A power supply circuit has a first MOSFET having a body region between the source and drain. The body region is connected so as to be at the same potential as the source. Application of a suitable potential to the gate causes the MOSFET to switch to a conductive on state. The power supply circuit also has signal generation circuitry, which generates a signal indicative of a conductive state of the first MOSFET. The signal generation circuitry generates a reference voltage of a predetermined potential difference from the source potential. The power supply circuit further comprises a second MOSFET having a body region connected so as to be at the same potential as the drain of the first MOSFET, and the second gate is connected to receive the reference voltage. When the potential of the drain of the first MOSFET falls a predetermined voltage below the reference voltage the second MOSFET is switched to a conductive on state between the second source and the second drain, the signal being dependent upon the state of the second MOSFET.