Physio Control LIFEPAK 9 - Page 26

LIFEPAK 9 defibrillator/monitor
Once a charge cycle initiates at U2A pin 2, the active low output
simultaneously performs two.separate functions: transformer driver QS
conducts, and Q7 shuts off. Timing for the transformer drive pulse-width is
provided by Q7, U2B, C17, and R23. While Q7 conducts, the voltage across
C17 is zero. When Q7 shuts off, the voltage at SWBATT starts charging
C17. When the voltage at C17 exceeds +2.5V, the output of U2B switches
low, forcing U2A output high and preventing current flow to the primary of
T1.
The power circuit to the primary of TI consists of two stages: Q5 and Q6
function as push-pull drivers, and Ol and 02 provide improved current
handling for the drive pulse.
The secondary winding of T1 supplies the energy stored on the energy
storage Capacitor for defibrillation. The output is therefore subject to
potentials of up to +4.5kV under normal operating conditions. Capacitor C1
limits the single pulse voltage rise to about +1.5kV, providing
circuit-response time for control of the output voltage.
Low Energy Charge Rate This circuit is centered around op amp U17A and is active only during low
battery voltage levels, or when an energy level below 30] is selected. When
a low battery voltage is detected, the Low Energy Charge Rate circuit slows
the charge rate for the energy storage capacitor, thus reducing the amount of
power required for charging. Similarly, the charge rate is slowed when a low
charge energy setting is selected, thereby making sure the energy storage
Capacitor will not charge too quickly for the Analog Voltage Evaluation
circuit to detect the energy level.
A signal from the System microprocessor at the energy request register, U20,
determines the output of U17B. When an energy level of less than 30J is
selected, low signals are applied to the inputs of U1SD from U20. The ULSD
output then switches low, forcing CR45 to reverse-bias. This pulls the
noninverting input of U17A to +5V, changing the output of U17A.
The signal from U17A controls the charge timing circuit (U2B, C17, and
R23) and, in turn, the output of U2A. During normal charging, the effect of
this signal is blocked by CR46. When U17A pin 1 is driven high, additional
Current is supplied to C17. This reduces the on-time of U2A which reduces
current to the primary circuit and slows the rate of charge of the energy
Storage Capacitor. This reduces the power consumption of the Energy
Storage Capacitor Charger to less than half of normal. At 360J, the charge
time is increased to 30 seconds.
The instrument can operate with low battery voltage if the power required for
charging the energy storage capacitor is reduced. Op amp U17A detects low
battery levels by monitoring the SWBATT voltage at the inverting input and
1-12 All Rights Reserved O August 1993 Physio-Cantrol