Switching Power Supplies A - Z

Chapter 5

initially at least, the discussion is more from the standpoint of the mosfet, not the topology

that it may be a part of.

Switching a Resistive Load

Before we take up inductors, it is instructive to first understand what happens when we

switch a resistive load.

For simplicity, we are considering an ideal situation. So we start with a “perfect” n-channel

mosfet in Figure 5-1. It behaves in the following manner

■

It has zero on-resistance.

■

With zero gate-to-source voltage “Vgs” applied at its gate, it is completely

non-conducting.

■

As we raise the gate-to-source voltage Vgs slightly above ground, it starts

conducting, and so a drain current ‘Id’