Transformer-Based Equalization Circuit Applied to n-Number of High Capacitance Cells

Over the last decade various high-capacitance devices have become available in the market such as supercapacitors, ultracapacitors, and recently, Li-ion capacitors. The cell voltage limit of each of these technologies is a small percentage of the system-level voltage so they must, therefore, be connected in series to attain a high voltage. During charging and discharging, manufacturing tolerances between the cells result in voltage mismatch across the stack. Mismatched voltages are an inefficient use of the energy storage medium and can lead to dangerous failures in the cells if voltages exceed safety limits. Transformer-based voltage equalization techniques are the preferred circuit topologies in applications with low system voltage due to simplicity of control and low number of switches. The drawback of these circuits is the number of isolated windings that are required on a single core. This paper describes for the first time a solution to that problem by using a classical two windings transformer that in principal can be applied to any number of capacitors. This paper describes the operation of the circuit, shows simulation results and practical results based on a prototype with five cells.