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Can you power an SMPS from another SMPS psu?

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anzic

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Hi everyone,

Something that isnt really covered in most power supply cookbooks. Can you power an SMPS from another SMPS and have it work?

The idea is to use a laptop power supply that puts out 17vdc@5a, and power a small custom made boost converter SMPS putting out 100vdc@40ma.

Will the two exist happily together or will one try and dominate the other and blow up? Also, impedances... a big issue?

Any replies or references to books covering this would be great!

Regards,
Anzic.
 

Hi

No problem it will work !

All the best

Bobi
 

Thanks Bobi. It's just that I vaguely remember reading somewhere a long time ago that it was bad news....

Input and output impedance, and minimum load would be the only things I thought would make an SMPS play up.....

Cheers,
Anzic.
 

you right to be worried.

2 smps can interfere with each other and "beat" with each other (potentially)

keep them well decoupled from each other.

keep their current loops apart
 

Hi eem2am,

By 'beating with eachother' do you mean that they interfere with eachothers switching frequency and 'lock' onto eachother?

Any suggestions for keeping the current loops apart and decoupling? Like totally seperate and no common grounds between the two?

Getting a little worried now.... wouldn't want them to blow up on runaway or anything.

Anzic.
 

Notebook SMPS have clean DC output (possibly in contrast to some self-elaborated supplies). You also have a large power reserve, I don't see a problem.

I'm often using similar off-mains SMPS (50-100 W, 12 -24V) to power portable instruments with internal secondary SMPS.
There's a (more theoretical) issue, that the inrush current of the secondary SMPS will trigger the current limit of the primary and
prevent a correct startup. A soft-start function of the secondary SMPS can reliably avoid this sitauation.
 

FvM got it right. the laptop itself has got about 2 or three built in switching Power supplies for 5V rail, 1.8V (some) and 3.3V for the board CPU and peripherals. So it wouldn't make a problem other than for the inrush as mentioned by FvM.

If it were a PC SMPS, the specification specifically requires them to be able to start with huge capacitive loads on all rails which will add up to huge inrushes. I do not know whether laptop power supplies are rated so.
 

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