A Simple Active Method to Avoid the Balancing Losses of DC Link Capacitors

摘要

DC voltage links of three-phase power converters very often are equipped with a series connection of two electrolytic capacitors due to the high voltage level. In general, resistors arranged in parallel to each capacitor are necessary for balancing the capacitor partial voltages. The balancing resistors have to be dimensioned regarding the worst-case condition of the capacitor leakage currents, resulting in high permanent dissipative losses. To avoid these losses to a large extent, a novel simple and low-cost active circuit to replace the passive balancing resistors by a cascode array of low-cost small-signal low-voltage bipolar transistors is presented. The paper describes the operating principle of the circuit, analyzes the fundamental relationships being relevant for the balancing characteristic and gives guidelines concerning component selection. Furthermore, simulation results as well as measurements taken from a laboratory prototype are presented.