在实际的汽车电子电路中，我们遇到一个问题，用efuse （VNF1048）去驱动电机，电机端会存在反向电动势，VNF1048驱动的外部的单NMOS无法做到防反功能，我们在实际电路上增加了一个NMOS，可以达到防反的目的（在efuse关闭外面NMOS时），想请教这样做有什么风险，在贵司规格书中未提及此种应用方案。 电路如如下：
Google translation:
In practical automotive electronic circuits, we encountered a problem: when using an efuse (VNF1048) to drive a motor, a reverse electromotive force (EMF) exists at the motor terminals. The external single NMOS driven by the VNF1048 cannot provide reverse protection. We added another NMOS to the actual circuit to achieve reverse protection (when the external NMOS is turned off by the efuse). We would like to inquire about the risks of doing this, as this application scheme is not mentioned in your company's specifications. The circuit is as follows.
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hi bensch
很高兴看到你的回信。
看到你提出的风险问题，从我们的使用场景角度，我们认为没有什么风险，为了更全面的说明问题，我在此补充一下，使用场景。如下图：
[English translation]
I’m glad to see your reply.
Regarding the risk issue you raised, from the perspective of our use case, we believe there is no risk. To explain the matter more fully, I’d like to add the following use-case details, as shown in the diagram below:
我们使用了两颗VNF1048FTR ,分别去驱动雨刮电机的高速模式和低速模式，因电机内存在磁感应情况，当我们使能高速efuse时，在低速efuse输出上会有许多峰值在72V的感应峰值电压(通道1)，这个电压会通过低速efuse外部驱动的NMOS回流到24V电源，我们实测这个感应平均电流有4.14A.坏的结果就是，电机会过热，触发过热保护。为了解决这个问题。我们需要在使能高速efuse，低速efuse关闭时，在低速efuse输出端加防反电流，雨刮电机正常运行的电流大约在，1.5~2.5A,但是一旦出现堵转，也需要通过efuse来实现过流保护。所以无法使用普通的二极管来实现，我们才想到在efuse外区的nmos后端再加一个背靠背的NMOS，在低速efuse关闭时实现 防反功能。因为雨刮是驾驶员机械切换，存在操作延时，所以不存在切换倒灌的风险。软件上也可以做最小时间限制。因为驱动电流比较小，所以我们认为2.6us的启动延时，不存在风险。不知道我的判断是否正确，还请指出不足。
[English translation]
We use two VNF1048FTR devices, respectively to drive the wiper motor’s high-speed mode and low-speed mode. Because the motor has magnetic induction, when we enable the high-speed eFuse, there will be many induced voltage peaks of 72 V on the low-speed eFuse output (Channel 1). This voltage will flow back to the 24 V supply through the externally driven NMOS of the low-speed eFuse. We measured the average induced current to be 4.14 A. The bad outcome is that the motor will overheat and trigger overtemperature protection.
To solve this problem, when the high-speed eFuse is enabled and the low-speed eFuse is off, we need reverse-current protection at the low-speed eFuse output. The normal operating current of the wiper motor is about 1.5–2.5 A, but once a stall condition occurs, overcurrent protection must also be achieved through the eFuse. Therefore, we cannot use a standard diode. That is why we thought of adding a back-to-back NMOS after the NMOS in the external stage of the eFuse to provide reverse-current blocking when the low-speed eFuse is off.
Because the wiper is mechanically switched by the driver, there is an operating delay, so there is no risk of backfeed during switching. This can also be handled in software by enforcing a minimum time interval. Since the drive current is relatively small, we believe that a 2.6 µs startup delay does not pose a risk. I’m not sure whether my assessment is correct, so I would appreciate it if you could point out any shortcomings.
