Switching pattern of a modular voltage balancing circuit for battery cells

Abstract

Switching patterns are described for balancing the voltages of series connected Li-ion polymer batteries using non-dissipative cascaded modular power electronics. A balancing circuit is connected to two battery cells and consists of an asymmetrical half-bridge and a high frequency transformer. This reduces the number of switches and diodes to an average of one per battery. A balancing circuit or module can balance the voltage of two batteries by using two magnetically coupled windings connected to the centre tap of the two batteries: intra-bridge voltage balancing. Each module is coupled to adjacent modules by connecting transformer windings. Two fundamental control methods are described for transferring electrical charge between the batteries in adjacent modules: inter-bridge voltage balancing. Both control methods, “2-switch flyback control” and “phase-shifted control”, assume that each module determines its own switching pattern by monitoring the dc voltages in each of its adjacent modules. The relative merits of both controllers are described. Simulated and experimental results are presented for both controllers to validate the charge transfer methods used.