Electronic – Why is AC earth connected to DC ground in laptop adapters

Any long cables with RF clocks/data will radiate noise and when connected to AC ground could also radiate via AC cable from path ground loops.

Smart idea is make generous use of common mode chokes for interface cables and DC outputs to raise impedance of the common mode signals from AC ground.

Beware that Hipot testing of AC/DC PSU may cause failure on secondary port when grounded. This is due to dielectric breakdown of Hipot from primary to secondary. Floating secondary is easier to protect if there is capacitive leakage and tester uses fast rising input. In this case modify Hipot tester to avoid discharge surge that would destruct PSU by using a current limiting series resistor.

Leakage can be minimized with prudent gaps, quality caps, and moisture protection on conductors. (moist dust causes leakage)

1. Usually for thermal management or mechanical compliance.

   • thermal -- placing the power supply away from the potentially hot device and away from the wall (better airflow) results in the most efficient operation.
   • mechanical -- large power supplies would block adjacent outlets if connected directly. They can also be heavier than can be safely suspended (hung) from wall outlets.

2. Basically no... except for the above mentioned reasons.