Research on Cascaded On-Board DC/DC Converter Based on Three-Phase Interleaved LLC

The on-board DC/DC converter proposed in this research consists of a Buck converter and a three-phase interleaved LLC resonant converter. This paper analyzes the average current and output current ripple characteristics of the rear-stage converter in addition to explaining the topology and working principles of the front-stage Buck converter and the rear-stage three-phase interleaved LLC resonant converter. To establish a single-phase fundamental equivalent circuit for the rear-stage converter, apply the fundamental wave analysis method, and it is essential for comprehending the impact of relevant system parameters on voltage gain characteristics and resonant operating regions. Conducting research on the two-stage on-board DC/DC converter's control strategy involves establishing small-signal circuit models for both front and rear stage converters, derivation of parameters for double closed-loop control, and determination of the control strategy employing phase-shifted current sharing for the rear-stage converter in cases where significant tolerance exists in resonant components. The simulation for a two-stage on-board DC/DC converter results indicates that under various conditions of resonant component tolerance, the converter is capable of achieving balanced phase currents and demonstrates minimal output current ripple prior to capacitor filtering. A 1.5-kW experimental prototype has been fabricated, and the experimental findings indicate that the output voltage remains stable at 13.8 V despite fluctuations in input voltage. The highest efficiency of the prototype is about 96.27%, and the utilization of phase-shifting current balancing control results in a notable reduction in output current ripple. This provides more evidence that the design of a two-stage on-board DC/DC converter is correct.