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Here is a circuit showing some effects of trying switch a high DC current at a reasonable frequency. the power cables from the power supply have resistance, inductance and capacitance.
when the input signal tries to turn the mosfet on, it doesn't turn on hard, because the voltage on this side of the circuit is sagging due to the resistance on the power cables. the voltage sag is transmitted to the gate by the capacitive coupling from the drain and source terminals. the voltage on the source terminal of the fet has increased, so the voltage across Vgs is lowered. This gives the fet a higher on-resistance, which heats it up quickly and causes it to turn on very slowly; even though it's rated for many MHz.
when the input signal tries to turn the mosfet off, it doesn't turn off quickly either. the inductance that created a magnetic field around the power cables collapses, causing the voltage to spike negative on the source terminal and positive on the drain terminal. even though the gate is low, the low negative source terminal keeps the Vgs high enough to keep the fet on. the high voltage spike on the drain terminal is also transmitted to the gate terminal. the fet stays on for quite some time; it doesn't turn off immediately like we want it to.
the faster you switch and the higher the current, the more prevalent the problems become.
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