Biomedical Engineering Reference
In-Depth Information
In operation, the UC3906 ensures that power is available from the dc/dc converter and
that the batteries are in a good state. Pin 5 monitors the supply voltage and enables the chip
when at least 4.5 V is available. Pin 12 senses the battery terminal voltage. If the voltage
is 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 watchdogs
to 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 across
series resistor R33 and limits it to 0.25 V by controlling the charging current. Thus, the bulk
charging rate is determined solely by the value of this resistor. As such, the bulk charging
current using a 1-
resistor is 250 mA. This current corresponds to a C/4.8 charging rate. A
current 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 reference
voltage. The actual terminal voltage is prescaled appropriately through R35 and R37. The
resistor divider values were selected such that when the critical voltage is reached, the volt-
age at pin 13 equals 2.3 V. At this point, pin 10 is latched and R36 no longer participates
in the circuit. When the terminal voltage rises to a level that is just below
Ω
float, the voltage
regulator takes control away from the bulk-current regulator and goes into the overcharge
state. The current then tapers as the voltage continues to rise toward 2.4 V per cell, the
point at which the
fl
float state is started.
As the current tapers, the voltage across R33 drops. Another watchdog looks at this
voltage to determine when it goes below 0.025 V. When 0.025 V is sensed, a latch is tog-
gled and pin 10 ungrounded. Float conditions are established and the battery voltage drifts
back to 2.3 V per cell, which is maintained until the battery becomes discharged or the
power is switched off
fl
and back on. A MOSFET (Q17) switches power to the high-voltage
power supply from the
ff
24-V battery. This switch is turned on by Q18 upon receipt of the
appropriate command from the shock-box microcontroller.
High-Voltage Capacitor Charger
Implantable de
yback converters to charge the energy-storage
capacitor 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 an
OEM module designed speci
fi
brillators typically use
fl
cally for charging capacitor banks: the Ultravolt model IC24-
P30 programmable high-voltage power supply. The high-voltage charge section is shown in
Figure 8.38. This module utilizes a dual-ended forward topology with a nominal switching
frequency of
fi
100 kHz. A soft-start circuit brings the converter to full power over a 1-ms
period. A constant-frequency PWM regulation system controls a MOSFET push-pull power
stage and HV transformer. The power stage is protected from output current overloads via
a secondary current limit circuit. The current limit is optimized for low-impedance capaci-
tor charging. HV ac is recti
ed and multiplied internally. The HV developed by this multi-
plier generates feedback voltage which is sent to the control circuit to maintain regulation.
The ac feedback network is con
fi
fi
gured for no overshoot into capacitive loads.
The module has high e
ciency (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 power
output is not isolated from the input. The module produces an output that is proportional
to the level presented to its control input. A voltage of 0 to
5 V at the input results in 0
to
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.
c module
used in our prototype is encased in plastic, the same module is available in an RF-tight case
with a six-sided mu-metal shield. The module was originally selected by using the fol-
lowing formula, used to calculate the rise time required to charge an external capacitor
The module's dimensions are 3.7 in.
1.5 in.
0.77 in. Although the speci
fi
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