A power supply circuit includes a first comparator, a second comparator, a first voltage regulator, an output terminal, a first path and a second path. The first comparator compares a first input voltage with a first reference voltage to generate a first control signal. The second comparator compares a second input voltage with the first reference voltage to generate a second control signal. A voltage level of the second input voltage is different from a voltage level of the first input voltage. The first voltage regulator is selectively enabled based on the first control signal and the second control signal, and generates a first voltage based on the first input voltage. A voltage level of the first voltage is substantially the same as the voltage level of the second input voltage. The output terminal is configured to output one of the second input voltage and the first voltage as a power supply voltage. The first path directly provides the first input voltage to the first voltage regulator. The second path directly provides the second input voltage to the output terminal. The second input voltage bypasses the first voltage.