Can I chain two isolated DC/DC supplies together for a higher output voltage? (Say, a 48V and a 12V to get 60V?) They'd be fed from the same 24V DC source.
I've asked a related question before (Step up DC voltage from 12-24V to 60V), and I have a schematic from the NA Simple Switcher tool for a marginally adequate 60V supply. (750mA, I will eventually need about twice that) I'm just getting back on this problem.
It occurred to me that there might be an easier solution, and it sounded like a good enough question on it's own... so here it is.
The simplest thing I can think of would be to take a 48V 2A isolated DC/DC brick and put a 12V 2A brick after it. If I disable the 12V supply until the 48V is up, would that be good enough? Would I even need to bother sequencing them?
Answer
The proper way to do this is with two diodes per brick. One diode has its anode connected to the brick's + output. The other diode has its anode connected to your brick's - output. Both diodes' cathodes are connected together and used as the + output. The - output is connected to the brick's - out.
The PSpice-like connection for two bricks would be:
V1 1 0 48V
D1 1 2 D1N4002
D2 0 2 D1N4002
V2 3 2 12V
D3 3 4 D1N4002
D4 2 4 D1N4002
R1 4 0 1k
Vicor has a good application note on how to do this. The nice thing about this topology is that you can sequence or disable bricks however you want. I have used this in an application with 4 95 V Vicor bricks to get an output of 90, 180, 270, or 360 V, depending on how many bricks were enabled. Remember, one diode per brick will always be forward-biased, and each diode will need to be rated for the maximum output voltage of the whole array.
Finally, make sure you know how much current your load will draw at each possible voltage. All bricks must be able to provide the full load current (due to Kirchoff's Current Law).
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