A switch mode power supply for igniting and operating a high-pressure gas discharge lamp includes an ignition sub-circuit which is coupled via a transformer to the circuit. The ignition sub-circuit has at least an ignition capacitor, and a switching element for discharging the capacitor at a desired moment in time having a control electrode. The control electrode of the switching element is connected to the commutator of the lamp circuit via a control sub-circuit having substantially passive elements.