Drivetrain integrated dc-dc converter utilizing the zero sequence current of the starter-generator in 48V network vehicles

Increasing power demand due to further electrification of internal combustion engine vehicles has taken the 12 V power supply to its limits. Therefore, some major automobile manufacturers including Audi, BMW, Daimler, Porsche and Volkswagen have agreed to implement a new two voltage-level network. A starter-generator, which may optionally assist acceleration and recuperate energy during braking, is fed from a 48 V battery. Components that do not require high power are fed from a 12 V battery. A bidirectional dc-dc converter is placed between these networks so that the 48 V generator can charge the 12 V system and a 12 V jump starter can start the internal combustion engine. This paper proposes a novel topology and a control algorithm which eliminate the dc-dc converter to save cost, weight and volume. The proposed topology is a conventional three phase inverter with an additional asymmetric leg, which connects the star point of the machine to the 12 V and 48 V batteries. Power transfer between these two networks is achieved by controlling the zero sequence current of the starter-generator. This machine can be an externally excited or permanent magnet synchronous machine. Using a multiphase machine is also possible. A conventional field oriented control is applied to the machine. An additional PI controller regulates the zero sequence current by changing the duty cycle of the asymmetric half bridge. The zero sequence current determines the amount of power flowing between the batteries. This power transfer is independent of the machine speed and torque; therefore, the integrated dc-dc converter can also operate when the machine is running. The paper provides the theoretical background of the control, the details of the proposed topology, and simulation results which prove its feasibility.