Mains (Ac) Led Operation - Mallinckrodt NELLCOR NPB-4000 Service Manual

Section 15: Drawings
15.7.3 Power Devices
15.7.4 Control Circuits

15.8 MAINS (AC) LED OPERATION

15.8.1 External (DC) LED Operation
15-8
Controller U4 is a two-loop controller, current and voltage. Input current
information is supplied by transformer T4 which with its 100:1 turns ratio
produces 1 volt across R31 at 15.4 amps of peak switch current. Current limit is
established by U4 at 1.1 volts or 17 amps peak. This peak current equates to
about 5 amps maximum average input current.
The voltage loop of U4 regulates the +3.3 volt output through attenuator R41 and
R40 which provides 2.5 volts at U4, pin 2. The internal U4 reference and error
amplifier regulates this voltage, while C35 and gain limiter R38 provide loop
compensation.
Controller U4 drives the switching FET Q8 through R37 (to prevent bipolar latch
up of U4). The switching FET builds current in the system transformer,
monitored by current transformer T4. When the main FET shuts off, C32 and
R36 provide rise-time limitation and leakage inductance snubbing. Leakage
current energy is caught by D17 and transferred to C25/C33. Transistor Q7 acts
as a switch that turns on when the supply is running to shunt the leakage energy
(through R34) to the +12 volts DC supply output. This output (+12 volts)
always has a load (from the NPB-4000/C isolated Front End).
The transformer secondary has four rectified outputs to produce +12, +5, +3.3
and -24 volts DC. The -24-volt winding has snubbing (R35, C31) to damp
secondary leakage inductance ringing.
The system supply is turned ON and OFF by the PWRUP/ signal that controls
Q11 and Q12. The supply is started when Q12 turns on, powering the voltage
doubler to start the controller. When the supply turns OFF, the voltage doubler
is stopped, but by that time, the supply is powered from the +12 output.
Transistor Q11 now comes into play, dropping the output of the error amplifier
(through the COMP pin of U4) to stop system supply operation.
The NPB-4000/C monitor has a front panel LED indicator that shows when AC
is connected to the unit and charging is underway. The DC input to the charger
powers comparator U14. When there is no external DC input (DCIN is low), the
comparator output is low. The low output turns on Q29, which powers the
MAINS LED indicator from the +5 reference (CVREF) of the charger IC.
The NPB-4000/C monitor has a separate front panel LED indicator that shows
when external DC is connected to the unit and charging is underway from this
source. When external DC is the source for the charger input voltage, the
section of D5 that is connected to the external DC is forward biased.
Comparator U14 senses that condition and opens its open collector output.
Resistor R98 pulls the output up to the +5 volt reference at CVREF and drives
the MAINS LED through emitter follower Q23.