enter image description here DC-DC converter outputs with bypass capacitors

Thank you for any help! This is my first question post, so let me know how I can improve it.

I'm working on my first Altium project, which is to remove several DC-DC converters that were causing noise in our atomic force microscope system -- two of them are shown in the first schematic. Note that the red writing is how we modified these manufactured boards to fix the noise problems. Ironically, I think they were meant to isolate the rest of the system from noise caused by the motor drivers.

I am wondering which of these capacitors and diodes are unnecessary now that the dc-dc converters are being removed from the design. My guess is that the input capacitors are all unnecessary, they are to smooth the input voltage to the dc-dc buck converters. I think that the smaller capacitors at the output are also for smoothing the higher frequencies of ripple caused by the converters. But do I need to keep some of the capacitors between 24V/15V (was previously 12V)/GND? The situation has changed in that I am no longer stacking "floating" voltages to provide 24V, so maybe I can just get rid of all of them? And I must admit I'm not actually sure what the diodes are for.

For context, these two lines are being used to power a row of motor drivers, which are each configured to run at either 12 or 24V -- the second schematic shows one of them... they are all identical except for which 0 ohm resistor is populated.

application: motor drivers


1 Answer 1


Tha diodes are there to absorb over-voltage (resulting in reversed current flow) shoved back into this power supply from the motors, I'm thinking. Any inductive load can "return energy back to the source" and those diodes are there to absorb such energy. Think of the reverse polarity diodes across relay coils, same thing.

Those input and output filter caps cover an impressive set of orders of magnitude. I wouldn't change them at all. This is an atomic force microscope, yes? That's an amazingly precise instrument. Those different caps are there to soak up noise and avoid self-resonance at different frequencies.

If your AFM works now, good enough!! Look at some atoms!!!

  • \$\begingroup\$ Thanks for the input! Yes I suppose they will still help isolate the system from motor noise. But it seems like I should at least remove the caps at the input of U22, unless I want to now connect them to the 15V line. Otherwise they are now a random row of capacitors a long ways away from anything. \$\endgroup\$ Oct 30, 2015 at 14:25
  • \$\begingroup\$ I'd imagine you could remove the bypass caps at the input to U22, but only also if you remove the trace or cabling feeding them. Otherwise, by removing them you're creating an antenna of sorts. \$\endgroup\$ Nov 1, 2015 at 0:04
  • 1
    \$\begingroup\$ Also, I'm re-thinking the utility of those diodes. They may be (inadequate?) fault protection in case the +12 / +15 supply comes up faster than the +24 supply, or the load on +24VMOT goes lower than +15 or +12 V in a fault scenario. I'm not getting this stacked supply design, frankly. I would connect the negative sides of all the +24VMOT bypass caps to ground, rather than the lower volt +12VMOT supply, particularly given the possible power-up issue, and that the loads themselves select one or the other supply thus don't source noise directly across those two rails. \$\endgroup\$ Nov 1, 2015 at 0:27
  • \$\begingroup\$ I like your re-thinking on the diodes, John. I actually found the answer (I think) on the website of the manufacturer of the DC-DC converter: blog.uk.tdk-lambda.com/uk/2014/07/18/… \$\endgroup\$ Nov 9, 2015 at 18:18
  • \$\begingroup\$ The diodes do appear to be in case the power supplies don't power up at the same time. Although these power supplies both turn on at the same time, different capacitance loads could probably cause one to come up slower than the other. I also was wondering about the wisdom of putting the capacitors between the power lines instead of both to ground. \$\endgroup\$ Nov 9, 2015 at 18:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.