This is an excerpt from D. Ladner, D. Huckaby and S. Cravens, " The 42V Automotive Converter," ECE 4512: Design I, Department of Electrical and Computer Engineering, Mississippi State University, Spring 2003:

In order to get to the 42-volt system, there must first be a transition stage. One problem caused by increasing the operating voltage is the compatibility with the 12-volt accessories such as headlamps, dome lamps, radio, etc. Therefore, a two-bus system implemented with a 42V/12V scheme will create a gradual increase to the 42-volt system [2]. This will allow production companies of automotive components time to design and implement the 42-volt accessories into the system. In this type of two-bus network, the 42V/12V DC-DC converter will allow the 12-volt accessories the voltage bus needed to operate normally. will design a 42V/12V quasi-resonant DC-DC converter that uses current technology in order to create a low-loss, bi-directional converter that will maintain a minimum load of 100-watts and a zero current switching condition.