The signal of the inverter in the microcontroller circuit is abnormal

The signal of the inverter in the microcontroller circuit is abnormal, and the MOSFET is damaged for no reason

To protect the microcontroller and optimize control, it is common to use an inverter to isolate the microcontroller and the controlled element. The inverter can not invert the signal but also boost the control signal amplitude to better drive the subsequent MOSFET.

However, in actual production, the circuit that uses an inverter control the MOSFET will have the situation that some boards are normal, but the transistors and MOSFET on some boards are damaged for no reason.

Why this happen?

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

In an ideal state, the inverter can theoretically perfectly output0V - 5V logic signals, but in actual work, it may output non-ideal signals such as pull-down signals.

Let's analyze the schematic diagram 1, if the inverter really outputs a signal, the push-pull circuit (Q1, Q2 and Q3, Q4) and MOSFET will easily have a short-circuit phenomenon.

This is because, when the power supply of the inverter and the input signal (i.., the output signal of the microcontroller) Vin = 9V, according to the internal principle of the inverter, the upper diodes is completely turned off the lower diodes  is fully turned on, and the output end of the inverter is normal at this time. However, once Vin drops from 9V to V, the upper diodes  cannot be completely turned off, which will cause the output end of the inverter Vout>Vss, and at the same time V<Vdd. That is, it is possible to output a signal of 2V - Vdd, which can easily cause a short-circuit problem in subsequent stage.

Therefore, when the amplitude of the input signal of the inverter is less than the power supply voltage of the inverter, there is a certain. Therefore, when the MOSFET or transistor in the circuit appears abnormal, it can be detected by experiments whether it is a problem of the inverter. Of course most of the inverters have been made with internal processing, so this situation only exists in a part of the inverters. But if this problem really occurs it can be solved by the following methods.

1. Set the amplitude of the control signal to be the same voltage as the power supply voltage of the inverter

2. Pull down the signal that is not low enough at the output end of the inverter through a resistor