Optimizing The Efficiency Of The Four-Switch Buck-Boost Converter

Focus:

A keen understanding of a converter’s operating modes and power losses is both essential and invaluable. A primary objective of this article is to look closely at the four-switch buck-boost converter’s efficiency. This converter is a convenient platform to study dc-dc converter efficiency as the analysis is primarily based on buck and boost switching modes. By deriving the relevant duty cycle in each mode, we can calculate efficiency and power losses over input voltage and output current ranges. The article begins by reviewing the structure of the four-switch (non-inverting) synchronous buck-boost power stage and discussing its benefits. Then the operation of the converter in its various operating modes is explained. Next, equations are derived for duty cycle in the buck, boost and buck-boost transition modes. The circuit model used to derive duty cycle equations for operation in the buck-boost region is presented. Duty cycle is plotted versus input voltage for an example converter. Then, expressions are given for switch losses in the three operating modes and for inductor and bias losses. An actual four-switch buck-boost converter design based on the LM5175 controller is shown and its efficiency and losses are plotted versus load and input voltage.

What you’ll learn:

• How to understand operation of the four-switch (non-inverting) synchronous buck boost converter
• How to derive duty cycle equations for the four-switch (non-inverting) synchronous buck boost converter
• How to calculate losses and efficiency for the four-switch (non-inverting) synchronous buck boost converter

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Author & Publication:

Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Sep 15 2015

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