How can MOS transistors be used as switching circuits?

How can MOS transistors be used as switching circuits?

1. How can MOS transistors be used as switching circuits?

2. Why do we add a pull-down resistor when making MOS transistor switch circuits?

3. Why not use GPIO control directly and use a transistor to control MOS on/off?

The first issue is that there are three situations where MOS transistors are used as switching circuits, namely controlling low-level conduction, controlling high-level conduction, and controlling negative level conduction.

Control low-level conduction universal NMOS;

Control high-level conductive universal PMOS;

Control negative conductance universal NMOS;

The second question is that the pull-down resistor must be connected for three main purposes:

1) When powering on, give a certain level to the G pole of the MOS transistor to prevent interference from the uncertain G pole level of the MOS when the GPIO is high resistance during powering on;

2) When power is off, if the MOS is in a conductive state and the parasitic capacitance between the GS does not have a discharge path, the pull-down resistor can provide a discharge path for the Cgs;

3) Prevent electrostatic breakdown;

The third question:

The GPIO port of a microcontroller can directly drive a transistor, but for many MOS transistors, it cannot be directly driven. It needs to be converted through a transistor or optocoupler because the commonly used working voltage of a microcontroller is generally 5V or 3.3V, which does not exceed 5V. For MOS transistors with higher power, the conduction voltage may exceed 5V, which cannot meet the conduction conditions. Therefore, a transistor must be used for conversion, as shown in the following figure:

In the above figure, if the microcontroller outputs high level, the transistor is on, and the collector is low level, then the MOS transistor is off; When the microcontroller outputs a low level, the transistor cutoff collector is at a high level, and the MOS transistor is on. You can also add a voltage regulator between the G and S poles of the transistor.

Taking a lightning triggered circuit as an example:

This circuit can achieve positive polarity triggering when SW_ When Positive polarity is present, Q8 conducts, resulting in a low Vg level on pin 1 of PMOS transistor Q7. Vgs<0, Q7 conducts, and outputs a high level, resulting in Q9 conducting. Q9 continues to maintain Q7 in a conducting state, resulting in Poweron_ P continuously high level. Simply put, SW_ Even if there is a very short positive pulse signal, Positive outputs Poweron_ P remains high, thus turning on the entire power supply and supplying power to the MCU. The function of Q10 is to turn off the power supply after the MCU starts normally, simply give POWEROFF_ P can provide a high level to make Q10 conductive, break this self-locking state, and then shut down.

However, the experiment found that this circuit is easy to be triggered by mistake and is very sensitive. The main reason here is that the Q9 grid is not connected with a pull-down resistor. If the power is turned off, the Q9 grid, that is, pin 1, is connected with GPIO and Q10 leakage levels, but when the power is turned off, these two pins are suspended. Therefore, Q9.1 is very tolerant of external level interference and electrostatic breakdown. So, the modification method is Q9, just like Q10, with one pin connected to a 10K pull-down resistor.