Bi-directional power converter for wide speed range integrated starter-generator

The paper outlines a case study on an integrated starter-generator-torque-booster (ISGtB) for automotive applications, representing an electric part of a hybrid propulsion system of a street scooter. Requirements for a high power density of the electrical machine led to a permanent magnet brushless DC machine (BLDC) with an external rotor. The complex functionality of the ISGtB, some deciding characteristics of modern two-stroke internal combustion (IC) engines and the nature of the selected electrical machine with linear dependence between back EMF and rotational speed dictate the properties and capabilities of the switch-mode power converter (SPC) that must support all operating modes of the ISGtB and bi-directional energy flow. The research work involved the extensive study on different suitable SPC topologies, including also control strategies, in the sense of starting torque, motor as well as generator operation mode of ISGtB. The selection of the SPC topology was made on the basis of theoretical statements, computer simulations and measurement results on several ISGtB prototypes. It consists of a bi-directional buck/boost converter and of a common known six-step inverter, as an executing stage. The topology of the bi-directional buck-boost converter is derived mainly with regard to the conversion ratio between the supply battery voltage level and DC link voltage level. The instantaneous value of the transferred electric power is tuned to the torque/speed characteristic of the IC engine and, in the opposite energy flow direction, to the requested output power in the generator operation mode. The deciding parameter when selecting the executing stage of the SPC was the torque/current ratio of the BLDC machine for different inverter topologies and phase winding connections. A particular attention was paid also to an efficient operation of the ISGtB on the boundaries of the operation range, such as providing a low impedance current path at the starting maneuver and extending the low-speed generator mode. The phenomenon of reverse conduction of MOSFET transistors was effectively employed, thus improving the overall SPC efficiency and extending the ISGtB operation range. Measurement results clearly reveal the functionality of ISGtB over the whole rotational speed range of the prototype set-up. The employment of reverse conduction of MOSFET transistors significantly influences the torque characteristic in the starter mode of operation. The proposed inverter control principle in the generator operation mode of the ISGtB evidently improve the transition from generator to torque-boosting operation mode.