Driving capacitive loads with an H bridge
in [Design]
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From: Tim Williams on 25 Jul 2010 14:38 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:4qso4618ig6ls9d2vq67vrh681l39mqe19(a)4ax.com... > 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. Hard to do for more than a few milivolts, though. For large signals, you have to use back-to-back MOSFETs and *lots* of floating gate drives. Incidentially, I recently made a synchronous rectifier with BJTs. PP, not bridge. It passes up to +/-200mV in either direction. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Fields on 25 Jul 2010 15:06 On Sun, 25 Jul 2010 10:38:53 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >This particular circuit can drive a triangle wave into the capacitive >load with, theoretically, zero power required after startup. --- Now, now... In the real world, 10 henry 100 milliampere chokes exhibit a winding resistance of somewhere in the vicinity of 250 ohms, so you're going to take a hit with every reversal. --- >In practise, it would be far more efficient than a linear amplifier. --- Specifically, Mr. fast-and-loose, how much more? (Show your work, please.) --- >Hmmm, you can also turn on all four switches in an h-bridge. --- Hmmm...??? New to H bridges? ;)
From: Jim Thompson on 25 Jul 2010 15:10 On Sun, 25 Jul 2010 14:06:10 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Sun, 25 Jul 2010 10:38:53 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>This particular circuit can drive a triangle wave into the capacitive >>load with, theoretically, zero power required after startup. > >--- >Now, now... > >In the real world, 10 henry 100 milliampere chokes exhibit a winding >resistance of somewhere in the vicinity of 250 ohms, so you're going >to take a hit with every reversal. >--- > >>In practise, it would be far more efficient than a linear amplifier. > >--- >Specifically, Mr. fast-and-loose, how much more? (Show your work, >please.) John "The Bloviator" Larkin show his work??? Dream on. > >>Hmmm, you can also turn on all four switches in an h-bridge. > >--- >Hmmm...??? > >New to H bridges? ;) Current driven? You could. Maybe to re-zero the drift in the capacitor voltage which is (supposed) to stay "centered". Of course, from John "The Bloviator" Larkin, you're NEVER going to see a real-world circuit. He only argues with Slowman... someone on his same mental-competency level... ZERO ;-) ...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 Larkin on 25 Jul 2010 15:24 On Sun, 25 Jul 2010 13:38:16 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:4qso4618ig6ls9d2vq67vrh681l39mqe19(a)4ax.com... >> 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. > >Hard to do for more than a few milivolts, though. For large signals, you have to use back-to-back MOSFETs and *lots* of floating gate drives. > >Incidentially, I recently made a synchronous rectifier with BJTs. PP, not bridge. It passes up to +/-200mV in either direction. > >Tim This is a cool chip, been around forever. http://www.national.com/mpf/LM/LMD18200.html#Overview I used it on a CAMAC module to drive stepper motors in microstep mode, for tuning the JLabs cavities. My customer used it to drive a 48-volt, 10 amp motor. It got really hot and terrified me, but it worked. John
From: John Larkin on 25 Jul 2010 15:46
On Sun, 25 Jul 2010 14:06:10 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Sun, 25 Jul 2010 10:38:53 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>This particular circuit can drive a triangle wave into the capacitive >>load with, theoretically, zero power required after startup. > >--- >Now, now... > >In the real world, 10 henry 100 milliampere chokes exhibit a winding >resistance of somewhere in the vicinity of 250 ohms, so you're going >to take a hit with every reversal. Which just goes to show what a handy word "theoretical" is. You could do a similar theoretical analysis on a conventional linear driver and demonstrate miserable efficiency and lots of power dissipated. After all, switching regulators are more efficient than linears, in theory and in the real world. >--- > >>In practise, it would be far more efficient than a linear amplifier. > >--- >Specifically, Mr. fast-and-loose, how much more? (Show your work, >please.) You can run Fred's nice Spice model and see for yourself. In fact, I believe you already have. >--- > >>Hmmm, you can also turn on all four switches in an h-bridge. > >--- >Hmmm...??? > >New to H bridges? ;) Four switches do have 16 states. There are all sorts of possibilities... pumping up my inductor at startup, for example. John |