Author Topic: Filtering DC input to board  (Read 1345 times)

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Offline pigrewTopic starter

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Filtering DC input to board
« on: November 06, 2017, 01:16:58 am »
I'm working a a board that will need 5V and 3.3V inputs (about 500 mA), and I'd like to (due to simplicity) use a 9VDC wall-adapter for its input. I've not decided if I'm going to use a linear or a buck converter for it. The board will have a few analog amplifiers, a battery charger, and a microcontroller.

Is there a general guidance for what sort of filtering there should be at the power input? I tried searching this forum, but didn't find anyone talking about how to filter DC input.

I found a design that looks reasonable in this automotive power supply project on HACKADAY. (see attached)

Some thoughts of mine:

  • Is this overkill, and I should be able to use only a fuse and schottky diode, and I should just add ferrite beads/inductors if I notice problems?
  • Should the fuse be closer to the DC input jack, so that things don't fry if the capacitor shorts?
  • Should I add another 100nF capacitor on the input side of the inductors?
  • How do I choose which ferrite bead to use? Is larger impedance at 100 MHz generally a good thing? Or would I need to do full EMC testing to know which one to use. (I know I need to look at the current and max DCR ratings, but there are still many choices.)
  • Should I be using a common-mode choke in addition to or instead of the ferrites?
  • Is it common to add some over-voltage protection, like a zener diode past the fuse?
  • Are there existing designs that I should copy?


Thanks!
 

Offline ebclr

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Re: Filtering DC input to board
« Reply #1 on: November 06, 2017, 04:40:25 am »
Yor design is quite good, but remember that the way you trace the power on your CPU board, will be 100 times more important than this input protection
 

Offline trophosphere

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Re: Filtering DC input to board
« Reply #2 on: November 06, 2017, 04:57:20 am »
Quote
Is this overkill, and I should be able to use only a fuse and schottky diode, and I should just add ferrite beads/inductors if I notice problems?
The fuse and schottky diode components are for protection versus the ferrite beads/low value capacitors which are more for noise filtering.

Quote
Should the fuse be closer to the DC input jack, so that things don't fry if the capacitor shorts?
What do you mean by "things"?

Quote
  • Should I add another 100nF capacitor on the input side of the inductors?
  • How do I choose which ferrite bead to use? Is larger impedance at 100 MHz generally a good thing? Or would I need to do full EMC testing to know which one to use. (I know I need to look at the current and max DCR ratings, but there are still many choices.)
  • Should I be using a common-mode choke in addition to or instead of the ferrites?
  • Is it common to add some over-voltage protection, like a zener diode past the fuse?
  • Are there existing designs that I should copy?
The selection of various ferrites, capacitors, and other components is usually made once you know what you want to filter. Putting components in just for the sake of filtering "noise" is a shotgun approach will pretty much be a tossup as to rather it is effective or not. I would recommend you select the wall adapter you want to use and look at its output to (1) gauge if filtering will be of any added benefit and (2) characterize the ripple/high frequency noise it has so that you may develop an idea of what you are trying to filter out.
 


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