Switch Mode Power Supplies

Many of these illustrations are from SWITCHMODE Power Supply Reference Manual, rev 3, 2002, by ON Semiconductor.

A switching regulator topology is very similar to that of a linear regulator.

In addition to a regulator, a switching power supply consists of four external components:

A switch – used to transfer a small portion of energy from the input

An inductor – to store energy when the switch is closed and to discharge energy when the switch is open

A capacitor – to store the energy from the inductor when the switch is closed and to provide energy to the load

A diode – to govern the direction of the charging and discharging currents

These simple components can be used to create a bewildering array of power supplies, not attainable by linear methods.

Although each of the following circuits is quite complex, the basic operation can be understood by applying the Kirchoff’s voltage and current laws to an RLC network. Each circuit type has only two states, one where the switch is ON and the other where the switch is OFF.

For the sake of simplicity, the circuitry need to control the switch will be considered later.

The voltage drop across the (transistor) switch is normally quite small (0.1 – 0.3 volts), but it will be included in the following analysis.

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