The buck—boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a transformer. Two different topologies are called buck—boost converter. Both of them can produce a range of output voltages, ranging from much larger in absolute magnitude than the input voltage, down to almost zero. Compared to the buck and boost converters, the characteristics of the inverting buck—boost converter are mainly:. Like the buck and boost converters, the operation of the buck-boost is best understood in terms of the inductor's "reluctance" to allow rapid change in current.
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The buck—boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a transformer. Two different topologies are called buck—boost converter. Both of them can produce a range of output voltages, ranging from much larger in absolute magnitude than the input voltage, down to almost zero.
Compared to the buck and boost converters, the characteristics of the inverting buck—boost converter are mainly:. Like the buck and boost converters, the operation of the buck-boost is best understood in terms of the inductor's "reluctance" to allow rapid change in current. From the initial state in which nothing is charged and the switch is open, the current through the inductor is zero.
When the switch is first closed, the blocking diode prevents current from flowing into the right hand side of the circuit, so it must all flow through the inductor. However, since the inductor doesn't allow rapid current change, it will initially keep the current low by dropping most of the voltage provided by the source.
Over time, the inductor will allow the current to slowly increase by decreasing its voltage drop. Also during this time, the inductor will store energy in the form of a magnetic field. If the current through the inductor L never falls to zero during a commutation cycle, the converter is said to operate in continuous mode. The current and voltage waveforms in an ideal converter can be seen in Figure 3.
The rate of change in the inductor current I L is therefore given by. D is the duty cycle. It represents the fraction of the commutation period T during which the switch is On. Therefore D ranges between 0 S is never on and 1 S is always on. During the Off-state, the switch S is open, so the inductor current flows through the load.
If we assume zero voltage drop in the diode, and a capacitor large enough for its voltage to remain constant, the evolution of I L is:. As we consider that the converter operates in steady-state conditions, the amount of energy stored in each of its components has to be the same at the beginning and at the end of a commutation cycle. As the energy in an inductor is given by:. From the above expression it can be seen that the polarity of the output voltage is always negative because the duty cycle goes from 0 to 1 , and that its absolute value increases with D, theoretically up to minus infinity when D approaches 1.
Apart from the polarity, this converter is either step-up a boost converter or step-down a buck converter. Thus it is named a buck—boost converter. In some cases, the amount of energy required by the load is small enough to be transferred in a time smaller than the whole commutation period.
In this case, the current through the inductor falls to zero during part of the period. The only difference in the principle described above is that the inductor is completely discharged at the end of the commutation cycle see waveforms in figure 4. Although slight, the difference has a strong effect on the output voltage equation.
It can be calculated as follows:. As can be seen on figure 4, the diode current is equal to the inductor current during the off-state. Therefore, the output current can be written as:. Compared to the expression of the output voltage gain for the continuous mode, this expression is much more complicated.
Furthermore, in discontinuous operation, the output voltage not only depends on the duty cycle, but also on the inductor value, the input voltage and the output current. As told at the beginning of this section, the converter operates in discontinuous mode when low current is drawn by the load, and in continuous mode at higher load current levels. The limit between discontinuous and continuous modes is reached when the inductor current falls to zero exactly at the end of the commutation cycle.
Therefore, using the expression of the output voltage in continuous mode, the previous expression can be written as:. These expressions have been plotted in figure 5. The difference in behavior between the continuous and discontinuous modes can be seen clearly.
The 4-switch converter combines the buck and boost converters. It can operate in either the buck or the boost mode. In either mode, only one switch controls the duty cycle, another is for commutation and must be operated inversely to the former one, and the remaining two switches are in a fixed position.
A 2-switch buck-boost converter can be built with two diodes, but upgrading the diodes to FET transistor switches doesn't cost much extra while due to lower voltage drop the efficiency improves. In the analysis above, no dissipative elements resistors have been considered. That means that the power is transmitted without losses from the input voltage source to the load. However, parasitic resistances exist in all circuits, due to the resistivity of the materials they are made from.
Therefore, a fraction of the power managed by the converter is dissipated by these parasitic resistances. For the sake of simplicity, we consider here that the inductor is the only non-ideal component, and that it is equivalent to an inductor and a resistor in series. This assumption is acceptable because an inductor is made of one long wound piece of wire, so it is likely to exhibit a non-negligible parasitic resistance R L.
Furthermore, current flows through the inductor both in the on and the off states. If we consider that the converter operates in steady-state, the average current through the inductor is constant. The average voltage across the inductor is:.
Therefore, the average voltage across the switch is:. The output current is the opposite of the inductor current during the off-state. Assuming the output current and voltage have negligible ripple, the load of the converter can be considered purely resistive. If R is the resistance of the load, the above expression becomes:.
If the inductor resistance is zero, the equation above becomes equal to the one of the ideal case. But when R L increases, the voltage gain of the converter decreases compared to the ideal case. Furthermore, the influence of R L increases with the duty cycle. This is summarized in figure 6. From Wikipedia, the free encyclopedia. This article is about the type of switched-mode power supply. For the autotransformer, see buck—boost transformer. Daniel W.
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The buck—boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a transformer. In this tutorial we will learn how to build and how a DC to DC buck-boost converter works. The circuit is very basic using just one diode, an inductor and a capacitor. But first let's study a little bit of theory. We have the buck-boost converter circuit in the next figure where we can see the switch, inductor and capacitor and of course we add a load to the output.
Arduino Buck-Boost Converter
Documentation Help Center. This example shows the operation of buck boost converters using the inverting and non-inverting topologies. It is comparable to a flyback converter where an inductor is used in place of a transformer. The theoretical transfer function of the buck boost converter is:. The inverting buck-boost topology produces an output voltage that is of the opposite polarity as the input voltage.
Buck Boost Converter