Unbalanced Three-Phase LLC Resonant Converters: Analysis and Trigonometric Current Balancing

Three-phase LLC resonant converters can handle very high power levels beyond the capabilities of half-bridge and full-bridge LLC topologies. Among other characteristics, three-phase LLC structures reduce output current ripple (small output filter), enable parallel power processing (low peak current), and provide good thermal distribution. However, all these key advantages can be severely compromised due to passive components tolerances, leading to undesired current balance issues in three-phase LLC resonant converters. Tolerances in resonant tank passive components are inevitable and lead to unequal peak currents between phases, uneven temperature distribution, and large output current ripple. This paper investigates the imbalances in three-phase LLC converters and proposes a novel trigonometric current balancing (TCB) technique using phasor analysis. In this strategy, the required input voltage phase angles are calculated to achieve balanced phase currents, even under severe unbalanced conditions. In some cases, the output filter current ripple is reduced to less than half. The methodology is verified with a 3-kW experimental prototype, which validates the analytical framework and effectiveness of TCB.