Research On Analysis And Solutions Of Key Issues In Single Stage Full Bridge APFC Converter

Active power factor correction(Active Power Factor Correction,APFC)technique is an effective method to increase power factor,to suppress harmonics of input current and to improve power quality.Relative to two stage APFC technique,single stage APFC technique can realize the function of PFC and DC/DC conversion in one stage circuit with the merits of simple construction,and lower costs in power density and efficiency,which conforms to the development of power electronics technology.Comparing with other single stage APFC techniques,single stage full-bridge APFC technique can be applied flexibly to the systems of single phase and three phase,and is more suitable for the application of medium and large power ratings.However,there are problems that,the bus voltage spike,the output voltage ripple in single phase single stage APFC converter,the more harmonics content of input current in three phase single stage full-bridge APFC converter and in combined three phase single stage full-bridge APFC converter when unbalanced grid conditions,existing in single stage APFC converter,which limits its development and application.Therefore,the problems mentioned above are deeply studied,and several novel methods and ideas are proposed.The larger bus voltage spike is produced during the commutation process,as the leakage inductance of the high frequency transformer exists in the single phase single stage APFC converter,which inluences the reliability of the system.A method based on the flyback auxiliary circuit link to suppress the bus voltage spike is proposed on the basis of analyzing the changing law of bus voltage and explaining the producing mechanism of bus voltage spike.The function of clamped capacitor and working process of flyback circuit in the flyback auxiliary circuit link are studied.The suppression effects of flyback auxiliary circuit link are analyzed,and the design demand for clamped capacitor is given.The bus voltage spike is not only suppressed,the absorbed energy of bus voltage spike is also transferred to the load to use with the method.Compared with the methods adopting RC or RCD snubber circuits to suppress the bus voltage spike,there is no hot and loss of the resistor,which improves the efficiency of the converter.There is a larger output voltage ripple whose frequency is twice line frequency in single phase single stage full-bridge APFC converter which influences the output characteristic of the converter.On the basis of analyzing the producing mechanism of output voltage ripple,a method based on the flyback auxiliary circuit link to compensate the output voltage ripple is proposed.The basic operation principle and feasibility of proposed compensation method is analyzed,including the effects on ripple compensation influenced by clamped capacitor.The method to detect output voltage ripple based on notch band stop filter and the control strategy for flyback circuit based on quasi proportional resonant(Quasi-Proportional Resonant,Q-PR)control is designed,which realizes controlling the output compensation current of flyback auxiliary circuit link effectively to achieve the purpose of reducing the output voltage ripple.The absorbed bus voltage spike energy is used to compensate the output voltage ripple by adopting the existed flyback auxiliary circuit link,which can suppress the bus voltage spike and compensate the output voltage ripple simultaneously without complicating the circuit.For the problem that larger bus voltage spike exists in three phase single stage full-bridge APFC converter,a suppression method based on a passive flyback auxiliary circuit link is proposed.The suppression mechanism of bus voltage spike is studied,and the parameters of passive flyback auxiliary link is analyzed and designed.Compared with the methods adopting active clamped circuit and active flyback auxiliary circuit link,the proposed method is easy to implement without control.There are low order harmonics(mainly 5th order harmonic)in the input current in the three phase single stage full-bridge APFC converter when adopting the conventional constant duty cycle control strategy.The harmonics content of three phase input current is analyzed to get the reason that makes the low order harmonics content vary,and a control strategy based on fitting duty cycle is proposed accordingly.The ideal duty cycle is achieved by weighting factor model to realize the maximum power factor and then fitted by least squares,which suppresses the low order harmonics of input current.The control strategy improves the power quality of input current and power factor compared with the conventional constant duty cycle control strategy.Under balanced grid conditions in combined three phase single stage full-bridge APFC converter,there are no low order harmonics of input current and no low order ripple of output voltage,and a fast dynamic characteristic of output voltage and a balanced three phase input average power are achieved.But under unbalanced grid conditions in combined converter with the conventional voltage and current double closed loop control strategy,there are more low order harmonic of input current,low order ripple of output voltage and unbalanced power among three phase single modular,which influences the operational characteristic.Accordingly,an improved control strategy for combined three phase single stage full-bridge APFC converter based on double scaling factor is proposed.Two scaling factors and one limiter are incorporated into the current inner loop of each single modular in the combined converter,and then the functions of each component part and the operational principle are analyzed.Compared with the conventional voltage and current double closed loop control strategy,the problems mentioned above are addressed by the proposed control strategy simultaneously,which improves the quality of input current and output voltage waveforms and balances the three phase power.