A method is disclosed for producing a corona discharge for igniting an air/fuel mixture in an internal combustion engine. An igniter is provided having a discharge tip that protrudes into a combustion zone. During a first stage of a combustion process, a first primary winding of a RF transformer is driven at a first predetermined voltage level and at a first resonant frequency that is based on a first impedance in the combustion zone prior to onset of combustion, for generating a corona discharge at the tip of the igniter. During a second stage subsequent to the first stage, a second primary winding of the RF transformer is driven at a second predetermined voltage level and at a second resonant frequency that is based on a second impedance in the combustion zone at a time that is subsequent to onset of the combustion process.