文件名称：Handling EMI IN Swithc Mode Power Supply Design
文件大小：224KB
文件格式：PDF
更新时间：2015-11-22 04:30:48
PWM Power EMC
Introduction When designing switch mode power supplies (SMPS), undesirable noise and Electromagnetic Interference (EMI) are always present. Their effects are even more severe as the switching frequency increases, especially in applications requiring the use of small size transformer and capacitors. This application note describes the nature and sources of EMI noise, and the design techniques used to reduce their shortcomings. All the examples of the circuitry used are referred to flyback topology however, they are also applicable to any other common SMPS topology. Noise Sources The most effective way to handle EMI is to minimize its source, then use filters to filter away the remaining noise. Therefore understanding the noise sources will greatly help to reduce the effects of EMI. Noise Generated in the Primary Side of the Switching Regulator In a linear regulator, output voltage regulation is achieved by dissipating the voltage drop in the form of heat. The switching regulator achieves output regulation by switching on and off the output transistor and varying the on time. However, switching the transistor on and off causes the generation of high frequency noise. Primary Voltage and Current Waveforms The Drain–to–Source voltage of the transistor, VDS, has a high dv/dt characteristics. At the same time, the leakage inductance results in voltage spikes and ringing, consequently generating noise due to the high dv/dt as shown in Figure 1 and 2. Typically, the collector or drain of the transistor is connected to the tab of the package. (TO–220, TO247 and etc.) The package’s tab becomes an antenna radiating noise. Its effects become even more severe when a heatsink is added on to it. In some high voltage regulator ICís like the MC3337X Series, a relatively quiet source is connected to the TO–220 tab to helps reduce the related common