Welch Allyn Atlas Service Manual page 80

1.3 +12V Boost Converter
The +12V boost circuit provides regulated 12VDC. The input to the 12V converter is either battery or
12V from the AC/DC converter. The circuit is configured as a boost PWM using current mode feedback.
The PWM controller is a UC3843A. The controller includes an internal 2.5V, 2% reference, and an
external 5V, 2% reference. Nominal output voltage for the boost converter is Vout = [2.5V*(1 +
173.8K/4.64K)] = 12.1V.
The UC3843A requires 8.5V minimum to power on. The maximum voltage allowed on the switching
transistor, Q305, is 15V. To meet both these requirements, the UC3843A is powered from 12.9V (+18V –
5.1V zener diode D303)
When operating from battery, the converter will run at duty cycles over 50%, which requires slope
compensation for a current mode controller. Slope compensation is achieved by summing in part of the
oscillator signal (pin 4) with the current sense line.
The PWM controller is current limited on a cycle to cycle basis by monitoring the voltage on the Isense
line, U302-5. Current limit is activated when the voltage at the sense line reaches 1V. The nominal DC
voltage at Isense is about 0.5V when operating from battery only, and 0.7V when operating from AC.
Then, current limit is set to 5Amps when Atlas in operating on battery and 3Amps when operating on
AC.
+V
+18V
R308
17.8K
U302-fb
R309 R310
4.64K 10K
C305-1
C305
5600pF
U302-comp
5VRef
C307
.047uF R311 U302-ct
10K
C308
2200pF 3
1
AC+12V-fuse
Appendix A
Q303-s
C335
.047uF
+12V Boost
(12.1V nominal)
3 1 U302-pwr
D303
5.1V + C306 +
1uF
U302
UC3843A
3
VFB
1 R314
COMP
10
14 10
VREF OUT
7 5 U302-sense
RT/CT ISENSE
R312
3.83K
2 Q304-e
Q304 R313 C309
5600pF
MMBT3904L 1.96K
D312
BAV99L R317
7.5K
2 D312-c
L303
Bead
C310 L302
1000uF 22uH
D304
MBRS340T3
Q305-d D304-C
+ C311
1000uF
Q305
MTP30N06VL
1
R315
196
R316
0.1
10/14/99
L305
Bead
+12V
+ C312 + C313
1000uF
C336 330uF
.047uF