Abstract

The demand for a bidirectional dc-dc converter with a flexible dc bus is driven by the fast development of renewable energy system, transportation electrification, and microgrid. In order to accommodate different dc bus, two-stage ac-dc-dc architecture has been widely used, and the dc output regulation was handled by a rear-end dc-dc converter. If the galvanic isolation is not required, the four-switch buck-boost (FSBB) converter with quadrangle control is a good candidate because of the bidirectional noninverting output, step-up/down capability, and zero voltage switching. However, to achieve the minimum rms current and soft switching, the calculation of quadrangle control is complicated and often requires the resource-consuming loop-up tables, or additional high-frequency current detection circuits. Moreover, due to the unbalanced circuit topology, the common-mode (CM) noise is another concern. In this article, a symmetric three-level (3-L) buck-boost converter was first proposed to suppress the CM noise. To increase the power density and efficiency, a planar coupled inductor was designed for this 3-L buck-boost converter with a 30% winding loss reduction. And then, to realize a simple close-loop output control, a real-time simplified minimum rms current calculation for quadrangle modulation was found without look-up tables or ZCD circuits. Based on this simplified output control, a decoupled mid-points balance control for both input and output sides were also proposed. Finally, the simplified close-loop control, the decoupled active balance control, and the CM mode noise reduction were all verified by a 30 kHz 50 kW 3-L buck-boost converter. Compared with the typical FSBB converter, the proposed 3-L buck-boost converter has a up to 25 dB CM noise reduction from 150 kHz to 30 MHz. This article is accompanied by two videos demonstrating the effect of decoupled active balance control.