The present invention provides a high efficiency power circuit. It includes an input transistor having a negative-threshold coupled to a voltage source for providing a supply voltage to the output terminal of the power circuit. An input detection circuit is coupled to the voltage source to generate a control signal when the voltage level of the voltage source is higher than a threshold voltage. A second transistor is coupled to the input detection circuit to turn off the input transistor in response to the control signal. An output detection circuit is connected to the supply voltage to generate a first enable signal when the voltage level of the supply voltage is higher than an output-over-voltage threshold. The first enable signal is used to switch off the input transistor. The output detection circuit generates a second enable signal when the voltage level of the supply voltage is lower than an output-under-voltage threshold. The second enable signal is used to turn off the output of the power circuit.