Four-switch buck-boost converter in buck or boost mode delivers the highest efficiency

May 26, 2015 //By Haifeng Fan
Four-switch buck-boost converter in buck or boost mode delivers the highest efficiency
Haifeng Fan examines the principles of operation of buck-boost converters and explains how these principles affect efficiency, and elaborates on how to improve efficiency by adopting synchronous rectification and optimizing operation modes.

Buck-boost converters are popular solutions to cope with a wide-input voltage range that fluctuates below or above the desired output voltage. The conventional two-switch buck-boost converter offers low cost and simple control. However, it suffers from high current stress and high conduction loss, resulting in low efficiency. The excessive power dissipation associated with low efficiency makes the conventional two-switch buck-boost converter impractical for high-power applications.
A four-switch buck-boost converter benefits from synchronous rectification and thus outperforms its two-switch counterpart in terms of efficiency and power capability. Moreover, as a cascaded combination of a buck converter followed by a boost converter, a four-switch buck-boost converter can operate in buck mode or boost mode rather than conventional buck-boost mode. As such, its efficiency can be further improved.
Two-switch buck-boost converter principles of operation
Figure 1 shows a conventional two-switch buck-boost converter. The power stage consists of two switches (Q 1 and Q 2), two diodes (D 1 and D 2), a single inductor (L 1), and input and output capacitors.
In buck-boost mode, MOSFETs Q 1 and Q 2 share a gate-control signal and turn on and off simultaneously. When Q 1 and Q 2 are turned on, the input voltage (V IN) is applied to the inductor (L 1) and the energy is stored in the inductor. In this stage, the output capacitor supplies the entire load current. When Q 1 and Q 2 are turned off, diodes D 1 and D 2 are forward-biased; thus the inductor current ramps down at a rate proportional to V OUT. In this stage, energy is transferred from the inductor to the output load and capacitor. Figure 2 shows the current waveforms in continuous conduction mode (CCM).



Figure 1: Two-switch buck-boost converter


Figure 2: Ideal current waveforms of a two-switch buck-boost converter in CCM
The two-switch buck-boost

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