Driving capacitive loads with an H bridge
in [Design]
|
From: Jim Thompson on 23 Jul 2010 10:42 On Fri, 23 Jul 2010 15:35:30 +1000, Grant <omg(a)grrr.id.au> wrote: >On Thu, 22 Jul 2010 19:11:06 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Fri, 23 Jul 2010 12:04:46 +1000, Grant <omg(a)grrr.id.au> wrote: >> >>>On Thu, 22 Jul 2010 21:56:35 -0500, legg <legg(a)nospam.magma.ca> wrote: >>> >>>>On Thu, 22 Jul 2010 00:53:47 +0100, "markp" <map.nospam(a)f2s.com> >>>>wrote: >>>> >>>><snip> >>>>>>>I'm not looking for a design as such, just a possible architecture. So far >>>>>>>the info you've been given is a triangle waveform across the cap, bipolar >>>>>>>so >>>>>>>no DC component, >40V AC(true RMS), a few hundred Hertz and a question >>>>>>>whether an H-bridge architecture could do it. I'm not sure how much more >>>>>>>information you want...:) >>>>>>> >>>>>>>Mark >>>>>>> >>>>>> A single ended load would require a single ended source - a >>>>>> half-bridge. >>>>>> >>>>>> A triangle wave is generated by a constant current, reversing, but >>>>>> residual DC can only be limited by the accuracy of the modulator. >>>>>> >>>>>> Driving purely inductive or purely capacitive loads is as efficient as >>>>>> driving a short circuit, no matter what the drive method. >>>>>> >>>>>> RL >>>>> >>>>>Come again? I can create a pair of constant current sources with transistors >>>>>and resistors and alternately charge and discharge the capacitor with them. >>>>>But that's not efficient... >>>>> >>>>>Mark. >>>>> >>>>Neither is any other method of driving a short circuit. >>>> >>>>Given a large enough and perfect enough inductor, and ideal switches, >>>>a short or a capacitor can be driven efficiently by reversing the >>>>inductor's polarity at the waveform peak. That would require 4 >>>>switches. >>>> >>>>Given imperfect and realistically sized components, a half bridge can >>>>reverse it's output inductor current in a finite time period while >>>>supplying approximately constant current of the correct polarity, with >>>>a modest ripple component and reasonable losses. >>> >>>Wouldn't it be easier to run the H bridge as switching current >>>direction only, with current through the bridge controlled by a >>>separate switching regulator? >> >>(*) >> >>> >>>Saves talking about ginormous inductors, for starters. Might >>>even be buildable ;) >>> >>>Grant. >> >>That would meet the "shape" requirement, but I still ponder what does >>"...it has to be efficient, i.e. some kind of energy retrieval" mean? > >Dunno, may be this is a piezoelectric thingy that wants to squish >the current back out when being relaxed at a controlled rate? > >Rotate the bridge 90' so the capacitor voltage see-saws and the >charge doesn't fall out? ;^) >> >>* You'd need some kind of loop to keep it "centered" also. > >Yeah, that too, unless that thing is taken rail to rail, self >centering? > >Grant. Yep, You do need to be careful to keep the charge from falling off the plates ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Only as good as the person behind the wheel.
From: John Fields on 23 Jul 2010 12:22 On Fri, 23 Jul 2010 10:57:14 +0100, "markp" <map.nospam(a)f2s.com> wrote: >>>"John Fields" <jfields(a)austininstruments.com> wrote in message >>>news:vbtg46ditdftdgl5hg3i06b84oj0irdhoc(a)4ax.com... >>>> Here's six circuits: >>>> >>>> On the top, an "H" bridge driving a capacitor, a series resonant >>>> circuit, and a parallel resonant circuit, and on the bottom a bipolar >>>> half-bridge driving the same circuits. >>>> >>>> Does it look like any of them will do what you want? >Thanks for that. Yes, one of these is going to solve it - my preference at >the moment is not actually an H bridge for noise reasons but rather a >resonant type oscillator like the one in the top right of the schematic, if >I can make the waveform triangular. --- That _is_ an H bridge; notice that the drivers are two voltage sources 180 degrees out of phase. --- >That actually *might* be possible by >using a class D oscillator and forcing additional current in and out of the >drive windings, not sure yet. The question is how the driving sources are >implemented. For example the top right (a capacitor driven by two voltage >sources either side) would need to have a current source as well, and that >current source mustn't be resistive so that the energy used to charge the >capacitor in one direction is recovered from the capacitor and not thrown >away by resistive losses. --- How much power are you willing to lose? After all, it's not likely that you'll be able to build a "perpetuum mobile." ;) As I see it you have few choices if you want to make the waveform across the capacitor triangular. Here are two: 1. Use a bipolar resistive current source and drive it with a square wave. 2. Change the inductance of the choke with respect to time so its increasing and decreasing reactance does the current limiting. Either way is going to cost you power, so what efficiency would you _like_ to see?
From: Fred Bartoli on 23 Jul 2010 12:46 Jim Thompson a �crit : > On Thu, 22 Jul 2010 21:56:35 -0500, legg <legg(a)nospam.magma.ca> wrote: > >> On Thu, 22 Jul 2010 00:53:47 +0100, "markp" <map.nospam(a)f2s.com> >> wrote: >> >> <snip> >>>>> I'm not looking for a design as such, just a possible architecture. So far >>>>> the info you've been given is a triangle waveform across the cap, bipolar >>>>> so >>>>> no DC component, >40V AC(true RMS), a few hundred Hertz and a question >>>>> whether an H-bridge architecture could do it. I'm not sure how much more >>>>> information you want...:) >>>>> >>>>> Mark >>>>> >>>> A single ended load would require a single ended source - a >>>> half-bridge. >>>> >>>> A triangle wave is generated by a constant current, reversing, but >>>> residual DC can only be limited by the accuracy of the modulator. >>>> >>>> Driving purely inductive or purely capacitive loads is as efficient as >>>> driving a short circuit, no matter what the drive method. >>>> >>>> RL >>> Come again? I can create a pair of constant current sources with transistors >>> and resistors and alternately charge and discharge the capacitor with them. >>> But that's not efficient... >>> >>> Mark. >>> >> Neither is any other method of driving a short circuit. >> >> Given a large enough and perfect enough inductor, and ideal switches, >> a short or a capacitor can be driven efficiently by reversing the >> inductor's polarity at the waveform peak. That would require 4 >> switches. >> >> Given imperfect and realistically sized components, a half bridge can >> reverse it's output inductor current in a finite time period while >> supplying approximately constant current of the correct polarity, with >> a modest ripple component and reasonable losses. >> >> RL > > The OP (markp) implies this is a _real_ situation. How do you meet > his requirement "...it has to be efficient, i.e. some kind of energy > retrieval" ?? > > ...Jim Thompson ROFL! You'd better not write such nonsense when you intend to blast everybody that don't fit your taste here... I take it that you didn't think much when writing this, which anyway shouldn't be with the posture you choose to display, but even, understanding this doesn't take much thinking, so? -- Thanks, Fred.
From: markp on 23 Jul 2010 12:58 "John Fields" <jfields(a)austininstruments.com> wrote in message news:ufej46l3bfjvvkrov55oi2d1sin73oiqof(a)4ax.com... > On Fri, 23 Jul 2010 10:57:14 +0100, "markp" <map.nospam(a)f2s.com> > wrote: > >>>>"John Fields" <jfields(a)austininstruments.com> wrote in message >>>>news:vbtg46ditdftdgl5hg3i06b84oj0irdhoc(a)4ax.com... > > >>>>> Here's six circuits: >>>>> >>>>> On the top, an "H" bridge driving a capacitor, a series resonant >>>>> circuit, and a parallel resonant circuit, and on the bottom a bipolar >>>>> half-bridge driving the same circuits. >>>>> >>>>> Does it look like any of them will do what you want? > >>Thanks for that. Yes, one of these is going to solve it - my preference at >>the moment is not actually an H bridge for noise reasons but rather a >>resonant type oscillator like the one in the top right of the schematic, >>if >>I can make the waveform triangular. > > --- > That _is_ an H bridge; notice that the drivers are two voltage sources > 180 degrees out of phase. > --- > >>That actually *might* be possible by >>using a class D oscillator and forcing additional current in and out of >>the >>drive windings, not sure yet. The question is how the driving sources are >>implemented. For example the top right (a capacitor driven by two voltage >>sources either side) would need to have a current source as well, and that >>current source mustn't be resistive so that the energy used to charge the >>capacitor in one direction is recovered from the capacitor and not thrown >>away by resistive losses. > > --- > How much power are you willing to lose? > > After all, it's not likely that you'll be able to build a "perpetuum > mobile." ;) > > As I see it you have few choices if you want to make the waveform > across the capacitor triangular. > > Here are two: > > 1. Use a bipolar resistive current source and drive it with a square > wave. No good, way to lossy. > > 2. Change the inductance of the choke with respect to time so its > increasing and decreasing reactance does the current limiting. Now that's interesting. A saturable core reactor that responds quickly enough could do it. Now there's I2R losses in doing that, you have to drive current through a coil, the big question is whether this would actually not be that lossy. The question becomes something like this: If you had a certain current flowing through a saturable core inductor the energy stored in it is (I^2L)/2. So if the current remains the same, and you change its inductance by changing the control current, what happens to the stored energy? > Either way is going to cost you power, so what efficiency would you > _like_ to see? > Something like 85% would be nice.
From: Jim Thompson on 23 Jul 2010 13:00
On Fri, 23 Jul 2010 18:46:13 +0200, Fred Bartoli <" "> wrote: >Jim Thompson a �crit : >> On Thu, 22 Jul 2010 21:56:35 -0500, legg <legg(a)nospam.magma.ca> wrote: >> >>> On Thu, 22 Jul 2010 00:53:47 +0100, "markp" <map.nospam(a)f2s.com> >>> wrote: >>> >>> <snip> >>>>>> I'm not looking for a design as such, just a possible architecture. So far >>>>>> the info you've been given is a triangle waveform across the cap, bipolar >>>>>> so >>>>>> no DC component, >40V AC(true RMS), a few hundred Hertz and a question >>>>>> whether an H-bridge architecture could do it. I'm not sure how much more >>>>>> information you want...:) >>>>>> >>>>>> Mark >>>>>> >>>>> A single ended load would require a single ended source - a >>>>> half-bridge. >>>>> >>>>> A triangle wave is generated by a constant current, reversing, but >>>>> residual DC can only be limited by the accuracy of the modulator. >>>>> >>>>> Driving purely inductive or purely capacitive loads is as efficient as >>>>> driving a short circuit, no matter what the drive method. >>>>> >>>>> RL >>>> Come again? I can create a pair of constant current sources with transistors >>>> and resistors and alternately charge and discharge the capacitor with them. >>>> But that's not efficient... >>>> >>>> Mark. >>>> >>> Neither is any other method of driving a short circuit. >>> >>> Given a large enough and perfect enough inductor, and ideal switches, >>> a short or a capacitor can be driven efficiently by reversing the >>> inductor's polarity at the waveform peak. That would require 4 >>> switches. >>> >>> Given imperfect and realistically sized components, a half bridge can >>> reverse it's output inductor current in a finite time period while >>> supplying approximately constant current of the correct polarity, with >>> a modest ripple component and reasonable losses. >>> >>> RL >> >> The OP (markp) implies this is a _real_ situation. How do you meet >> his requirement "...it has to be efficient, i.e. some kind of energy >> retrieval" ?? >> >> ...Jim Thompson > > >ROFL! > >You'd better not write such nonsense when you intend to blast everybody >that don't fit your taste here... > >I take it that you didn't think much when writing this, which anyway >shouldn't be with the posture you choose to display, but even, >understanding this doesn't take much thinking, so? Where's the "energy retrieval"? An H-bridge driven from an inductor doesn't "retrieve". I see nada of substance in your comments. Another "Bloviator" ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Friday is Wine and Cheeseburger Day |