Optimal LLC Converter Design With Topology Morphing Control for Wide Voltage Range Battery Charging Applications

Abstract

LLC converters benefit from soft switching and sinusoidal currents over dual active bridge (DAB) converters. The design process of an LLC converter involves the selection of resonant tank inductor, capacitor, magnetizing inductance, resonant frequency, and transformer turns ratio for proper operation within the desired range of voltages and power. However, the design and control of frequency-modulated LLC converters in wide voltage range applications is challenging due to the wide range of switching frequencies. Topology morphing control is an established technique utilized for countering the challenges of wide voltage range LLC operation. This work provides a design framework for an LLC converter with topology morphing for wide voltage range applications. The proposed design framework uses time domain analysis and a power loss model to evaluate the optimal converter parameters for efficiency maximization over the entire voltage range. Methodology of implementing online topology morphing with closed-loop control in a digital signal processor (DSP) considering an on-board battery charger (OBC) application is also provided. The design optimization process and control methodology are validated through a 300–700 V input, 250–450 V output, 3.3 kW hardware demonstrator. An experimental peak efficiency of 97.72% is achieved compared to a calculated 97.63% efficiency, proving the accuracy of the analytical model. Time weighted averaged efficiency above 96.7% is observed over the entire voltage range.

Keywords

• DC-DC converter
• design optimization
• frequency modulation
• LLC
• online topology morphing
• resonant converter