a controller 456 the overvoltage protection circuit 452 may compare the voltage level of the dc power v bus with a predetermined level v ref to generate a comparison result dr the controller 456 is coupled to the overvoltage protection circuit 452 and is arranged for generating the protection signal s p according to the comparison result dr by way of example but not limitation a detection circuit 455 may compare a voltage v d generated by a voltage divider circuit 453 with the predetermined level v ref to generate a detection result and an optical coupler circuit 454 may generate the comparison result dr according to the detection result when the voltage v d is too high i e the voltage level of the dc power v bus is too high the optical coupler circuit 454 may couple a voltage v m to ground i e a resistor r 3 is grounded hence the controller 456 may generate the protection signal s p according to the comparison result dr the voltage v m thereby instructing the primary side control circuit 330 shown in fig3 to turn off the dc dc converter 110 in the embodiment shown in fig4 the voltage divider circuit 453 maybe implemented by a resistor r 1 and a resistor r 2 the detection circuit 455 may be implemented by a comparator cp and a switch sw and the optical coupler circuit 454 may be implemented by a photodiode d 1 and a transistor.