04-01-2013, 04:41 PM
Understanding Buck-Boost Power Stages in Switchmode Power Supplies
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ABSTRACT
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 report addresses the buck-boost power
stage only and does not cover control circuits. Detailed steady-state and small-signal
analysis of the buck-boost power stage operating in continuous and discontinuous mode
is presented. Variations in the standard buck-boost power stage and a discussion of
power stage component requirements are included.
Introduction
The three basic switching power supply topologies in common use are the buck,
boost, and buck-boost. These topologies are nonisolated, i.e., 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.
Buck-Boost Stage Steady-State Analysis
A power stage can operate in continuous or discontinuous inductor current mode.
Continuous inductor current mode is characterized by current flowing
continuously in the inductor during the entire switching cycle in steady-state
operation. Discontinuous inductor current mode is characterized by the inductor
current being zero for a portion of the switching cycle. It starts at zero, reaches
a peak value, and returns to zero during each switching cycle. The two different
modes are discussed in greater detail later and design guidelines for the inductor
value to maintain a chosen mode of operation as a function of rated load are
given. It is very desirable for a converter to stay in one mode only over its
expected operating conditions because the power stage frequency response
changes significantly between the two different modes of operation.
For this analysis, an n-channel power MOSFET is used and a positive voltage,
VGS(ON) , is applied from the Gate to the Source terminals of Q1 by the drive circuit
to turn ON the FET. The advantage of using an n-channel FET is its lower RDS(on)
but the drive circuit is more complicated because a floating drive is required. For
the same die size, a p-channel FET has a higher RDS(on) but usually does not
require a floating drive circuit.
Buck-Boost Steady-State Continuous Conduction Mode Analysis
The following is a description of steady-state operation in continuous conduction
mode. The main goal of this section is to provide a derivation of the voltage
conversion relationship for the continuous conduction mode buck-boost power
stage. This is important because it shows how the output voltage depends on duty
cycle and input voltage or conversely, how the duty cycle can be calculated based
on input voltage and output voltage. Steady-state implies that the input voltage,
output voltage, output load current, and duty-cycle are fixed and not varying.
Capital letters are generally given to variable names to indicate a steady-state
quantity.
In continuous conduction mode, the buck-boost converter assumes two states
per switching cycle. The ON State is when Q1 is ON and CR1 is OFF. The OFF
State is when Q1 is OFF and CR1 is ON. A simple linear circuit can represent
each of the two states where the switches in the circuit are replaced by their
equivalent circuit during each state. The circuit diagram for each of the two states
is shown in Figure 2.
Critical Inductance
The previous analyses for the buck-boost converter have been for continuous
and discontinuous conduction modes of steady-state operation. The conduction
mode of a converter is a function of input voltage, output voltage, output current,
and the value of the inductor. A buck-boost converter can be designed to operate
in continuous mode for load currents above a certain level, usually 5% to 10% of
full load. Usually, the input voltage range, the output voltage, and load current are
defined by the converter specification. This leaves the inductor value as the
design parameter to maintain continuous conduction mode.
The minimum value of inductor to maintain continuous conduction mode can be
determined by the following procedure
Buck-Boost Power Stage Small Signal Modeling
We now switch gears, moving from a detailed circuit oriented analysis approach
to more of a system level investigation of the buck-boost power stage. This section
presents techniques to assist the power supply designer in accurately modeling
the power stage as a component of the control loop of a buck-boost power
supply. The three major components of the power supply control loop (i.e., the
power stage, the pulse width modulator, and the error amplifier) are shown in
block diagram form in Figure 7.
Summary
This application report described and analyzed the operation of the buck-boost
power stage. The two modes of operation, continuous conduction mode and discontinuous
conduction mode, were examined. Steady-state and small-signal
were the two analyses performed on the buck-boost power stage. The flyback
power stage was presented as a variation of the basic buck-boost power stage.