RESEARCH
Multi-Step Simplified Optimal Trajectory Control (SOTC) for LLC Resonant Converter
Year: 2015
Fig. 1. Four quadrant operation and switching loops for the new modulation for ANPC phase leg
In this paper, first, implementation of SOTC is optimized and can be applied to the LLC converter with a maximum switching frequency of 250kHz with 60MHz MCU. To further improve switching frequency, multi-step SOTC is proposed to apply a low-cost MCU to a high frequency LLC converter. The basic concept is that instead of using 2-step SOTC to settle the resonant tank, which is the optimal method for the LLC converter, n-step SOTC is instead used to settle the resonant tank, in which n is determined by delay of controller and switching frequency of power stage (n = 1, 2, 3, 4). The comparison of 2-step SOTC and 6-step SOTC is shown in Fig. 1(a) and Fig. 1(b). Fig. 1(c) is an example of implementation of multi-step SOTC for a 500kHz LLC converter, in which digital delay takes 3 switching cycles, and fast load transient response is achieved within 6 steps.
Multi-step SOTC for fast load transient response is verified on a 500kHz LLC converter, which is designed based on Matrix Transformer for LLC Resonant Converters. The hardware is shown in Fig. 2(a). The experimental results of multi-step SOTC for load step up from 40A to 80A are shown in Fig. 2(b).
In summary, the proposed multi-step SOTC settles resonant tank within n-step instead of fixed 2-step, where n is selected based on delay of controller. Multi-step SOTC makes it feasible to use a low-cost MCU to control the high frequency LLC converter, while simultaneously achieving nearly the same performance as simplified optimal trajectory control.
Fig. 2. Inverter side current ripple waveform
