LifePak Defibrillator Energy Charge Rate

Once a charge cycle initiates at U2A pin 2, the active low output simultaneously performs two separate functions: transformer driver Q5 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 T1 consists of two stages: Q5 and Q6 function as push-pull drivers, and Q1 and Q2 provide improved current handling for the drive pulse.

The secondary winding of Ti 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 C l 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 30J 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 U 15D from U20. The U 15D 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 C 17. 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 BATT voltage at the inverting input and comparing it with the +2.5V at the noninverting input. When the threshold limit set on the noninverting input of U 17A exceeds the BAIT voltage on the inverting input, it indicates a failing battery. The output of U 17A then functions as described in the previous paragraph.

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