Research On The Control Strategy Optimization Of CRM PFC Converters

With the popularity of the consumer electronics,most AC-DC converters are forced to adopt power factor correction(PFC)technology to eliminate harmonic pollution in power grid and improve power factor.At present,several standards are introduced in many countries and regions to regulate the power factor(PF)and harmonics of AC-DC converters with different power levels,such as IEC61000-3-2,Energy Star and DesignLights Consortium(DLC).In low power applications,such as laptop adapters and LED drivers,the commonly used isolated PFC topology is Flyback,and non-isolated topologies are Buck-Boost,Buck and Boost.Since the power level is low,the inductor current is always in critical conduction mode(CRM).In order to decrease the input current harmonics and improve the power factor of CRM Flyback(Buck-Boost)and Buck PFC converters,based on the conclusion of present control schemes,an analog divider circuit is proposed firstly in this paper.The analog divider consists of an operational amplifier,an RC filter and two signal switches.Appling the analog divider circuit into Flyback(Buck-Boost)PFC converter,two novel control strategies are presented to make the theoretical input current be sinusoidal;Appling the analog divider circuit into Buck PFC converter,a new control strategy is introduced to reduce its input current harmonics,and it is also suit for hybrid PFC converters to further improve its power factor and reduce its current harmonics.The theoretical input current of CRM Boost PFC converter is sinusoidal,but with the influence of the input differential mode(DM)capacitor,it always has a leading phase,which will deteriorate the PF and THD,especially in high input and light load conditions.In order to solve this problem,two simple and effective methods are presented to compensate the capacitive current of the DM capacitor,as a result,the PF and THD are both improved in high input and light load conditions.All the control strategies above are effectual,easy to design,simple and low-cost,which are very attractive in commercial applications.