A hybrid powertrain system may include an electric machine, a battery back, a boost converter and at least one controller. The boost converter may include an inductor and be configured to receive input voltage from the battery back and provide an output voltage to the electric machine. The at least one controller may generate a duty cycle command for the boost converter based on a magnitude of input current to the boost converter and a magnitude of ripple associated with a current through the inductor such that for a given commanded voltage, an actual duty cycle for the boost converter changes depending on a direction of the current through the inductor to drive the output voltage to the given commanded voltage.