A switching power supply consists of the power stage and the control circuit. The power
stage performs the basic power conversion from the input voltage to the output voltage
and includes switches and the output filter. This paper addresses the buck power stage
only and does not cover control circuits. Detailed steady-state and small-signal analysis
of the buck power stage operating in continuous and discontinuous mode are presented.
Variations in the standard buck power stage and a discussion of power stage component
requirements are included.

 The three basic switching power supply topologies in common use are the buck,
boost, and buck-boost. These topologies are nonisolated, that is, the input and
output voltages share a common ground. There are, however, isolated
derivations of these nonisolated topologies. The power supply topology refers to
how the switches, output inductor, and output capacitor are connected. Each
topology has unique properties. These properties include the steady-state
voltage conversion ratios, the nature of the input and output currents, and the
character of the output voltage ripple. Another important property is the frequency
response of the duty-cycle-to-output-voltage transfer function.
The most common and probably the simplest power stage topology is the buck
power stage, sometimes called a step-down power stage. Power supply
designers choose the buck power stage because the output voltage is always
less than the input voltage in the same polarity and is not isolated from the input.
The input current for a buck power stage is discontinuous or pulsating due to the
power switch (Q1) current that pulses from zero to IO every switching cycle. The
output current for a buck power stage is continuous or nonpulsating because the
output current is supplied by the output inductor/capacitor combination; the
output capacitor never supplies the entire load current (for continuous inductor
current mode operation, one of the two operating modes to be discussed in the
next section).
This report describes the steady state operation of the buck power stage in
continuous-mode and discontinuous-mode operation with ideal waveforms
given. The duty-cycle-to-output-voltage transfer function is given after an
introduction of the PWM switch model.

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