Hello Paul:
One, and only one, of the considerations in the design of a device like an ESC is power dissipation in the output FETs caused by operation in the linear region. Ideally the FET should be either on or off, in either case it dissipates relatively little power. When the FET is off the current is 0 while the voltage is max resulting in 0 Watts dissipated in the FET When the FET is on the current is max and the voltage is low (the on voltage being current times the on resistance of the FET) for a relatively small powere dissipation. But, as the FET switches from on to off or vice versa, the FET goes through the "linear region" where it is conducting current and is dropping voltage across it. This results in relatively high power dissipation, consider 1/2 voltage * 1/2 current, which gives a much higher value than either the full on or full off states. Therefore much of the power dissipated by the FET occurs during on-off transitions. So, how does this relate to switching frequency? Since the on-off transition (technically the rise and fall times) is finite, as the frequency is increased the FET spends a larger and larger percent of it's time in the linear region and thus power dissipation increases. Therefore there is a limit (only one of several) on how fast we can switch the output FET.
Kevin
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