Research On DC UPS System Based On Fixed Frequency LLC Converter

To ensure uninterrupted power supply of important load, AC UPS system has been widely used in financial, medical, military, communication and many other fields. However, with the rapid development of world economy, the demands of AC UPS power systems’ reliability, flexibility and capacity are increasing, especially in the communication industry. With the continuous development of the communication network and business needs, higher security requirement of power supply system is demanded in communication equipment, particularly in data center. The problems of AC UPS in low reliability, expansion difficulty and the energy consumption are increasingly obvious. Therefore, DC UPS is becoming more and more appealing for its higher reliability, more convenient way of expansion and higher efficiency.Based on the analyses of DC UPS designing requirements, a power system prototype with 1k W rated power is designed. On the topology, interleaved Boost converter for power factor correction is chosen in the first stage, and full-bridge LLC resonant converter for voltage conversion and electrical isolation is taken in the second stage. Considering weakness of traditional two-loop control and the problem of low efficiency in light load, wide range of frequency change and other issues which LLC brings, a fixed frequency LLC control scheme is applied in this paper. Directly sampling the output voltage of LLC converter and using average current control method, a double-loop with voltage and current control strategy is designed to control the switch duty cycle of pre-interleaved Boost converter, achieving the power factor correction and the stability of output voltage. The control scheme not only simplifies the control structure of the system, but also makes the LLC stage has the highest efficiency, thereby enhancing the efficiency of the entire system. In the design of hardware parameters, considering the ZVS of the switches and improving the LLC stage efficiency, the resonance parameters are designed to be optimal. In terms of software, based on the analysis and modeling of LLC fixed frequency control DC UPS system,the program design of double-loop with voltage and current control is completed.A DC UPS system with 1k W rated power is built in this paper. The TMS320F2810 DSP control chip of TI has been used for digital control. The prototype achieves power factor correction and a stable output voltage. Experimental results are consistent with theoretical analysis and simulation results.