Average Inductor Current Measure and Control Strategy for Multimode Primary-Side Flyback Converters

Primary-side flyback converter (PSR) is widely used among small and medium power applications for its simplicity and cost effectiveness. However, it is hard to measure the current of secondary side due to the special structure characteristic of PSR. Conventionally, a peak current measurement strategy is adopted to measure the current of primary inductor. However, the sampling method is just for one conduction mode and is affected by the propagation delay between the control signal and actual turn-off instant of mosfet. Thus, a compensation circuit is needed to improve the accuracy, which leads to a large area and cost of the controller. In this work, a kind of average inductor current measure and control strategy for multimode is introduced. The average inductor current measurement strategy is based on the Miller stage to recognize the state of the mosfet, which eliminates the error caused by the mosfet. The average current of primary inductor is obtained and there is no need to estimate the rising slope and the peak value of the inductor current. To verify the feasibility of the proposed method, a controller for charging application is fabricated integrated with the proposed method. Hardware experimental results show that the control system can properly operate in multimode. The accuracy of constant output current is +/- 1% and the line regulation of the system is 1.6% under the input line voltage ranges from 85 to 265 Vac.