An Isolated Output-Feedback Scheme With Minimized Standby Power for SMPS

A feedback network for isolated switch-mode power supplies that automatically reduces the currents flowing through the optocoupler to nearly zero under the no-load condition is presented. This feedback network uses a proposed reverse-type shunt regulator to generate an error signal for optical coupling, and a modified PWM controller is adopted to receive the feedback signal. In comparison to the conventional topology, this proposed one exhibits much lower standby power loss. Besides, light-load efficiency can be improved as well. For implementing the proposed scheme, the PWM controller and the reverse-type shunt regulator are designed and fabricated in VIS 0.5- $\mu$ m 5-V/40-V high-voltage CMOS technology. Experiments reveal that a 12-V/18-W flyback converter prototype which adopts the proposed feedback technique will save a power of at least 27 mW under the no-load condition and improve the system efficiency by 2.2 $\%$ under the 10 $\%$ -load (1.8-W output) condition.