This SBIR proposal aims to research on a novel non-resonant DAB Converter (DABc) with cycle-by-cycle peak current control for interfacing multiple Li-ion prismatic cells in either series or parallel configuration to an intermediate DC bus of 400 V which can be used to exchange power between single phase AC grid and the prismatic cells. Natural soft switching of the proposed DABc enhances efficiency to above 98% and facilitates high frequency operation for high power density. The proposal will also investigate a novel matrix transformer which can be used to integrate multiple outputs on the low voltage (LV) DC side and to facilitate a height of 3 cm of the proposed DABc. Overall power density of 700 W per cu.in. will be achievable for the proposed DABc. Conventional system consists of multiple individual Li-ion battery cells connected in a matrix as shown in Fig.1(a) forming a battery pack. Often passive resistive voltage balancing circuits are implemented for balancing of the voltage of each cell in such a matrix. This battery pack is eventually connected to a DAB converter that provides bi-directional energy exchange between an intermediate DC bus and the battery pack and provides galvanic isolation for safety. Such conventional method, battery pack formation and integration to the power grid has significant drawback of efficient and effective cell voltage balancing in the battery pack specially when higher voltage battery packs are desired in high power (>1 kW) applications. Cell level integration using DABc integrating individual prismatic cells to an intermediate DC bus have been researched. The main drawback that remains is as follows: (1) Resonant and non-resonant DABc (Fig. 1(b)) are mostly implemented which controls only the power flow between the battery and the grid but not the instantaneous current flowing into or out of an individual cell which is essential for maintaining the terminal voltage across the cells protection against overcharging or discharging of each cell; (2) A large number of DABcs are required to interface individual cells to the intermediate DC bus which increases the cost, complexity of control and packaging (3) Large height and low power density of the overall converter for a complete battery pack due to large number of individual converters and their high frequency transformers which poses significant challenges in packaging and thermal management of the power electronics close to the individual cell MAM Inc. in this proposed SBIR phase-1 research in collaboration with Professor Pritam Das aims to mitigate the major problems to give rise to a novel method of integrating individual cells to the grid through an intermediate DC bus. The proposed research is based on a novel non-resonant DAB with the following: with multi low voltage (LV) output each of which interface with a cell a single high voltage (HV) output the LV and HV outputs which are coupled magnetically by a novel matrix type transformer wherein the HV winding turns are split equally on each pillar of the matrix transformer which also has one of the multiple secondary windings BT1,1BT2,2BT3,1BTn,1BT1,2BT2,2BT3,2BTn,2BT1,pBT2,pBT3,pBTn,pC1VBus+-1 turnVBattiCell (a) (b) patented cycle by cycle peak current control enabled by boundary conduction mode of the leakage inductor current The advantage arising from the above mentioned properties of the proposed DABc include: Soft Switching, high efficiency and high-power density: Natural soft switching reducing frequency dependent losses and enabling high frequency operation that facilitates reduction of the size of the high frequency transformer and increases the power density and also increases the efficiency toward 98% Protection and cell fault tolerance: Equal Current Cycle by cycle peak current control along with the proposed configuration of the high frequency transformer allows equal current output or input to each cell of the BESS irrespective of whether one or more cells have any type of failure Low height and high-power density: The high frequency transformer is the part that is the most bulky and tallest. The proposed matrix transformer splits the primary winding into multiple pillars which allows height reduction to less than 3mm as per preliminary design shown in Fig. 2. Combined high frequency operation and the proposed matrix transformer along with available modules of Wide band gap GaN devices a height of 3mm and a power density of 700 W per cu.in can be achieved Further one can extend this concept to realize the plug and play storage which can be useful for small residential application where storage can be expanded to depending on the power need.
