EMC Question of the Week: December 18, 2023
In order to be galvanically isolated, two circuits cannot share
- a DC current path
- any current path
- any wires or components
- a ground
Answer
The best answer is “a.” Galvanic isolation requirements prevent the sharing of any conduction path that would allow significant amounts of direct current to flow. They are generally imposed for safety reasons, but they can also be used to limit the flow of DC currents that produce unwanted magnetic fields or facilitate corrosion.
Components used to convey power and/or signals across the boundary, while still providing galvanic isolation, include transformers (magnetic-field coupling), relays (magnetic-field coupling), capacitors (electric-field coupling), opto-isolators (light), and hall-effect sensors (magnetic-field coupling). Each of these devices blocks the flow of DC conduction current.
When capacitors are used to provide galvanic isolation, high-frequency current is intentionally conveyed across the boundary. Capacitors connecting the DC current returns of two galvanically isolated circuits can help them to maintain the same high-frequency reference potential.
If both sides have a conductor labeled "ground" that carries intentional power or signal current, then those "grounds" typically can't be shared. However, it's important to recognize that (from both an EMC and safety perspective) circuits on both sides of the galvanic isolation should have the same ground. In other words, they should have the same zero-volt reference. Two isolated safety grounds could be unsafe. Two isolated EMC grounds could result in compliance issues.
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