A main power load signal and a main voltage reference signal correlated to a main power signal. A sub power load signal and a sub voltage reference signal correlated to a sub power signal. A reference voltage is provided and a boost voltage is determined according to a control signal. The main voltage reference signal and the sub voltage reference signal are compared, and when the first comparison result complies with a predetermined condition, a difference between the main voltage reference signal and the sub voltage reference signal is obtained to be further compared with the reference voltage. Then the control signal is determined. The main power load signal and the sub power load signal are compared, and then one of the sub voltage reference signal and the boost voltage is selected as a power source voltage of the sub power control circuit or the load sharing control circuit to generate an adjustment signal for determining current and voltage of the sub power signal.