Flyback converter and slope compensation circuit based on UC3842

The UC3842 has high performance fixed frequency and current mode PWM controller.They are specifically designed in switching power supplies curcuits,with low cost by using minimal external components.But flyback converters run in continuous current mode with a duty cycle greater than 50%,then the system will disperse if disturbed or exhibit subharmonic oscillations when in the two-closed-loop regulation,forming inevitable risks in this kind of power supply.The paper analyses the causes of instability in the system control,develops a slope compensation circuit to enable the disturbance to converge,and the power can be in stable condition with less than 0.1% voltage regulation,or lower than 3% load-adjustment.