Relevant Background Information

As part of my Bachelors Thesis I have to design IOT integrated control Electronics for a Pick-And-Place Machine. This specific machine has modules that can be attached to its Z axis that allows it to do different tasks. Here comes the part relevant to the question. The power supply for these modules come at 48V and have to be reduced to 24V,12V,5V,3.3V inside the module. I am deducing from the large size of the machine and the distance between the power source and the module there should be a lot of parasitic noise induced. I was thinking it would be a good idea to implement high-frequency noise filtering and then regulating the 48V to the respective voltages using buck converters. Here is a block diagram of what I had in mind:

enter image description here

Here comes the first question, does this sort of an implementation make sense?

What sort of a Low-Pass Filter to choose

The second question: What sort of a low pass filter should one choose in such an application? A circuit that I am familiar from Uni and was also recommended here is the RLC Low pass filter:


enter image description here

Since R_L is the inner resistance of my circuit it isn't relevant to calculate. However, I have no idea how to choose the cutoff frequency in such an application. If I am not mistaken the relation of the output AC signal of such an RLC should be like:

U_RL = 1/(1-(w^2)LC)*U_in

How to choose the inductor and capacitor type and values?

Now I don't know the cutoff frequency, I have no idea how to choose the L and C values either so I cant draw a Bode Diagram. I read that I could choose the cutoff frequency anywhere from 7kHz to 5Mhz, but that is an awfully large range. I have no idea on what type of capacitor or inductor to use. So I am stuck. Any help on the matter would be much appreciated, I would love to get some insight from more experienced engineers.

  • \$\begingroup\$ Beware of the Middlebrook criterion. \$\endgroup\$
    – John D
    Nov 3, 2020 at 20:10
  • 3
    \$\begingroup\$ LC filters at the input of an SMPS are problematic. See this excellent answer for more details. \$\endgroup\$ Nov 3, 2020 at 20:10
  • \$\begingroup\$ @aconcernedcitizen I see, so should I dump the idea and go with simply bypass capacitors near voltage ICs? \$\endgroup\$
    – Emre Mutlu
    Nov 3, 2020 at 20:18
  • 1
    \$\begingroup\$ Generally bigger inductors and capacitors are better. Practical matters limit their size. Also, you have to avoid resonant effects. Learn about “ferrite beads” on the web. \$\endgroup\$
    – user69795
    Nov 3, 2020 at 21:06
  • 1
    \$\begingroup\$ It is not clear at all to me that you do need filtering. A buck converter is, to some extent, a noise filter that will mostly avoid passing noise on the 48V rail to the lower-voltage regulated rail. A common mode choke where 48V goes into the module might be a decent idea. And a bulk 48V capacitor will be required on each module. You could put a small resistor in series with 48V to make sure you squash any resonances. A PTC could be used instead of a resistor to serve as both over-current protection and a dampener. \$\endgroup\$
    – user57037
    Nov 4, 2020 at 4:47

1 Answer 1


Several suggestsion

  • Use a GROUND plane under your filtering. Otherwise the SHUNTING element will be severely degraded because of Return_Path inductance.

  • Plan on inclusion of dampening; a series resistor of size SQRT( L / C ) is a good start.

  • inductors generate an external field; sensitive circuits may be upset

  • inductors also respond to external fields, thus high_current high dI/dT wires and motors and transformers may be a problem, ruining your attempt to filter


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