ZOLL PD 2000 - Page 74

FUNCTIONAL DESCRIPTION
To provide safety and to improve battery operating time, the stimulator
circuits are only powered when the main selector switch is in the Pace or
Defib position. A high level on the signal XPWR_RESET and XSAFE will
close Q100 providing power (+12) to the stimulator board.
The defibrillator capacitor is shunted for safety reasons with a resistor
and relay to internally dissipate any energy remaining. When charging is
initiated by the CPU, this relay is opened by providing a low level on
signal XSAFREL.
P/XD sets the defib mode by disabling the pace energy driver, enabling
the defib high voltage transformer driver (Q7) and setting the basic ramp
time for the PWM at U1 pin 7 to set the operating frequency for flyback
transformer T3.
The CPU can control the charging rate of the high voltage capacitor to
improve battery efficiency by changing XLORATE. This signal affects the
ramp time for the PWM by changing the capacitor to the ramp timing
circuit.
When the CPU has completed set up for initiating high voltage capacitor
charging, the CPU enable signal XPWRENB is set low. This signal will
initiate the PWM controller. The three signals XPWRENB, XPATREL, and
XSAFE are logically interlocked so that the proper combination must be
present for charging to occur.
Charging Operation
The high voltage capacitor is charged by converting the system battery
voltage to a pulsed high voltage by way of transformer T3. Control of this
function is provided by the PWM controller (U1). It provides a basic
operating frequency signal that is used to switch transistor Q7 providing
current in the primary windings of the transformer T3. Efficiency is
improved in charging by providing feedback of the current into the high
voltage capacitor on every cycle of Q7 switching. The PWM controller is
inhibited on each cycle until the high voltage capacitor current is nearly
zero. This control is provided by the U3 op amp circuit. Ul pin 3 is
inhibited with a high level when significant capacitor current is present.
When the high voltage capacitor is charging, the EPU independently
monitors the capacitor voltage through signal VMON. If an improper
level is detected, the EPU will halt operation by setting XSAFE low. This
stops the PWM controller and removes power (+12) from patient relay
coil, safety relay, and flyback drive.
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