Driving capacitive loads with an H bridge

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

In my sentence "A parallel resonant LC circuit does just this by shifting the stored energy from the capacitor to the inductor and back again", the 'just this' bit refers to energy transfer, it was not specific to any waveform. The first sentence gave an example of a waveform that charges up a capacitor and then discharges it, as would any, including sinusoids.

I do have very good reasons indeed for wanting a triangle wave and at some point I'll need to make a real unit to test. However I cannot discuss the actual application, so unfortunately (however much I'd like to) I can't elaborate on it. I wish I could because I'm sure you and others may have ideas.

Mark.

Send me an NDA and then some real information.

I'm not into guessing.

I can do a Larkinesque ideal machine that will run forever, and take no energy to work ;-) (And produce a triangle wave :) ...Jim Thompson

in

Yep, that happens. The current waveform and voltage waveform into the capacitor are out-of-phase, the energy flows both ways for a net zero loss (but some stray resistances, of course, suck power).

Like this...

On Thu, 22 Jul 2010 18:05:46 +0100, "markp" wrote:

--- Nope, the circuit list:-(

Version 4 SHEET 1 960 680 WIRE -272 -224 -336 -224 WIRE -144 -224 -208 -224 WIRE 16 -224 0 -224 WIRE 112 -224 96 -224 WIRE 192 -224 176 -224 WIRE 384 -224 384 -320 WIRE 384 -224 336 -224 WIRE 400 -224 384 -224 WIRE 464 -224 464 -320 WIRE 528 -224 464 -224 WIRE -336 -176 -336 -224 WIRE -144 -176 -144 -224 WIRE 0 -176 0 -224 WIRE 192 -176 192 -224 WIRE 336 -176 336 -224 WIRE 528 -176 528 -224 WIRE -336 -48 -336 -96 WIRE -144 -48 -144 -96 WIRE -144 -48 -336 -48 WIRE 0 -48 0 -96 WIRE 0 -48 -144 -48 WIRE 192 -48 192 -96 WIRE 192 -48 0 -48 WIRE 336 -48 336 -96 WIRE 336 -48 192 -48 WIRE 528 -48 528 -96 WIRE 528 -48 336 -48 WIRE -336 16 -336 -48 WIRE -160 96 -336 96 WIRE 80 96 16 96 WIRE 192 96 160 96 WIRE 480 96 352 96 WIRE 544 96 480 96 WIRE -336 144 -336 96 WIRE 16 144 16 96 WIRE 352 144 352 96 WIRE 480 144 480 96 WIRE -160 160 -160 96 WIRE 192 160 192 96 WIRE 544 160 544 96 WIRE -336 272 -336 224 WIRE -160 272 -160 224 WIRE -160 272 -336 272 WIRE 16 272 16 224 WIRE 16 272 -160 272 WIRE 192 272 192 224 WIRE 192 272 16 272 WIRE 352 272 352 224 WIRE 352 272 192 272 WIRE 480 272 480 224 WIRE 480 272 352 272 WIRE 544 272 544 224 WIRE 544 272 480 272 WIRE -336 352 -336 272 FLAG -336 352 0 FLAG -336 16 0 SYMBOL voltage 16 128 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMATTR InstName V1 SYMBOL ind 496 240 R180 WINDOW 0 36 80 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName L1 SYMATTR Value .845 SYMBOL ind 64 112 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L2 SYMATTR Value .845 SYMBOL cap 176 160 R0 SYMATTR InstName C2 SYMATTR Value 3e-6 SYMBOL cap 528 160 R0 SYMATTR InstName C1 SYMATTR Value 3e-6 SYMBOL voltage 352 128 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMATTR InstName V2 SYMBOL voltage -336 128 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMATTR InstName V3 SYMBOL cap -176 160 R0 SYMATTR InstName C3 SYMATTR Value 3e-6 SYMBOL cap -208 -240 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 3e-6 SYMBOL voltage -336 -192 R0 WINDOW 0 -42 0 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL voltage -144 -80 R180 WINDOW 0 -43 116 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V5 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL cap 112 -208 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C5 SYMATTR Value 3e-6 SYMBOL voltage 0 -192 R0 WINDOW 0 -42 -2 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V6 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL voltage 192 -80 R180 WINDOW 0 -44 115 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V7 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL cap 464 -240 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C6 SYMATTR Value 3e-6 SYMBOL voltage 336 -192 R0 WINDOW 0 -44 -3 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V8 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL voltage 528 -80 R180 WINDOW 0 -38 106 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V9 SYMATTR Value PULSE(-35 35 0 .005 .005 0 .01) SYMBOL ind 368 -304 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L3 SYMATTR Value .845 SYMBOL ind 0 -208 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L4 SYMATTR Value .845 TEXT -320 304 Left 0 !.tran .1

JF

You must put an inductor in series with the capacitor. otherwise, when the H-bridge switches, the current will be very high. The inductor value should be chosen based on the frequency, capacitor value, etc. if the triangle wave frequency is high, you can just drive the bridge at the desired frequency, and a properly chosen inductor will give a very close approximation to a triangle wave (or sine wave, or what have you).

If it is a low frequency, then you need to provide a PWM drive to the bridge to get the waveform you desire at the capacitor, or a very large inductor.

Jon

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

Brilliant! Why didn't I think of that? Shouldn't cost much to make. All I need to do is find out where I can get component F1 and a 1 picoohm resistor. The latter part might require a cryogenic chamber and some liquid helium.

I think you have far too much time on your hands sir ;)

Naaaah! I whip that kind of stuff out in seconds.

Definitions:

F is a current controlled current source, i.e. an ideal current mirror

The OP (markp) implies this is a _real_ situation. How do you meet his requirement "...it has to be efficient, i.e. some kind of energy retrieval" ?? ...Jim Thompson

Wouldn't it be easier to run the H bridge as switching current direction only, with current through the bridge controlled by a separate switching regulator?

Saves talking about ginormous inductors, for starters. Might even be buildable ;)

Grant.

(*)

That would meet the "shape" requirement, but I still ponder what does "...it has to be efficient, i.e. some kind of energy retrieval" mean?

• You'd need some kind of loop to keep it "centered" also. ...Jim Thompson

Neither is any other method of driving a short circuit.

Given a large enough and perfect enough inductor, and ideal switches, a short or a capacitor can be driven efficiently by reversing the inductor's polarity at the waveform peak. That would require 4 switches.

Given imperfect and realistically sized components, a half bridge can reverse it's output inductor current in a finite time period while supplying approximately constant current of the correct polarity, with a modest ripple component and reasonable losses.

RL

Dunno, may be this is a piezoelectric thingy that wants to squish the current back out when being relaxed at a controlled rate?

Rotate the bridge 90' so the capacitor voltage see-saws and the charge doesn't fall out? ;^)

Yeah, that too, unless that thing is taken rail to rail, self centering?

Grant.

Thanks for that. Yes, one of these is going to solve it - my preference at the moment is not actually an H bridge for noise reasons but rather a resonant type oscillator like the one in the top right of the schematic, if I can make the waveform triangular. That actually *might* be possible by using a class D oscillator and forcing additional current in and out of the drive windings, not sure yet. The question is how the driving sources are implemented. For example the top right (a capacitor driven by two voltage sources either side) would need to have a current source as well, and that current source mustn't be resistive so that the energy used to charge the capacitor in one direction is recovered from the capacitor and not thrown away by resistive losses.

Mark.

transistors

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

ROFL!

You'd better not write such nonsense when you intend to blast everybody that don't fit your taste here...

I take it that you didn't think much when writing this, which anyway shouldn't be with the posture you choose to display, but even, understanding this doesn't take much thinking, so?

No good, way to lossy.

Now that's interesting. A saturable core reactor that responds quickly enough could do it. Now there's I2R losses in doing that, you have to drive current through a coil, the big question is whether this would actually not be that lossy.

The question becomes something like this: If you had a certain current flowing through a saturable core inductor the energy stored in it is (I^2L)/2. So if the current remains the same, and you change its inductance by changing the control current, what happens to the stored energy?

Something like 85% would be nice.

transistors

Where's the "energy retrieval"?

An H-bridge driven from an inductor doesn't "retrieve".

I see nada of substance in your comments.

Another "Bloviator" ?:-) ...Jim Thompson