I am feeding the output of an isolated ACDC converter (this one) into a DCDC converter (that one), and the ripple out of the ACDC supply jumps by a factor of 5 when the DCDC converter is connected - which apparently is nearly entirely passed through to the output of the DCDC.
Ripples when the ACDC supplies the DCDC:
(blue is ACDC's output and DCDC's input, yellow is DCDC's output)
The fact that both increase when they are connected together means to me that they are interacting.
Both oscillations seem to be at the same frequency as the DCDC switching (about 360kHz), so my current theory is that as the DCDC downstream switches the rail ON, which discharges the ACDC's output cap voltage, the ACDC's controller starts to compensate for that and passes some current through - but the DCDC controller downstream switches off and the rail voltage starts to increase before the ACDC acts on it at the next loop cycle.
It's the only explanation I have for the fact that both waveforms are so in-sync and close in amplitude whereas I'd normally expect the DCDC to show on the waveform clearer signs of attempting (in vain, by limited bandwidth) to regulate out some of the ripple.
If it is true, heavy filtering could improve this. To calculate what's necessary though, I need to confirm - or at least increase confidence - that this is what's happening. What's your take? Any other ideas?
Note: When connected together, 10x4.7uF are already present at the input of the DCDC. I'd be surprised if this is too high a capacitance for the stability of the ACDC.