Research On Digital Control Stragtegy Of Single-phase Power Factor Corrector

With widely application of switching power supply system including rectifier and filter circuits, the input current distortion problem emerges in the equipments which connect to power supply system. In order to improve the power quality of the power grid, PFC(Power Correction Power, PFC) is required in switching power supply system to increase the PF(power factor, PF) and to comply with THD(Total Harmonics Distortion, THD) standard. Traditional PFC controller is usually designed basing on analog technology, but with the development of integrated circuit technology, digital control technology basing on DSP(Digtial Signal Processor, DSP), microcontroller and FPGA(Field Programmable Gate Array, FPGA) has been widely used in power electronic field.Accurate digital control algorithm of single-phase PFC implemented in simple structure is researched in this thesis. Controller structures of PFC based on FPGA are proposed, implementation issues of control alogrithm are analysed in this thesis. The research results of this dissertation are listed as below:(1) The control strategy based on triangle PWM(Pulse width modulation, PWM) is is intensely analyzed. Implementation strategy of one-cycle digital control PFC based on triangle PWM is proposed. This proposed sampling algorithm avoids tranditional one-cycle control strategy’s shortcomings such as performance depending on ADC(Analog to Digital Converter,ADC), complex control structure and difficulty in the acquisition of average inductor current. The digital one-cycyle control PFC converter utilizing triangle PWM has the merits including simple structure of controller, be capable of outputting high PF and low THD, accurate control performance and ease of fully digital implementation.(2) The digital predictive PFC control strategy based on emulated input admittance is proposed. According to the principle of predictive PFC operating under CCM, a DCM(Discontinuous Conduction Mode,DCM) correction factor Δ[n] based on emulated input admittance is calculated. A control strategy applied to both CCM and DCM is derived by correcting the sampling inductor current iL by the DCM correction factor Δ[n]. The proposed digital control scheme is achieved with minimal changes to the CCM predictive control strategy. The detailed analysis of experimental results verifys the proposed digital controller could achieve low THD, high PF value operating under CCM and DCM condition.(3) The SFM(Switching Frequency Modulation, SFM) digital PFC control strategy is proposed. The proposed PFC controller could switchover between two different switching frequencies to avoid PFC operating in DCM. The FSM(Finite State Machine,FSM) of proposed SFM is presented, the principle of SFM is explained. The proposed controller could automatically detect load condition of PFC converter and switch or keep the switching frequency accordingly so as to ensure that the system is always operating under CCM in full load range,which guarantees the the proposed PFC could achieve high PF and low THD under light load condition.(4) The sampling algorithm for average input current of one switching period based on the leading triangle PWM or tailing triangle PWM is utilized to verify the proposed PFC control strategy. This sampling algorithm utilizes a simple current sensing circuit and a low speed ADC to obtain the average input current directly at a fixed frequency(switching frequency), this sampling algorithm assures the accuracy of ADC’s acquisition. The sampling instant of ADC triggered by the falling edge of conversion signal avoids the EMI(Electro-Magnetic Interference, EMI) caused by the on-offs of power transistor. Direct acquisition of the average input current through sensing circuit could make the structure of control circuit simple and less hardare resource consuming. All the three digital PFC control strategies proposed in thE dissertation untilize this current sampling algorithm. Experimental results verify untilizing the current sampling algorithm could acquire accurate average value of inductor current.The experimental platform and the system prototype prove the validity of digital control strategies proposed in the thesis.