voltage inverter? Split supply from single?

I challenged myself to find an IC somewhere out there that will create a split supply from a single sided supply WITH high current capability... I can't find one.

Sure there are plenty of chips that create virtual grounds so you can split a single supply in half. There are also switched capacitor voltage inverters that give you the negative voltage you need to go with your positive voltage and then there are op-amps that will work very close to the rail voltages, BUT none of these options are good enough.

The inverters and virtual ground ICs don't have the current carrying capacity that I need (5A or more) and the op-amps that get close to the rail voltages don't get close enough. I need 0-5V, no ifs ands or buts and while I can exceed 5V quite easily, I can't go lower than 0 without a split power supply so I'm back to the same problem: How the heck do I run a power op-amp (I have nice 10a amps) that need less than 0V on the low side, if all I have to work with is a single power supply? My initial answer to this was a transformer with a center tap - nice elegant solution except the damn thing weighs a million pounds and is the size of a softball so it can run off AC power. A tiny xformer would be nice but I need high frequency switching circuitry just to make that work which is almost as bad after you've built that.

How do conventional smart battery chargers work? They can apply any voltage from 0 to whatever. I have to assume they are NOT using linear devices like power op amps? Can I then assume that they use FET like devices to simply chop a supply voltage and filter? If so, how do they deal with load impedance and isolation? That's what I love about amplifiers... they are isolated from the load. I never figured out how to deal with a FET chopper style supply without having to factor in the impedance of the load as well. But I digress... my real concern is how to get a split supply in a small package that can pass a few amps.

Gahhhrrrlic:
That's what I love about amplifiers... they are isolated from the load.

Really? In my experience, that hasn't been the case since vacuum tubes were around.

I don't know, man... I did a quick check on Digikey and [u]this[/u] is the "biggest" +/-5V DC-DC converter I could find. 8.5 Amps, and it requires 36V in.

What's your actual need/requirement and what's your power source... Why are you limited to a "single supply"?

You may be able to find a single DC-DC inverter for the negative side if you already have the positive side available. (When I needed -12V low-current for an op-amp in a car, I just used a single-ended/isolated DC/DC converter and I reversed the outputs.)

My initial answer to this was a transformer with a center tap - nice elegant solution except the damn thing weighs a million pounds and is the size of a softball so it can run off AC power.

You said you wanted to run it from a "single sided supply", so I assume you can't use AC?

There are also switched capacitor voltage inverters that give you the negative voltage you need to go with your positive voltage

You're not going to get 10A from a switched capacitor circuit.

No, I can use AC. I'm rectifying and filtering it to + DC. But that's where the big-ass transformer comes into play. Is there a way to somehow exploit both ends of the sine wave without all that bulk? Like an IC or something that will spit out + and - from the sine wave but still give 5+ amps?

My power source is flexible. House power, battery, DC-DC converter, whatever, so long as I can get about 25V or so DC at the end to run my charger. I would really prefer to stick with my 10A op-amp if possible because I don't know how to properly use high side FETs and the low side FET causes problems with feedback and load impedance and stuff like that. It's just that the op amps need the bloody split power supply or they don't work right. My charger would be ginormous.

Most smart battery chargers these days are simply a switchmode power supply driving a ferrite transformer with the secondary feeding a rectifier bridge which forms the output.
The output is isolated from the input by the transformer.
Not too dissimilar from a PC power supply, but with only 1 output.

Gahhhrrrlic:
I challenged myself to find an IC somewhere out there that will create a split supply from a single sided supply WITH high current capability... I can't find one.

Not surprisingly as switched-capacitor devices are limited to low currents and low powers.

If you want a high current virtual ground then add a high current buffer into the feedback loop of an opamp,
but usually you'll need to build that buffer yourself. And its a dissipative way to do it (lots of waste heat).

So DC-DC conversion is the way to go I think. Surely you can find a 25W isolated 5V -> 5V dc-dc converter?

Gahhhrrrlic:
How do conventional smart battery chargers work? They can apply any voltage from 0 to whatever. I have to assume they are NOT using linear devices like power op amps? Can I then assume that they use FET like devices to simply chop a supply voltage and filter? If so, how do they deal with load impedance and isolation? That's what I love about amplifiers... they are isolated from the load. I never figured out how to deal with a FET chopper style supply without having to factor in the impedance of the load as well. But I digress... my real concern is how to get a split supply in a small package that can pass a few amps.

Yes, that's a big lot of digressions! No idea what you mean by "conventional smart battery charger" for
a start...

Modern power electronics is pretty much exclusively switch-mode, with linear regulators added when quiet
analog supplies are needed (switch-mode is too noisy for many analog and RF circuitry).

That's what I love about amplifiers... they are isolated from the load

huh?

I have browsed and purchased many dc-dc converters of various power ratings but they are not isolated. My last attempt to take 2 such converters, tie the + and - terminals together and ground that node to make a split power supply, ended in a smoke show that nearly blew up my PSU and totally cooked the converters. I like the transformer approach but, as mentioned, modern applications exploit high frequency switching to shrink the ferrite needed. Its not out of the question for me to do the same I guess but it's just an extra hassle to then build a circuit to convert house power to high frequency ahead of the transformer.

When I said amps were isolated, what I meant was that changing the load impedance on the output side has no effect on the input side of the amp because the input side of the amp is high impedance and responding to voltages only. With switched power, you get high efficiency and little heat as an advantage but when the switch is "ON", it's like it's not even there, and your load becomes part of the resistive network of whatever is ahead of the switch, creating loading problems. Hooking up a load with 10mOhm would have a different effect on the entire circuit than say a 1Ohm load would. In my design, I have current flowing in different directions along the same wire (charge/discharge) so it really makes a mess if the output side where the load is, is not isolated from the input side.

Gahhhrrrlic:
When I said amps were isolated, what I meant was that changing the load impedance on the output side has no effect on the input side of the amp because the input side of the amp is high impedance and responding to voltages only. With switched power, you get high efficiency and little heat as an advantage but when the switch is "ON", it's like it's not even there, and your load becomes part of the resistive network of whatever is ahead of the switch, creating loading problems. Hooking up a load with 10mOhm would have a different effect on the entire circuit than say a 1Ohm load would. In my design, I have current flowing in different directions along the same wire (charge/discharge) so it really makes a mess if the output side where the load is, is not isolated from the input side.

I have read this several times and it just does not make sense. Am I the only one?

P.S. PC power supplies normally convert mains frequency to high frequency so that they can use a smaller transformer.

Gahhhrrrlic:
I have browsed and purchased many dc-dc converters of various power ratings but they are not isolated.

Well now is the time to get isolated ones. This sort of power level isn't very cheap alas, I found these: http://uk.farnell.com/xp-power/jtk3024s05/dc-dc-converter-5v-6a/dp/2545439

Hi,
What transformer do you have?
You can rectify and filter a positive and negative supply if you have a transformer with suitable secondary winding(s).

Tom... :slight_smile:

Tom: I actually built something precisely like that. It's on a plank of melamine, weighs about 20 lbs and provides up to 30VDC (variable) or +/- 30VDC (fixed) that runs on a big toggle switch. The xformer is a mains type that I think is 48 VRMS. It's big and was custom made for me when I first needed something to prototype with.

MarkT: That's cool! I'd sure fork out the dough for that if I could find one that supplies 25V. 5 unfortunately isn't enough but I'll see if I can find a beefier one.

K, wow, I need like 2 of these:

http://uk.farnell.com/xp-power/rdq150110s24/dc-dc-converter-150w-24v-6-5a/dp/2422845

Your first post implied you wanted +/-5V at 5A, then later you mentioned 25V / 10A without reference to
split-supply.

Perhaps you should be more clear about what actual hardware you have and what you are trying to do,
it seems my time spent researching 5V supplies was probably pointless...

No, not at all. Had you not introduced me to these isolated dc dc converters I wouldn't have discovered them myself in all likelihood. Finding a 25V version was pretty trivial once I followed your link so you gave me a shortcut.

My control logic is obviously coming from an Arduino which is 0-5V but the amp output depends on my supply voltage which I need to be about 25V. The Concern is not with the top end... I can rectify mains voltage for as much + supply as I want but I can't go any lower than 0 with a single supply and that happens to also be the bottom limit of the Arduino outputs so the amplifier will never actually reach 0V, which means I have no way of charging say a single ni-cd at 1.2V. Having the - supply merely gets me sufficiently far away from 0V to allow the amplifier to work all the way down to 0V.

So it looks like these isolated converters may be the most compact way to do what I want to do if I plan to stick with linear amps. A small xformer could work too but after making the high frequency switching circuit I may end up adding just as much bulk to the entire assembly. There seems to be no other easy way of doing what I want to do unless there is a single IC solution out there that converts mains to high frequency that I can then hook up to a small xformer, thus saving me the work of making one myself out of discrete components on a proto board.

MarkT:
Perhaps you should be more clear about what actual hardware you have and what you are trying to do,

+1

I don't see why you need a negative supply to generate a voltage down to 0.0volt.
And why you need zero volt to charge batteries.

Opamp outputs can't go down to 0.0volt, but they can drive a power transistor that can do that.
And there are opamps that can measure to (and below) zero volt on a single rail supply.

Post the diagram of your "charger".
Leo..

I remember now why I didn't want to use power transistors. I'd need to filter the output side of the FET to get DC from the square wave that the switching would produce and the size of the cap I'd need would destroy the time constant. Charging and discharging would take so long that I wouldn't be able to get high frequency pulses when I desired them (which for some batteries is desirable) but have smooth DC when I wanted it (desirable for other batteries). The only way I could think to do it was to control the waveform in front of some isolated current source, such as a linear amp.

...and I'm willing to be proven wrong but every piece of literature I've ever read about amps since someone here pointed it out to me, has said that you can't achieve rail voltage with an amp. There's a dead zone of a couple of volts for even the best amps. I can't live with that. I'd need to get down to at least 0.8V which means on the - side I'd need to at least feed -3V or so into my amp to get 0 at the output reliably.

Hi,
A picture is worth a thousand words, can you post a diagram of the concept of your charger, please?

What sort and voltage batteries do you want to charge?

Thanks.. Tom... :slight_smile:
PS Sorry I found it, please read how to use the forum.

This thread seems to be a continuation of this one.

Leo…

Hmm,
Why are you concerned about amplifiers and how close they can output to 0V and Vcc.
If you are going to charge a cell, 1.25V Nicad by the looks of it, why does the amplifier need to go to 0V?
If your charge voltage is going to be near your circuit Vcc, then increase the Vcc on the amplifier.

I looked back over more than 23 pages of your posts and you seem to have a puzzling idea of electrical theory and in one of my reply's I mentioned you are over thinking every aspect of your projects.

I will ask again, what are you charging, why so much current, and have you built your electric car yet.
Have you started to experiment in real life, not just your head.

Can you edit your diagram and add some in names and voltages.

C1 and R6 has me worried as to what they do.

Tom..... :o :o :o