NMOS管应用于高边开关(high-side driver)的分析

NMOS 是控制电路中常用的开关器件，当然也有用于放大电路的场景。NMOS 作为开关时，常用于低边控制(low-side driver)，如下所示的电路图。

但是，最近在研究某一产品时，发现将 NMOS 管用于高边开关(high-side driver)，即类似下图的情况。

这个电路在应用时，一定要注意NMOS 的栅极(Gate)电压高于漏极(Drain)电压且满足导通状态下 V_GS > V_GS(th)，否则，当 NMOS 导通时，V_GS < V_GS(th)，不满足开启条件，NMOS 管就会关断。这样，NMOS 管就会处于开启-关断的临界状态，从而不能用于开关。

除此之外，NMOS 用于高边开关时，还会有其他的问题。顺便找了相关的资料，对 NMOS 管应用于高边开关的场景做一下分析。

N-MOS and P-MOS power stages

文档中描述：

Many competitors’ products use N-channel MOSFETs also as high-side driver. In this configuration, Source and Drain are swapped when compared to the P channel MOSFET. The MOSFET then operates in a voltage follower circuit. Driving this MOSFET is more complex, because its gate now is related to the motor driver output, i.e. to a floating level rather than to a fixed level like a power supply rail. Additionally, its gate must be driven to a voltage higher than the positive supply voltage in order to switch on the MOSFET completely.

NMOS 用于高边开关时，由于源极(Source)接负载，就会存在两个问题：

源极(Source)悬空；
栅极(Gate)电压要高于漏极(Drain)供电电压。

而且还会带来不稳定的情况：

A P channel MOSFET is the natural choice for a high-side driver. It is efficient, rugged and easy to control. Operation is possible up to the maximum supply voltage of the driver chip. A high-side NMOS gate driver is more complex and subject to issues at high dU/dt, because the driver itself is floating, i.e. its relative GND potential is identical to the driver output. Sometimes this is perceived when an unforeseen event (e.g. ESD event on the outer side of the motor) leads to an accelerated slope. As the slope is seen by the N channel high side driver control chip directly, it might exceed the maximum allowable dU/dt. This can destroy the driver and lead to a sudden defect.

相比较而言，PMOS 管就相当适合高边开关的应用，控制起来较为简单。以下分别是 N&N-MOS 和 N&P-MOS 控制电路的对比。