Hi, been lurking, now I actually have a question.

One DC/DC converter is not enough for a project I'm working on. Load current is too close to maximum output current, so I figure I'll just put two DC/DC converters in parallel.

As I look into it, it seems that I may need some ballast resistors at the output of each parallel converter to ensure that both converters supply current and not just one? Are ballast resistors all I need? What are good low values to use for ballast resistors in this application?

Converters being paralleled are two Datel TWR-5/1000-12/210-D24A. Input 18-36 V DC, Outputs: 5V@1A, +/- 12V@210mA

Datasheet:

http://www.cd4power.com/data/power/twr8-11w.pdf

I'm just not sure what values to use for ballast resistors. Small values of course, but how low can I go?

 

there are a couple of things you need to consider...

first, do you want to have the regulators share current equally? I would imagine that you do, so the current sharing resistors will be the same value. you could set up the resistor values so that one will source, say 30% of the load current while the other converter supplies 70%.

second, adding a resistor will cause the output voltage to droop as the load current increases. this droop will obviously be a function of the load current and the value of the resistor being used. It is up to you to decide how much droop you can tolerate.

third, obviously the smaller the value of resistor, the better - or else you'll be burning power for no good reason.

there may be a number of issues that can pop up by doing this. If one regulator starts up a little earlier, then the second regulator may actually be forced to sink current. if that is the case, then sequencing them may be necessary - that is if they can be started into a pre-biased output.

you may also run into problems with current sharing. you'd be hard pressed to find two regulators that are going to regulate to the exact same voltage. This will cause imbalances - especially at no load - that can cause the converter that is regulating at a lower voltage to sink current.

 

Ah, looks like it's even more complicated than I thought.

Yes, both converters will be required to output the same current, so the ballast resistors will have to be the same. Thank you for mentioning initial conditions... I hadn't thought of that at all and it could present a problem. I'm just going to have to try this on the bench and record everything.

 

It would probably be a whole lot simpler and buggy if you just used a properly rated regulator, I wouldn't trust a system even for home use that was built like that simply because that's what you have around, too much of a hack.

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