# DC/DC isolated supplies in series for higher voltage?

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?

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).

• I was going to use a Vicor brick, but after ordering samples and spending hours specing the part into my design they cancelled and refunded the order because I am using a residential address and told me to use a distributor I never heard where the part number doesn't even show up. It really pissed me off. I'm now totally against ever using anything from them, and I'll relate this story to everyone I can. – darron Jun 14 '11 at 16:11
• I'd rather have the two diodes over the four. The two would almost never be on. The four diode configuration may prevent the small 0.5V back driving, but it creates another heat source headache in my sealed unit. Since another vendor shows two being acceptable, I'll try to go with that. – darron Jun 14 '11 at 16:24
• Yeah, Vicor is no fun. (They drive us nuts with their acoustic emissions; the frequency content varies with the load.) The good news is that there are lots of companies with compatible products, and you'll be able to use the two-diode-per-brick technique with any of them, so long as they are isolated supplies. – Mike DeSimone Jun 14 '11 at 16:25
• Make sure the other vendor doesn't have the "this only works if all supplies are the same model" caveat before dropping the "extra" diodes. Also, if you're talking about Mr. Kessner's solution, the reason you put a diode in series with the brick's output is to prevent a disabled brick from being a load, so you'll need to guarantee that both brick are turned on and off simultaneously. – Mike DeSimone Jun 14 '11 at 16:43

As usual, the answer is yes and no.

At the theoretical text-book level the answer is yes. At the practical level, there are some things that you'll need to check.

The #1 thing to check is what happens when one of the power supplies is on and the other is off. This will for sure happen as you'll never be able to get both supplies to come up at the same time. Let's say that your 48v supply comes up first. That would essentially be "backfeeding" the +12v supply with -48 volts. Some supplies might be OK with that, but I will guess that most will die a horrible death. Without knowing the details of the supply, I would assume it would die.

Even if things were reversed, the +12v was on and the +48 was off, the +48v supply would be fed with -12v. Even though +12 is smaller than +48, +12 is not the same as -12. Again, without knowing more about the power supply I would assume you'd let the magic smoke out.

You could limit this somewhat by using schottky in parallel with each power supply. This would limit the "backfeeding voltage" to Vf of the diode (about 0.5v). Most supplies would survive this, but I wouldn't trust my life to it.

• I did manage to find one family of bricks that includes a datasheet section on doing just what I ask (wiring outputs in series), and they show putting diodes in parallel as you say.. but in the context of shorts, not power sequencing. So, at least with those bricks it's possible. – darron Jun 14 '11 at 3:45