A power supply apparatus for supplying electric AC power to a capacitive load having a capacitive load impedance, the apparatus comprising - a transformer (110, 500) with a first coil (120, 510) and a second coil (130, 520), the first coil having first and second half-coils with a common centre tap, and - a positive half-period driver (160, 151, 170) and a negative half-period driver (160, 151, 180) arranged to alternatingly supply positive half-periods of voltage to the first half-coil and negative half-periods of voltage to the second half-coil, respectively, of the first coil (120, 510), - the second coil (130, 520) connectable to the capacitive load so as to form an electric resonance circuit having a resonance frequency, and to supply the electric AC power at the resonance frequency to the load, and - a device for determining zero crossings of the voltage supplied to the first coil (120, 510) and for causing alternation between positive and negative half-periods of voltage supplied to the first coil (120, 510) at the zero crossings of the voltage supplied to the first coil (120, 510), the device for determining zero crossings comprising a separate third coil on the transformer (110, 500), and each of the positive and negative half-period drivers (160, 151, 170, 180) being arranged to feed the respective half-periods of voltage through an inductive element to the centre tap of the first coil (120, 510), and - a device (151, 160) for controlling the duration (t) of the voltage fed to the centre tap to durations between zero and one quarter of a period corresponding to a predetermined highest resonance frequency of the resonance circuit and thereby control the AC power delivered to the load. - a transformer (110, 500) with a first coil (120, 510) and a second coil (130, 520), the first coil having first and second half-coils with a common centre tap, and - a positive half-period driver (160, 151, 170) and a negative half-period driver (160, 151, 180) arranged to alternatingly supply positive half-periods of voltage to the first half-coil and negative half-periods of voltage to the second half-coil, respectively, of the first coil (120, 510), - the second coil (130, 520) connectable to the capacitive load so as to form an electric resonance circuit having a resonance frequency, and to supply the electric AC power at the resonance frequency to the load, and - a device for determining zero crossings of the voltage supplied to the first coil (120, 510) and for causing alternation between positive and negative half-periods of voltage supplied to the first coil (120, 510) at the zero crossings of the voltage supplied to the first coil (120, 510), the device for determining zero crossings comprising a separate third coil on the transformer (110, 500), and each of the positive and negative half-period drivers (160, 151, 170, 180) being arranged to feed the respective half-periods of voltage through an inductive element to the centre tap of the first coil (120, 510), and - a device (151, 160) for controlling the duration (t) of the voltage fed to the centre tap to durations between zero and one quarter of a period corresponding to a predetermined highest resonance frequency of the resonance circuit and thereby control the AC power delivered to the load.