A Hybrid Bidirectional Converter With Always-Half-Inductor Current Using Adaptive Target Current Tracking for USB-to-2-Cell Applications

This article presents an always-half-inductor-current hybrid bidirectional converter (AHI-HBC) for USB-to-2-cell bidirectional power transfer. The converter achieves always-half-inductor current (AHI) in both directions compared to conventional bidirectional converters (CBCs) while using only 5-V transistors. This enables the elimination of high-voltage (HV) transistors, which have poor on-resistance ( $R_{\mathrm { ON}}$ ) performance and increase process costs. In addition, to address the challenge of varying charging current caused by changes in a battery voltage ( $V_{\mathrm { BAT}}$ ), we propose a normalized inductor current ( $N_{\mathrm { IL}}$ )-based adaptive target current controller that leverages the relationship between the inductor current ( $I_{L}$ ) and output current to effectively regulate the battery charging current ( $I_{\mathrm { BAT}}$ ). By utilizing the proposed controller, $I_{\mathrm { BAT}}$ adaptively tracks the target charging current regardless of $V_{\mathrm { BAT}}$ , which varies depending on the state-of-charge. The prototype chip was fabricated in a 180-nm CMOS process with a 4.7- $\mu $ H inductor, two 10- $\mu $ F flying capacitors, and a 10- $\mu $ F output capacitor. With the greatest $I_{L}$ reduction within the target USB-to-2-cell range, the converter demonstrated peak efficiencies of 97.4% (96%) in the forward mode and 97% (96.2%) in the on-the-go (OTG) mode with an inductor dc resistance ( $R_{\mathrm { DC}}$ ) of 11.5 m $\Omega $ (295 m $\Omega $ ).