Transient Control and Soft Start-Up for 1-MHz LLC Converter With Wide Input Voltage Range Using Simplified Optimal Trajectory Control

The 48-V bus converters are widely used in telecommunications and network power architectures. A single-stage regulated$LLC$bus converter can achieve high efficiency and high power density. This article addresses the control challenges of a regulated$LLC$converter for 48-V bus converter applications. Simplified optimal trajectory control (SOTC) is used to control a 1-MHz$LLC$converter with an input voltage range of 40–60 V and a frequency range of 0.8–1.8 MHz. A digital implementation of two-step SOTC is proposed for the high-frequency$LLC$using a low-speed microcontroller (MCU). The proposed implementation can achieve the optimum transient performance and minimize settling time. SOTC can achieve the optimum transient response for an$LLC$operating at resonant frequency but has a limited impact at a wide gain range. As a result, a feedforward compensation is proposed, in conjunction with SOTC, to achieve optimum transient response at a wide input voltage range or wide gain range. Moreover, a soft start-up with adaptive switching frequency is proposed to achieve start-up under limited current stress over the whole input voltage range. The experimental results are demonstrated on a 1-MHz$LLC$48-V bus converter using a 90-MHz TMS320F28069 Piccolo MCU.