DC power supplies in parallel

[Toby Corkindale]

Hi,

Intuitively, I think this should be fine, but I thought I'd ask in case I am missing something.

If I have two (or more) DC switching power supplies, can I use them in parallel to pull more amps than a single unit could supply on its own?

(eg. Pulling 8 amps from two 5 amp sources)

For my purposes, I'm looking at DC-DC boost transformers (joule thief style), and they would have a common battery source.

However just for my knowledge, I am interested to hear if this theory would work differently if AC->DC switch-mode power supplies were used instead.

The way I think about it, the DC-DC converters should be safe to use together, as they have a solid common ground. Whereas the AC-DC converters don't have the same definite ground reference.

Thanks in advance,

Toby

[Bharanee Rathna]

Things to consider is how the combination will behave in parallel with ripple (phase ?) and if you're using unshielded inductors, how close they're to each other.

But in principle it should work, diode OR the outputs and add a smoothing capacitor after that. 

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[Clifford Heath]

On 21 Oct 2017, at 10:59 AM, Bharanee Rathna <deep...@gmail.com> wrote:
> But in principle it should work, diode OR the outputs and add a smoothing capacitor after that.

That might be enough, but I doubt it. You will probably need some resistance as well
to equalise the current flow from the two supplies, but even that might not be enough.

The two SMPS are each executing a control algorithm, but neither is in full control
of the controlled variable. The controllers will quite likely oscillate, possibly wildly. It
might be stable at some load points, and terrible at others. You might get lucky, but
in general, you should not just expect this to work.

Clifford Heath.

[Toby Corkindale]

I agree it's not ideal, but sometimes you don't have time to wait for an appropriately-sized module to get shipped from China, and wonder if you could get away with parts you already have lying around :)

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[Clifford Heath]

On 24 Oct 2017, at 11:47 AM, Toby Corkindale <to...@dryft.net> wrote:
> I agree it's not ideal, but sometimes you don't have time to wait for an appropriately-sized module to get shipped from China, and wonder if you could get away with parts you already have lying around :)

If it works, it works. But you're equally likely to end up waiting for the burnt
smell to waft away while your new modules ship from China anyhow.

Test it with resistors in between, say 1 ohm or less, and look at the waveform
on a 'scope before reducing the resistors. Make sure you take it through a
range of load conditions.

Clifford Heath.

> On Sat, 21 Oct 2017 at 11:06 Clifford Heath <cliffor...@gmail.com> wrote:
> On 21 Oct 2017, at 10:59 AM, Bharanee Rathna <deep...@gmail.com> wrote:
> > But in principle it should work, diode OR the outputs and add a smoothing capacitor after that.
>
> That might be enough, but I doubt it. You will probably need some resistance as well
> to equalise the current flow from the two supplies, but even that might not be enough.
>
> The two SMPS are each executing a control algorithm, but neither is in full control
> of the controlled variable. The controllers will quite likely oscillate, possibly wildly. It
> might be stable at some load points, and terrible at others. You might get lucky, but
> in general, you should not just expect this to work.
>
> Clifford Heath.
>
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[Toby Corkindale]

Thanks, that's a good plan.

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