This disclosure relates to power converters that are capable of providing smooth transitions between multiple output voltage levels. The converter's output may need to be changed from, e.g., 5V to 12V, 15V, or 20V—based on the charging device's request. By using improved power converter designs comprising both a flyback converter circuit and variable-frequency buck converter circuit that may each be selectively coupled to an output load, a more smooth, e.g., monotonous, transition between output voltage levels may be achieved. In particular, by varying the switching frequency of the buck converter in a controlled way, the output voltage of the power converter may rise monotonically during the transition between output voltage levels. According to some embodiments, once the output of the buck converter has reached its maximum value, the buck converter may be disabled, and the flyback converter may be enabled to begin supplying the output voltage to the load.