Voltage Balancing of a Pi-Type Multilevel Converter for Charging Electrocaloric Capacitors

Electrocaloric solid state heat pumps provide a sustainable solution for cooling and heating systems powered by electricity. The electrocaloric effect is a reversible temperature change when the electric field changes in multilayer capacitors made of special dielectric functional materials. It can be transferred into an efficient solid-state heat pump system but requires highly efficient power electronics. High efficiency has already been achieved with multilevel converters with ideal capacitive loads. However, when operating with real, lossy and non-linear electrocaloric components as a load, the internal buffer voltages drift away due to uneven charge distribution across the capacitors. The main idea of this paper is to investigate and solve the unbalancing problem with a purely software-based control technique: Intentional unsymmetrical level-skipping can effectively charge or discharge the buffer voltage. The new balancing operation is experimentally demonstrated with a four-level Pi-type GaN-based multilevel converter, hysteretic current control for zero voltage switching, and an electrocaloric ceramic capacitive load (10µF, 120V, 0.5Hz system frequency). The 4 level balancing operation reduced the power loss by 24% (compared to 2 level), and enabled continuous operation with stabilized buffer voltages.