A Multiphase Series-Capacitor Buck Converter with Reduced Flying Capacitor Volume and Auxiliary Start-Up Circuit

This paper introduces a modification to the conventional multiphase series-capacitor (SC) buck converter topology that transforms it into a more suitable candidate for emerging high step-down, high current point-of-load (PoL) applications. The introduced topology reduces the total energy storage of the flying capacitors (FC), compared to an equivalently-phase conventional SC topology, as a result of lowering their respective steady-state voltages. This translates into easier procurement of capacitors and greater than 33% smaller FC volume due to lower capacitive derating. The converter also includes an unobtrusive auxiliary switch to facilitate the pre-charging of each FC to near their steady-state voltage such that the critical high current-carrying synchronous rectifiers (SR) never experience a voltage stress equal to the supply voltage. This advantageous property subsequently enables the designer to safely use lower voltage / higher figure-of-merit (FoM) SRs to boost operating efficiency. An interleaved 4-phase discrete prototype was developed for 48-V nominal input and 1.0-1.8-V output at 140-A load to confirm diminished SR voltage stress during start-up that ultimately improves efficiency at heavy-load.