In my current project, I need voltages of +/- 30V and +/- 15V for analog control purposes. The current on the 30V rails is fairly limited, ca. 70mA max on one rail and <10mA on the other. The 15V rails will only see <5mA.
I couldn't find a (reasonably priced) converter to generate +/- 30V directly, so I now plan to stack two +/- 15V DC/DC converters. However, I read that stacking converters this way will increase the need for external filtering, so I tried to figure out how to go about that.
Here is my current schematic:
The DC/DC converters are TEL-3 0523 from Traco Power. I selected them because of their good regulation and constant current limiting, which helps in my application.
The schottky diodes at the outputs are supposed to protect the converters from reverse voltage, in the case where one or the other converter would turn on more quickly.
The capacitors are all Panasonic FR series low ESR types, specifically EEUFR1H121L for the 120µF, EEUFR1V271L for 270µF and EEUFR1C471 for 470µF. The ferrite beads are all high-current types from Würth Elektronik (this one for the outputs, this one for the input). LTSpice simulation shows that the output ferrite together with the 120uF capacitor make a very well damped LC filter due to their parasitic resistances, with a -3dB point at ~6kHz and almost -50dB at the 300kHz switching frequency of the regulator.
The common mode noise (between input and output, due to capacitive coupling) will be conducted through the ground connection.
Is this filter OK? Should I add ceramic capacitors in parallel to the aluminum electrolytic caps, or should those be fast enough? LTSpice simulation suggests that adding 10nF with a series resistance of 15 Ohms would improve the response in the high megahertz range, but at that frequency I don't really trust the simulation anymore because so many parasitic effects become important.
Finally, will my ground plane become noisy from the high frequency return currents (mostly via C6, and between the converter outputs and inputs), and can/should I try to do something about that?