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Common-mode (CM) electromagnetic interference (EMI) noise of a power converter is mostly due to the displacement current of the parasitic CM capacitance distributed between earth and the corresponding dynamic circuit node with very large dv/dt. In the layout of the printed circuit board (PCB) of a power converter, the power ground (GND) is an important factor to mitigate CM EMI, because it could act as a shield between earth and the dynamic circuit node. Taking a buck converter as an example, this paper proposes a method to evaluate the incomplete shielding effectiveness of GND on CM EMI emission. It first builds up a CM EMI model for the converter. The CM model is then used to analyze the shielding mechanism of GND on CM EMI. Numerical simulations are performed for comparison of shielding effectiveness of GND with different layouts. The simulation results show that the shielding effectiveness of GND increases conditionally with the mutual capacitance between the GND and the dynamic circuit node. The evaluation method and the simulation results are validated through experimental results.