Paralleling of 2 power supplies with different voltages
Paralleling converters isn't trivial.

If the voltages aren't equal, there won't be sharing going on - the supply with the higher voltage will deliver the entire current and the other(s) will deliver none. Setting the voltages identically is often impractical and as things drift and age over time, setpoint precision won't be maintained.

There needs to be a feedback mechanism in place to make sharing work and keep it working. In order to force converters to share, there are a few techniques commonly used in industry:


Droop sharing - the output current of each converter is sensed and scaled and is fed into the voltage loop of the converter, causing the output voltage to decrease as the current being supplied increases. Lighter load means higher voltage which means it takes more of the load, which reduces the voltage - the system will settle into (relative) equilibrium
Forced sharing - each converter generates a signal proportional to its output current and these signals are all tied together externally. The highest signal dominates. Each unit senses its output current and compares it with the forced sharing signal, feeding into the voltage loop to slightly increase or decrease the output voltage until the internal current signal matches the external bus. Once this happens, all the converters are equally sharing the load.


The current loops in both these examples must be designed not to interfere with the voltage loops (usually by setting their crossover frequencies an order of magnitude lower than that of the voltage loops).

Now if you want to get fancy, you can force a minutely small imbalance in the sharing so that the converters don't share equally - this can work when paralleling a lower capacity supply with a higher one (so long as you don't care about redundancy).

Putting converters in series is a little simpler (usually) - so long as neither rail collapses, you'll get the sum of the voltages out. No fancy feedback needed.

Some references: Unitrode/TI, Blogspot.com.