How to use MOSFET as a Switch ? MOSFET as a Switch Explained

About this video

• Video Title: How to use MOSFET as a Switch ? MOSFET as a Switch Explained
• Channel: ALL ABOUT ELECTRONICS
• Speakers: Not explicitly mentioned, but likely a single narrator.
• Duration: 00:18:08

Overview

This video explains how to use MOSFETs as switches, comparing them to BJTs and detailing the advantages of MOSFETs, particularly enhancement-type MOSFETs. It covers the operational regions, design considerations for N-channel and P-channel MOSFETs, the importance of datasheet parameters like RDS(ON) and gate charge for switching speed, and the impact of switching losses.

Key takeaways

• MOSFET vs. BJT as a Switch: MOSFETs are voltage-controlled devices, generally easier to drive than current-controlled BJTs, especially for high currents and high-frequency switching. BJTs can be more cost-effective for low-drive current applications like driving LEDs.
• Enhancement-Type MOSFETs (E-MOSFETs): These are preferred for switching applications because they are normally off, meaning they conduct only when the gate-source voltage (VGS) exceeds a threshold voltage. This contrasts with depletion-type MOSFETs, which are normally on.
• N-channel MOSFET as a Switch: In a typical circuit, the load is connected between the supply and the drain. The gate-source voltage controls the MOSFET's on/off state. For switching, MOSFETs operate in either the linear (ohmic) region (on-state) or the cut-off region (off-state). RDS(ON) is a critical parameter representing the on-state resistance, which should be as low as possible to minimize conduction losses.
• P-channel MOSFET as a Switch: The configuration is reversed compared to N-channel MOSFETs. The source is connected to the positive supply (VDD), and the load is between the drain and ground. A negative VGS is required to turn it on, meaning the gate voltage must be lower than the source voltage.
• Switching Speed and Gate Charge: The total gate charge (Qg) determines how quickly a MOSFET can be switched on or off. A lower gate charge and a higher current from the gate driver circuit result in faster switching. This is crucial for high-frequency applications to minimize switching losses.
• Design Considerations: A series resistor is recommended between the control input and the gate to limit transient current during charging of the gate capacitance. A pull-down resistor (for N-channel) or pull-up resistor (for P-channel) is also important to ensure the MOSFET stays in the off state during power-up or when the control signal is inactive.
• Switching Losses: These occur during the transition between the on and off states when both voltage and current are present across the MOSFET. Faster switching (lower gate charge, higher gate drive current) reduces these losses.