A Battery-Connected Inductor-First Flying Capacitor Multilevel Converter Achieving 0.77W/mm<sup>2</sup> and 97.1% Peak Efficiency
Abdullah Abdulslam, Patrick P. Mercier
Abstract
Modern mobile and IoT devices require power management solutions that are efficient, compact, and can operate directly from Li-ion battery voltages (2.8-4.5V). Unfortunately, it is difficult to achieve both high power density and high efficiency from a conventional buck converter, in part since the inductor, which must be miniaturized for power density reasons, has a large DCR and thus has large conduction losses when placed on the high-current side of the converter (Fig. 1, top left). To reduce inductor DCR losses, an inductor-first buck was shown in [1] that split the output inductor into two half-sized inductors placed in series with the input, where they process lower current and thus have lower net conduction losses (Fig. 1, bottom left). Importantly, this arrangement provides a continuous, non-chopped input current, and thus reduces EMI which is an important concern in power management [2], [3]. However, processing Li-ion voltages with conventional or inductor-first buck structures require either ~5V transistors, which are not available in all CMOS processes, or requires transistor stacking. It is now well known that if transistor stacking is needed, then a flying capacitor multilevel (FCML) structure can reduce the voltage swing seen by the inductor, which offers size, ripple, and loss advantages (Fig. 1, top right) [4], [5]. Despite the advantages, however, the FCML structure still places the inductor at the high-current side of the converter, where its high DCR can contribute significant losses. In addition, the pulsated input current results in the same EMI issues as conventional buck converters.