Design Of A High-precision Constant Current And Constant Voltage Control Chip For Primary-side Regulation Flyback Converter

In recent years,with the popularity of portable electronics,lithium batteries have been widely used.The primary-side regulation flyback converter is becoming one of the preferred solutions for lithium battery chargers because of its simple structure,low cost,high-precision constant current(CC)and constant voltage(CV)output and other excellent characteristics.Due to the good market application prospect,a high-precision constant current and constant voltage control chip for primary-side regulation flyback converters is designed in this paper.In terms of CC,a high-precision demagnetization detection circuit and a high-precision closed-loop constant current control scheme are adopted in this paper.Compared with the common detection method which takes the first resonant zero crossing of the feedback voltage as the demagnetization ending point,the voltage comparator with input offset designed in this paper produces less lag when detecting the demagnetization ending point,which has higher demagnetization detection accuracy and lays a foundation for the realization of high-precision constant current.Due to the adoption of closed-loop constant current control scheme,the switching period of the system can accurately converge at twice the demagnetization time.Compared with the common open-loop scheme,the closed-loop scheme adopted in this paper can control the switching period more precisely,so it has higher output current accuracy.Aiming at the problem that the linear adjustment rate of the output current decreases due to the switching-off delay of the power switch,a built-in integrated line voltage compensation circuit is adopted in this paper.The voltage drop formed by the compensation current on the internal compensation resistor is adjusted according to the change of the line voltage,so as to reduce the change of the primary-side peak voltage under different input voltages,which improves the accuracy of the output current.In terms of CV,a constant voltage control scheme is proposed in this paper,which is different from the traditional one that requires an external compensation capacitor.The designed control loop takes the output pole of the converter as the main pole,the loop adopts proportional adjustment and does not introduce low-frequency zeros and poles,the DC gain of the loop is designed to be moderate to ensure that the steady-state error of the system is small enough while ensuring the loop stability without needing an external compensation capacitor.Aiming at the problem that the change of the cable voltage drop under different loads causes the change of the load voltage,a built-in integrated cable compensation circuit is put forward in this paper to improve the accuracy of the load voltage.A transmission gate with extremely low on-duty cycle is designed to realize a RC low-pass filter network with a large time constant,greatly compressing the bandwidth of the cable compensation loop,and ensuring the loop stability without the need of an external compensation capacitor.In this paper,the designed control chip has been taped out by the HHGRACE 0.35 um BCD process,and a 5V/2A prototype has been built to verify the performance of the control chip.The test results show that within the input range of 90Vac/60Hz～265Vac/50 Hz,the CC deviation of the system is ±1.4%,the CC linear adjustment rate is 1.25%,and the CC load adjustment rate is 1.5%;The CV deviation is ±1.08%,the CV linear adjustment rate is 1.12%,and the CV load adjustment rate is 1.22%.The maximum standby power consumption of the system is 102 m W,and the system efficiency is controlled within the range of 72.59%～80.24%.It can be seen that the control chip designed in this paper can realize the high-precision output of the primary-side regulation flyback converter,and its performance is relatively excellent,which satisfies the requirements of pratical application.