Bidirectional DC-DC converter modeling and unified controller with digital implementation

In this paper, a unified current controller is introduced for a bidirectional dc-dc converter which employs complementary switching between upper and lower switches. The unified current controller is to use one controller for both buck and boost modes. Such a controller may be designed with analog implementation that adopts current injection control method, which is difficult to be implemented in high power applications due to parasitic noises. The averaged current mode is thus proposed in this paper to avoid the current sensing related issues. Additional advantage with the unified digital controller is also found in smooth mode transition between battery charging and discharging modes where conventional analog controller tends to saturate and take a long delay to get out of saturation. The unified controller has been designed based on a proposed novel third- order bidirectional charging/discharging model and implemented with a TMS320F2808 based digital controller. The complete system has been simulated and verified with a high-power hardware prototype testing.

I. Introduction

The high power bidirectional dc-dc power converter has been widely used in hybrid electric vehicle and fuel cell applications [1]–[8]. To increase its power density, the design can adopt small inductor with multiphase interleaving to operate in discontinuous conduction mode (DCM) while reducing the ripples [9]. The problem with DCM operation is the related parasitic ringing caused by the inductor and the device output capacitance during current turn-off condition [9], resulting poor efficiency and significant EMI noises. A gate signal complementary control scheme was thus introduced to allow the inductor current continuously flow through the opposite direction such that the parasitic ringing is avoided, and the zero voltage soft switching (ZVS) is achieved [10]–[12]. This complementary switching scheme is also defined as “synchronous conduction” mode and is adopted here for a high-power bidirectional charger application.