无电流传感复合前馈控制无桥功率因数校正变换器

Bridgeless Power Factor Correction Converter without Current Sensing and Compound Feedforward Control

有源功率因数校正(APFC)是AC-DC变换器的重要组成部分,其目的是改善输入侧电能质量,提高系统功率因数。传统的有源电流控制功率因数校正(PFC)变换器需要准确地提取升压电感电流,而双二极管式无桥PFC变换器升压电感位于整流部分之前,电感电流不再是单一方向流动,这使得其电流采样电路相比更复杂。为此该文提出一种基于观测器的无电流传感方案。方案中针对占空比误差的不确定影响修正了无桥PFC电流观测器算法,实现了占空比误差自适应的电流观测器,进一步基于所提观测器实现对负载信息的观测,设计出无电流传感的复合前馈控制方案,运用负载观测信息改进电流内环参考形式,同时利用输出电压纹波观测值进行2次谐波前馈补偿,最终实现无电流传感控制条件下变换器高动态响应设计。该文搭建了225 W单相双二极管式无桥PFC的实验样机,实验结果表明,在开关频率100 kHz时电流观测精度达到96.75%,所提控制方案输入电流的总谐波畸变率(THD)含量约为5%,功率因数(PF)约为99.85%,半载动态响应时间10 ms,相比于传统PI控制无桥PFC变换器提高了约90%。

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.