Electromagnetic Compatibility (EMC) debugging of power electronics systems depends heavily on the experience of specialists due to the complex mechanisms of electromagnetic noise. In this paper, the time domain approach is used instead of the conventional frequency domain method to identify noise propagation path. The proposed test mode switches the upper or lower transistors of the converter legs simultaneously on and off at a 50% fixed duty ratio. It is called common mode test mode. Because there is no significant functional current flowing, the differential mode noise is minimized and the remaining common mode noise and mixed mode noise are maximized. Therefore, the common mode noise in this test method gives its upper boundary of Electromagnetic Interference (EMI) level when the system is normal operated. A motor drive system is measured to show how this test mode works.@en