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Dual output voltage from two single output power supplies

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mike buba:
Hi all,

I found a Traco Power schematics, which has the same connection diagram as mentioned in this topic, but I still have several questions. The idea is to use two AC/DC +15Vdc power supplies to create +/-15Vdc power supply.

1. My PCB has three inputs: +15Vdc, -15Vdc and GND. Where should I connect common connection from a 'new' power supply (-Vout from PS1 to +Vout from PS2)?

2. In my case +15Vdc will have a higher load (+5Vdc circuits, LEMs and IGBT driver supply) and -15Vdc will have a smaller load (only IGBT driver supply) which will lead to common point being not zero. Will this affect +/-15Vdc values, i.e. will they drift or osculate?

3. Should I connect a power supply common point to GND PCB input? But the PCB's GND is connected to metallic enclosure GND and in that case the voltage will be different between GND and common point (because of load unbalance). Should I disconnected enclosure from the PCB, connect GND to enclosure and use common for GND on PCB?

Thank you in advance.

David Hess:

--- Quote from: mike buba on September 23, 2021, 02:52:51 pm ---1. My PCB has three inputs: +15Vdc, -15Vdc and GND. Where should I connect common connection from a 'new' power supply (-Vout from PS1 to +Vout from PS2)?
--- End quote ---

That is right.


--- Quote ---2. In my case +15Vdc will have a higher load (+5Vdc circuits, LEMs and IGBT driver supply) and -15Vdc will have a smaller load (only IGBT driver supply) which will lead to common point being not zero. Will this affect +/-15Vdc values, i.e. will they drift or osculate?
--- End quote ---

There will be no effect unless one side if overloaded.  Observe the shunt diodes in your example which prevent one power supply from reversing the voltage on the other when overloaded.


--- Quote ---3. Should I connect a power supply common point to GND PCB input? But the PCB's GND is connected to metallic enclosure GND and in that case the voltage will be different between GND and common point (because of load unbalance). Should I disconnected enclosure from the PCB, connect GND to enclosure and use common for GND on PCB?
--- End quote ---

The unbalanced load is irrelevant.  Usually it is best to use a single point ground.  What this means is that the common connection for the power supplies should be the same point on the board where the chassis is connected.

mike buba:
Do I connect Com. to GND input?

What about grounding the PCB or the system after the power supply as I have only live (+/-15V) and neutral wire (Com.) coming out.

I have attached a simplified drawing of the system. If I connect Com. to GND, I do not have GND (earth) connection on PCB, enclosure or anywheres else on the right. I cannot assume Com. is equal to GND, only 0V?


PCB expects a GND input.

David Hess:
The outputs from the two power supplies should be floating with respect to their AC inputs and the AC ground.  So the common point for your +/-15 volt supply is connected to the common of the circuit, and that point is connected to the chassis and presumably earth ground.

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