Beckhoff AX5125-0000
Beckhoff AX5125-0000當igbt在高性能應用中高速接通和斷開時，總會發生過壓。例如，當關閉負載電流電路時，集電極 - 發射極電壓突然上升，達到非常高的峰值。由開關引起的過電壓會嚴重損壞甚至破壞開關晶體管。
常見的過電壓保護方法是“有源鉗位（active clamPIng）”。在這種情況下，雪崩二極管用作直接反饋。如果關斷導致電感負載過壓峰值，則由雪崩二極管傳導至IGBT柵極，并且IGBT重新接通。
上圖顯示了基本原理：當電壓上升時，二極管被阻斷（A）。在耗盡區中，一個自由電子觸發雪崩的瞬間，電壓突然下降到低于30V的擊穿電壓電平，雪崩二極管立刻擊穿（B）。在重新啟動之前，有時只能保持雪崩電流在短時間內穩定，并且電壓再次上升（C）。擊穿延遲（D）即兩次擊穿事件之間的時間，是不能預測的。
建議將具有改善噪聲性能的雪崩二極管用于有源鉗位過壓保護，因為它們能夠：
˙ 在快速上升的反向電壓下，更快擊穿
˙ 在低電流（低于~1mA）時具有更穩定擊穿電壓，因此：
˙ 延長其它器件的壽命，例如 IGBT或Mosfet，結果：
˙ 為變頻器或電機控制器等應用節省成本，因為組件較少需要更換。
雪崩二極管的噪聲是如何產生的？
雪崩二極管的噪聲來自雪崩的不斷接通和斷開，即電壓峰值的不斷產生及其突然擊穿（見圖）。觸發雪崩擊穿有兩個先決條件：
1. 存在足夠的擊穿電壓以產生用于碰撞電離的臨界電場強度。
2. 存在自由電子，因而形成漏電流。
例如，1.6pA = 1.6 x 10-12A漏電流等于通過阻擋層的電子流速為每秒107電子，這意味著在統計上每100ns只能觸發一次雪崩。然而，由于不是每個電子都會觸發雪崩，實際上觸發時間會更長。因此，觸發雪崩擊穿的概率與泄漏電流成比例。換句話說：漏電流越大，觸發雪崩擊穿的概率越高或擊穿延遲時間越短（圖中：D）。
在兩個沖擊漏電流電子之間，二極管處的反向電壓可以顯著上升到高于擊穿電壓電平。只有當下一個沖擊電子觸發雪崩時，二極管的電壓才會突然下降到擊穿電壓水平。
如果電壓源提供足夠的電流，例如 1mA，雪崩擊穿可以通過連續的碰撞電離保持自身運行，從而產生穩定的雪崩電流。
但是，如果源電流太低，例如100μA，低于擊穿電壓電平的雪崩電壓突然下降，使得二極管放電，將導致雪崩擊穿立即再次停止。這時，需要一定的時間來使二極管和線電容充電，使低源電流達到所需的電壓電平，然后下一個電子才能觸發新的雪崩。這種雪崩的不斷接通和斷開導致雪崩二極管擊穿的典型噪聲。
二極管噪聲性能的差異在圖中也可見：圖中顯示了兩個Z二極管（齊納二極管）的擊穿電壓范圍，在100μA的反向電流（IR）下測得的擊穿電壓為30V。其中一個二極管基于標準技術，使用極低的漏電流，另一個則采用“低噪聲技術”。具有“低噪聲技術”的齊納二極管具有更穩健的電壓特性，優于僅能在短時間內保持恒定雪崩電流的另一個二極管（C）。
威世提供采用“低噪聲技術”的Z二極管，這些新一代產品包括SMF、BZD27、BZG 03、BZG04、 BZG05、PLZ 和 VTVS系列，由于適度增加漏電流（IR~10nA）而明顯增加了觸發雪崩擊穿的可能性，從而降低了噪聲，并為用戶提供了在低電流（低于~1mA）時更穩定的擊穿電壓以及快速上升反向電壓的更快擊穿。
二極管噪聲的更進一步影響因素
漏電流隨溫度增加而呈指數上升，即噪聲隨溫度升高而降低；光還可以釋放二極管耗盡區中的自由電子，從而降低噪聲水平。這意味著：四周環境越暗越冷，噪音水平越高。
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