Why is External Grounding Necessary in DC/DC Converters?

[fonz]

I have two identical DC systems fed from separate DC/DC isolating converters, one converter is fed from a negatively earthed DC supply and the second is fed from a floating DC supply. In both cases the negative of the DC/DC converter supply is tied externally to the 0V or 'common' of the output side.

My question is what reasons would there be to do this in either case?

Also depending on the size of the converter, what current would be expected to pass through this connection?

 

[meBigGuy]

Can you draw this for us. What you said can be interpreted in different ways.

 

[fonz]

See attached

 

[Jeff Rosenbury]

Safety?

If the converter is indeed isolated, there's no electrical reason, but also no reason not to. I would link the two as a matter of course unless I had a reason not to. It's usually a good idea to make all of your grounds connected if possible.

Don't expect any current through the link. If there is, you have an isolation problem.

 

[Finny]

It's usually a good idea to make all of your grounds connected if possible.

This is generally good advice. Grounds provide a common reference point; without a connection there is potential [no pun intended] for variations. Grounds in general provide for protection in the event of a equipment failure [like deteriorating insulation], the build up of static electricity and even corrosion protection.

For another thing, there is little guarantee that the dc input negatives are at the same ground reference...so the outputs may not be.

I recently spent a lot of time working on an older boat with 12v dc, 24v dc, 120v ac and 240v ac. In addition there was an isolation transformer and a polarization transformer between the shore supply power and the on board. Each is kept separate and the proper connection between the systems is via a separate grounding wire
for each connected at a common point.

If a 'ground' wire actually carries current, one end will not be at the same potential as the other, due to the power loss in the conductor.

Oh, It just occurred to me, if you want more. https://en.wikipedia.org/wiki/Ground_(electricity)

 

[fonz]

I can understand for the negatively earthed system but I have the same system that is isolated from Earth on both sides of the converter but the link is still there?

 

[Finny]

the link is still there?

How about just thinking of electronic components inside the dc/dc converter being between the input and output 0v...wires [resistance wires, capacitors,inductors, who knows what...silicon semiconductor components...

 

[fonz]

How about just thinking of electronic components inside the dc/dc converter being between the input and output 0v...wires [resistance wires, capacitors,inductors, who knows what...silicon semiconductor components...

Never thought about it that way before, thanks.

 

[Finny]

I looked at wikipedia for dc/dc converter schematics to see if that REALLY made sense...they did not have any,
but if you check around I'll bet you can find some.

Ground wires and especially grounding wires are a science unto themselves...in sensitive electronic circuits to avoid noise, and in wet environments [such an underground fuel storage tanks and underwater metals on boats, even wood boats] to avoid galvanic corrosion.

 

[Bystander]

https://www.google.com/?gws_rd=ssl#q="grounding+and+shielding+techniques"
I'm remembering this book as McG.-H. rather than J.-W., but put many miles on the copy I no longer have.

 

[meBigGuy]

Your drawing doesn't show the two converters mentioned in your original post.

From a functional power supply perspective "earthing" is not required. From a safety or regulatory perspective I would need more information to comment.

As for the input and output common:
A dc to dc converter generally needs a common ground between input and output unless it is a transformer isolated system (which is very possible). Look at google images for "buck converter". Most simple converters depend on a common ground. If you actually have two input terminals and 4 output terminals as you have drawn, it could be isolated. Do you have a part number or schematic for the converter? Does it have a transformer?

Measure the impedance between the input common and output common.

 

[fonz]

Your drawing doesn't show the two converters mentioned in your original post.

From a functional power supply perspective "earthing" is not required. From a safety or regulatory perspective I would need more information to comment.

As for the input and output common:
A dc to dc converter generally needs a common ground between input and output unless it is a transformer isolated system (which is very possible). Look at google images for "buck converter". Most simple converters depend on a common ground. If you actually have two input terminals and 4 output terminals as you have drawn, it could be isolated. Do you have a part number or schematic for the converter? Does it have a transformer?

Measure the impedance between the input common and output common.

There is 1kV rms isolation between the input and any output on this particular power supply. It's old, 1983 I think and therefore don't have or can't find a data sheet. Its made by a company now owned by Rockwell Automation but it was discontinued years ago.

About the drawing, I've shown a connection to Earth with scenario one being the first system I have which is negatively earthed. Scenario two is unearthed, like my other system. Both have the link between input 0V and output 0V (common).

Thanks