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> I'm currently working on a project where we need to get 12V and 24V 
> from
> a 18-36V hydrogen fuel cell. I have two of these
> <http://www.tracopower.com/products/ten20wi.pdf> 12V DC/DC 
> converters,
> and I want to hook them up as shown in the attached circuit. I'm a
> little worried that I'm asking for a free lunch with this design, so 
> I'd
> like some feedback on it. Can I get away with shifting the ground on 
> the
> second output up to 12V to get 24V with respect to the ground of the
> first unit and the fuel cell?

Yes. You can get there from here.
But it may be less efficient than alternatives.

The main essential requirement is that the 24v output converter must 
have a fully isolated output - which your target converter does with 
1450+ volts to spare.

The 24v load draws power from both the lower and the higher supply so 
the lower supply must supply half the watts for the 12v output and all 
of the 24v output watts.

As the 12v output is always below the supply range you may wish to 
consider a buck converter as these are inherently more efficient than 
most alternatives (although a bad buck may be worse than a good 
forward converter or whatever).

Given the cost of fuel cells I'd be tempted to use two specialist 
converters (FC->12 and FC-> 24) that do the best job possible (unless 
this is a project where the FC cost is a given and other costs 
matter)(eg a student project MAY be like that).

That's a nasty variation in fuel cell voltage. If you could persuade 
it to be eg always >24 or always close to 18v you may get better 
efficiencies.




        RM


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