I have two Recom R24-150B dc-dc converters (24V in, adjustable up to 220V out) and I want to have +/-220V output rails. The converters need a resistor between the - (COM) output and their adjust port in order to set the output voltage. This acts to regulate the output.
According to the data sheet it is ok to tie the outputs of these converters together to obtain higher voltages because they're isolated, as long as some capacitors are attached across the outputs of each unit to prevent beats appearing between the switching frequencies of the two units.
I tied two together and attached the filtering capacitors. I grounded the middle to create the bipolar output rails, and this produces the desired dc voltages on each rail, +220V and -220V. However I am seeing quite a lot of noise with my oscilloscope around 1 MHz on the load attached between the two HV rails, around 200-300 mV peak-to-peak. The switching frequencies should be around 200 kHz so I guess this is some higher harmonic content. This noise appears to be on the ground rail between the two converters because the noise on each of the HV rails is smaller and in phase. The individual output ripple from each converter is less than 100 mV peak-to-peak so clearly something is ringing up and increasing the output noise.
Is there some reason why bridging two converters together like this to form differential outputs is a bad idea? Or is there some additional filtering required somewhere?