DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

420 CARDIAC PACING AND DEFIBRILLATIONFigure 8.35 Block diagram of the 50-J defibrillator. Power is supplied by a medical-grade power supply. A current of 15 V from this supplyoperates a dc/dc converter to charge two 12-V gel-cell batteries. Battery power is supplied to the microcontroller constantly. The batteryis also used to power a high-voltage power supply which charges the energy-storage capacitor bank (165 µF) up to 50 J, as selected through aDAC. A defibrillation pulse is generated by commuting the capacitor bank onto the defibrillation load through an H-bridge switch matrix.Capacitor bank voltage and current can be monitored by way of isolation amplifiers. Parameter information and commands are entered via acontrol computer through isolated RS232 line. The instrument includes circuitry to measure load impedance without delivering shock currents.fast, float, and trickle-charge phases to charge the gel cells safely. Two Yuasa NP1.2-12batteries are connected in series to generate approximately 24 V for the high-voltage powersupply. Each battery has a nominal voltage of 12 V and a capacity of C 1.2 Ah.Lead–acid batteries with a gelled electrolyte are best kept charged and maintained by acharger at a float voltage of 2.25 to 2.3 V per cell. To obtain a full charge, the battery ischarged to about 2.4 V per cell. Therefore, the charger IC switches back to the float levelwhen necessary. Terminal voltages that exceed the float level place the battery in what isknown as the overcharge region. During fast charge, the charger limits the current to a safelevel known as the bulk rate and then tapers off to the overcharge region at about one-tenthof the bulk rate.