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Digital Implementation of Light-load Efficiency Improvement for High-frequency LLC Converters with Simplified Optimal Trajectory Control

Comparison of OTC and SOTC
Fig. 1. Comparison of OTC for burst mode and SOTC for burst mode with multi-step
The light load efficiency of an LLC converter with frequency control cannot meet the increasing efficiency requirements, such as 80 Plus and Energy Star. In recent years, a high frequency LLC converter has emerged, due to its high power density and integrated magnetics, which reduce the total cost. Digital controllers, especially cost-effective microcontrollers (MCUs), are gradually replacing analog controllers for controlling the LLC resonant converter. Hence, it is desirable to extend burst mode to the high frequency LLC converter with a low-cost microcontroller (MCU). Due to the dynamics of the resonant tank, a conventional burst control for an LLC converter has a problem; the resonant tank cannot keep to the efficiency-optimal state trajectory. The optimal trajectory control (OTC) for burst mode with a fixed 3-pulse pattern can solve this problem, based on state-plane analysis. However, when the OTC for burst mode is applied to a high frequency LLC converter with an MCU, the burst mode operation range is limited. This is because in burst off-time, the digital controller must leave enough time to blank sensing noise, sample, and update the control signal. With minimum off-time Toff_min, the maximum average power delivered to the secondary side is limited, as shown in Fig. 1(a). To extend the burst operation range for a high frequency LLC converter, a simplified optimal trajectory control (SOTC ) for burst mode, with adaptive burst on-power and adaptive multi-step, are proposed in this paper. The first step in the process is to increase the burst on-power, but the desired trajectory is not fixed to the efficiency-optimal trajectory. The next step is to increase the burst pulses, as shown in Fig. 1(b). The principles are as follows: Assuming the load increases from an empty load, initially a 3-pulse pattern is applied. When the load increases to the limit of the 3-pulse pattern, then a 5-pulse pattern is applied. The same analogy is used for the rest. The SOTC for burst mode with an adaptive multi-step is verified on a 500 kHz LLC converter controlled by a 60 MHz MCU. The efficiency curve is shown in Fig. 2. The light efficiency improvement is significant, compared with conventional burst mode.
Efficiency
Fig. 2. Efficiency of proposed Burst mode for high frequency LLC converter
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