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
power supply subtelties
Started by ●June 24, 2022
Reply by ●June 24, 20222022-06-24
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
Reply by ●June 25, 20222022-06-25
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. 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Reply by ●June 27, 20222022-06-27
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
Reply by ●June 27, 20222022-06-27
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
Reply by ●June 27, 20222022-06-27
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
Reply by ●June 27, 20222022-06-27
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 HobbsThis 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.
Reply by ●June 27, 20222022-06-27
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
Reply by ●June 27, 20222022-06-27
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.
Reply by ●June 27, 20222022-06-27
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.
