A Modular Balancing Bridge for Series Connected Voltage Sources

The operation of a nondissipative cascaded modular balancing bridge is described for automatically balancing series connected voltage sources, such as lithium-ion battery cells or capacitors. Automatic voltage balancing is achieved using coupled inductors; modularization and voltage balancing is achieved across N cells using cascaded transformers with coupled windings. The elementary modular bridge consists of four windings placed on a single core and is excited by an asymmetric half-bridge. Two of these windings are used to automatically balance voltages across two voltage sources, while the remaining two windings magnetically cascade one bridge with its neighboring bridges to balance N voltage sources. This configuration allows for the voltage balancing of N voltage sources using N/2 identical transformers, and N/2 asymmetric bridges. The voltage balancing action of the resultant magnetically coupled cascaded asymmetric bridges can be broken into two categories: intrabridge voltage balancing (within a single bridge) and interbridge voltage balancing (between neighboring bridges). Design parameters relating to automatic voltage balancing are highlighted, as the approach is more cost effective than methods using directed, or individualized, voltage balancing of each voltage source. The physical size considerations of the four winding coupled inductor are discussed and a lithium-ion battery-based experimental prototype is used to verify simulated results.