A boost converter has a main energy storage inductor and a main GaN boost switch with a node between them. A pre-charging switch is connected at the output side of the main boost switch and the inductor, between the node and an output capacitor. The pre-charging switch comprises a body diode in the forward direction from the output capacitor to the node. The output capacitor is charged by the pre-charging switch in a pre-charge phase and this provides a soft start-up to avoid large inrush currents. A third switch, which is also a GaN switch, is in series with the pre-charging switch to provide synchronous output rectification. The third switch has a body diode in a forward direction from the node to the output capacitor. In the pre-charge phase, both the boost switch and third switch are turned off whereas in a subsequent boost phase, the pre-charging switch is turned constantly on, and alternately the boost switch and the third switch are turned on and off to implement the boost conversion.