Switching a Resistive Load

Before we take up inductors, it is instructive to first understand what happens when we switch a resistive load. 

The analysis proceeds as follows ( Vds is the drain-to-source voltage at any given moment, Vgs is the gate-to-source voltage, and Id is the drain-to-source current)  

So the drop across the 1 Q resistor is 2 V (using Ohm s law). Therefore Vds equals 

We proceed ramping up the gate voltage progressively in this manner. When 10 s have elapsed, Vgs is 10 V, Id is 10 A, and Vds is 0 V. After 10 s, no further change in Vds or Id can occur, even if Vgs is increased further. 

Note In general, if the gate voltage is increased beyond what it takes to dehver a specified maximum load current, we say that in effect, we are applying overdrive. This is usually considered wasteful in that sense, but in practice, overdrive helps reduce the on-resistance of the mosfet, and thereby decrease its conduction losses. 

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Figure 5-2 The Voltage and Current Waveforms when Switching a Resistive Load...

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