In some embodiments, the ranging circuit 130 compare the average or peak of the voltage at the output of the rectifier 40 with a threshold (V_th) to determine if the AC input voltage is within a first range, for example, the 100 VAC to 120 VAC range, or within a second range, for example, the 240 VAC to 277 VAC range, and produces an output signal that is low or high according to the input range. In some embodiments, when the input AC signal is within the 100 VAC to 120 VAC range, the ranging circuit 130 may output a low signal (e.g., 0 V), which deactivates (e.g., turns off or opens) the bypass switch SW and maintains the second current limiting resistor R_CL2 in the signal path from the bias windings 110 to the reference pin REF. This in turn leads to greater attenuation of the negative rectified voltage at the cathode of the diode D and a lower magnitude for the negative injection voltage ?V_DC. Within this input range, the negative injection voltage may be about ?0.25 V to about ?0.5 V.

Further, when the input AC signal is within the 240 VAC to 277 VAC range, the ranging circuit 130 may output a high signal (e.g., 12 V), which activates (e.g., turns on or closes) the bypass switch SW and bypasses the second current limiting resistor R_CL2. This in turn leads to lower attenuation of the negative rectified voltage at the cathode of the diode D and a higher magnitude for the negative injection voltage ?V_DC. Within this input range, the negative injection voltage may be about ?0.5 V to about ?1.0 V.

The ranging circuit 130, the bypass switch SW and the second current limiting resistor R_CL2 together ensure that negative injection voltage ?V_DC is appropriately adjusted according to the input range of the power supply system 30.