GB 45
8.
CDR600
Faultfinding Guide
Circuit description of the current mode power supply
Blockdiagram
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
.
7201
7131
6220
6250
6240
6230
6210
5131
7125
2643
Start
Protection
Overvoltage
REGULATION
EMI
FILTER
MAINS
+5V
+5V
+12V
+12V
-8V
VFTD
VDC2
VDC1
0V
R
Q
S
CLOCK
+
+
-
-
Vsense
Rsense
v
t
2.5V
Error
Amplifier
Verror
PWM
comparator
latch
Vcc
Vcc
Output
Isense
Comp
Vfb
Lightning
Protection
Rectifier
Circuit
CONTROL
2
5
7
9