Driving capacitive loads with an H bridge

[ehsjr]

Yep, You do need to be careful to keep the charge from falling off the
plates ;-)

...Jim Thompson

That does seem to be happening around here lately.

Maybe you could saw the capacitor open and put some
Coulomb Glue on the plates.

Ed

 

[Tim Williams]

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

 

[Grant]

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?

Anyway, we don't know if the 'capacitor' is the load, or something
there to swamp it, in order to define and make a known waveform?

This particular circuit can drive a triangle wave into the capacitive
load with, theoretically, zero power required after startup. In
practise, it would be far more efficient than a linear amplifier.
Yes.

Hmmm, you can also turn on all four switches in an h-bridge.

Yeah, that's to pop the main fuse and protect the load from EMP

Grant.

 

[Grant]

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.

PP, whazzat? Mind blank.

Grant.

 

[Grant]

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.

 

[Tim Williams]

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

 

[Grant]

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.

 

[Winfield Hill]

markp wrote...

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?

 

[Tim Williams]

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?

Typical of automotive stuff, for the same reason: conduction loss dominates over copper loss at low voltages. And for really low voltages, you use MOSFETs instead of schottkies, in a synchronous rectifier.

PP at line voltages would be silly, because you have twice the peak voltage (requiring 800V+ transistors for 240V input) and a lot more turns, increasing capacitance that you don't need in a hard switching forward converter.

Half bridge is great at line voltages because it only sees full rated voltage and needs one winding.

Tim