An electronic ballast for a fluorescent lamp has a single-switch flyback inverter including a magnetizing inductance, a resonant circuit connected to the output of the inverter including a tank inductor, and a clamp circuit including a diode coupled to the primary winding of the flyback transformer for limiting the voltage across the magnetizing inductance when the switch is non-conductive. With the inverter switch conductive the resonant circuit has a first resonant frequency. With the inverter switch non-conductive, the resonant circuit, combined with the magnetizing inductance, has a second resonant frequency lower than the first. The magnetizing inductance is chosen such that the second resonant frequency is close to the first resonant frequency. The operating frequency of the inverter is controlled to be at or about the second resonant frequency, whereby zero current switching is achieved in the inverter switch.