Classic Application of MOSFETs - Switch Circuit Design

Hardware engineers know that MOSFETs are voltage-controlled devices. By applying a voltage to the gate of a MOSFET, when the gate-source voltage difference exceeds Vgs(th), the MOSFET can be turned on (it should not be too high, otherwise it will burn out). Once the source voltage is determined, we can control the gate voltage to turn the MOSFET on or off, thus achieving the switching of loads connected to the drain or source of the MOSFET.

NMOS Transistor

The source is grounded, and the gate-source voltage equals the gate voltage. By applying a PWM signal to the gate voltage at this point, we can achieve the turning on and off of the MOSFET. When on, the LED operates normally. When off, the LED stops working.

The LED is not connected to the gate because it is inconvenient to determine the gate-source voltage in that configuration. Typically, when using an NMOS as a switch, the load is connected to the drain.

PMOS Transistor

The source is connected to a voltage, and the gate is connected to PWM. When the gate-source voltage is greater than Vgs(th), the MOSFET turns on, and the LED operates. When the gate-source voltage is less than Vgs(th), the MOSFET turns off, and the LED stops working.

The LED is not connected to the source because it is difficult to determine the source voltage. It is not as convenient as connecting it directly to the drain.

In summary, when using a MOSFET as a switch, the load is connected to the drain.

In some scenarios, the circuit may also incorporate a soft-start design. An example is shown below:

The resistor R and capacitor C constitute the soft-start function.

Soft-start makes the voltage at the MOSFET gate ramp up more smoothly and increases the turn-on time.

The following is a comparison from another project with and without the delay.

With Soft-Start

 

Without Soft-Start

It can be seen that the soft-start effect is superior to not implementing the soft-start measure. Therefore, the design of soft-start is particularly important when using MOSFETs as switches.