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I have used pi filters in the past with the USB powers and it worked great. This time I need the pi filters to smooth out the 24V supply from a regular SMPS power adapter.

There is a load switch/fuse for safety and some current limiting, but I'm not sure if I need to place the pi filters before or after the load switch. Below is my thoughts on the placements, but I really can't decide which one is better. It's assumed that the adapter has large switching noises.

Supply -> Pi Filter -> Load Switch

This placement can filter out noises from the supply and pass it to the load switch, but I think in case of a high transient current drain, currents can be limited by the load switch. Maybe a simple capacitor after the load switch can handle this issue? enter image description here

Supply -> Load Switch -> Pi Filter

This placement might be better in transient loads, but I was told that the filters need to be close to the noise source. enter image description here

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  • \$\begingroup\$ What sort of load switch are you using? \$\endgroup\$
    – vtolentino
    Jul 2, 2020 at 7:05
  • \$\begingroup\$ @vtolentino I'm using efuse load switch from TI TPS2590 \$\endgroup\$
    – BrianH
    Jul 2, 2020 at 7:25
  • \$\begingroup\$ Schematic please. \$\endgroup\$
    – Andy aka
    Jul 2, 2020 at 7:29
  • \$\begingroup\$ @Andyaka I added the schematics and I deleted all the parallel capacitors to make the image small. \$\endgroup\$
    – BrianH
    Jul 2, 2020 at 7:42
  • \$\begingroup\$ The need to filter using an inductor hasn't exactly been established. What is it about the 24 volts supply output that requires it to be filtered and/or, what is it about the load that requires it's supply to be filtered? What are you trying to prevent? Where's the problem? BTW you are exceeding the maximum recommended supply voltage in this circuit (20 volts) and that means the data sheet information cannot be relied upon. You are also within 1 volt of exceeding the maximum operating voltage. Not a good choice for 24 volts. \$\endgroup\$
    – Andy aka
    Jul 2, 2020 at 7:46

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You are exceeding the maximum recommended supply voltage for the TPS2590 (20 volts) and that means the data sheet information cannot be relied upon. You are also within 1 volt of exceeding the maximum operating voltage. Not a good choice for 24 volts. It would be OK for a 12 volt power supply I reckon: -

enter image description here

The addition of an LC circuit (i.e. a resonant low pass filter) can make this especially worse: when switching the load on then off the current through the inductor that feeds the switch and load is interrupted by said load switch and, this can nearly double the peak voltage seen at the input to the load switch. Been there but never again!

Realistically, I wouldn't expect the load switch to survive. Mine didn't so I dumped the pi filter.

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  • \$\begingroup\$ If I have a TVS diode that clamps at the max recommended operating voltage before the switch, do you think this will work? I think I need to make some options before I test. At the end I might just have to remove the filter just like you did. \$\endgroup\$
    – BrianH
    Jul 3, 2020 at 0:33
  • \$\begingroup\$ What model number for the TVS diode? What maximum continuous load current do you have? What are the upper voltage limits for the "stock smps" power supply? What full-load current can the power supply deliver? \$\endgroup\$
    – Andy aka
    Jul 3, 2020 at 7:37
  • \$\begingroup\$ Firstly, I found another load switch with the 60V max input.TPS1663. My supply only specifies output of 24V/3A. I'm not sure about the tolerance. The diode is ESDA25SC6Y. Breakdown 25V, Clamping 51V, Current rating 10A. \$\endgroup\$
    – BrianH
    Jul 4, 2020 at 1:40
  • \$\begingroup\$ @BrianH that looks a whole lot better. Not sure what diode you mean. \$\endgroup\$
    – Andy aka
    Jul 4, 2020 at 7:41
  • \$\begingroup\$ TVS diodes for ESD protection are only suitable for low joule ESD protection - they'll offer little protection against power surges. \$\endgroup\$
    – Andy aka
    Jul 4, 2020 at 7:47

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