采用频率抖动技术降低开关电源芯片的EMI设计

Design of Controller for Off-Line Supplies with Low EMI Using Frequency Jittering

介绍了一种能有效抑制开关电源芯片应用中,功率开关管电磁辐射干扰的频率抖动技术原理及其电路实现。采用调频技术,让调制信号去控制开关电源的工作频率,使其原有的频率发生偏移。这样,原先集中在狭窄频谱上的电磁辐射能量得以分布在较宽频谱范围,实现了对EM I的有效抑制。基于3μm的高压BCD工艺模型,在Cadence设计环境下,采用差分电路控制开关电源中振荡电容的充放电;用镜像电流选通电路调控电容充放电电流的大小,完成了带频率抖动振荡器的电路设计,给出了实验仿真的结果和电路拓扑结构。并使用MATLAB工具对功率开关管驱动信号中有/无频率抖动功能的输出波形进行了频谱特性的对比分析,得到了采用频率抖动技术大致可以减小EM I辐射幅值近一倍的结论。

The technologies and schematic of frequency jitter to reduce the electromagnetic radiation of controller for off-line supplies were introduced. According to the principle of the frequency jitter based on frequency modulation, the switching power supply＇s operating frequency was controlled by the modulated signal to shift the original frequency.. Thus, the electromagnetic radiation energy originally concentrated in the narrow spectrum was distributed in a wide spectral range to achieve effective suppression of EMI. Based on 3 μm high-voltage BCD process model in the Cadence design environment, the switching power supply capacitor charging and discharging was controlled by the differential osc＇illation circuit ; Current mirror circuit with control gate controlled the current of capacitor charging and discharging to complete oscillator circuit design with frequency jitter. The experimental results of simulation and circuit topology were given. Using MATLAB tools, the comparison with the spectrum of the driving waveform of the power switching transistor without frequency jitter was analyzed. The technology of using frequency jitter can reduce almost double the effect of EMI.