原边串联型LLC变换器拓扑及其频率自适应追踪方法

Primary-side Series-connected LLC Converter Topology and Its Frequency-adaptive Tracking Method

为了拓宽传统LLC变换器的增益范围,多采用脉冲宽度调制(pulse width modulation,PWM)变换器与LLC变换器级联的两级式拓扑,其中LLC变换器开环工作于谐振频率点,PWM变换器负责闭环稳压。由于PWM变换器承担了全部负载功率,效率损失较大,且LLC谐振腔参数受电路杂散电感、器件批次一致性等影响,谐振频率点常偏离于预设值,导致整机效率进一步下降。针对上述问题,提出了一种原边串联型LLC变换器拓扑,将同步Buck变换器的输出端与全桥LLC变换器的任一桥臂串联,通过调节Buck输出电压改变LLC谐振腔输入电压幅值,Buck仅承担部分负载功率即可实现整机宽增益范围调节。对于谐振频率点偏离问题,提出了一种频率自适应追踪方法,结合LLC在不同谐振状态下的换流特性,根据副边整流管通流时间,无需谐振状态判断便可直接计算频率调节值,进行谐振频率追踪控制。最后搭建一台28 V/1 kW实验样机,在240~380 V输入条件下,Buck变换器最多仅承担45%的负载功率,同时可以准确追踪变换器的谐振频率点。变换器整机峰值效率为97.39%,相较传统两级式拓扑效率最大提升2.32%。

To broaden the voltage gain range of the conventional LLC converter,a two-stage topology consisting of a PWM converter and an LLC converter in cascade is commonly used.The LLC converter operating at resonant frequency and the PWM converter provides a stable output voltage.However,there is a significant losses due to the PWM converter providing the full load power.In addition,the resonant frequency of the LLC converter usually deviates from the preset value because of the stray inductance,device consistency and other factors,resulting in the converter efficiency will be further reduced.For the above problem,a primary-side series-connected LLC converter topology is proposed.The output port of the Buck converter is connected in series with any bridge of the full-bridge LLC converter,and adjusting the out-put voltage of the Buck converter can change the input voltage amplitude of the resonant tank of the LLC converter.The Buck converter only needs to provide part of the load power to achieve wide voltage gain range regulation of the overall converter.For resonant frequency deviation,a frequency adaptive tracking method is proposed.Combining the operating characteristics of the LLC converter in different resonant states,the frequency regulation value can be calculated based on the secondary rectifier on-time.Finally,an experimental prototype with 240~380 V input and 28 V/1 kW output was built.The Buck converter provides only up to 45%of the load power,and the LLC converter can accurately track to the reso-nant frequency.The peak efficiency of the experimental prototype is 97.39%,and the efficiency is improved by up to 2.32%compared to the conventional two-stage converter.