Multi-Step Simplified Optimal Trajectory Control (SOTC) for LLC Resonant Converter
Simplified Optimal Trajectory Control (SOTC) can solve the challenge of fast load transient response based on state-trajectory analysis. SOTC senses load current and modifies the pulse widths of driving signals, which works as a load current feed-forward loop and improves transient response significantly. In addition, digital controllers have superior advantages over analog controllers, and cost-effective microcontrollers (MCU) are preferred in industrial applications. High frequency LLC converters have also become more popular in recent years due to their high power density and integrated magnetics, which reduce total cost. It is therefore useful to apply SOTC to the high frequency LLC converter with low-cost MCU.
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.