A high voltage power supply for an electrostatic precipitator comprises a high voltage transformer, a high voltage bridge rectifier and a semiconductor switch controller configured to control an output power level of the high voltage power supply. A control unit (50) is configured to determine a firing angle of firing pulses for the semiconductor switch controller. During normal operation, a peak voltage value is measured and stored in each half period. When a breakdown is detected, a residual voltage (Ur) over the electrostatic precipitator is measured. A firing angle (α1) of a firing pulse to be provided to said semiconductor switch controller in the first half period after the breakdown is determined from the latest stored measured peak voltage value (Ubef) and the measured residual voltage (Ur) based on a predetermined relationship between the firing angle (α1) and said measured voltages.