As the figure below shows, when the MOSFET is ON, current mainly flows through the coil and the MOSFET. Energy is accumulated in the coil during this time. The coil emits accumulated energy and suppresses changes of current. Current does not flow at the moment when the MOSFET turns OFF, but owing to the characteristic of this coil, current flows through a diode, and the capacitor is charged. Because of this phenomenon, output voltage exceeds input voltage.