The invention relates to methods of controlling operation of a resonant power converter and to controllers configured to operate according to such methods. Embodiments disclosed include a method of controlling a power output of a resonant power converter comprising first and second switches (S1, S2) connected in series between a pair of supply voltage lines, a resonant circuit connected to a node between the first and second switches and to an output connectable to an output electrical load, the resonant circuit comprising an inductor and a capacitor, the method comprising: closing the first switch (S1) to start a first conduction interval; setting a first voltage level (902); setting a first time period; and opening the first switch (S1) to end the first conduction interval when a voltage (901) across the capacitor crosses the first voltage level (902) and when a time period from closing the first switch exceeds the first time period (904).