Match your power supply to the motion controller input voltage instead of trying to use a converter. If your psu is within the listed range you're ok.
You may need control relays that work at the psu voltage, so consider that too. Look at voltage requirements for the board outputs, perhaps having a voltage limit applied to them.
In other words, my motion controller requires 24vdc input, and will only take 24v across the output terminals switching. So I got 24vdc relays for safety latching the stepper drivers.
I had to provide a 5vdc psu for the drivers excitation ckt. I also added a 12vdc psu for my exhaust fan on the control box I made.
You can use a motion controller with more axes than you need and simply don't connect them.