Physio Control LIFEPAK 9 - Page 25

Switching Power Supply
External Control Input
Energy Storage Capacitor
Charger
© August 1993 Physio—Control
Description 1
of USB to ground which turns off Q11, disabling the Power Control circuit.
Electrical isolation is provided by CR24 and R39.
A time delay function in the Logic Power Monitor circuit prevents the Power
Control circuit from shutting down while operating voltages are still rising
during initial power-on. Capacitor C26 accomplishes this for the high-limit
detector (USA) and C18 for the low-limit detector (USB).
The low- and high-limit detector inputs to the test connector are used to
check circuit performance by serving as test points to measure voltage.
Voltage measurements check the accuracy of voltage dividers R41, R42, R43,
and R56 (relative to the +5V supply).
The Switching Power Supply uses a pulse-width regulator (U3) and a
flyback-coupled inductor (T2) to produce the +5V to +15V supply voltages
used by the other circuits.
The pulse-width regulator U3 contains an oscillator, reference voltages, an
error amplifier, and drive circuitry for Q12. Oscillation frequency is set at
90Hz by R30 and C12. The +15V output feedback at U3 pin 2 determines
the outgoing pulse-width. The pulse output at U3 pin 6 provides a constant
frequency pulse by driving an N-channel FET (Q12) and dumping the
V BATT voltage into the primary of T2. Two diodes, CR28 and CR29,
protect Q12 from high voltages caused by leakage inductance from the
primary.
The Energy Storage Capacitor Charger circuit is externally controlled by the
N-channel MOSFET, Q9. Two resistors (R58 and R59) provide
forward-biasing for Q9 whenever power is applied to the instrument. The
System microprocessor on the Main PCB initiates a charge cycle for the
energy storage capacitor by setting the CHARGE INHIBIT line at low
voltage, turning O9 off. The inverting input of U2A rises to the required
+5V minimum.
The Energy Storage Capacitor Charger circuit uses a flyback transformer to
charge the energy storage capacitor through the Transfer Relay. The core of
the circuit consists of the following components: T1, U2A and U2B, 01, 02,
05-07, απά (17.
Amplitude comparators U2A and U2B determine the drive pulse for the
primary of Tl. The input to the energy storage capacitor at U2A pin 2 is
controlled by the External Control, Low Energy Charge Rate, and Capacitor
Charge Control circuits. If any of these incoming signals fall below the
+2.5V reference at U2A pin 3, the output switches high, inhibiting the drive
to the primary of T1.
All
Rights
Reserved
1-11