許多高性能、高頻率的PWM控制芯片，無論是數(shù)字類型還是模擬類型，都不具備或只有有限的直接驅(qū)動功率MOSFET的能力。因為功率MOSFET對柵極驅(qū)動電流有較高的要求，驅(qū)動芯片就相當(dāng)于PWM開關(guān)控制芯片與功率MOSFET之間的橋梁，用來將開關(guān)信號電流和電壓放大，同時具備一定的故障隔離能力。一旦確定了選用某種開關(guān)電源方案后，接下來就要選擇合適的驅(qū)動IC，而選好驅(qū)動芯片，就需要硬件工程師對電路特性有一定的了解。
由于開關(guān)電源經(jīng)常需要硬開關(guān)驅(qū)動大功率負(fù)載，在硬開關(guān)以及布局限制的情況下，功率MOSFET往往會對驅(qū)動芯片的輸入和輸出端形成較大的地彈電壓和振蕩尖峰電壓。地彈電壓會造成驅(qū)動器輸入端等效出現(xiàn)負(fù)電壓，因為內(nèi)部等效體二極管，大多數(shù)柵極驅(qū)動器能夠承受一定的負(fù)壓脈沖。然而，亦有必要采取預(yù)防措施，以防止驅(qū)動器輸入端的過沖和欠壓尖峰過大，而對驅(qū)動芯片造成損壞，或產(chǎn)生誤動作。
驅(qū)動輸入端負(fù)壓尖峰的形成原因
仍以PFC拓?fù)錇槔瓦咈?qū)動器用在控制芯片與功率MOSFET之間，以幫助減小開關(guān)損耗，并為MOSFET提供足夠的驅(qū)動電流，以跨過米勒平臺區(qū)域，實現(xiàn)快速打開。在開關(guān)MOSFET的時候，有一個高di/dt的脈沖產(chǎn)生，這種快速變化與寄生電感共同作用，產(chǎn)生了負(fù)電壓峰值，可以用Vn = Lss* di/dt公式估算。Lss代表寄生電感。寄生電感值約等于功率MOSFET的內(nèi)部鍵合線和PCB回線接地回路中的電感量之和，其值可以從幾nH至十幾nH不等，寄生電感大小主要取決于PCB布局布線。
從上面等式可以看出，負(fù)向電壓與寄生電感和電流變化率均成正比。在典型的低邊柵極驅(qū)動電路中，雖然控制器和功率MOSFET使用同一個直流地平面作為參考，但一些情況下，由于驅(qū)動器和控制器有一定距離，所以總會存在寄生電感。高di/dt的電流在流經(jīng)MOSFET及其板級回路時，寄生電感存在會導(dǎo)致驅(qū)動器的地電位相對于控制器地電位瞬間抬升，驅(qū)動器的輸入和地之間就相當(dāng)于出現(xiàn)一個瞬間負(fù)壓。在極端情況下，可能造成驅(qū)動器內(nèi)部輸入ESD器件受損，驅(qū)動器出現(xiàn)失效。
另一個常見的出現(xiàn)輸入負(fù)壓的場景與對MOSFET進(jìn)行電流采樣相關(guān)。為了實現(xiàn)更精確的控制，有時在功率MOSFET和大地之間會接一個采樣電阻，用這個采樣電阻來檢測流過MOSFET的電流，從而使控制器能快速做出響應(yīng)。而為了使MOSFET的驅(qū)動環(huán)路足夠小，會將驅(qū)動器的GND引腳與MOSFET的源極連接在一起，而控制芯片的GND與真正的地平面在一起，這樣驅(qū)動器的GND和控制芯片GND之間就會存在一個偏置電壓，因此控制芯片輸出低電平時，相對于驅(qū)動器的輸入端，則有一個負(fù)向的偏置電壓。
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