This paper investigated the common mode (CM) noise model for multiwinding switching transformers. Based on the proposed model, a Flyback transformer is investigated. A measurement technique is used to characterize and evaluate a switching transformer's CM noise performance. Only a signal generator and an oscilloscope are needed in this measurement technique. Because there is no any in-circuit tests are needed, the technique can help quickly design and evaluate transformers in mass testing. CM noise reduction techniques, including the balance capacitor technique, core shielding technique, balance winding technique, and winding design technique, are investigated based on the developed two-capacitance model and measurement technique without introducing extra power loss. The near-field capacitive coupling due to transformer magnetic cores is analyzed, and a core shielding technique is applied to reduce the coupling. Experimental results validated the proposed model and the developed CM noise reduction techniques. The Flyback converter can finally meet electromagnetic interference (EMI) standards without using CM EMI filters and Y-capacitors.