power supply subtelties...

[server]

I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.



--

Anybody can count to one.

- Robert Widlar

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

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

 

[Fred Bloggs]

On Friday, June 24, 2022 at 9:35:47 AM UTC-4, jla...@highlandsniptechnology..com wrote:

I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

I didn\'t go thru the analytics, but the output Z for the buck is proportional to sqrt(L/C) or something. Low impedance at high frequency requires only small C. It\'s up to the user to do their own decoupling anyway. It\'s a lost cause to try to do that with a general purpose power supply. I analyzed more than few HP bench tops ( 30 years ago) and don\'t recall them do anything arcane, manufacturing success correlates strongly with simplicity.

--

Anybody can count to one.

- Robert Widlar

 

[Fred Bloggs]

On Friday, June 24, 2022 at 12:45:51 PM UTC-4, Phil Hobbs wrote:

jla...@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.
There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

National invented a more than few unconditionally stable circuit topologies for their voltage regulator and power op amp product families. They go back to Widlar\'s day.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

--
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 Mon, 27 Jun 2022 08:00:18 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

Cheers

Phil Hobbs

This is my current thinking. I can get my AC feedback from a local
node that I can control the dynamics of, and get DC fb from the nasty
remote sense. The notch filter really helps kill 250 KHz and above,
and its impedance actually helps the control loop a little.

https://www.dropbox.com/s/wf8rq1ziypt1vjn/ACDC_sense.jpg?dl=0

https://www.dropbox.com/s/g4qba0cjly7blbq/PS_Filt_3.jpg?dl=0

https://www.dropbox.com/s/m6pg94dxrmencq3/PS_Filt_3.asc?dl=0

And I thought power supplies were simple.

I guess my HF filter could be un-notched too, with a bigger L maybe.
I\'ll try that.

 

[Phil Hobbs]

Fred Bloggs wrote:

On Friday, June 24, 2022 at 12:45:51 PM UTC-4, Phil Hobbs wrote:
jla...@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power
supplies, but now we have to design some.

A power supply has two knobs (or SCPI commands in our case),
voltage and current limit.

A power supply should have low impedance at high frequencies, so
after whatever current limit circuit is has, there must be a real
capacitor. When you short a bench supply, you get a spark from
the energy in the output cap. So for a while, it\'s not really
current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz
ripple but allow reasonable programmable voltage slew rates.
We\'ll close a feedback loop from the voltage sensor ADC into the
bridge PWM drive, so the filter has to be well behaved. Maybe we
need the R3C3 damper to kill the Q of L1C1 so the loop doesn\'t go
bonkers.

As if that isn\'t bad enough, the customer load could be most
anything, a short or a resistor or a box with big input caps. Or
a big DC bus. Or even a battery. So our filter gets messed with
by the customer.

And a buck switcher is a boost switcher backwards. If the
customer gadget sources more voltage than our setpoint, we
extract power from the customer and charge C9 and blow everything
up. We can sense the +60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output
caps are like and how they managed the voltage/current dynamics.
Those would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps.
L1 is 180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at
250K, to un-compromise the main L1C2 filter, but that won\'t
affect than main loop dynamics.
There\'s a chapter(*) in one of Jim Willams\' books about a guy who
built big SMUish things using a \'1/2 pole\' rolloff--a bunch of
lead-lags that approximated a 10 dB/decade, 45-degree phase shift
network. At that point it didn\'t matter what the load capacitance
was, the loop was always stable. It\'s probably possible to make a
digital version of that.

National invented a more than few unconditionally stable circuit
topologies for their voltage regulator and power op amp product
families. They go back to Widlar\'s day.

Sure, as far back as (iirc) the 80s I used to use a fair number of
LM6361As that were like that. It rather involves putting the
compensation cap in the output stage, so that the capacitive loading
appears in parallel with it.

Since JL is rolling his own, it might be possible to do that. It\'s
tougher to do with an internally-compensated regulator that somebody
else designed.

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

 

[Don]

Fred Bloggs wrote:

jlarkin wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

I didn\'t go thru the analytics, but the output Z for the buck is proportional
to sqrt(L/C) or something. Low impedance at high frequency requires only
small C. It\'s up to the user to do their own decoupling anyway. It\'s a lost
cause to try to do that with a general purpose power supply. I analyzed more
than few HP bench tops ( 30 years ago) and don\'t recall them do anything
arcane, manufacturing success correlates strongly with simplicity.

Fred, you confirm my intuition about how responsibility for proper
power supply operation ultimately rests with its user. On the other
hand, the use of a ground return for current sense remains nonintuitive.

The use of ground symbols in schematic diagrams (actually
just a convenience for avoiding more lines in the drawing)
lulls us into thinking that they\'re all at the same
potential. That’s the essence of the fantasy ... but far
from the truth. Until room-temperature super-conductors
become a common reality, \"grounds\" are connected by
wires, PCB traces, or sheets of metal - all of which have
both resistance and inductance. So much for the fantasy!

- Bill Whitlock

Danke,

--
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

 

[server]

On Mon, 27 Jun 2022 06:40:43 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

On Friday, June 24, 2022 at 9:35:47 AM UTC-4, jla...@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

I didn\'t go thru the analytics, but the output Z for the buck is proportional to sqrt(L/C) or something. Low impedance at high frequency requires only small C. It\'s up to the user to do their own decoupling anyway. It\'s a lost cause to try to do that with a general purpose power supply. I analyzed more than few HP bench tops ( 30 years ago) and don\'t recall them do anything arcane, manufacturing success correlates strongly with simplicity.

HP did often include a big final electrolytic cap that could be
jumpered in or out.

We want fast programmable voltage slew rates, clean fast current
limiting, and stable remote sense no matter how far away or how stupid
the wiring and the load may be. We could expect the load to be some
decent fraction of a mile away.

 

[server]

On Mon, 27 Jun 2022 15:35:54 -0000 (UTC), \"Don\" <g@crcomp.net> wrote:

Fred Bloggs wrote:
jlarkin wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

I didn\'t go thru the analytics, but the output Z for the buck is proportional
to sqrt(L/C) or something. Low impedance at high frequency requires only
small C. It\'s up to the user to do their own decoupling anyway. It\'s a lost
cause to try to do that with a general purpose power supply. I analyzed more
than few HP bench tops ( 30 years ago) and don\'t recall them do anything
arcane, manufacturing success correlates strongly with simplicity.

Fred, you confirm my intuition about how responsibility for proper
power supply operation ultimately rests with its user.

We have to answer the phone when something doesn\'t work as expected.
So we prefer to design a power supply that is maximally tolerant of
customer wiring and loads.

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Mon, 27 Jun 2022 08:00:18 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

Cheers

Phil Hobbs

This is my current thinking. I can get my AC feedback from a local
node that I can control the dynamics of, and get DC fb from the nasty
remote sense. The notch filter really helps kill 250 KHz and above,
and its impedance actually helps the control loop a little.

https://www.dropbox.com/s/wf8rq1ziypt1vjn/ACDC_sense.jpg?dl=0

https://www.dropbox.com/s/g4qba0cjly7blbq/PS_Filt_3.jpg?dl=0

https://www.dropbox.com/s/m6pg94dxrmencq3/PS_Filt_3.asc?dl=0

And I thought power supplies were simple.

I guess my HF filter could be un-notched too, with a bigger L maybe.
I\'ll try that.

I sometimes do the split AC/DC feedback thing wrapped round a cap
multiplier. It does need a buffer to break the sneak path from the
output reservoir cap to the output via the RC diplexer.

The ESR on the 1000 uF cap is probably on the high side. I\'m using some
nice 220 uF alpos with 25 mohm ESR.

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

 

[John Larkin]

On Mon, 27 Jun 2022 15:37:08 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Mon, 27 Jun 2022 08:00:18 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

Cheers

Phil Hobbs

This is my current thinking. I can get my AC feedback from a local
node that I can control the dynamics of, and get DC fb from the nasty
remote sense. The notch filter really helps kill 250 KHz and above,
and its impedance actually helps the control loop a little.

https://www.dropbox.com/s/wf8rq1ziypt1vjn/ACDC_sense.jpg?dl=0

https://www.dropbox.com/s/g4qba0cjly7blbq/PS_Filt_3.jpg?dl=0

https://www.dropbox.com/s/m6pg94dxrmencq3/PS_Filt_3.asc?dl=0

And I thought power supplies were simple.

I guess my HF filter could be un-notched too, with a bigger L maybe.
I\'ll try that.

I sometimes do the split AC/DC feedback thing wrapped round a cap
multiplier. It does need a buffer to break the sneak path from the
output reservoir cap to the output via the RC diplexer.

The ESR on the 1000 uF cap is probably on the high side. I\'m using some
nice 220 uF alpos with 25 mohm ESR.

Cheers

Phil Hobbs

I need that ESR to tame the phase shift at node MID, so we can close a
reasonable loop. It will probably be an actual resistor.

--

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 Mon, 27 Jun 2022 15:37:08 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Mon, 27 Jun 2022 08:00:18 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

Cheers

Phil Hobbs

This is my current thinking. I can get my AC feedback from a local
node that I can control the dynamics of, and get DC fb from the nasty
remote sense. The notch filter really helps kill 250 KHz and above,
and its impedance actually helps the control loop a little.

https://www.dropbox.com/s/wf8rq1ziypt1vjn/ACDC_sense.jpg?dl=0

https://www.dropbox.com/s/g4qba0cjly7blbq/PS_Filt_3.jpg?dl=0

https://www.dropbox.com/s/m6pg94dxrmencq3/PS_Filt_3.asc?dl=0

And I thought power supplies were simple.

I guess my HF filter could be un-notched too, with a bigger L maybe.
I\'ll try that.

I sometimes do the split AC/DC feedback thing wrapped round a cap
multiplier. It does need a buffer to break the sneak path from the
output reservoir cap to the output via the RC diplexer.

The ESR on the 1000 uF cap is probably on the high side. I\'m using some
nice 220 uF alpos with 25 mohm ESR.

Cheers

Phil Hobbs

I need that ESR to tame the phase shift at node MID, so we can close a
reasonable loop. It will probably be an actual resistor.

Better be a honking big pulse rated job, then. A short could
potentially dump

0.5 * 57V **2 *0.001F = 1.65 J

into that poor little resistor in under a millisecond. Toasty!

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 Tue, 28 Jun 2022 12:34:30 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Mon, 27 Jun 2022 15:37:08 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Mon, 27 Jun 2022 08:00:18 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Fri, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


snip circuit

Interesting. I use notch filters in feedback loops for resonant
actuators. They\'re the bomb for that, because the resonance is usually
simple and isolated, so notching it out lets you use a much wider
feedback BW.

I\'ve played with them for switchers, but have never used one because
they don\'t work that well with harmonic-rich waveforms (especially
highly asymmetric ones). I\'m usually happier keeping the extra two
poles at high frequency.

Cheers

Phil Hobbs

This is my current thinking. I can get my AC feedback from a local
node that I can control the dynamics of, and get DC fb from the nasty
remote sense. The notch filter really helps kill 250 KHz and above,
and its impedance actually helps the control loop a little.

https://www.dropbox.com/s/wf8rq1ziypt1vjn/ACDC_sense.jpg?dl=0

https://www.dropbox.com/s/g4qba0cjly7blbq/PS_Filt_3.jpg?dl=0

https://www.dropbox.com/s/m6pg94dxrmencq3/PS_Filt_3.asc?dl=0

And I thought power supplies were simple.

I guess my HF filter could be un-notched too, with a bigger L maybe.
I\'ll try that.

I sometimes do the split AC/DC feedback thing wrapped round a cap
multiplier. It does need a buffer to break the sneak path from the
output reservoir cap to the output via the RC diplexer.

The ESR on the 1000 uF cap is probably on the high side. I\'m using some
nice 220 uF alpos with 25 mohm ESR.

Cheers

Phil Hobbs

I need that ESR to tame the phase shift at node MID, so we can close a
reasonable loop. It will probably be an actual resistor.


Better be a honking big pulse rated job, then. A short could
potentially dump

0.5 * 57V **2 *0.001F = 1.65 J

into that poor little resistor in under a millisecond. Toasty!

Cheers

Phil Hobbs

It kills efficiency too. AoE X-chapters has nice data on exploding
various resistors.

The other approach is to use an LCLC filter with an effective
bandwidth in the KHz region, and let it flail the phase all it wants
up there, as long as it doesn\'t wreck our maybe 350 Hz control loop.

Something roughly like

https://www.dropbox.com/s/95jmjayj0cykykg/PS_Fast_Filt_1.jpg?raw=1

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

--
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, 24 Jun 2022 12:45:41 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
I never thought a lot about general-purpose bench type power supplies,
but now we have to design some.

A power supply has two knobs (or SCPI commands in our case), voltage
and current limit.

A power supply should have low impedance at high frequencies, so after
whatever current limit circuit is has, there must be a real capacitor.
When you short a bench supply, you get a spark from the energy in the
output cap. So for a while, it\'s not really current limited.

Our supply will be a buck switcher

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

so we need an LC lowpass filter. It has to kill the 250 KHz ripple but
allow reasonable programmable voltage slew rates. We\'ll close a
feedback loop from the voltage sensor ADC into the bridge PWM drive,
so the filter has to be well behaved. Maybe we need the R3C3 damper to
kill the Q of L1C1 so the loop doesn\'t go bonkers.

As if that isn\'t bad enough, the customer load could be most anything,
a short or a resistor or a box with big input caps. Or a big DC bus.
Or even a battery. So our filter gets messed with by the customer.

And a buck switcher is a boost switcher backwards. If the customer
gadget sources more voltage than our setpoint, we extract power from
the customer and charge C9 and blow everything up. We can sense the
+60 and shut off both fets, I guess.

We also need a well-behaved current-limit loop.

When I get time, I might prowl the web for old power supply
schematics, HP or Kepco or whatever, and see what their output caps
are like and how they managed the voltage/current dynamics. Those
would be mostly linear supplies, I guess.

Wild guesses: switch at 250 KHz. Output 0 to 48v at 0 to 6 amps. L1 is
180 uH. C2 could be 10 to 300 uF. Loop bandwidth 1 KHz.

We will probably add a secondary lowpass filter with a notch at 250K,
to un-compromise the main L1C2 filter, but that won\'t affect than main
loop dynamics.

There\'s a chapter(*) in one of Jim Willams\' books about a guy who built
big SMUish things using a \'1/2 pole\' rolloff--a bunch of lead-lags that
approximated a 10 dB/decade, 45-degree phase shift network. At that
point it didn\'t matter what the load capacitance was, the loop was
always stable. It\'s probably possible to make a digital version of that.

Cheers

Phil Hobbs

(*) Phil Perkins, \"My approach to feedback loop design\", Ch 22 of Jim
Williams, _Analog Circuit Design: Art, Science, and Personalities_

Here\'s a possible filter.

The ESR could be native to some electrolytic caps, but probably added.
They will get warm from the 250 KHz ripple current from our
half-bridge switcher, which encourages a big inductor.

https://www.dropbox.com/s/enuvum2gt0nzbf2/Isol_PS_1.jpg?raw=1

Of course we don\'t know how much load capacitance we\'d ever see; could
be a farad. I was thinking that we\'re measuring the current, so we can
use that info to help compensate big caps. Maybe differentiate it and
squirt into the loop or something. After/if I wake up I might close
the loop and play with that.

I have the Williams books; I\'ll look that up.


Version 4
SHEET 1 1348 680
WIRE -112 96 -160 96
WIRE -48 96 -112 96
WIRE 144 96 32 96
WIRE 288 96 224 96
WIRE 432 96 288 96
WIRE 528 96 432 96
WIRE 672 96 608 96
WIRE 720 96 672 96
WIRE 880 96 800 96
WIRE 1040 96 880 96
WIRE 1088 96 1040 96
WIRE 1248 96 1088 96
WIRE 1344 96 1248 96
WIRE 288 144 288 96
WIRE -160 176 -160 96
WIRE 672 192 672 96
WIRE 736 192 672 192
WIRE 880 192 880 96
WIRE 880 192 800 192
WIRE 432 208 432 96
WIRE 1088 208 1088 96
WIRE 1248 224 1248 96
WIRE 288 256 288 208
WIRE 880 256 880 192
WIRE -160 384 -160 256
WIRE 288 384 288 336
WIRE 432 384 432 272
WIRE 880 384 880 320
WIRE 1088 384 1088 288
WIRE 1248 384 1248 288
FLAG 288 384 0
FLAG -160 384 0
FLAG 1088 384 0
FLAG 1248 384 0
FLAG 432 384 0
FLAG 1040 96 OUT
FLAG -112 96 GEN
FLAG 880 384 0
SYMBOL ind 128 112 R270
WINDOW 0 -33 52 VTop 2
WINDOW 3 -38 53 VBottom 2
SYMATTR InstName L1
SYMATTR Value 50µ
SYMBOL cap 272 144 R0
WINDOW 0 51 21 Left 2
WINDOW 3 48 50 Left 2
SYMATTR InstName C1
SYMATTR Value 1m
SYMBOL res 272 240 R0
WINDOW 0 51 44 Left 2
WINDOW 3 46 75 Left 2
SYMATTR InstName Resr
SYMATTR Value 250m
SYMBOL res -64 112 R270
WINDOW 0 -33 55 VTop 2
WINDOW 3 -41 54 VBottom 2
SYMATTR InstName Rgen
SYMATTR Value 100m
SYMBOL voltage -160 160 R0
WINDOW 0 32 11 Left 2
WINDOW 3 38 73 Left 2
WINDOW 123 41 42 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 1 1K)
SYMATTR Value2 AC 1
SYMBOL res 1072 192 R0
WINDOW 0 -78 35 Left 2
WINDOW 3 -68 66 Left 2
SYMATTR InstName Rload
SYMATTR Value 100
SYMBOL cap 1232 224 R0
WINDOW 0 -81 2 Left 2
WINDOW 3 -68 37 Left 2
SYMATTR InstName Cload
SYMATTR Value 1m
SYMBOL cap 416 208 R0
WINDOW 0 56 21 Left 2
WINDOW 3 51 49 Left 2
SYMATTR InstName C3
SYMATTR Value 10µ
SYMBOL res 512 112 R270
WINDOW 0 -39 65 VTop 2
WINDOW 3 -47 59 VBottom 2
SYMATTR InstName Rshunt
SYMATTR Value 25m
SYMBOL ind 704 112 R270
WINDOW 0 -30 34 VTop 2
WINDOW 3 -2 95 VBottom 2
SYMATTR InstName L2
SYMATTR Value 10µ
SYMBOL cap 800 176 R90
WINDOW 0 67 63 VBottom 2
WINDOW 3 40 -4 VTop 2
SYMATTR InstName C2
SYMATTR Value 40n
SYMBOL cap 864 256 R0
WINDOW 0 48 20 Left 2
WINDOW 3 49 49 Left 2
SYMATTR InstName C4
SYMATTR Value 5µ
TEXT -16 288 Left 2 !.ac dec 20 100 300k
TEXT -24 328 Left 2 ;Power Supply Filter
TEXT -16 360 Left 2 ;JL Jun 24 2022
TEXT 728 272 Left 2 ;250 KHz
TEXT 752 304 Left 2 ;trap