Optimum Phase Count in a 5.4-W Multiphase Buck Converter Based on Output Filter Component Energies

This article presents an analysis of low-power, multiphase interleaved buck converters to illustrate the extent to which adding more phases is beneficial for reducing the passive components’ sizes. The analysis focuses on a typical converter application with a wide operating duty cycle range and requires good load transient. It is shown that by restricting the phase current ripple, the theoretical reduction in total inductor peak energy predicted for increased phase numbers is limited. The article assesses the benefit of inductor coupling and presents guidelines for coupling factor selection to avoid steady-state inductance roll-off over the wide input voltage range. Standard printed circuit board (PCB) manufacturing design rules are shown to place a practical phase count limit considering the reduction in total inductor size. PCB integrated, air-core solenoid and spiral inductors are implemented in both single- and two-phase 5.4 W, 20 MHz buck converter prototypes. When compared with single-phase designs, two-phase spiral inductors are 44% smaller, and the two-phase solenoid has a 54% lower loss. The two-phase prototype converter achieves better overall efficiency and peaks at almost 90% at V_IN = 4.5 V, V_OUT = 1.8 V, and F_SW = 20 MHz.