Research On Three Phase LLC Resonant Converter

LLC resonant converter has a simple structure, high power density and high efficiency, which achieves zero voltage switching at full load over a wide input voltage range.It is more and more widely adopted in flat-panel TVs, laptops adapters and other applications. However, subject to its output filter circuit only with capacitor, no output filter inductor, the output current ripple of conventional LLC resonant converter is large, which leads to the more volume of the filter capacitor, when used in the high current applications. And the life of the device is also limited by capacitor life. The output current ripple frequency is only twice the frequency of the switching, so it is not easy to reduce the volume of capacitor, which limits the power stage. Three-phase interleaved LLC resonant converter using transformers primary windings star connection could achieve current sharing and reduce the current flowing in device, apart from the excellent performance of single-phase LLC resonant converter.For three-phase LLC resonant converter topology, this paper adopted a feasible phase-shift control after analyzing the basic operating mode, which could realize current auto balancing and mitigate the output capacitor ripple offset with 120° phase shift for bridges. The realization of natural Zero Current Switch (ZCS) for secondary diodes in fm<fs≤fr area, will lower the loss on ESR of capacitor, reduce the size of the required capacitor and longer the life of equipment, resulting in lowest rate of current ripple.On the basis of the theoretical analysis and working principle for single-phase and three-phase topology, this paper presents the specific design standards and calculation process of important parameters for power supply. The voltage closed-loop of three phase LLC resonant converter is simulated in steady and transient state by using PSIM. By contrast to single phase LLC resonant converter topology under the same input and output conditions, the correctness of the theoretical analysis is proved.Finally, the article designs the hardware and software digital control systems based on the DSP chip TMS320F2812 and the prototype has been trial-produced, according to above theories. Typical experimental waveforms of three phase topology are given in fm<fs< fr, fs=fr,fs>fr working range.The primary resonant voltages, the resonant currents, the line voltages and the output currents of the secondary side rectifiers all show a delayed 1/3 cycle phase relationship and the ripple situations are focused on particularly. Compared with the PSIM simulation results, the consistency of the theory and experimental waveforms results proved the accuracy and the validity of phase-shift control adopted.Experimental results show that the current ripple rate used to reflect the improvement degree of the output ripple is nothing to do with the load, but the parallel number of phases. Three phase LLC resonant converter can indeed effectively reduce current ripple.