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Is it okay to short input and output negative terminal of an isolated DC-DC converter? This kind of situation is not mentioned in the datasheets. The problem arises when a load has negative terminal connected to shield (or chassis).

For example lets take this converter:DCM3414B50M31C2T09 Together with the filter: MFM1714x50M50C5yzz

If designed according to manual, the converter conforms to following standards:

  • MIL STD 704 (withstand of overvoltage and surge conditions)

  • MIL STD 461 (radiated and conducted EMI)

Do I lose those standards if i connect the load as shown on the image (orange)?

enter image description here

The fundamental problem is with loads that naturally have grounded enclosures (I.e almos all RF transmitters, PC etc)

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  • \$\begingroup\$ MIL-STD-704F (I don't know if conforming to this standard is a requirement for your application) 5.4.1 indicates that the negative DC terminal should not be connected to the chassis ground: All electrical power input terminals, including AC neutral and DC negative terminals, shall be electrically isolated from case ground. Equipment chassis shall not be used for power returns. If I didn't misunderstand, you are not allowed to do what you have shown. Source. \$\endgroup\$ Sep 30 at 8:56
  • \$\begingroup\$ In your schematic, the requirement is violated by 1) tying the chassis of the converter to the input negative directly, and 2) connecting the output negative to the chassis i.e. making the chassis a return path. Again, I don't know if MIL-STD-704F is a compliance requirement for your application. Just wanted to indicate. \$\endgroup\$ Sep 30 at 9:00
  • \$\begingroup\$ Yes those MIL standards are requirements for me. However Since the input and output is totally isolated there should not be any meaningful voltage potential between IN- and OUT+ so the return current will not flow through ground, in my opinion. \$\endgroup\$
    – tarmogr
    Sep 30 at 9:42
  • \$\begingroup\$ Yes, you're right. The chassis will be kept at ground potential and the return currents may not flow through the chassis. Nevertheless, the first sentence of item 5.4.1 is still violated: The chassis of the converter is tied to the input negative. \$\endgroup\$ Sep 30 at 10:10
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Is it okay to short input and output negative terminal of an isolated DC-DC converter?

Yes, that is OK with regard to the isolated DC-DC converter. That DCDC allows for a certain voltage difference between inputs and outputs. For example, the voltage must not exceed +/- 1000 V.

You intend to short the input and output side so there's 0 V (zero Volt) between them so that's perfectly OK.

MIL STD 704 (withstand of overvoltage and surge conditions)

That deals mainly with the input voltage which will be the same wether you short the input and output grounds or you don't. So no change expected.

MIL STD 461 (radiated and conducted EMI)

It gets a bit more complicated regarding the EMI emissions. However, you mention that this is for an RF system where I would expect everything to be shielded making it hard / impossible for any EMI emissions to "escape". Also here I would not expect any issues.

In case that you do get EMI issues, what I would then try is to open the connection for RF but not for DC. This can easily be done by adding an inductor in series with one of the two (or both) ground connections. Then for RF, you did not short the grounds. But for DC you did. Then check if that makes a difference regarding EMI.

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