Differential Power Processing Three-Port Dual Active Bridge Converter for Active Balancing in Large Battery Packs

This paper presents an approach to combine modular cell-level active balancing systems with a new three-port dc/dc converter topology. The proposed topology adds a significant value to the active balancing system by improving the volume and cost of cell-level balancing converter and reducing the total component count. These benefits are achieved by balancing two cells using only one three-port converter in contrast to a conventional two-port converter per cell. The three-port dc/dc topology is derived as a variation of the two-port dual-active bridge (DAB) converter by dividing the input into two halves using a center-tapped transformer and an additional inductor. No additional switches or auxiliary components are needed. The two-ports on the input side lets active control of currents from the two cells or cell strings connected to these ports. The converter uses both pulse-width modulation (PWM) and phase-shift modulation (PSM) simultaneously. To achieve active balancing the proposed converter injects a differential current into the mid-point of the cell sub-strings. PWM on the primary side devices is used to control this differential current, where as the traditional PSM for DAB is used to regulate the power to load bus. The proposed topology is such that there is no strong coupling between the PWM operation and PSM operation, and hence simpler single input single output approach is enough to achieve the control objective. The concepts for proposed topology, modulation scheme, and decoupled control are developed and hardware results are demonstrated on a 480 W hardware prototype. Detailed steady-state waveforms with mismatched cell voltages validate the converter topology and transient responses for the closed-loop are provided that validate the proposed control approach.