DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

SHOCK BOX PROTOTYPE 423In operation, the UC3906 ensures that power is available from the dc/dc converter andthat the batteries are in a good state. Pin 5 monitors the supply voltage and enables the chipwhen at least 4.5 V is available. Pin 12 senses the battery terminal voltage. If the voltageis too low (indicative of a dead battery or reverse polarity), the charger is disabled.Upon detection of charging power and a good battery, the UC3906 puts two watchdogsto work: One regulates the charge current and the other looks at the battery terminal voltage.The current regulator (which uses Q19 as the power transistor) senses the voltage acrossseries resistor R33 and limits it to 0.25 V by controlling the charging current. Thus, the bulkcharging rate is determined solely by the value of this resistor. As such, the bulk chargingcurrent using a 1-Ω resistor is 250 mA. This current corresponds to a C/4.8 charging rate. Acurrent of 250 mA is also well within the 300-mA output range of the dc/dc converter.Battery terminal voltage sensed at pin 13 is compared to the IC’s internal referencevoltage. The actual terminal voltage is prescaled appropriately through R35 and R37. Theresistor divider values were selected such that when the critical voltage is reached, the voltageat pin 13 equals 2.3 V. At this point, pin 10 is latched and R36 no longer participatesin the circuit. When the terminal voltage rises to a level that is just below float, the voltageregulator takes control away from the bulk-current regulator and goes into the overchargestate. The current then tapers as the voltage continues to rise toward 2.4 V per cell, thepoint at which the float state is started.As the current tapers, the voltage across R33 drops. Another watchdog looks at thisvoltage to determine when it goes below 0.025 V. When 0.025 V is sensed, a latch is toggledand pin 10 ungrounded. Float conditions are established and the battery voltage driftsback to 2.3 V per cell, which is maintained until the battery becomes discharged or thepower is switched off and back on. A MOSFET (Q17) switches power to the high-voltagepower supply from the 24-V battery. This switch is turned on by Q18 upon receipt of theappropriate command from the shock-box microcontroller.High-Voltage Capacitor ChargerImplantable defibrillators typically use flyback converters to charge the energy-storagecapacitor bank. Crude feedback loops are used in these devices to control the charge level.Instead of designing a custom high-voltage converter, the shock-box prototype uses anOEM module designed specifically for charging capacitor banks: the Ultravolt model IC24-P30 programmable high-voltage power supply. The high-voltage charge section is shown inFigure 8.38. This module utilizes a dual-ended forward topology with a nominal switchingfrequency of 100 kHz. A soft-start circuit brings the converter to full power over a 1-msperiod. A constant-frequency PWM regulation system controls a MOSFET push-pull powerstage and HV transformer. The power stage is protected from output current overloads viaa secondary current limit circuit. The current limit is optimized for low-impedance capacitorcharging. HV ac is rectified and multiplied internally. The HV developed by this multipliergenerates feedback voltage which is sent to the control circuit to maintain regulation.The ac feedback network is configured for no overshoot into capacitive loads.The module has high efficiency (up to 92%) and requires 23 to 30 V dc to operate.The module will remain operational (derated performance) down to 9 V. The HV poweroutput is not isolated from the input. The module produces an output that is proportionalto the level presented to its control input. A voltage of 0 to 5 V at the input results in 0to 1 kV (at 30 W) at the output. The module also has a TTL-controlled enable function.When disabled, the module remains on standby mode at 30 mA.The module’s dimensions are 3.7 in. 1.5 in. 0.77 in. Although the specific moduleused in our prototype is encased in plastic, the same module is available in an RF-tight casewith a six-sided mu-metal shield. The module was originally selected by using the followingformula, used to calculate the rise time required to charge an external capacitor