Philips CDR600/00/17 Service Manual page 49

Faultfinding Guide
Circuit description of the current mode power supply
Blockdiagram
Lightning
Protection
EMI
MAINS
FILTER
Comp
2.5V
Error
Amplifier
+
-
Vfb
Figure A: Blockdiagram power supply
Function description
MOSFET 7125 is used as a power switch controlled by the controller IC 7110. When the switch is closed, energy is
transferred from mains to the transformer. This energy is supplied to the load when the switch is opened. Through control
of the switched-on time, the energy transferred in each cycle is regulated so that the output voltages are independent of load
or input voltage variations. The controlling device UC3842 is an integrated pulse width modulator. A clock signal initiates
power pulses at a fixed frequency. The termination of each output pulse occurs when a feedback signal of the inductor
current reaches a threshold set by the error signal. In this way the error signal actually controls the peak inductor current on
cycle-by cycle basis
.
Rectifier
2643
Start Overvoltage
Circuit Protection
Vcc
v
t
CLOCK
PWM
comparator
Verror
S
- Output
R Q
+
latch
Isense
CONTROL
6210
5131
2
6230
5
7
6240
9
6250
7125
6220
Vsense
Rsense
+12V +5V
Vcc
7131
7201
REGULATION
CDR600 8.
+5V
+12V
-8V
VFTD
VDC2
VDC1
0V
GB 45