The integrated starter generator (ISG) is used for an idling-stop scooter, which can effectively reduce both air pollution and fuel consumption during idle periods. The ISG, that is made using permanent magnet synchronous motor (PMSM), should provide enough torque for engine cranking, which causes the output voltage of a three-phase rectifier from the ISG to be higher than the battery voltage (12V) in generating mode. Hence, the output voltage control of the ISG cannot be performed by a conventional boost operation. This paper proposes a constant voltage control method for the PMSM type ISG in generating mode to generate a 12V dc-link voltage over a wide speed range of the engine. First, a phasor diagram is used to explain the principle of how the controlling of phase voltage lagging behind its back-EMF can achieve power generation. Furthermore, the relationships among generating dc-link voltage, generating power, phase voltage lagging angle, and current angle are derived for developing the control methodology. Due to the small lagging phase angles of the ISG in generating mode from light to rated load, a dc-link voltage controller with a feedforward angle control for the light load is adopted to yield voltage control with or without connected 12V battery. In addition, a dc-link current feedforward module is proposed to enhance the voltage controller without the 12V battery. Consequently, the voltage variation under step load change is further reduced to nearly 0V on the test bench and on a 150cc scooter. Finally, a DSP-based ISG drive is implemented to verify the effectiveness of the proposed method on both the test bench and the 150cc scooter. INDEX TERMS Integrated starter generator (ISG), idling stop, six-step voltage operation, phasor analysis, PMSM, scooter.