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Energy Transformation Advantages of buck-boost converters

| Author / Editor: Luke James / Erika Granath

The buck-boost converter is a type of DC-to-DC converter with an output voltage magnitude that is either greater or less than the input voltage magnitude, and it offers several benefits to design engineers.

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Buck-boost converters are equivalent to flyback converters that use a single inductor instead of a transformer.
Buck-boost converters are equivalent to flyback converters that use a single inductor instead of a transformer.
(Source: Unsplash)

The basic schematic of an inverting buck–boost converter.
The basic schematic of an inverting buck–boost converter.
(Source: CC)

Buck-boost converters are equivalent to flyback converters that use a single inductor instead of a transformer. They have two different topologies, inverting and non-inverting, and are used as switching regulators which use a switching element (typically one or two MOSFETs) and an energy storage device (such as an inductor) to regulate input voltage to lower or higher output voltage efficiently.

Schematic of a buck–boost converter.
Schematic of a buck–boost converter.
(Source: CC)

In the inverting type of buck-boost converter, output voltage polarity is different than input. In the non-inverting type, output voltage is the same as input voltage. They are primarily used where a negative output is required with respect to the ground terminal.

Buck-boost converter applications

There are many applications for buck-boost converters, including battery-powered systems where input voltages can vary widely, starting at full charge and then decreasing over time as the battery charge depletes.

The two operating states of a buck–boost converter: When the switch is turned on, the input voltage source supplies current to the inductor, and the capacitor supplies current to the resistor (output load). When the switch is opened, the inductor supplies current to the load via the diode D.
The two operating states of a buck–boost converter: When the switch is turned on, the input voltage source supplies current to the inductor, and the capacitor supplies current to the resistor (output load). When the switch is opened, the inductor supplies current to the load via the diode D.
(Source: CC)

At full charge, where the battery’s voltage is often higher than is needed by the circuit being powered, a buck regulator steps in to steady supply voltage. As this charge diminishes, however, input voltage can fall below the required level for the circuit, and so the boost regulator comes into play. By combining the two in a single circuit, it is possible to have a regulator circuit that can cope with a range of input voltages that are either higher or lower than that needed by the circuit at a given point in time.

In short, buck-boost converters combine the principles of the buck converter and the boost converter in a single circuit, providing a regulated direct current (DC) output voltage from either altering current (AC) or DC input.

Buck-boost converter advantages and disadvantages

Buck-boost converters offer a more efficient solution with fewer, smaller external components. They are able to both step-up or step-down voltages using this minimal number of components while also offering a lower operating duty cycle and higher efficiency across a wide range of input and output voltages. Buck-boost converters are also a lot less expensive when compared to other converters.

They are not perfect solutions, though; there are some drawbacks. For example, they cannot achieve high gain because efficiency is too poor for it (i.e. very small or very large duty cycle only – no in-between). There is also no isolation from the input to output side, rendering it unsuitable for certain applications.

Buck-boost circuit variations

The basic buck-boost circuit can have many variations, with some designs working at lower frequencies or higher voltages that use bipolar transistors instead of MOSFETs, for example. Or, as seen in high voltage designs, silicon diodes may be used due to its higher reverse voltage capabilities.

Buck-boost converters may also carry out over current and voltage protection and carry out ‘pulse skipping’ where charging is prevented on one or more oscillator pulses when it senses that the load current is low. This reduces power supply current draw, prolonging battery life.

Buck-boost converters are very versatile, often used to carry out control unit functions. These can range from low power, high efficiency integrated circuits for portable electronics to large industrial high power DC-DC converters.

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