Overvoltage protection for SMPS based on demagnetization signal

摘要

An overvoltage protection circuit is provided for a switched-mode power supply of the type having a transformer with a primary winding, an auxiliary winding, and a secondary winding, with a switching transistor coupled in series with the primary winding. The overvoltage protection circuit includes a sensing resistor having a first terminal directly connected to the auxiliary winding for generating a current proportional to the voltage across the auxiliary winding, with a second terminal of this resistor being coupled to an input of a current mirror. The reflected current output of the current mirror is compared to an overvoltage reference current generated by an overvoltage reference current source by means of a current comparator, and a circuit is provided to generate a signal to turn off the switching transistor when the reflected current exceeds the overvoltage reference current. Such an overvoltage protection circuit provides fast, efficient and accurate overvoltage protection for the switched-mode power supply.