As charge is injected into the gate, more and more current is able to flow from drain to source, until the gate capacitance is fully charged. Before any charge is injected into the gate, it is fully turned off, with no current able to flow from the drain to the source. This gate charge is one thing governing the speed in which you can switch a MOSFET. The most useful specification on a MOSFET datasheet for this is the “Total Gate Charge.” “Injecting” this amount of charge into the gate of a MOSFET fully turns it on. The same is true of the capacitance of a MOSFET gate. It takes a certain amount of time to fully charge a capacitor. Once the capacitor is charged, there is essentially zero current that flows, with the circuit viewing the capacitor as an open circuit. As with any capacitor, an applied voltage is seen as a short circuit initially.
#GATE DRIVE CIRCUIT FOR MOSFET HOW TO#
Understanding that the gate of a MOSFET acts as a capacitor is crucial in understanding how to design MOSFET circuits. As a result, they are usually used for on/off switching applications where they are turned on and off quickly.
They also don’t have a well defined linear region like NPN/BJT transistors do. Since a MOSFET based circuit only requires there to be a voltage applied, they tend to be easier to implement. This is in contrast to a BJT/NPN which needs a current flow to conduct. A MOSFET gate essentially acts as a capacitor, that when charged, allows the source and drain to conduct. The fundamental difference between MOSFETs and a BJT/NPN transistor is that a MOSFET turns on based on an applied voltage instead of current. I intend to focus more on real-world applications and circuits using MOSFETs. There is a lot of great information available on the fundamentals of MOSFETs, so I will only briefly discuss that here.
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