A typical switch control circuit combining triode and MOS tube

In the switch control circuit, MCU often controls MOS tube indirectly through transistor instead of driving it directly

. Why?

When I/O is high, the transistor is turned on, the gate of the MOS tube is pulled down, and the 1.8V power supply is turned on;

When I/O is at a low level, the transistor is cut off, the gate voltage of the MOS tube is close to the power supply voltage, the MOS tube is cut off, and the 1.8V power supply is disconnected.

transistor only needs the base voltage (Ube) to be higher than the threshold (usually 0.6V for silicon transistors and about 0.3V for germanium transistors) to be turned on, which perfectly matches the 3.3V IO output of the MCU to achieve the switching function. However, its current amplification capability is limited and it cannot directly drive large loads.

MOS tubes are voltage-controlled devices, and their conduction depends on the gate voltage (Ugs) being higher than the threshold, usually 3V~5V, and saturated conduction often requires a 5V~10V drive voltage. The 3.3V MCU output may not be able to fully conduct it, resulting in heating or reduced efficiency. However, its on-resistance is extremely low (milliohm level), which can withstand large currents and is suitable for driving high-power loads.
The MCU indirectly controls the high and low levels of the MOS tube gate through the switching state of the triode, which not only solves the problem of insufficient MCU driving capability, but also takes advantage of the low loss of the MOS tube.