Chapter 5: General Theory of Two-Level DC-DC Converters Boost and Buck/Boost Converters

INTRODUCTION

The design equations developed for the buck converter in Chapter 3 are a subset of a general set of equations that are applicable to a great variety of DC-DC converters. These types of converters are called two-level DC-DC converters because of the two voltage levels of the inductor voltage waveform. In this chapter the principles of analysis applied to the buck converter are generalized as a theory from which the design equations for other converters are formulated. The general theory is developed and applied to the boost and buck/boost converters, both in CCM and DCM operation.

The terms buck and boost come from electrical transformer jargon. A bucking, or buck, transformer is a step-down transformer that outputs a lower secondary voltage than the primary source. The rated secondary current is greater than the primary current by a factor of the ratio of the primary to secondary voltages. A boost transformer steps the source voltage up to a higher voltage but requires a primary current that is greater than the secondary current by a factor of the ratio of the secondary to primary voltages. These voltage and current relationships are also true for the buck and boost converters, except that the voltages and currents are DC rather than AC. Indeed, DC-DC converters are often called "DC transformers." As the name implies, the buck/boost converter can output voltage magnitudes that are greater or less than the source.

GENERAL THEORY OF CCM CONVERTERS

The primary energy-transfer element in...