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| Bridgeless Power Factor Correction Converter without Current Sensing and Compound Feedforward Control |
| Lu Weiguo, Hu Linghe, Luo Haikui, Bai Yue, Zhang Tingting |
| State Key Laboratory of Power Transmission Equipment & System Security and New Technology Chongqing University Chongqing 400044 China |
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Abstract Accurate sensing of inductor current is necessary for closed-loop control of PFC converters. However, in bridgeless PFC converters, the inductor current is no longer unidirectional, which needs expensive and complex current sampling devices. Sensorless techniques using current observers are regarded as a cost-effective solution. However, existing current observers do not consider the influence of duty cycle errors, resulting in poor observation accuracy at high switching frequencies. This paper proposes an improved adaptive current observer with a duty-cycle error correction function. Additionally, a compound feedforward control without current sensing is designed for the bridge PFC converter.
Firstly, by introducing duty cycle error into the current observer's expression, the specific influence of the duty cycle error on the observed current is obtained. The error value of the observed current increases linearly in every half of the industrial frequency cycle until it resets to zero at zero crossing time. When the switching frequency is low, the influence of duty cycle error can be negligible. With the increase of switching frequency, the influence on the observation accuracy becomes significant. Based on theoretical analysis, an improved observer with adaptive duty cycle error is designed. The proposed approach is to find an alternative to inductor current using the output voltage. The natural zero crossing point of the inductor current is utilized to obtain a duty cycle error adaptive observer for the bridgeless PFC converter. Finally, the proposed current observer is utilized to construct a compound feedforward control scheme, using observations of load information and output voltage ripple information to achieve a high dynamic response of the bridgeless PFC converter.
An experimental prototype of a 225 W bridge-free PFC is built. The experimental results show that the proposed duty-cycle adaptive observer achieves a 96.75% observing accuracy of the inductor current at 80 V input voltage and 100 kHz switching frequency. The load dynamic response of the proposed compound feedforward control scheme requires only about 2 IF cycles; in contrast, the conventional control scheme requires about 8-9 IF cycles. Meanwhile, the PF of the proposed scheme is 99.85%, and the THD is 5.09%.
The following conclusions are obtained in this paper. (1) Compared with the traditional current observer, the proposed duty-cycle adaptive observer corrects the duty cycle error of the switching tube at high switching frequency in software and improves the observation accuracy of the inductor current. Meanwhile, it has potential applications in other PFC topologies. (2) The proposed compound control scheme avoids the complexity of inductor current sampling in the bridgeless PFC converter, demonstrating a better dynamic response than the traditional double-loop control.
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Received: 09 June 2023
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2024, Vol. 39 
