An apparatus suitable for use in an implantable automatic defibrillation system for automatically generating a multiphasic defibrillation pulse waveform in response to sensed fibrillation has first and second series charge-storing capacitors having a common terminal and two other terminals each at different potentials. A controller senses cardiac fibrillation and generates a control signal which causes a charging circuit to charge the capacitors to selected voltage levels in sequentially alternating charge generation and charge coupling cycles. A voltage level detector senses the stored voltage level, disables the charging circuit when the sensed voltage reaches a predetermined level, and informs the controller that the capacitors are fully charged. The controller then communicates control signals indicative of pulse magnitude, duration, and polarity to a multiphasic pulse generator having a number of high-power switches and corresponding switch drivers interposed in circuit between the heart and the terminals of the charge-storing capacitors. The drivers control the conduction states of the switches according to the control signals to establish selected circuit paths between the three terminals and the heart, and to thereby deliver to the heart a multiphasic waveform having pulses with the selected parameters of magnitude, duration, and polarity.