DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

SHOCK BOX PROTOTYPE 419Figure 8.34 This prototype instrument is a demonstrator of the internal workings of an implantabledefibrillator. This circuit is capable of delivering defibrillation energies of up to 50 J to the heart. Thisenergy level is suitable when at least one of the electrodes is in direct contact with the ventricularmuscle.Lead impedance is estimated prior to delivery of a shock or under command from thecontrol computer. Inappropriate lead impedances cause the defibrillation command to beaborted since delivering high energies into inappropriately low loads can be dangerous,because the developed currents pose fire or explosion risks. The different modules of thecircuit are presented in the following sections. The interconnection between these modulestakes place as depicted in Figure 8.36.Power Supply SectionA Condor medical-grade (low-leakage, redundantly insulated) power supply is used to generate15 V dc for the defibrillation module. The power line is applied to the power supplyinput terminals through a medical-grade connector/switch/fuse/filter/voltage-selector module.The power supply has low leakage and a redundant isolation on the power transformer,and its linear regulator is capable of producing 15 V at 0.4 A.As shown in Figure 8.37, a Power Convertibles HB04U15D15Q (C&D Power Technologies)isolated dc/dc converter powers the battery charger circuitry. The isolation barrierof this converter is rated at 3000 V (continuous) and tested at 8000 V with a maximum60-Hz leakage of 2 µA. This dc/dc isolation rating is the main power line leakage barrierfor the applied part of the circuit. The output of the dc/dc converter is filtered through aMurata BNX002 filter block and a 470-µF capacitor (C22). A TIP42 transistor and a 33-Vzener diode are configured as an “amplified zener diode” to clamp the dc/dc output of thedc/dc converter at approximately 34 V.A single-chip gel-cell charge controller is used to charge two sealed, leakproof gel cellsin series. Charging of the two 12-V batteries is controlled through a Unitrode (now ownedby Texas Instruments) UC3906 IC, which implements an optimal-charge algorithm with