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Switching power supply driven from bipolar input?

I have a design problem that I thought would be easy but is turning out to be more difficult than I thought. I am working on a program to insert an FPGA, A/Ds, D/As etc. into an existing, all analog, system. The system power is sourced from both +12V and -12V with equal and limited current available in each supply. I am going to need 3.3V and around 1.5V and possibly some other voltages. The power distribution to the needed voltages is not evenly distributed so I cannot see a way to generate 3.3 from +12 and 1.5 from -12 and not overdrive one or the other of the inputs. The overall power requirements are doable withing the total power budget.

I am not an expert on switching supplies but I know enough to be dangerous so I began to wonder how easy it would be to generate a switching DC converter that takes in both 12V and -12V and generates one or more output voltages. The power should be drawn roughly equally between the two input supplies and the output voltages need to be referenced to the system ground. The total output power would be around 10W.

I searched through the usual vendors components and application notes and didn't find anything so I talked to a couple of application engineers. They pretty much blew me off and said to use to separate converters, one on the +12 and one on the -12.

This seems like a relatively doable concept. Am I missing something?

Reply to
nonone
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Buy a small isolated DC-DC converter brick, 24 volts in to 3.3 out. Mouser and Digikey have them, not very expensive. To get the Vcc_core voltage, use a LDO linear regulator 3.3-to-1.5 if the current is low, or a switcher from 3.3 to 1.2 if not. Given that you're not a switcher jock, buy that too, or copy a reference design from a synchronous switcher IC datasheet.

The DC-DC bricks are available with all sorts of input and output voltages, regulated and not.

John

Reply to
John Larkin

how about a transformer coupled one that operates on 24 volts.

Bob

Reply to
<castlebravo242

It is doable. Use a half-bridge between +12V and -12V. If you are really brazen you could even do away with the coupling cap(s) and connect one side of the xfmr to ground. But woe to him who veers off with the duty cycle just a wee bit or lets the +/-12V become uneven ... zzzt ...

*POOF* ... so maybe keep the cap.

Half-bridges do not like to be regulated although they can be. Especially if you want to get away with some catalog part for the transformer (the VersaPac series comes to mind here) you could then hang a 3.3V and 1.5V buck converter behind it. Plus others if you later find out you also need 5V or whatever.

However, as John wrote, if you aren't a switcher jock and this system isn't produced in the hundreds or thousands per month buy the stuff as modules. One downside of buying is that such modules can become discontinued out of the blue.

--
Regards, Joerg

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Reply to
Joerg

At that voltage, isolation is not necessary, source it from the rails and regulate to the common "ground". .

Reply to
JosephKK

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