Study And Design Of Single Stage AC/DC Converter With PWM/PFM Hybrid Modulation

With the increasing use of electronic equipment,AC/DC converters as the source of electronic equipment are getting the attention by researchers increasingly.In order to comply with the relevant harmonics standard such as IEC 61000-3-2,power factor correction function has to be included in AC/DC converters.Accordingly,the single-stage AC/DC converter is a hotspot because of its simple control and high efficiency.A PFM control strategy with a duty cycle of 0.5 is used in the traditional single-stage Boost-LLC AC/DC converter.Meanwhile,the boost PFC cell is operated at DCM mode.As a result,the intermediate bus voltage has to be greater than twice the peak value of the ac input voltage,resulting in the narrow range of the input voltage with the power device of 650V voltage rated.In order to solve this problem,a PWM/PFM hybrid control strategy is proposed in this thesis.Furthermore,a proportional feed-forward control scheme and a dual-loop feedback control scheme are designed respectively.The proportional feedforward control strategy is composed of the PWM-based bus voltag feed-forward block and the PFM-based output voltage feedback block.The PFM-based output voltage feedback block is mainly in charge of regulating the output voltage at the desired value.The PWM-based bus voltag feed-forward block is employed to decrease the duty cycle of MOSFET Q2 while the bus voltage is higher than the desired value.Thus,the low bus voltage can be achieved.However,with increasing of input voltage,the bus voltage still increases.For this reason,a dual-loop feedback control scheme is proposed.The dual-loop feedback control scheme is composed of two closed-loops.One is the PFM-based output voltage feedback block.The other is APWM-based bus voltage control block.The PFM-based output voltage feedback block is mainly in charge of regulating the output voltage at the desired value,and the APWM-based bus voltage control block is mainly in charge of regulating the bus voltage at desired value.The bus voltage is stabilized at the desired value when the input voltage is higher than the setted reference.Hence,the range of bus voltage variations is garetly reduced.A proportional feed-forward control scheme prototype and a dual-loop feedback control scheme prototype are designed respectively.The experimental results show that the bus voltage is reduced and the input voltage range is increased in both of the control schemes.The efficiency and power factor of the proportional feed-forward control scheme is higher than the dual-loop feedback control scheme when the input voltage is higher.The efficiency and power factor are greater than 90%and 0.96 in the proportional feed-forward control scheme.Compared with the proportional feed-forward control scheme,the bus voltage is charged at desired value in the dual-loop feedback control scheme,which is more favorable for the design of the magnetic element and the choice of the power device.