GB 46
8.
CDR600
Faultfinding Guide
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
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 capacitor at the Vcc pin will discharge because
the primairy auxiliary voltage, coming from winding7-9
is below the Vcc voltage. At some moment during t2, the
primary auxiliary voltages reaches the same level as Vcc.
This primary auxiliary voltage now determines the Vcc
voltage
t3: regulation
The output voltage of the power supply is in regulation
t4: overload
When the output is shortened, the supply voltage of the
circuit will decrease and after some time drop below the
lower threshold voltage. At that moment, the output will
be disabled and the process of charging the Vcc capacitor
starts again. If the output is still shorted at the next t2
phase, the complete start-and stop sequence will repeat.
The power supply comes in a hiccup mode
6
8
2.5V
ERROR AMP
I SENSE COMP
S
R
5
16V
4
OSC
Rt/ct
GND
VI
2
Vfb
COMP
OUTPUT
Vref
1
LATCH
I sense
1
3
2R
R
1V
-
+
+
-
-
+
6V
+
BIAS
S/R
5V
2.5V
REF
Figure B : Blockdiagram UC3842
Figure C: Start-up sequence
1mA
t1
Vo
0
OUTPUT
t2
t3
t4
short
Vcc
Vc2616
Icc
20mA
0V
10V
16V
12V
