Charge pumps in parallel?

[armdnrdy]

Hey all,

Over the weekend I was pondering how one could achieve creating a higher output current, 15 volt, bipolar supply using a charge pump in place of a transformer. Higher voltage charge pumps such as the LT1054 max out at around 100ma.

I had the thought that if one were to run two or more LT1054 charge pumps in parallel, as inverters, that it would increase the current output.

The data sheet for the LT1054 states "devices can be paralleled" the data sheet for the 7660S also states that they can be run in parallel in this fashion, but is vague on how much the current is actually increased.  (Any number of ICL7660S voltage converters may be paralleled to reduce output resistance.)

I would imagine that the oscillator pins would have to be synchronized to reduce ripple.

I know that most stompboxes use much less than 100ma, but I'm working on something from the past that's a bit larger and I would like to ace the ac transformer to put it in a smaller enclosure.

Any thoughts on parallel charge pumps?

[Cliff Schecht]

It'll work. You do increase the current output by about double. Also you are right to synch the oscillators, not necessarily to reduce ripple but to prevent the two from subharmonic oscillations or creating beat frequencies in the audio range (think two PWM waves near the same frequency mixed together, you get junk in the audio range).

Have you looked into real inductor based switching power supplies at all? You can get much more current output, lower ripple, very accurate voltage regulation (charge pumps aren't regulated) and a smaller overall footprint compared to what charge pumps offer. I won't push the issue, but charge pumps aren't great and should be used as an exception, not a rule!

[armdnrdy]

Hey Cliff,

Thanks for the response. Inductor based switching power supplies? I'm open to whatever serves the purpose. I don't have stock in charge pump companies!!

I looked around the net for a bit and could only find inductor based bipolar supplies that incorporated charge pumps for the negative voltage.

Do you know of any good links on the subject?

[a soBer Newt]

I have had luck with the Recom Power Inc RD-0515D and using more than one and not having issues with beat frequencies.  I know some one that designed a mic pre with 4 one for each channel and 2 more to supply the 48V 

[armdnrdy]

Hey soBer Newt,

Thanks for the insight. I've never seen those before. The average frequency looks pretty good for audio use and the 66ma is pretty good for most stompboxes. I'll definately take a closer look.

[Cliff Schecht]

This is a good little overview of switching power supplies (at the bottom of this post). Actually a charge pump is a switching power supply as well, it just uses a capacitor as the energy storage device instead of an inductor. Since you probably want to adapt 9V to +/-15V then you can go with a few different solutions.

The most elegant solution would be the flyback topology but you have to find a flyback transformer that will work for your needs (not easy, there aren't a lot of of the shelf solutions) or wind a flyback transformer yourself (not too hard). The word transformer is actually sort of a misnomer here as flyback transformers are actually specially wound coupled inductors. The difference is simple enough, transformers don't store energy (just transfer it) while coupled inductors are used as energy storage elements. Thus while at first glance they look the same, one can't just be used in place of the other. This is why the flyback transformers are also hard to find off the shelf, every design requires different windings and it's hard to make a one-fits-all solution (so nobody even really tries to).


Then there are solutions that use a readily available off-the-shelf coupled inductor in a boost design or inverting design. What you do is hook up one side of the inductor as it would normally would be in the boost or inverting topology. The second winding on the coupled inductor is then used to generate a second voltage (along with a diode and capacitor) that will be the compliment of the primary voltage. If you use this in a boost design to go from 9V to 15V then the other winding on the coupled inductor can give you -15V (and vice versa if you use an inverting design).

Another solution is to boost 9V to 15V and use an inverting switching power supply (charge pump or inductor based) to go from 15V to -15V. It's usually good to sync the oscillators but actually is not necessary if you filter each supply well enough.

I've used all of the solutions in the past but the last one is probably the simplest to implement with off-the-shelf components. Definitely not the most compact solution but IME it's easier to get a boost supply working than it is to get a flyback working, especially if you aren't experienced in these sort of circuits.

Here's a nice little overview of switching power supply topologies. It doesn't cover everything (it barely scratches the surface!) but it's a good overview. Don't be intimidated by the math here either, most chips that you use will have a lot of example circuits and simplified design methods in the datasheet that make applying their chip easy to do.
http://www.maxim-ic.com/app-notes/index.mvp/id/2031

[armdnrdy]

Thanks Cliff,

I've got  some reading to do and data sheets to absorb!