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I have a long (20 - 50 m) copper wire for supplying DC power. One one end there is a AC-DC converter supplying 600W-1000W of power at 60V output. At the other end I have a switching type (100-140 kHz) DC-DC converter out-putting 24/12 Volts for a power requirement of 400W-800W.

I understand that there will be large line losses due to the DC transmission at such low voltages and high power. But I am more worried about the EMI effects of the long wire on the switching type DC-DC converters.

The wire will have significant inductance and resitance. How can I choose appropriate bypass or decoupling capacitors for the DC-DC converter in order to mitigate the EMI effects due to this lengthy wire?

As of now I have planned to use the following capacitors near the DC-DC converter:

1) 4.7 nF between all the input/output terminals and the baseplate/ground to eliminate high frequency common mode noise,

2) 0.2 uF between the input terminals to handle high frequency differential mode noise,

3) 100 uF between the input terminals to handle low frequency components and current droops on the power supply,

4) 0.01 uF between the output terminals to handle high frequency differential mode noise,

Are these capacitor values appropriate? Also I need some guidance on what can be done on the AC-DC converter end to improve the EMI filtering?

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  • \$\begingroup\$ Are there external filter recommendations from the DC-DC converter manufacturer? Usually there will be (at least for reputable manufacturers). \$\endgroup\$ – Spehro Pefhany May 5 '15 at 17:45
  • \$\begingroup\$ I agree with Spehro. However, I would like to point out that it may not be enough to put bypass caps on the two supplies. You may also need to run the power wire through a ferrite (common-mode choke) of some sort on both ends. This is to prevent common-mode conduction and radiation. It is also possible that the modules already have this type of protection built-in (a common mode power choke). \$\endgroup\$ – mkeith May 5 '15 at 18:06
  • \$\begingroup\$ @SpehroPefhany, yes some of the values I mentioned above are recommended by the manufacturer. But I belive those values are suited for general applications where the distance between the supply and the converter is not so much. I might have to do something as mkeith suggested. \$\endgroup\$ – vineet May 7 '15 at 4:24
  • \$\begingroup\$ I feel you have done an adequate job with the filtering you are providing. My only suggestion is to use a 'scope to look at the lines and get a feel for the amount of "noise" left still. If too much is present, then go ahead and provide more filtering, if it is acceptable, then you are done. \$\endgroup\$ – Guill May 8 '15 at 23:48

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