the voltage sampling feedback circu

the voltage sampling feedback circuit is shown in figure 4.5, which is mainly composed of optocoupler PC817 and voltage regulator TL431. TL431 as a reference and feedback error amplifier, the output signal is sampled to produce the corresponding error voltage. the error voltage is converted from the optocoupler PC817 to the error current, coupled to the primary, and then converted to the voltage as the VFB port UC3843 the input to the control chip. here, the optocoupler C poles are directly connected to VCC,2 feet grounded through a 1 K, and the voltage is sampled to VFB feet of the UC3843 by 4.7 resistance.