Mallinckrodt NELLCOR NPB-4000 Service Manual page 184

Section 15: Drawings
15.2.3 Input filter and Rectifier
15.2.4 Controller
15.2.5 Power Devices
15-4
volts DC to the flyback circuit. A control IC runs the FET switch, while the
transformer (used as an inductor) stores energy and transfers it to the load.
The input EMI filter consists of an external "canned" EMI filter assembly
(common-mode choke plus X capacitors) that is external to the NPB-4000/C
power supply PC board, plus an on- board filter consisting of an L1
common-mode choke with C1 and C2 as X capacitors and C7 and C8 as Y
capacitors. Resistor R1 bleeds off any residual voltage on C1 and C2 after the
connection to input AC is broken. Surge Limiter R16 limits the initial current
through the AC Front End on a cold start when C3 is completely discharged.
Fuses F1 and F2 offer safety breaks for both sides of the input line. Diode
bridge D1 rectifies the incoming AC to produce a DC voltage from 100 to 350
volts DC at C3.
The input controller (U1) is a two-loop (current and voltage) controller that
forms a fixed frequency variable duty cycle PWM flyback power supply by
operating the main switching FET Q1 to regulate the voltage across C12.
Controller U1 draws minimal current before operation starts. Initial controller
power builds up on C4 from the current supplied by R14 and R15. When U1
begins to operate (at about 10 volts DC), the primary tap (T1, pin 3) supplies
power through D4 for the controller. The primary tap also creates the voltage on
C12 which is regulated by the controller to 11 volts DC. The transformer
secondary is indirectly regulated by turns ratio and magnetic coupling to provide
about 18 volts DC.
Resistor R11 and capacitor C14 set the fixed operating frequency (about 100
kHz) of the converter. Current loop feedback enters the controller from current
shunt R6, which produces a peak signal of 1 volt with a 1.5 amp primary current.
Current limiting (short protection) occurs when the ISNS pin of the controller
reaches 1.1 volts. The current waveform is filtered by R8 and C15 to drive the
ISENSE input of the controller. Feed-forward resistor R13 provides a bias that
lowers the current limit setpoint as the primary input voltage rises . The
controller voltage error amplifier works with an internal 2.5 volt reference to
regulate the voltage at pin 2 at 2.5 volts. Attenuator R10 and R12 scale the
11 volts DC down to 2.5 volts DC for the error amplifier. Error amplifier
compensation is provided by C11, C17, and R9.
The switching FET is driven through R4 (to eliminate the possibility of bipolar
latch-up of the output stage). Leakage inductance energy from the transformer
primary is caught by D2 and C9, and dissipated in R2. Capacitor C16 and
resistor R7 help to damp spurious ringing and lower EMI radiation.
The transformer output is rectified and filtered by D5 and C18. Capacitor C10
and resistor R17 serve to damp output leakage inductance ringing.