切換式穩壓器的電磁干擾研究

Abstract

本論文選用“致新科技”公司，型號G5684的Step Down DC-to-DC converter IC為例，探討改變切換頻率對輸出電壓漣波(Ripple)和電磁干擾(Electromagnetic Interference, EMI)問題的影響。 由Switching Regulator的結構原理可知，輸出電壓漣波與頻率的平方成反比，提高切換頻率可降低漣波，電感亦可選用較低的電感值，而使印刷電路板(Printed circuit board, PCB)的使用面積下降。但是副作用就是其內部結構造成PWM週期切換時不是一完美的方波，在high-low急遽變化的地方，波形會產生震鈴(Ringing)，此ringing會疊加在輸出電壓波形上，造成雜訊干擾，實際量測EMI，結果隨著頻率升高而變糟。研究結果顯示，在輸出電壓端加上可抑制ringing雜訊的ferrite bead可達到抑制EMI的效果。

In this paper, we used the GMT model G5684 Step Down DC-to-DC IC to explore the effect of changing the switching frequency on the output voltage ripple and electromagnetic interference (EMI). From the structural principle of switching regulator, we can see that the output voltage ripple is inversely proportional to the square of the frequency. Increasing the switching frequency reduces ripple and allows the use of lower-valued inductor which occupies smaller area on the printed circuit board (PCB). But the side effect is when the wave changes rapidly in the high-low transition, ringing occurs. The ringing will be superimposed on the output voltage waveform, causing noise interference. When we test EMI, the result becomes worse as the frequency increases. The result shows that adding ferrite bead on the output voltage terminal can suppress EMI associated with the ringing noise.