I tried to get a LC filter at the output working, using the link I provided above. This resulted in a 10µH coil, 220µF for the first cap and 22µF for the second (although this one could be smaller from the formulas). It did work quite well to reduce the ripple from the switcher (at 67kHz), but there is an additional 1kHz ripple super-imposed on the output. And that one I could not get reduced to where I wanted it.
The scope signals (using a 47µF coil and two 220µF caps, in addition to a damping RC-combination) look like this (filter input at the bottom, output at the top) are attached. The DPS5005 was set to 5V, with a 700mA load.
It might be that the control loop of the module runs at this frequency.
I haven't actually attempted to repair the noise from these modules yet (I'll be looking into this very soon, I hope). But I wanted to point out a few things:
There are at least two noise problems:
- the noise imposed on the load (which you are combatting)
- the noise sent back toward the input power source
Obviously this first kind of noise has to be dealt with no matter what, if it's important to you to reduce the noise. But if you want to make more sophisticated power supplies using this module (for instance, I am trying to combine two units to have either a pair of isolated supplies, or a single tracking (or not) bipolar supply). In my case, I'm using a big audio transformer to provide the two isolated supplies and this means all of the noise that is sent back from one DPS5005 is picked up by the transformer and sent into the second DPS5005.
I don't have much to say about this yet, other than I presume at the very least the standard double choke setup (2 capacitors, 1 common choke, 2 capacitors, 1 common choke, 2 capacitors) is probably a good starting point here. I plan to put one of these between each power supply and the transformer -- and one between the transformer and the wall input.
As for the second kind of noise (applied to the load)...
1) It might be worth it to see where exactly one could bodge in filtering capacitors and chokes inside the supply. Any filtering that can be done in-situ is far more effective than any filtering done after the fact. I wonder what kind of absurd things are possible, like lifting legs of ICs and putting ferrite beads over them. I know there are critical current loop paths that are fundamental to reducing noise, and I don't know if these critical paths were analyzed in the design of the modules.
2) More importantly: the noise of these supplies changes. The worst noise comes when V_out is at the extremes: near zero, or near V_in. In the next week or so I'll try to get captures demonstrating this. I am building my supplies to maximize the range of the unit, so I'm feeding them with 55-57V input. This is probably a bad idea for lots of reasons, but I have a bunch of these modules so I'm not to afraid to break some eggs to make an omelette.