A Bidirectional Three-Level Converter with Single Point Sensing Technique for Flying Capacitor Balance

Abstract

In this paper, a novel bidirectional three-level (TL) converter control scheme is presented with a flying capacitor (FC) control loop and adaptive constant on-time (ACOT) control. Due to the FC balance issue in TL converters, the FC control with single point sensing (SPS) formed by only two pairs of sample and holds (S/Hs) and operational transconductance amplifiers (OTAs) is proposed not only to balance FC at half of high-side voltage, but also to have simpler design. The sensing method additionally minimizes inductor ripple. Moreover, the proposed ACOT design solves the switching frequency variation issue, and the variation is only 5kHz, which is much smaller than previous works especially in boost converter. Besides, both buck mode and boost mode share the same control elements except for compensator. Consequently, the controller die area can be minimized. The proposed scheme is realized with a 1.5 MHz switching frequency bidirectional TL converter IC in TSMC 0.18um CMOS process. Due to the characteristics of TL converter, the stress of switch is chosen with half of high-side voltage. Full transistor simulation result verifies that the functions are worked well on both directions and there are no FC voltage runaway and mismatch issues. The efficiency up to 96% at load 1A in high conversion ratio.