Omron CV500-IC301 如果你對EMC有一定的了解，你一定聽過EMC測試主要測兩大類干擾，一是傳導，二是發射。傳導噪聲又可以分為兩大類，一種是差模噪聲，另一種是共模噪聲。
共模干擾與差模干擾
什么是差模噪聲和共模噪聲
在下圖中，表示一個線路板裝在金屬外殼中，我們具體來具體分析一下。在差分模式噪聲中，噪聲源出現在電源線兩端并與電源構成串聯的關系，在圖中我們明顯可以發現共模噪聲電流的方向與電源電流的方向相同。那么我們為什么叫它差模電流呢？之所以稱為“差模”，是因為輸出電流和返回電流的方向相反。
差模噪聲與共模噪聲
那么什么是共模噪聲呢？共模噪聲其實就是通過雜散電容等泄漏的噪聲電流通過地返回到電源線的噪聲。之所以稱之為“共模”噪聲，是因為電源的正（+）側和負（-）側上的噪聲電流方向相同，這樣電源線的正負噪聲壓差為0。
如上所述，無論是共模噪聲，還是差模噪聲，這些類型的噪聲是屬于傳導輻射。但是，由于噪聲電流在電源線中流動，因此仍然會輻射出噪聲。
共模噪聲和差模噪聲的差異
由差模噪聲引起的輻射電場強度Ed可以使用下圖中的左邊的公式表示，Id是差分模式下的噪聲電流，r是到觀察點的距離，f是噪聲頻率。差模噪聲會產生噪聲電流環路，因此環路面積S成為影響差模噪聲強度的重要因素。由公式可知，如果其他元素不變，較大的環路面積，電場強度會更高。
由共模噪聲引起的輻射的電場強度Ec可以由下圖中右邊的公式表示。電纜長度L是一個重要的影響因素。
計算實例
下圖中是一個計算實例，當然，由于實際應用中的線纜一般比較復雜，一般只能定性分析，通過這個例子我們可以得出重要的結論，對于相同的噪聲電流值，共模噪聲引起的輻射干擾強度要大得多在這個示例中，共模噪聲干擾強度大約是差模噪聲強度的100倍。所以在考慮解決輻射的措施時，應記住，應對共模噪聲的措施尤為重要。當然這不是說差模干擾就不用管了，只是要有側重點。
計算實例
總結
總結一下，為了減小差分模式噪聲，我們采取的最基本措施是通過使用雙絞線電纜來減小環路面積S，對于共模噪聲，盡可能縮短電纜長度。聯想我們在進行PCB布線時，經常看到的一句話就是走線要避免形成大的地環路，其實基于減弱差模噪聲干擾的考慮。
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