Small-signal analysis and control design of isolated power supplies with optocoupler feedback

Optocouplers are widely used in isolated power supplies to transfer the feedback signal from the secondary to the primary side. In many power supplies, the feedback amplifier is supplied from the output voltage that creates additional feedback path which should be taken into account in the control design. The paper presents a detailed analysis and control design for each of the two possible feedback loops that can be identified. In addition, dynamic limitations of the TL431 shunt regulator and of the optocoupler are discussed. Practical guidelines for the error amplifier design and a design example are presented for both voltage-mode and current-mode controls.