How to calculate the MOSFET switching frequency

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The switching on and off of a MOSFET does not occur in an instant. The voltage across the MOSFET decreases, while the current flowing through it increases. During this period, the loss in the MOSFET is the product of the voltage and current, and this is known as switching loss. Typically, switching losses are much greater than conduction losses, and the higher the switching frequency, the greater the losses. Based on the parameters of IRF840, assuming a gate voltage of 10V, the capacitance is 63nC/10V = 6.3nF. With a 10 kΩ discharge resistor, the time constant is 63 us. However, the MOSFET does not turn off after a certain time constant has passed; instead, it turns off only when the gate voltage drops below Vg(th). This period is related to the MOSFET model and to the voltage reached during gate charging (in fact, the gate capacitance is not a linear capacitor); it’s an approximate estimate, but the discharge time of the gate capacitance can be estimated as twice 63 us, that is, 0.12 ms. The MOSFET gate charging resistance is low (3 kΩ in the image from the first post), so the charging time is estimated to be 0.06 ms. So the total charging and discharging time is 0.18 ms. The switching frequency of this MOSFET in this circuit is 5.5 kHz.