Joining two isolated SMPS outputs is ok?

[grizedale]

If i have two separate SMPS's, each with isolated 40V 4A, outputs

...can i join them together to give a 80V 4A output?

...and will i suffer any interaction problems with the feedback loops of each SMPS
clashing with each other, resulting in overall instability of the 80V, 4A rail?

 

[cks3976]

Parallel connection - with OR Diodes- good. But series connection- NOT AT ALL GOOD.

SMPS will keep sampling their outputs (feedback) and keep adjusting their output voltages accordingly. Now when you short two samples the supplies will clash with each other while trying to stabilize their outputs.

 

[keith1200rs]

I disagree. Parallel power supplies with diodes will not allow the load to be shared equally if the output voltages of the two power supplies are not exactly equal.

I wouldn't expect connecting in series to be a problem. Each output will have its own stable control loop. Try it and see.

Keith

 

[milind.a.kulkarni]

I think you need to have total 8 power supply of 40v 1amp in that case if you need to get 80v 4amp - 4 are in parallel and such two units are in seires....

Sorry.....I miss read it.....It is only 2 power supply of as 40 V 4amp so you need only 2 in series....thanks keith

Good Luck

 

[keith1200rs]

Luckily the original poster has 4A power supplies so only needs two, not 8.

Keith

 

[cks3976]

I disagree. Parallel power supplies with diodes will not allow the load to be shared equally if the output voltages of the two power supplies are not exactly equal.

Keith

Yes, you are correct. Parallel combination is not for giving the 80V source, or sharing the current.... yes definitely not. What I meant to say is parallel connection of supplies through OR diodes (such as Vishay MBRB20100CT) can be used for redundancy or backup supply. This is what normally done in ATCA systems.

Well... on series connection... I have not tried it myself... and I feel stability might be an issue..

 

[keith1200rs]

Well... on series connection... I have not tried it myself... and I feel stability might be an issue..

Me neither, but I know you can connected two floating outputs of the same SMPS in series. I cannot see why it should be a problem with two separate supplies - the only way to find out is to try it.

Keith.

 

[FvM]

Usually the conditions for stable operation of a SMPS aren't exactly specified. Asking about series connection without specifying the load is even more problematic, I think.

One possible problem is different startup delay with large capacitive load, because it may force the delayed device into output diode reverse conduction.

My personal assumption is however, that the series circuit will be O.K. in most cases.

 

[mtwieg]

As long as the two supplies have similar bandwidth and compensation, small signal instability shouldn't be an issue. I base that on the assumption that the output of one supply is a relatively low impedance compared to the load impedance, meaning its presence won't significantly affect the small signal impedance seen by the other supply.

Other types of instability are more of a concern, like what happens during startup, and during overcurrent events. Especially if they have foldback current limiting.

 

[cks3976]

Thanks all for your valuable inputs. grizedale thanks for opening up nice topic. I am eager to see the results. Please share the same