Driving capacitive loads with an H bridge

Assuming you mean triangle voltage waveform (=3D 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

--=20 Deep Friar: a very philosophical monk. Website:

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I can't really say as I'm under NDA, suffice it to say the load is mostly capacitive (in the order of a few uF) and musn't have any DC component across it.

Mark.

what kind of frequencies, voltages?

class-d audio amplifier?

-Lasse

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.

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

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

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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.

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.

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

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

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.

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

"...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

be

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?

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Sorry Jim, what I meant was if you drive a triangle wave across a capacitor then the capacitor will store energy when it's charged up, but when it is discharged the energy has to be recovered back (retrieved) so it can be used again in the next cycle. A parallel resonant LC circuit does just this by shifting the stored energy from the capacitor to the inductor and back again, so for ideal components no power is needed to sustain oscillation. An H brigde would do it by temporarily storing energy in the inductor part and dumping that energy back to the DC supply reservoir.

I actually thought that by stating it was to be efficient was somewhat equivalent to that, which is why I put 'i.e. some kind of energy retrieval'. But you're right, it could have been stated better.

Mark.

be

Use a generator and flywheel; by shaping the poles of the generator, you can get the output to be triangle-wave-like. The flywheel is your energy-storage item, here.

A motor driving the generator/flywheel makes up the energy losses. Because the generator is a simple winding like a transformer, your DC zero is assured.

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.

Mark.

Did you forget to include the link?