Flyback Power Supply EMI Signature and Suppression Techniques

Abstract

Flyback power supplies have a distinctive EMI signature caused by superposition of the transformer primary current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PRI</sub> ), drain voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Drain</sub> ), diode voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Diode</sub> ) and transformer secondary current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SEC</sub> ) waveforms; each generate emission currents which may exceed the desired EMI specification limits without proper EMI design technique. The primary current waveform causes primarily differential mode emission currents to circulate between the AC mains and the power supply input. It can also create common mode emissions due to radiated magnetic fields if the current path defined by the PC board layout encircles a large physical area. The drain voltage, diode voltage and secondary current waveforms are mainly responsible for the generation of common mode emission currents. Controlling EMI in SMPS requires attentions to following areas: 1. Differential mode filtering 2. Common mode filtering 3. Power cord damping 4. Transformer construction This paper addresses the concept of EMI signature in SMPS and presents conceptual techniques for EMI suppression. Differential mode filter analysis and common mode filter analysis are discussed in detail and an effective high frequency circuit model of the differential mode filter is obtained. Important aspects of power cord damping and transformer construction are also briefly covered. This paper is intended to be an efficient helping tool for the EMI filter designers.