This paper presents the design of a digital controller for a 600 W Boundary-Conduction-Mode (BCM) interleaved boost converter used in an ac-dc Power-Factor-Correction (PFC) power supply. The output voltage of a typical boost PFC rectifier contains a 2nd harmonic ripple. Therefore to achieve good power factor, the output voltage compensator must have a low bandwidth, in order to sufficiently attenuate this 2nd harmonic component. As a result, the output voltage can suffer from poor transient response, especially at low input voltage. In this paper, a small-signal model of the output voltage dynamics is derived, where it is shown the open-loop gain of the system decreases with low input voltage. As a result, the output voltage transient response also deteriorates at low levels of input voltage. This can lead to poor voltage transient response for boost PFC converters that are designed to have an operating input line voltage range from 85 to 265 V. Having poor output voltage transient response is undesirable as it can lead to high peak over shoot on the output capacitors, or even worse, lead to an output voltage tracking failure. To correct this behaviour, an adaptive digital voltage compensator is designed to change gain at low input voltage, to ensure the output voltage maintains good transient response. Finally, the derived small-signal analysis, and output voltage compensator performance are experimentally verified on a 600 W interleaved BCM converter.