Three-Level DNPC-LLC Converter With Hybrid Modulation Method for Wide Output Voltage Applications

Three-level diode-neutral-point-clamped (DNPC)-LLC resonant converter is usually utilized for high input voltage applications, whereas its output voltage is regulated using three-level (3L) modulation method. However, 3L modulation makes it difficult to address the requirement of a wide output voltage range. Therefore, this paper presents a hybrid modulation method for the DNPC-LLC converter, incorporating the traditional 3L modulation and a proposed two-level (2L) modulation, which allows lower output voltages, reduces the switching frequency and the number of switching operations of the primary-side switches. Furthermore, the operating principles and voltage gains of both modulation methods are analyzed. Finally, a 3.7 kW SiC-based DNPC-LLC converter with synchronous rectification at an 800 V input is implemented. The output voltage ranges from 20 V to 450 V, and smooth transitions between different modulation modes are verified. The proposed converter achieves a maximum efficiency of 97.3% at an output of 360 V and 10.3 A. Furthermore, compared to the 3L method, the 2L method improves efficiency by at least 0.5% when the output voltage is below 200 V. Additionally, the voltage balance between two series dc-link capacitors is fulfilled after applying voltage balancing control.