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1 MHz, 48 V LLC Bus Converter with Simplified Optimal Trajectory Control

Block diagram of simplified trajectory control
Fig.1. Block diagram of simplified trajectory control with feedforward compensator for LLC operating at wide switching frequency range
LLC resonant converters have been trending as high efficiency dc-dc converters. LLC resonant converters feature soft switching for primary and secondary devices and operate at high switching frequency with high efficiency and power density. In some applications like bus converters, the dc-dc is required to provide regulation over a range of input voltage, typically 40 V to 60 V for datacenter applications and 35 V to 75 V for telecom applications. LLC resonant converters provide regulation by using frequency modulation. However, controlling LLC with fast transient response is challenging. Simplified optimal trajectory control (SOTC) achieves fast load transient response by simple measurement of output voltage and load current signals. However, SOTC assumes operating at fs=fo where LLC resonant converters run at resonant frequency for all loading conditions. This paper presents digital implementation of SOTC for 1 MHz LLC with a low cost and low speed 90 MHz microcontroller. We propose the implementation of two step SOTC to provide fast transient response. The delay from transient instant to settling point in the proposed implementation is reduced to only six cycles and faster transient response is achieved. Moreover, since the calculation time is reduced, the maximum switching frequency is pushed from 1.6 MHz to 2 MHz and a wider gain range is obtained. This work also discusses the challenges of controlling LLC resonant converters operating at wide gain and frequency range. The SOTC control scheme assumes the operation at resonant frequency and unity gain. However, when the switching frequency drifts far from the resonant frequency, the transient performance is highly degraded. A feedforward compensation is proposed in conjunction with the SOTC control, as shown in Fig. 1, to obtain fast transient performance over the entire frequency range. Furthermore, trajectory control is used for soft startup of LLC with limited current stress. The LLC starts with a fixed switching frequency profile where the switching frequency is calculat-ed as a function in output voltage to soft start-up under a predetermined current band limit (± ILMAX). However, with a fixed switching frequency profile, higher input voltage than the nominal value causes extra current stress that can damage the converter. At the same time, lower input voltage leads to insufficient energy transfer to the output capacitor for start-up. For LLC operating at a wide input voltage range, the switching frequency (fs=2πfo/4&aplpha;) is adapted with VIN to ensure the same current stress is applied and to guarantee successful start-up at all input voltage ranges.
Load transient response
Fig.2. 50% to 100% Load transient response at 48 V (a) 130 KHz commercial 800 W full-bridge phase-shift converter. (b) Proposed 1 MHz -1 kW LLC resonant converter using SOTC control