A unipolar PWM strategy for hybrid cascaded multilevel converters

The hybrid cascaded multilevel converters, compared with the traditional cascaded converters, can generate more levels with the same number of power cells. They have obvious advantages of reducing the size and weight of the system, increasing power density, and reducing cost. However, with the general hybrid modulation, the reverse current of the low-voltage cell will make the capacitor voltage boost. In order to avoid the problem, a controlled rectifier is employed in the dc-bus of the low-voltage cell, increasing the complexity of the equipment and limiting its applications. In order to use the diode rectifier instead of the controlled rectifier, the unipolar carrier disposition pulse width modulation (UCD-PWM) strategy is presented in this paper. The UCD-PWM strategy enables the output voltages of both cells to have the same polarity in the whole period. Therefore, the dc-bus capacitor of the low-voltage cell does not have reverse current and the diode rectifier is also available. Computer simulations accompanied with experimental verification are presented in this paper.