Simplified Optimal Trajectory Control for 1 MHz LLC Converter with Wide Input Voltage Range

48 V bus converter is widely used in telecom and networks power architecture. This paper presents LLC converter as single stage solution for isolated and regulated bus converter, and addresses the control challenges for this converter. In this paper, Simplified Optimal Trajectory Control (SOTC) is used to control 1 MHz LLC converter with input voltage range 40 V:60 V and frequency range 0.8 MHz: 1.8 MHz. Digital implementation of 2-step SOTC is proposed for the high frequency LLC using a low-speed microcontroller. SOTC can achieve optimum transient response for an LLC operating at resonant frequency but has limited impact at wide gain range. A feedforward compensation is proposed in conjunction with SOTC to achieve optimum transient response at wide input voltage range. Moreover, soft-start-up with adaptive switching frequency is proposed to achieve start-up under limited current stress over the whole input voltage range. Experimental results are demonstrated on a 1 MHz LLC 48 V bus converter using 90MHz TMS320F28069 piccolo MCU.

I. Introduction

The LLC resonant converter has been trending as a high efficiency DC-DC converter. The LLC features soft switching for primary and secondary devices and can operate at high switching frequency with high efficiency and high power density. Many applications utilize resonant converters such as front-end converters [1], battery chargers, and bus converters. For the 48 V/12 V bus converter, the DC-DC is required to provide regulation of output voltage at 12V over a range of input voltage, typically 40 V:6o V for datacenter and 36 V:72 V for telecommunication.