In order to protect the microcontroller and optimize the control, an inverter is usually used to isolate the microcontroller and the controlled element. The inverter can not only invert the signal, but also increase the amplitude of the control signal to better drive the subsequent MOS tube.

However, in actual production, when using inverters to control MOSFETs, some boards may function normally, while transistors and MOSFETs on other boards may be inexplicably damaged.

Why does this happen?

After research we can find that the problem lies in the inverter.

 Ideally, the inverter can theoretically perfectly output a 0V - 5V logic signal to invert the input signal, but in actual operation, it may output non-ideal signals such as 1V - Vdd or 2V - Vdd.

From the analysis in the schematic diagram 1, if the inverter really outputs a 2V-Vdd signal, the push-pull circuit (Q1, Q2 and Q3, Q4) and the MOSFET will easily be connected in series.

 This is because, when the inverter power supply Vdd = 9V, and its input signal (i.e., the output signal of the microcontroller) Vin = 9V, according to the internal principle of the inverter, the upper tube Pmos will be completely turned off, and the lower tube Nmos will be fully turned on. At this time, the output end Vout of the inverter = Vss, and everything is normal. However, once Vin drops from 9V to 5V, the upper tube cannot be completely turned off, which will cause the output end Vout of the inverter to be > Vss, and Vout < Vdd at the same time. That is, an output signal of 2V - Vdd may appear, which is very likely to cause the problem of series connection in the subsequent stage.

Therefore, when the amplitude of the input signal of the inverter is less than the supply voltage of the inverter, there is a certain risk. So when the MOS tube or transistor in the circuit is abnormal, you can use experiments to detect whether it is a problem with the inverter. Of course, most inverters have internal processing inside, so this situation only exists in some inverters. But if this problem really occurs, it can be solved by the following method.

1. Set the amplitude of the control signal and the supply voltage of the inverter to the same voltage;

2. Use a pull-down resistor at the output of the inverter to force the signal that is not low enough to be pulled down.