel
>>>>programmable power module. The TI and ADI switching regs are just too
>>>>smart. We'll use the reg chips when we just want a fixed power supply.
>>>>
>>>>I was thinking that we could use a DDS architecture to generate the
>>>>PWM into the switching half-bridges. We could wobble the frequency
>>>>setting to spread the spectrum.
>>>>
>>>>Maybe replace some of the LSBs of the frequency-set register with a
>>>>pseudorandom pattern, a different one for each power supply channel.
>>>>Change those LSBs at some rate, 20 KHz or something, to fool an
>>>>EMI-test spectrum analyzer.
>>>>
>>>>A pseudorandom pattern will average to 0.5, which affects the average
>>>>switcher frequency, but we can deal with that.
>>>>
>>>>I suppose I could draw a diagram.
>>>>
>>>>We'd like the fine-grain spectra to not overlap, across all 8
>>>>channels. Fun.
>>>>
>>>>Given eight unipolar half-bridges, we'll allow users to use a pair as
>>>>a full bridge to drive one bipolar load, or three to drive a 3-phase
>>>>load like a torque motor. In those cases, I think we can still allow
>>>>each phase to have its own independent spread-spectrum thing. The
>>>>motors won't care.
>>>
>>>
>>>The STM32F4 that I use in my inverter/charger has a SS clock option.
>>>I have not enabled that yet but intend to eventually for lower peaks
>>>from the PWM outputs driving all the power circuitry.
>>>
>>>boB
>>
>>That's cool. I'm designing a bunch of plugin modules that would all
>>get a 50 MHz clock from the backplane. Since my PWM frequencies will
>>be fairly low, we should be able to fuzz up the spectra in the FPGA on
>>each board.
>>
>>We could code a generic ss PWM block and use it everywhere.
>>
>><https://www.dropbox.com/scl/fi/2ypg6qhnalmixv6kx44if/Spread_Spectrum_Apr_19.jpg?rlkey=d3hiwl4mj57erk82629fyouse&raw=1>
>
>This dithers the frequency, which is a valid approach. But dithering
>the phase before going to the DAC core to generate the waveform is
>also widely used. And one can do both at the same time, particularly
>with different sequences, so the peaks are spread out in 2D.
>
>Joe Gwinn

Pseudo-random frequency is probably best as long as the average
frequency is the specified and wanted center frequency. So the PRN
source may need to be massaged to do that ?

What I see most often in processors and an old SMPS chip I seem to
remember all use triangle wave modulation to the clock frequency.
This way, it averages to the center frequency.
Those chips also (IIRC) have some other options where the modulation
is biased to one side or the other of that spectrum. Top or bottom.
I don't remember exactly.

I remember hearing about a 12V switchmode power supply for ham radio
where you could adjust the switching frequency up or down slightly so
that the EMI could be tuned away from the operator's radio frequency
he was operating on so it would not interfere. I guess if you can't
make the EMI go away by design, that might be the next best thing.

I have yet to find a 12V SMPS supply that I could not hear on my HF
radios so I just use a linear PS for that at home.

boB

On Sun, 21 Apr 2024 12:32:01 -0700, boB <boB@K7IQ.com> wrote:

>On Fri, 19 Apr 2024 17:33:48 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Fri, 19 Apr 2024 13:43:59 -0700, John Larkin
>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>
>>>On Fri, 19 Apr 2024 11:08:36 -0700, boB <boB@K7IQ.com> wrote:
>>>
>>>>On Fri, 19 Apr 2024 08:22:44 -0700, John Larkin
>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>
>>>>>On Fri, 19 Apr 2024 10:30:45 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>wrote:
>>>>>
>>>>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>>
>>>>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>>>wrote:
>>>>>>>
>>>>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>>>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>>>>>>
>>>>>>>>>I'm designing a switching power supply module and could reduce EMI by
>>>>>>>>>going spread-spectrum on the switching frequency. The simple one below
>>>>>>>>>reduces things by 20 dB. Probe the SS node and FFT.
>>>>>>>>>
>>>>>>>>>The ss inside switching reg chips is no doubt more sophisticated. In
>>>>>>>>>an FPGA, we could do some sort of pseudo-random thing.
>>>>>>>>>
>>>>>>>>>On a multi-channel power supply, there may be some small advantage to
>>>>>>>>>have a separate spread per channel. That would be easy.
>>>>>>>>
>>>>>>>>I'd check for cross-correlation as well, so no ganging up in systems
>>>>>>>>using multiple channels in some signal path.
>>>>>>>
>>>>>>>When my engineers get too fussy about stuff like that, I remind them
>>>>>>>"it's just a power supply."
>>>>>>
>>>>>>In my world, we have multiple parallel components (like array
>>>>>>sections) in the signal path powered by independent power supplies
>>>>>>that are required to have independent noise, to prevent correlated
>>>>>>gain when these parallel paths are summed, say in a radar beamformer.
>>>>>>
>>>>>>Telling the power-supply folk that it's just a power supply is a good
>>>>>>way to get buried in details.
>>>>>>
>>>>>
>>>>>My intent was to keep it simple and get it done.
>>>>>
>>>>>>
>>>>>>>>Depending on details, the problem could manifest itself as peaks or
>>>>>>>>ripples in the time domain, your beloved homeland.
>>>>>>>>
>>>>>>>>Joe Gwinn
>>>>>>>>
>>>>>>>
>>>>>>>TI has a couple of interesting appnotes
>>>>>>>
>>>>>>><https://www.ti.com/lit/pdf/slyt809>
>>>>>>>
>>>>>>><https://www.ti.com/lit/SLVAF18>
>>>>>>>
>>>>>>>
>>>>>>>Their little TPS54302 type parts have radical looking PWM, but the
>>>>>>>final DC is super clean. Nice trick.
>>>>>>>
>>>>>>><https://www.dropbox.com/scl/fi/8rytjiwp4hmt2ypgk9bk4/DSC06826.JPG?rlkey=4qipduct0ptrhei07ijdxpsca&raw=1>
>>>>>>>
>>>>>>><https://www.dropbox.com/scl/fi/kf2kxbxih6xjbx8uv2o0d/TPS54302_spectrum.JPG?rlkey=rd3diu5nvhasfn7228m8yk665&raw=1>
>>>>>>>
>>>>>>>We may get some EMI from switching rise/fall ringing too, in the
>>>>>>>hundred-MHz ballpark. It would help to de-phase that too.
>>>>>>
>>>>>>TI stuff is widely used in radar, but in the most capable radars the
>>>>>>dithering is provided by bespoke radar firmware, and not left to the
>>>>>>converter chip. But those chips do work well.
>>>>>>
>>>>>>Joe Gwinn
>>>>>
>>>>>We've decided to use home-made half bridges in the 8-channel
>>>>>programmable power module. The TI and ADI switching regs are just too
>>>>>smart. We'll use the reg chips when we just want a fixed power supply.
>>>>>
>>>>>I was thinking that we could use a DDS architecture to generate the
>>>>>PWM into the switching half-bridges. We could wobble the frequency
>>>>>setting to spread the spectrum.
>>>>>
>>>>>Maybe replace some of the LSBs of the frequency-set register with a
>>>>>pseudorandom pattern, a different one for each power supply channel.
>>>>>Change those LSBs at some rate, 20 KHz or something, to fool an
>>>>>EMI-test spectrum analyzer.
>>>>>
>>>>>A pseudorandom pattern will average to 0.5, which affects the average
>>>>>switcher frequency, but we can deal with that.
>>>>>
>>>>>I suppose I could draw a diagram.
>>>>>
>>>>>We'd like the fine-grain spectra to not overlap, across all 8
>>>>>channels. Fun.
>>>>>
>>>>>Given eight unipolar half-bridges, we'll allow users to use a pair as
>>>>>a full bridge to drive one bipolar load, or three to drive a 3-phase
>>>>>load like a torque motor. In those cases, I think we can still allow
>>>>>each phase to have its own independent spread-spectrum thing. The
>>>>>motors won't care.
>>>>
>>>>
>>>>The STM32F4 that I use in my inverter/charger has a SS clock option.
>>>>I have not enabled that yet but intend to eventually for lower peaks
>>>>from the PWM outputs driving all the power circuitry.
>>>>
>>>>boB
>>>
>>>That's cool. I'm designing a bunch of plugin modules that would all
>>>get a 50 MHz clock from the backplane. Since my PWM frequencies will
>>>be fairly low, we should be able to fuzz up the spectra in the FPGA on
>>>each board.
>>>
>>>We could code a generic ss PWM block and use it everywhere.
>>>
>>><https://www.dropbox.com/scl/fi/2ypg6qhnalmixv6kx44if/Spread_Spectrum_Apr_19.jpg?rlkey=d3hiwl4mj57erk82629fyouse&raw=1>
>>
>>This dithers the frequency, which is a valid approach. But dithering
>>the phase before going to the DAC core to generate the waveform is
>>also widely used. And one can do both at the same time, particularly
>>with different sequences, so the peaks are spread out in 2D.
>>
>>Joe Gwinn
>
>
>Pseudo-random frequency is probably best as long as the average
>frequency is the specified and wanted center frequency. So the PRN
>source may need to be massaged to do that ?
>
>What I see most often in processors and an old SMPS chip I seem to
>remember all use triangle wave modulation to the clock frequency.
>This way, it averages to the center frequency.

Yes, triangle wave modulation is common, but if center frequency is
important, they modulate phase, and maybe amplitude.

>I remember hearing about a 12V switchmode power supply for ham radio
>where you could adjust the switching frequency up or down slightly so
>that the EMI could be tuned away from the operator's radio frequency
>he was operating on so it would not interfere. I guess if you can't
>make the EMI go away by design, that might be the next best thing.

It's far easier to move the frequency than to suppress the EMI
everywhere. Ham operator budgets are not large.

>I have yet to find a 12V SMPS supply that I could not hear on my HF
>radios so I just use a linear PS for that at home.

That's the cheaper approach.

I don't know if this would be good enough, but a standard dodge is to
have a switcher go from AC line in (120 or 240 Vac) to 48 Vdc, and
then use linear regulators fed from that 48-V bus for all DC loads.

Joe Gwinn