On-board charger housekeeping power supply with automatic source transition

A power supply system for providing uninterrupted low voltage electrical power to control circuitry in an electric or hybrid-electric vehicle, the system comprising:

• a transformer configured to receive at least one of a low voltage input from a vehicle battery and a rectified AC high voltage input and convert one of the low voltage input and the rectified AC high voltage input to a low voltage output for use in powering the control circuitry, the transformer including a first primary side configured to receive the low voltage input from the vehicle battery and a second primary side configured to receive the rectified AC high voltage input;
• a first controller coupled to the low voltage input for operating a first switching device associated with the first primary side of the transformer, the first controller being operational to drive the first switching device to regulate the low voltage output using energy from the low voltage input when the rectified AC high voltage input is absent;
• a second controller connectable to the rectified AC high voltage input for operating a second switching device associated with the second primary side of the transformer, when the rectified AC high voltage input is present, the second controller being operational to (a) drive the second switching device to regulate the low voltage output using energy from the rectified AC high voltage input and (b) deactivate the first controller using a switching signal;
• and
• an opto-coupler in communication with the second controller for receiving the switching signal and transmitting the switching signal from the second primary side across an isolation boundary to the first controller on the first primary side.