Novel AC-DC converter control principle for automotive BLDC generator in low-speed range

The paper outlines a case study on an Integrated Starter-Generator-torque-Booster (ISGtB) for automotive applications, with a special focus on low speed generator operation mode. The discussed hybrid propulsion system consists of an internal combustion (IC) engine and a BLDC machine and is in its origin intended to drive the street scooter. The complex functionality of ISGtB, some deciding characteristics of modern two-stroke IC engines, stringent demands for the generated supply voltage and the nature of the selected electrical machine with linear dependence between the back EMF and rotational speed make the generator operation mode a point of interest. A sophisticated AC-DC converter and its control are introduced that enable different rectification possibilities, mainly due to the used power MOSFET switches and their characteristics. Two improvements of the low speed generator operation were studied, the synchronous rectification through activation of the MOSFET channels during the reversal of the drain voltage and the operation of the AC-DC converter as a BOOST converter with the use of BLDC phase windings. Measurement results reveal that the proposed AC-DC converter and its control, where each converter leg operates as autonomous BOOST converter with synchronously triggered upper transistors (T1, T3, T5) instead of the use of body diodes (D1, D3, D5), lead to an improved low speed generator characteristic and offer simple, yet effective energy flow control.