Philips CDR775 - Faultfinding Guide (Continued); Power Supply Unit Description and Tests

Faultfinding Guide
GB 53CDR775 8.
8.2 Power Supply Unit 20PS317
8.2.1 Description of PSU 20PS317
MOSFET 7125 is used as a power switch controlled by the
controller IC7110. 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 switch-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
MC44603 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.
Figure 8-10 ‘Blockdiagram PSU 20PS317’
Description of controller MC44603
The MC44603 is an enhanced high performance controller that
is specifically designed for off-line and DC-to-DC converter
applications. This device has the unique ability of automatically
changing operating modes if the converter output is
overloaded, unloaded or shorted. The MC44603 has several
distinguishing features when compared to conventional SMPS
controllers. These features consist of a foldback facility for
overload protection, a standby mode when the converter output
is slightly loaded, a demagnetization detection for reduced
switching stresses on transistor and diodes, and a high current
totem pole output ideally suited for driving a power MOSFET. It
can also be used for driving a bipolar transistor in low power
converters. It is optimised to operate in discontinuous mode but
can also operate in continuous mode. Its advanced design
allows use in current mode or voltage mode control
applications.
Pin connections
Figure 8-11
2220
Feedback
Voltage
Rsense
7201
6240
6230/6232
6210/6211
5131
7125
2121
REGULATION
EMI
FILTER
MAINS
+12V
RQ
S
OSC.
+
+
-
-
2.5V
Error
Amplifier
latch
Lightning
Protection
Rectifier
2
5
7
9
Soft-Start
Control
Overvoltage
Mgmt
Mgmt
Demagnet.
Output
Sense
Current
Sense
Input
Buffer
Supply Init
Out
Gnd
7110
6129
6132
Vc
-8V
VDC2
2210
+5V
VFTD
GND
GND
7249
2230
2240
6250
7221
17
15
11
13
10
14
18
16
VDC1
6102
7200
6220
2250
7251
7252
6201
CL 96532076_029.eps
290799
Rref
R Frequency Standby
Voltage feedback Input
Error Amp Output
R Power Standby
Soft-Start/Dmax/
Voltage Mode
Sync Input
Overvoltage Protection (OVP)
Current Sense Input
Demag. Detection
Foldback Input
107
CT
8 9
6
5
11
12
Output
Gnd
VC
VCC
3
4
2
14
13
15
1 16
CL 96532076_030.eps
290799