The flyback converter is the isolated version of the buck-boost converter. The circuit that has become very widespread is where the TL431 is used in conjunction with an optocoupler to provide feedback loop isolation. Although the TL431 is advertised as a transconductance amplifier, it can be used as a standard Type II error amplifier if it is connected properly. The TL431 and an optocoupler play an important role for stable feedback and control loop, and this application report describes the design considerations to make the entire loop stable for achieving proper operations on flyback converter.