ECL: Why'd they use negative voltages?

Check out the big fat audio amp hybrids. Of course, one has to be careful and pick one that isn't going obsolete next year because a particular car stereo was discontinued.

Heck, if you want to go green on this you might even consider class-D. Then claim your carbon offset :-)

A non-H-bridge class D amp can have circulating-current problems... it takes current out of one supply rail and dumps it into the other. Conservation of energy.

John

Its been a while since I made upside down units.

I was just thinking about the logics I have worked on, but I don't remember all the details. I think Collins logic used some negative voltages, and I think I have worked on negative logic as well.. There was Dec logic which I never really learned. There was Zerox logic, and I did learn General Dynamics dynamic logic, pulse or no pulse.

greg

Quite so. I encountered this problem while doing research on class D tube amplifiers:

This makes it a class D-A amplifier: the switch is constantly running, and the output network is resistive coupled, drawing bias current and, for the most part, wasting it.

Since it's class A, RC coupled, it has 25% maximum efficiency. I got somewhere around 22% overall efficiency. The plate efficiency was quite high, over 80%. It also had remarkably low output impedance, about 100 ohms, despite having no global NFB. This is due to the pentode's "Rpk(on)".

Tim

One of the old Motorola books has a class-D amp that uses bipolar supplies, half-bridge mosfets, LC filter, DC coupled to a grounded loudspeaker. The problem is that, if you're swinging the load, say, positive, you take power out of the + supply and pump power *into* the

- supply, and blow up its filter caps. Their fix was cute: an idler circuit off to the side, a pair of mosfets switching at 50% duty cycle, pumping V+ and V- into a dummy grounded inductor. That automagically equalized the supply voltages.

John

The problem is known as "rail pumping".

There is one small problem with this solution: it doesn't work. If anything is slightly off balance, that creates virtually unlimited current. It could be managed if the idler is controlled by some sort of intelligent loop, however that makes things complicated and expensive. (Remember, auduo is extremely cost sensitive!). Better solution is the synchronous rectifier in the power supply which dumps the energy back to the primary. But this is expensive, too. The only practical solution is having enough of capacitance on the power rails, and provide for overvoltage shut off just in case.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Nothing is off balance, because the idler forces things to balance. The "virtually unlimited current" is what pulls down the higher supply and boosts the lower one. In fact, it transfers energy from the unused, higher voltage side of the supply to the actively-loaded, lower-voltage side. Sorta cute.

They wouldn't have published it if it didn't work.

John

Current-mode control would likely cost too much as well?

Now think what happens if the duty cycle is not exactly 50/50. Or if the /+/ and /-/ supplies are slightly different.

Heh, haven't you seen published circuits which don't work?

That things could work for low power stuff where the stray resistances allow for some slack.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

If the power supplies are slightly different, the inductor sees a net average DC. So the inductor current rises. The inductor then extracts energy from one supply (the one with the higher magnitude) and pumps energy into the other one. That tends to equalize the supply voltage magnitudes.

These are soft supplies, especially in the absorbing-energy direction. They aren't stiff, absolute voltages. Which is why it works.

Of course, a true h-bridge solves the same problem more efficiently.

John

Thou shalt use an inductor towards the load :-)

Essentially this is how synchronous buck converters work, and also class-C push-pull RF power stages. There is always an inductor first. I remember a design of such a power amp where the designer had followed it with a Pi-filter meaning cap first ... *PHOOMP* ... *POP* ... both FET literally pulverized upon power-up, looked like the ones from your FET multilation photo the other day.

for

popular

Are you the guy on the right, with the wide-rimmed glasses?

So the lower rail is getting powered from the higher rail through the idler circuit. With the corresponding circulating high currents, losses and asymmetry. It looks worse if you consider the waveform of the idler current.

BTW, once I made similar circuit. FETs were controlled by the CPU; that allowed to optimize the operation (For any considerable currents and voltages, don't think of running this in uncontrollable mode). It worked reasonably well, however I didn't like it.

H-bridge needs twice as many FETs, drivers, protections and feedbacks; that is pricey. Also, it is impossible to connect two channels in a bridge configuration to double the output power. For those reasons, half bridge is usually preferred in audio. Synchronous rectifiers in the power supplies are good solution for this problem and for the efficiency reasons, but they come with a price also.

Bottom line is everybody just uses big capacitor tanks; this is dumb, cheap and good enough for the job.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Only as much current as needed to equalize the supply voltages. That turns out to be less current than the half-bridge is pumping into the speaker.

losses

It's a triangle. The more the L, the smaller the triangle.

A synchronous rectifier in the supplies doesn't help. If you push power back into a supply, as the half-bridge likes to do, then energy has got to go somewhere. A synchronous rectifier just pushes it uphill into the caps of the bulk supply.

For AC loads, yes.

John

Of course. The inductor causes the problem. If a bipolar half-bridge drove a resistive load, there would be no circulating current.

They use unipolar bulk supplies, and can dump their circulating current into ground.

John

Pumping is normally only an issue with low frequency AC drive. There is another trick: If you have two amps (usually the case with audio) operate them 180 degree out of phase down there.

I've never had any issues there and driven quite big loads. Except in cases where motors began generating because forces kept acting on the shaft while I had the half-bridge in coast. That can make the rail voltage rise. So I didn't use coast.

They don't dump anything, the inductor current just keeps flowing. Unless you leave CCM, then it just stops.

Hah! The American solution: Use an inverter and sell that power back to the utility :-))

Full H-bridge!

John