使用頻率調變壓低切換式電源供應器的EMI

Abstract

使用頻率調變(Frequency modulation)去壓低切換式電源供應器的EMI技術，已行之有年，而且市面上也有產品使用此項技術；有許多調變方法如正弦波、三角波、亂數波以及雙頻調變已有人提出，以上這些方法大都只強調數學上的分析，以及探討頻譜上的展頻形狀，沒有人提出使用此項技術對電路的性能會有何影響。 在本文中將對這些調變方法重新評估，並實際做出硬體以實驗的方式加以驗證，由諸多實驗發現使用頻率調變後，輸出端電壓會有漣波發生，對量測的數據作圖分析，發現頻率調變所造成的擾動，猶如輸入端電源的擾動，做開回路測試，發現與輸入輸出(Audio-suspectibility transfer function)變化相似，但尚未經數學理論驗證，閉回路測試在本文亦有實驗結果討論。

Employing frequency modulation techniques for EMI reduction in switching converters was reported some years ago and the techniques have been gaining some momentum in actual product applications. There were different modulating schemes reported, including sine-wave modulation, triangular wave modulation, random modulation and bi-frequency modulation. However, the results reported were mostly focused on the mathematical analysis of the resulting EMI spectrum without much discussion on the side effects of the schemes on converter performances. In this thesis, these modulating schemes were reevaluated and compared. A hardware circuit to modulate the converter switching frequency was designed and built for the experiments. From the experiments, it’s observed that significant output voltage ripple can be caused by the modulating schemes. The transfer function between the output ripple and the modulating signal seems to follow the input-to-output (audio susceptibility) transfer function of a converter power stage but this has not been theoretically derived. The effects of feedback control loop gain on the output ripple are also discussed in the thesis.