Startup and control of high efficiency 48/1V sigma converter

Abstract

48V voltage regulator modules (VRMs) are critical for telecom power supplies, and is becoming popular for future data centers. By using a novel sigma converter topology, an outstanding performance in terms of efficiency (94%) and power density (420 W/in3) have been demonstrated in [11]. Both the efficiency and power density are much higher than state-of-art solutions. The sigma converter is a quasi-parallel converter comprises an unregulated LLC converter (LLC_DCX) and a regulated buck converter connected in series from the input side and in parallel from the output side. However, the start-up and control is quite challenging for sigma converters and have not been solved properly yet. The soft startup of LLC-DCX is challenging due to excessive stress in the resonant tank, in addition, the regulated buck converter uses low voltage devices to guarantee high efficiency. This require a proper startup mechanism for the sigma converter to avoid these excessive stresses in the LLC-DCX and any over voltages across the buck converter devices. In this paper, a soft startup mechanism is proposed combining the simplified optimal trajectory control (SOTC) for soft startup of the LLC-DCX and digital voltage mode controller for the buck converter implemented with a low cost microcontroller. The proposed mechanism was experimentally tested on a 48/1V prototype using 60MHZ MCU TMS320F28025 achieving a fast and soft startup with limited stresses on both LLC-DCX and buck converter.