Power Supply - Philips CDR779 Service Manual

8.2 Circuit description of the current mode

power supply

8.2.1 Blockdiagram
Lightning
Protection
MAINS
8.2.2 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.
8.2.3 Description of UC3842
The input voltage Vcc(pin 7) is monitored by a comparator
with hysteresis, enabling the circuit at 16V and disabling the
circuit below 10V. The error amplifier compares a voltage
Vfb(pin 2) related to the output voltage of the power supply,
with an internal 2.5V reference. The current sense
comparator compares the output of the error amplifier with
the switch current Isense(pin 3) of the power supply. The
output of the current sense comparator resets a latch, which
is set every cycle by the oscillator. The output stage is a
totem pole, capable of driving a MOSFET directly
Rectifier
EMI
FILTER
2643
Start Overvoltage
Circuit Protection
Comp
Vcc
v
2.5V t
CLOCK
Error
Amplifier PWM
comparator
Verror
+
S
-
- R Q
+
latch
Vfb
CONTROL
Figure 8-10
Faultfinding Guide
6210
5131
2
6230
5
7
6240
9
6250
7125
Output
6220
Isense
Vsense
Rsense
+12V +5V
Vcc
7131
7201
REGULATION
7
VI
6V
+
-
+
16V
5
GND
4
Rt/ct OSC
2.5V
ERROR AMP
+ 2R
2
Vfb -
1
COMP
3
I sense
Figure B : Blockdiagram UC3842
8.2.4 Start up sequence
t1: Charging the capacitor at Vcc
C2129 wiil be charged via R3123 and R3134, C2133 and
C2111 via R3129. The output is switched off
During t1.
t2: Charging of output capacitors
When the input voltage of the IC exceeds 14,5V, the circuit is
enabled and starts to produce output pulses. The current
consumption of the circuit increases to about 17mA,
depending on the external loads of the IC. At first, the
CDR779 8.
+5V
+12V
-8V
VFTD
VDC2
VDC1
CL 06532151_018.eps
271100
5V
S/R REF
2.5V BIAS
1
S
I SENSE COMP
-
R
R 1V +
LATCH
CL 06532151_019.eps
Figure 8-11
GB 59
0V
8
Vref
6
OUTPUT
271100