A DC/DC converter converts an input voltage source to a desired voltage level. When the input voltage is higher than the desired output voltage, you need a buck converter. Conversely, when the input voltage is lower than the output voltage, you need a boost converter. In applications where the input voltage could be either higher or lower than the output voltage, what you need is a buck-boost converter.
A variety of topologies exist for realizing non-inverting buck-boost (step-down and step-up) conversion, such as single-ended primary-inductor converters (SEPICs), Zeta converters, two-switch buck-boost converters and four-switch buck-boost converters. However, buck-boost conversion using these topologies is not as efficient as a basic buck or boost converter. What is the reason behind that? Is there any way to improve efficiency?
Let’s answer these questions using a four-switch buck-boost converter as an example. Taking a closer look at the four-switch buck-boost converter topology, you can tell that it is actually a cascaded combination of a buck converter followed by a boost converter. Somehow, you should be able to operate it as either a buck converter or a boost converter. Figure 1 shows the comparison between buck or boost mode and conventional buck-boost mode.
Figure 1: Operation modes comparison: buck-boost mode versus buck or boost mode
When operating in conventional buck-boost mode, Q1 and Q2 share a gate-control signal, while Q3 and Q4 share another one. These two gate-control signals are complementary to each other. In buck or boost mode, when VIN is higher than VOUT, Q2 is kept off while Q4 is always off; thus it works like a typical buck converter. In contrast, when VIN is lower than VOUT, Q1 is always on while Q3 is kept off; it then works as a typical boost converter.
In buck-boost mode, all four switches are switching in each period. In contrast, only two switches are switching in each period in buck mode or boost mode. More switches switching in each period essentially generates more switching loss. Furthermore, the average or pedestal current flowing through inductors and switches in buck-boost mode (IIN+IOUT) is higher than that in both buck mode (IOUT) and boost mode (IIN), which results in higher conduction loss. As a result, the efficiency in buck or boost mode is much higher than that in buck-boost mode.
You can implement optimized buck- or boost-mode control using the LM5175, the latest wide-VIN four-switch buck-boost controller from TI. The efficiency measured from the evaluation module shows very high efficiency over the entire VIN range.
Figure 2: Efficiency and power loss of a four-switch buck-boost converter
So in order to achieve high efficiency, do not operate a four-switch buck-boost converter in buck-boost mode. Instead, operate it in buck mode or boost mode. What is your choice?
