A Dynamic Control Method for Buck + $LLC$ Cascaded Converter With a Wide Input Voltage Range

DC–DC converters with a wide input voltage range have earned numerous daily applications in the industry. An example is power systems for electric vehicles, where <inline-formula><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converters with remarkable features have also been deployed to provide isolation. It is, therefore, common to find a front buck converter cascaded to a back <inline-formula><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converter, where their intermediate bus voltage is set low to accommodate the lowest input voltage to the buck converter. Unfortunately, this causes efficiency to drop at a high input voltage. To avoid the drop, a reconfiguring strategy has been proposed to convert the front buck converter from full bridge to half bridge, as its input voltage rises above a specified threshold. Furthermore, to ensure a smooth and fast reconfiguration, a dynamic control method has been proposed to direct the intermediate bus voltage to track a reference trajectory with a lower current surge. Besides, the method employs pulse-shift modulation to keep a stable output voltage and proper control initialization to prevent oscillation during the transition. These favorable features and an overall higher efficiency have subsequently been verified through experimental testing with a wide input voltage range.