This applet investigates the switching behaviour of a power MOSFET and the influence various components have on the switching time. The applet also allows you to see in detail the current flow and operating point of the MOSFET through out the entire switching cycle.
The switching speed of the MOSFET is controlled by the value of the gate resistor. In this applet the value of the gate resistor can be changed. The value of the drain to gate capacitance of the MOSFET also has an impact on the switching speed although this value can not be externally adjusted as it depends on the MOSFET construction.
By sliding the red time line to the left hand side the MOSFET is in the off state since the gate voltage is zero. To turn the MOSFET on the gate driver output produces a positive voltage. Current then starts to flow from the gate driver and begins to charge the MOSFET gate-to-source and gate-to-drain capacitances. Once the gate-to-source voltage reaches the threshold voltage the MOSFET starts to conduct current. The gate-to-source voltage keeps on increasing and this allows the drain current to increase up to the load current value. The drain-to-source voltage now must be reduced and during this time the gate current is only used to discharge the gate-to-drain capacitance (the gate-to-source voltage is clamped during this time). When the drain-to-source voltage is reduced, near to zero, the MOSFET now operates like a low resistance between the drain and the source. At this point the gate-to-source voltage can now rise up to the voltage level of the gate driver output.
To turn off the MOSFET the reverse process must occur.
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ECPE European Center for Power Electronics e.V.
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