Classic application of MOS transistor circuit design (2) - switch circuit design

Previous section , We talked about it. MOS The application of tube in bidirectional level conversion . This application can be said to be very classic . Today I'm going to talk about MOS The tube is used as a switch .

Hardware everyone knows MOS Tubes belong to voltage control components . That is to say MOS When the grid of the tube is applied with voltage , When the grid - The voltage difference of the source voltage is greater than Vgs(th) when ,MOS The tube can be connected （ Not too big , Otherwise it will burn down ）. When the source voltage is determined , We can control the grid voltage to achieve MOS The conduction and cut-off of the tube , And that's how it works MOS On and off of the tube drain or grid load .

1. NMOS tube

Source grounding , The grid source voltage is equal to the grid voltage , At this time, we add PWM, That is to say MOS The conduction and cut-off of the tube , Conduction time , The LED works normally . At the deadline , Led stops working .

The LED is not connected to the grid , reason , It is not convenient to judge the gate source voltage at this time . Usually in NMOS Make a switch , The load is connected to the drain .

2. PMOS tube

Source voltage , Grid connected PWM. The gate source voltage is greater than Vgs(th) when ,MOS The pipe is open . Led works . When the gate source voltage is less than Vgs(th) when ,MOS Pipe cut off , Led stops working .

The LED is not connected to the source , Because the source voltage cannot be judged . It is not directly connected to the drain electrode for convenience .

In short, when making switches , The load is connected to the drain .

In some scenarios , The circuit will also be designed for soft start . As follows ：

resistance R And C Constitute the function of soft start .

Soft start makes MOS The power on of the tube grid is more gentle , At the same time, time increases .
The following is a comparison of whether delay is used in other projects .

Soft start

Non soft start

It can be seen that the effect of soft start is better than that without soft start measures , So the design of soft start , stay MOS It is also particularly important when making switches .