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.
power supply subtelties
Started by ●June 24, 2022
Reply by ●June 27, 20222022-06-27
Reply by ●June 27, 20222022-06-27
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
Reply by ●June 27, 20222022-06-27
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 HobbsI 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
Reply by ●June 28, 20222022-06-28
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
Reply by ●June 28, 20222022-06-28
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 HobbsIt 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
