Driving capacitive loads with an H bridge
in [Design]
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From: Grant on 25 Jul 2010 21:33 On Sun, 25 Jul 2010 16:00:54 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 26 Jul 2010 08:49:12 +1000, Grant <omg(a)grrr.id.au> wrote: > > >>>The four switches here >>> >>>ftp://jjlarkin.lmi.net/Triangle_Cap.JPG >>> >>>make an h-bridge. The a-a and b-b switch pairs are alternately turned >>>on. So the power source can be connected to the load (the capacitor >>>here) in one direction or the other. Or the two bottom switches could >>>be turned on to short the load, or all four turned off to open it. The >>>switches are usually transistors of some sort, and fully integrated >>>h-bridge chips are common. >>> >>>H-bridges are commonly used to drive motors and speakers, using >>>pulse-width modulation to control how much drive goes into the load. >>>The power source would ususlly be a voltage, not a current like in my >>>circuit. They allow you to, theoretically, make a 100% efficient >>>amplifier. >> >>But then one may consider delivering a constant current into / from >>a cap as not being efficient, since not a tuned circuit? Can you >>really have this both ways? > >Sure. The sim works with no lossy parts at all, just one L and one C >and the switches. An actual implementation could have only minor >losses. > >This *is* a resonant circuit. The switches just time-warp it in big >jumps so that we only use the segments of the sine wave that appeal to >us. Just skip over the parts you don't like. It's amazing what a tuned circuit can do, I remember driving big inductive loop (think doorway size) as tuned circuit with a bullet proof RS485 diff. driver chip and a seeing a whopping great signal in the loop, tuned with roughly binary value sequence caps with an 8way dip switch. Grant. > >John >
From: Tim Williams on 25 Jul 2010 22:22 "Grant" <omg(a)grrr.id.au> wrote in message news:jtfp46pvifns5kho0up43joc6bfo926gep(a)4ax.com... > PP, whazzat? Mind blank. Push-pull. Instead of driving both ends of the load, as H bridge, or just one end, as half bridge, you drive a tapped winding from each side. You can only pull down on the winding from either end, so transformer action fills in the rest: a pull on one end looks like a push from the other. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Grant on 26 Jul 2010 05:57 On Sun, 25 Jul 2010 21:22:35 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Grant" <omg(a)grrr.id.au> wrote in message news:jtfp46pvifns5kho0up43joc6bfo926gep(a)4ax.com... >> PP, whazzat? Mind blank. > >Push-pull. Instead of driving both ends of the load, as H bridge, or just one end, as half bridge, you drive a tapped winding from each side. You can only pull down on the winding from either end, so transformer action fills in the rest: a pull on one end looks like a push from the other. Yeah, know what push-pull is :) But PP recently for me is polypropylene film caps. Rarely see push-pull transformer these days, more common to see half or full bridge drive, to utilise copper all the time instead of half the time? Though centre-tap output windings are common, to save on expensive diodes, so copper utilisation perhaps a poor argument? Grant.
From: Winfield Hill on 26 Jul 2010 08:35 markp wrote... > > On Jul 21, "markp" wrote: > >>> 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. > > Oh dear! Just to be clear, by energy retrieval I meant the stored energy > in the capacitor when charged needs to be recovered back when discharging > so the cycle can repeat and process is efficient, much like a resonant LC > oscillator but with triangle waves instead of sine waves. See my post to > Jim. It's a lot of fuss to save a few watts. For 2uF, 138.5Vpp and 200Hz I calculate you need a roughly 110mA square-wave drive current. A resonant inductor would be 0.32H, which is a high inductance, and to insure a linear ramp, rather than a sine wave, you'd need a much higher inductance than that (did you say how much nonlinearity you can tolerate?). What's more, the H-bridge involved must be made from floating bidirectional switches. Ouch. So it appears any full-cycle energy-storage idea is going to be very painful. OTOH, a +/-110mA class-D chopper current-source drive with +/-70V compliance would be relatively easy, using components created for the high-power audio market. Class D also uses energy storage you know. But you said you'd not like chopper noise, so what's so bad about less than 8 watts of dissipation in a simple linear circuit? -- Thanks, - Win
From: Jim Thompson on 26 Jul 2010 10:50
On 26 Jul 2010 05:35:02 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >markp wrote... >> >> On Jul 21, "markp" wrote: >> >>>> 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. >> >> Oh dear! Just to be clear, by energy retrieval I meant the stored energy >> in the capacitor when charged needs to be recovered back when discharging >> so the cycle can repeat and process is efficient, much like a resonant LC >> oscillator but with triangle waves instead of sine waves. See my post to >> Jim. > > It's a lot of fuss to save a few watts. For 2uF, 138.5Vpp and 200Hz > I calculate you need a roughly 110mA square-wave drive current. > > A resonant inductor would be 0.32H, which is a high inductance, and > to insure a linear ramp, rather than a sine wave, you'd need a much > higher inductance than that (did you say how much nonlinearity you > can tolerate?). What's more, the H-bridge involved must be made > from floating bidirectional switches. Ouch. So it appears any > full-cycle energy-storage idea is going to be very painful. > > OTOH, a +/-110mA class-D chopper current-source drive with +/-70V > compliance would be relatively easy, using components created for the > high-power audio market. Class D also uses energy storage you know. > > But you said you'd not like chopper noise, so what's so bad about > less than 8 watts of dissipation in a simple linear circuit? Agreed! Look back at... Message-ID: <pshp46dqpnrd688pdia6hlm25iqvat7nl0(a)4ax.com> From the OP's loose description (he says a few amps, which I took as 2A), I got 36uF :-( ...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. |