When the driving capability of MOS tubes is insufficient, we use push-pull circuits to amplify the current. But MOS tubes are voltage-controlled devices, so why do we need to consider current size?

Principle of push-pull operation: The power supply IC sends out a duty cycle signal, which is this square wave, and it flows through resistors to reach the base of the transistor: the upper transistor is NPN, and the lower transistor is PNP.

When the signal reaches the base of the transistor at a high level, Q1 conducts (NPN conducts at a high level), and then the MOS tube conducts.

When the output is low, the upper transistor Q1 cuts off, Q2 conducts (PNP conducts at a low level), and then the MOS tube discharges to the ground through the resistor, and finally cuts off.

In other words, two transistors work alternately to perform push-pull, achieving push-pull functionality.

The role of R1 here is to determine the initial potential during startup, to prevent the MOS tube from being accidentally triggered during startup, and to discharge during shutdown.

What about R2? Any friends who know can answer in the comments~

Push-pull circuits are usually used because sometimes the current of some ICs or CPUs is relatively small and insufficient to drive MOS tubes, so push-pull is added to increase the driving capability.

But why do we need to consider the current size for driving even though MOS tubes are voltage-controlled devices?

This is a direct drive circuit from an IC:

First, it's necessary to understand the maximum peak driving current from the IC manual (different IC chips have different driving capabilities), and C1, C2 determine the conduction speed of the MOS tube when driven.

When the IC current is relatively small, C1 and C2 will be relatively large, and the driving speed of the MOS tube will be slower, which is not suitable for some circuits.

The reason is simple, this is because the input capacitance CGS and CGD of the MOS tube are related to the area of the chip. Although the MOS tube is essentially a voltage-controlled device, the rise and fall of the gate potential are essentially the charging and discharging speed of the CGS and CGD capacitances.

Therefore, when the driving capability of the MOS tube is insufficient, it is necessary to use push-pull to amplify the current.