為什麼總是某顆電容被燒毀?

作者 : Samuel Kerem,美國約翰·霍普金斯大學應用物理實驗室(APL)

這5個未發生故障的電容佈局都是相同的,而發生故障的總是那顆正極端子上有一個過孔而負極端子上有一條較粗走線的電容…。此時,我知道問題出在哪了…

我曾在一家光通訊公司擔任設計工程師,該公司生產的1,000台設備安裝在全世界各地。由於用於現場的模組很多,送回公司維修的也很多,我的工作便是搞清這些模組出了什麼問題。其中一次的故障查找經驗讓我上了精彩的一課,至今記憶猶新。

在收到一組客戶寄回來的模組後,我很輕鬆地就找到故障原因了:一顆燒焦的鉭電容。這顆電容因此短路了,導致價值數萬美元的模組無法正常運作。這種表面黏著電容——採用7343封裝,額定電壓為20V,黏著於12Vdc電源層。在這段時間內,大約10,000個電容中只有一個電容出現故障,這一數據遠低於統計資料預測的故障率。因此,我為這顆燒焦的電容拍了照留存後,就把這個案子結了。

幾週後,另一家客戶退回了一個類似的模組,在這個模組的同一位置,一顆電容燒焦並短路了。即便算上這個模組,故障率仍然低於統計預測值。我知道電路板上還有5顆相同的電容,並聯在同一個12Vdc電源層。除了模組的故障率,現在電路板上電容的故障率是六分之一。所以,我又拍了一張照片。雖然我還寫了一份報告來安撫高層管理人員,但後來覺得最好還是進行一下可靠性計算,尤其是鉭電容的可靠性,越快越好。

又過了幾週,再次收到另一個故障模組。而且還是相同位置的電容,看起來很糟。那時我已經完成了可靠性計算,對於可靠性進行長篇大論的複雜解釋可能會嚇倒其他人,但為什麼燒毀的總是相同位置的電容?過電壓?突波電壓?都不太可能。同一個12Vdc電源層上有很多靈敏元件,都可能在電容甚至還沒問題之前就被燒毀。除了過大的紋波電流,我想不到其他更好的解釋…

…繼續閱讀請連結EDN Taiwan網站

 

 

 

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