# Calculating power supply current output

I have a 15V power supply powering 2 Dc/dc converters that are the same. The converters are connected so that the outputs are 10V and -10V (the - terminal of the 10V converter is tied to the + terminal of the -10V converter . the 10V converter has a 10ohm load. the -10V converter also have a 10ohm load. What's the output current for the the 15V supply? My guess would be 2 amps (1 amp from each converter). Or is it dependent on a spec of the dc/dc converter?

• While I think I know, what circuit you are thinking of, I have great difficulties being sure that I think of the same circuit. So please, could you add a schematic for us? It puts away all ambiguities in an instant. Commented Feb 12, 2018 at 18:42

Your converters should have a spec for efficiency. Better yet, efficiency curves showing efficiency vs. output current.

Each of the 10V converters is putting out 1A, or 10W. The input power is the output power divided by the efficiency at 1A.

Suppose each converter is 90% efficient at 1A. Then the input power is 11.11W for each converter. Total input power is 22.22W.

Now the 15V supply has to supply 22.22W, meaning 1.48A.

Since these are DC-DC converters, output current doesn't necessarily equal input current like it would for an LDO. The important thing to remember is that input power = output power/efficiency.

• Thanks, the numbers work out. The current direction/numbers were throwing me off. Looking at where power goes makes sense and simplifies things. Commented Feb 12, 2018 at 17:51