Battery Charger; Buck Converter Operation - Mallinckrodt NELLCOR NPB-4000 Service Manual

15.3 BATTERY CHARGER

15.3.1 Overview

15.4 BUCK CONVERTER OPERATION

15.4.1 Controller
The NPB-4000/C power supply contains a battery charger circuit that accepts
DC input from either the isolated AC mains flyback circuit or an externally
supplied DC input (10 to 16 volts DC). The output of the battery charger
provides a current limited, voltage regulated, temperature compensated output to
charge a 6 volt, 8 AH lead-acid battery.
Figure 15-5: Buck Converter Circuit
See Figure 15-5. The battery charger is a buck-switching converter design. In a
buck converter (which provides a reduced, or bucked, output voltage) a series
switch applies the input voltage to the output through an inductor. When the
inductor current has built to a level sufficient to satisfy the load, the switch is
opened. The current flowing in the inductor causes the input lead to fly down
until it is caught by the catch diode at a voltage slightly below ground. The
inductor current then diminishes until the switching cycle is repeated.
The battery charger receives input DC either from the external DC supply (10 to
14 volts) or the AC main supply which provides 18 volts DC. Dual diode D5
essentially acts as an OR gate, allowing the highest voltage to charge C18. Thus
the complete input DC range on C18 is from about 10 to 18 volts DC.
Controller U2 receives its power directly from the input source. Controller U2
will begin operation when the available input voltage is above 9 volts DC. The
frequency of PWM operation (about 100 kHz) is set by R26 and C22. Capacitor
C69 charges slowly and provides soft start operation by slowly raising the
current limit of the controller by clamping the COMP pin through D35.
Section 15: Drawings
15-5