Designing Common-Mode (CM) EMI Noise Cancellation Without Y-Capacitor

It is well known that common-mode (CM) conducted electromagnetic interference (EMI) is caused by the common-mode current flowing through the parasitic capacitance of transistors, diodes, and transformers to ground in the power circuit. We need to attenuate this noise to avoid the interface with the other system. Conventionally it should be achieved from adding a magnetic choke and capacitors into across input power line, which will be affecting cost increase for power supply area. To reduce total cost of power supply, there has been in recent years a desire to introduce noise cancellation technique. Also recent safety requirement has been tightened. So, medical equipment, handheld, and portable equipment should be able to have a limitation of leakage current. Connecting a Y-capacitor between the primary return line terminal and the secondary return line terminal is a very common measure in EMI design for an isolated switching mode power supply. However, it has still a cost increase and too much leakage current flowing through transformer. This paper shows how to reduce leakage inductance with noise cancellation technique and EMI performance data with test result for less than 10W application.