In a capacitor charging apparatus, an on-time in which the primary circuit of a transformer is energized is continued until a primary current detection signal reaches a predetermined peak detection level, and an off-time in which the primary circuit is de-energized is clocked from the time when the primary current peak detection signal is received. At the end of the clocking, an ending signal is generated to terminate the off-time. The length of an off-time is inversely proportional to the charging voltage across the capacitor. The length of an off-time may be altered using a regulation resistor. Thus, an on-time and an associated off-time of the primary circuit of the transformer are controlled independently to optimize charging time and power efficiency of the charging.