Investigation Of Pulse Train Control Technique For Boost Power Factor Correction Converter

To reduce the line current distortion and improve the power factor(PF),Power factor correction(PFC)converters,which also feature small size and low cost,have been utilized for most of AC-DC power conversion applications.Boost converter exhibits many advantages,such as easy design for the EMI filter due to series connection of inductor at the input side.Depending on whether the inductor current is continuous or not,the boost PFC coverter can be designed to operate in three modes: discontinuous conduction mode(DCM),continuous conduction mode(CCM)and critical conduction mode(CRM).Constant duty cycle control is usually utilized for boost PFC converter operating in DCM.With fixed switching frequency and constant duty cycle,the input current of constant duty cycle controlled DCM Boost PFC converter can be automatically shaped nearly sinusoidal.However,when input voltage is high,the input current distortion of constant duty cycle controlled DCM Boost PFC converter is high and its PF is low.To increase PF and to decrease the input current distortion,the duty cycle should be varied within a half-line period according to the rectified input current and output voltage,however the variable duty cycle requires complex operation circuits,such as multiplier,divider and root-square ciruits.In this dissertation,a pulse train(PT)control technique for DCM boost PFC converter is proposed.The PT controller can generate control pulses with different duty cycle in each switching cycle,and uses appropriate control pulse to control the converter according to the instantaneous rectified input voltage.The duty cycle varies discretely over a half-line period to approximate the variable duty cycle.The PT controlled DCM boost PFC converter has following advantages: High PF(>0.996)over universal input voltage range of 90?264V AC;Only some simple operations,such as comparison,are required to perform.Thus the proposed digital PT controller significantly reduces computation requirements,and is suitable for general-purpose MCUs with low processing speed and low cost.Average current mode(ACM)control is usually utilized for Boost PFC converter operating in CCM.In the ACM controller,the current control loop is coupled with the voltage current,as a result,the twice-line frequency output voltage ripple will be brought into the current control loop and cause high input current distortion and low input power factor.In order to minimize the influence of twice-line frequency output voltage ripple,the bandwidth of voltage loop should be designed significantly lower than twice-line frequency,usually lower than 20 Hz.As a result,the compensator of voltage control loop acts as a low-pass filter to attenuate twice-line frequency output voltage ripple.However,low bandwidth brings about slow dynamic response speed,which may cause large output voltage overshoot and undershoot.In this dissertation,a PT controlled CCM Boost PFC converter is proposed and studied.The current reference of PT controller is directly obtained from input voltage and output current,which has no relation with voltage control loop.Therefore,low frequency output voltage ripple will not affect the current loop,i.e.voltage control loop and current control loop are decoupled.Thus,voltage loop can be designed at relatively high band-width to improve dynamic response,and by utilizing PT control,the inductor current can be regulated to follow current reference to achieve high PF and low total harmonic distortion(THD).The experimental results show that power factor of PT controlled PFC converter is high,and the dynamic response speed is significantly mproved.Constant on-time(COT)control is usually utilized for Boost PFC converter operating in CRM.In order to realize the regulation of output voltage,COT control adjusts the on-time of switch continuously.Since the on-time of COT control is determined by the error compensator in the voltage control loop.While the bandwidth of the voltage control loop is ususally designed at 10 Hz to obtain high PF and low input current THD,which brings very slow dynamic response and high output voltage overshoot and undershoot.In this dissertation,a line cycle adjusting(LCA)controlled CRM Boost PFC converter is proposed.The LCA controller can achieve the regulation of output voltage by adusting the on-time of switch discretely.With the no requirement of error compensator,the LCA controlled CRM boost PFC converter features fast dynamic response and easy design for control loop.Meanwhile,the computation requirement of LCA controller is low,and is suitable for low cost and low processing-speed digital controller.The experimental results show LCA controlled CRM Boost PFC converter can obtain both high power factor and fast dynamic response.