Miscellaneous Control; System Power Supply - Mallinckrodt NELLCOR NPB-4000 Service Manual

15.6 MISCELLANEOUS CONTROL

15.7 SYSTEM POWER SUPPLY

15.7.1 Voltage Doubler
15.7.2 Controller
to the basic buck-converter diagram to prevent back driving of the circuit from
the battery when not charging. Diode D20 acts as a lightweight "catch" diode for
the small inductance of the current transformer.
The voltage regulation attenuator R28/R25 loads the battery when the circuit is
not powered, so voltage sensing of the output voltage occurs using Q6 as a
switch to connect attenuator resistor R28 to the output voltage. When the
controller is running, it produces a reference voltage (+5 volts) from VREF.
This turns on Q3, which pulls 4 ma. from R24, saturating Q6.
A characteristic of buck regulators is an "oscillation" that occurs when the duty
cycle exceeds 50 percent because of the flattening of the effective output
inductor current slope. The classic method of solving this problem is called
"slope compensation" whereby an enhancing slope is summed with the inductor
current slope. Slope compensation is applied here by emitter follower Q4 which
injects a portion of the oscillator timing ramp into the current loop.
The system power supply is a flyback design like the AC mains supply, but it is
non-isolated, runs from a DC input of 5.2 to 7.5 volts, and produces multiple
output voltages.
The main switching FET (Q8) in the system supply requires +12 volts gate drive
for low on resistance (efficient operation), so the controller IC (U4) is powered
from the +12 volts DC output generated by the operating supply.
In order to get started, we have a voltage doubler to pump up the battery voltage
until supply operation has commenced. When the /PWRUP signal goes low, the
startup sequence begins. Transistor Q12 goes on, providing the battery voltage
to doubler U5. Doubler U5 is a dual-inverting power driver, and one section acts
as an oscillator with negative feedback provided through R44 and C41. The
output of the oscillator (/OUTA) is applied to the second inverting driver section
resulting in the /OUTB signal. Capacitor C68 provides positive feedback
hysteresis to ensure consistent high duty cycle oscillation . When /OUTB is low,
C39 is charged to the battery voltage through R42 and D19. When /OUTB is
high, the top of C39 is at twice the battery voltage and dumps its charge into C30
through D16. In a brief period, C30 will have built up enough voltage for
controller U4 to start, and thereafter U4 receives its power from the +12 volt
system output through D16.
The oscillator in U4 runs at about 40 kHz, set by C37 and R39. When the entire
system is running, an external 50 kHz sync pulse is applied to C45, which
creates a pulse on R47 to synchronize supply operation to the external signal.
Capacitor C34 charges slowly and provides soft start operation by slowly raising
the current limit of the controller by clamping the COMP pin through D18.
Section 15: Drawings
15-7