Voltage Balance of the series-connected dc-link capacitors is the key problem of three-level diode-clamped converters. In this paper, an auxiliary voltage balance circuit (VBC) is presented to maintain the voltage balance of dc-link capacitors. The voltage balance circuit consists of a clamping capacitor and a current-limiting inductor. With the clamping capacitor, the voltage stresses of the auxiliary circuit devices are restricted to one half of the bus voltage, and the current ripple of the inductor is also effectively suppressed. The operating principles of the auxiliary circuit are studied, and a time-domain model is derived. Accordingly, a control algorithm of the auxiliary circuit is proposed based on finite-control-set model predictive control. Simulation results validate the effectiveness of the auxiliary voltage balance circuit. The auxiliary circuit is applicable for three-level DCI-based energy conversion systems, static reactive power compensations and medium-voltage highpower motor drives.