Driving capacitive loads with an H bridge
in [Design]
|
From: John Larkin on 21 Jul 2010 12:19 On Wed, 21 Jul 2010 17:04:49 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:cb3e46lt9jra176n20k902menqcsnv9vej(a)4ax.com... >> On Wed, 21 Jul 2010 09:44:35 -0500, "Tim Williams" >> <tmoranwms(a)charter.net> wrote: >> >>>Assuming you mean triangle voltage waveform (= square current waveform), >>>you can do this with an H bridge and an arbitrarily large inductor in >>>series with the supply. You will actually have an arc segment of the LC >>>oscillation, so you need a big L to make the frequency low enough that the >>>arc looks straight. >>> >>>Tim >> >> The system becomes an LC lowpass filter, so there is in theory a >> higher-order filter that makes a better triangle wave. The first step >> would be a smaller LC ahead of the main LC, such as to tend to >> increase the drive to the output LC during the cycle. Tapped >> inductors, or a compensating series transformer, would be interesting >> too. >> >> A little saturation could be interesting, too, like the linearity >> correction inductor in an old teevee set. >> >> But, as usual, the problem is underspecified. >> >> John >> > >A little more info then, >40V AC (true RMS) across the load capacitor (a few >uF) and at a few hundred Hertz. > >I'm wondering whether a single inductor with a half-bridge PWM'd might do. I >could ground one end of the cap and use +/- V DC supplies to provide the >high and low DC to the half-bridge. One problem with that might be the >voltage swings at the PWM from a noise point of view, and like you say might >require another filter prior to the load cap. Thoughts? > >Mark. > Is frequency constant? Full-bridge sounds nicer to me, because you'd have twice the voltage to work with, for better linearity, and zero inherent DC offset if you use square-wave drive. If tiny DC offset is a problem, use feedback to tweak the duty cycle, or AC couple (!) into the capacitive load. To get a mostly linear triangle, using just one inductor, you'll need a big ratio of power supply voltage to cap swing voltage. Interestingly, the driver, in theory, dissipates no power. A more precise way to do this would be a switchmode (class D) amp, an output lowpass filter, then your cap. Close a voltage feedback loop around all that, tracking a triangle generator. Or even use a linear amp, if the power requirements aren't excessive. A linear amp would be the most precise. It all depends on the kind of linearity, dc offset, and amplitude stability requirements you have. I doesn't seem to me like a precise statement of the waveform requirements, load, and frequency range would give away any secrets, given that we have not a clue as to why anyone would want to do this to a capacitor. This ng is plagued by paritally-stated problems posed by people who think they have secrets. John
From: legg on 21 Jul 2010 20:35 On Wed, 21 Jul 2010 17:13:23 +0100, "markp" <map.nospam(a)f2s.com> wrote: > >"Nunya" <jack_shephard(a)cox.net> wrote in message >news:836af8b3-6b92-4292-bbc5-63fabc53c3c4(a)s24g2000pri.googlegroups.com... >On Jul 21, 8:45 am, "markp" <map.nos...(a)f2s.com> wrote: >> <langw...(a)fonz.dk> wrote in message >> >> news:a02cd398-9bd8-4315-8c89-4b86481a609e(a)f33g2000yqe.googlegroups.com... >> >> >> >>> > On 21 Jul., 13:48, "markp" <map.nos...(a)f2s.com> wrote: >>> >> Hi All, >>> >>> >> I need to drive capacitor with a triangle wave with no DC across the >>> >> capacitor (i.e. a symmetrical bipolar drive but triangular) but it has >>> >> to >>> >> be >>> >> efficient, i.e some kind of energy retrieval. >>> >>> >> Is it possible to use standard H bridge circuits to do this? Does >>> >> anyone >>> >> have any links or app notes? >>> >>> >> Thanks! >>> >> Mark. >>> >>> > what kind of frequencies, voltages? >>> > class-d audio amplifier? >>> > -Lasse >>> >>> I can't say much for NDA reasons, but assume >40V AC (true RMS) and a few >>> hundred Hertz, so there are potentially a few amps flying about in the >>> load >>> capacitor. >>> >>> Mark > >> Talking about driving a capacitive load with HVDC is pretty easy >> when >> particulars are few or missing. Driving one with AC would require a >> bit >> more info. And you say it is in the area of a couple uF?! Sorry, NDA >> or >> not, you would need to explain more to get viable responses. > >> You could float the voltage up just enough that it goes negative only >> for >> a tiny fragment of the waveform. It would still be AC then, >> technically. > >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
From: markp on 21 Jul 2010 19:53 "legg" <legg(a)nospam.magma.ca> wrote in message news:fb4f469jdpnke03g33q9f4aqgdrfk5lk8g(a)4ax.com... > On Wed, 21 Jul 2010 17:13:23 +0100, "markp" <map.nospam(a)f2s.com> > wrote: > >> >>"Nunya" <jack_shephard(a)cox.net> wrote in message >>news:836af8b3-6b92-4292-bbc5-63fabc53c3c4(a)s24g2000pri.googlegroups.com... >>On Jul 21, 8:45 am, "markp" <map.nos...(a)f2s.com> wrote: >>> <langw...(a)fonz.dk> wrote in message >>> >>> news:a02cd398-9bd8-4315-8c89-4b86481a609e(a)f33g2000yqe.googlegroups.com... >>> >>> >>> >>>> > On 21 Jul., 13:48, "markp" <map.nos...(a)f2s.com> wrote: >>>> >> Hi All, >>>> >>>> >> I need to drive capacitor with a triangle wave with no DC across the >>>> >> capacitor (i.e. a symmetrical bipolar drive but triangular) but it >>>> >> has >>>> >> to >>>> >> be >>>> >> efficient, i.e some kind of energy retrieval. >>>> >>>> >> Is it possible to use standard H bridge circuits to do this? Does >>>> >> anyone >>>> >> have any links or app notes? >>>> >>>> >> Thanks! >>>> >> Mark. >>>> >>>> > what kind of frequencies, voltages? >>>> > class-d audio amplifier? >>>> > -Lasse >>>> >>>> I can't say much for NDA reasons, but assume >40V AC (true RMS) and a >>>> few >>>> hundred Hertz, so there are potentially a few amps flying about in the >>>> load >>>> capacitor. >>>> >>>> Mark >> >>> Talking about driving a capacitive load with HVDC is pretty easy >>> when >>> particulars are few or missing. Driving one with AC would require a >>> bit >>> more info. And you say it is in the area of a couple uF?! Sorry, NDA >>> or >>> not, you would need to explain more to get viable responses. >> >>> You could float the voltage up just enough that it goes negative only >>> for >>> a tiny fragment of the waveform. It would still be AC then, >>> technically. >> >>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.
From: Jim Thompson on 21 Jul 2010 21:50 On Wed, 21 Jul 2010 17:13:23 +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 > "...but it has to be efficient, i.e some kind of energy retrieval." What does that mean? Some particulars could get you some suggestions, vagueness will lose our interest. ...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: amark on 21 Jul 2010 22:12
On Jul 21, 9:48 pm, "markp" <map.nos...(a)f2s.com> wrote: > Hi All, > > I need to drive capacitor with a triangle wave with no DC across the > capacitor (i.e. a symmetrical bipolar drive but triangular) but it has to be > efficient, i.e some kind of energy retrieval. > > Is it possible to use standard H bridge circuits to do this? Does anyone > have any links or app notes? > > Thanks! > Mark. There is a classic circuit which is an op-amp integrator, followed by an inverting comparator whose o/p is fed back to the integrator. What's an H bridge? |