A circuit includes a transformer with a primary, secondary, and auxiliary. A first control circuit actuates a first switch circuit based on a wake-up signal to cause the primary to transmit power to the secondary. A second control circuit actuates a second switch circuit based on an output voltage at the secondary being less than a threshold to generate the wake-up signal to the secondary for transmission to the auxiliary. The second switch circuit has a transistor with a source coupled to a reference node, a gate coupled to the second control circuit. A first diode has an anode coupled to the source and a cathode coupled to the drain, and blocks flow of current from the drain to the source. A second diode has an anode coupled to the substrate and a cathode coupled to the drain, and blocks flow of current from the drain to the substrate.