Mitigation of EMI in a flyback converter

M.Ing. (Electrical & Electronic Engineering Science)This study investigates the mitigation of conducted electromagnetic interference (EMI) in a flyback DC-DC converter. Without the use of filters, the maximum mitigation of EMI possible without significantly decreasing converter operating efficiency is investigated. The following parameters are found to influence EMI: · Switching speed: Decreasing switching speed (increasing rise and fall times of the MOSFET) effectively reduces both common mode (CM) and differential mode (DM) EMI above a certain frequency. Series gate resistors up to a certain value were found to not increase power dissipated in the MOSFET. Series gate resistors greater than this value, further reduce CM and DM EMI at the cost of larger amounts of power being dissipated in the MOSFET. · Leakage inductance and inter-winding capacitance: The dominant component of the flyback coupled inductor in terms of EMI generation is the inter-winding capacitance. Increasing inter-winding capacitance increases both CM and DM EMI. Reducing inter-winding capacitance increases leakage inductance. Increasing leakage inductance however, results in reduced converter efficiency. Coupled inductor design is therefore a compromise between leakage inductance and inter-winding capacitance. · Layout inductance: Reducing layout inductance in certain parts of the circuit is an effective method for reducing DM EMI. This is shown to also decrease CM EMI but not to the same extent as DM EMI. · Snubbing: Snubbing is shown to effectively reduce both CM and DM EMI by reducing the magnitude of the voltage overshoot and ringing on the drain of the MOSFET. Snubbing however reduces converter efficiency. This study gives important guidelines to the engineering trade-offs in reducing EMI versus efficiency in a flyback converter