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 accounted in the control design. The paper compares two possible loop gains corresponding to breaking the feedback loop at different locations. These loop gains are analyzed and are shown to have an unequal value for the design. Dynamic limitations of the TL431 shunt regulator and the optocoupler are discussed. Practical guidelines for the error amplifier design and a design example are presented.