Philips CDR600/00/17 Service Manual page 53

Faultfinding Guide
Circuit description
Input circuit
The input circuit consists of a lightning protection circuit and
an EMI filter.
The lightning protection comprises R3120, gasarrestor
1125 and R3124.
The EMI filter is formed by C2120, L5120, C2125 and R3124.
It prevents inflow of noise into/from the mains.
Primary rectifier/smoothing circuit
The AC input is rectified by rectifier bridge 6102 and smoothed
into C2121. The voltage over C2121 is approximately 300V. It
can vary from 100V to 390V.
Start circuit and Vcc supply
This circuit is formed by R3123, R3134, C2129, D6129,
R3129, R3111, C2133 and C2111.
When the power plug is connected to the mains voltage, the
stabilised voltage over D6129(24V) will charge C2133 via
R3129. When the voltage reaches 14,5V across C2111, the
control circuit of IC7110 is turned on and the regulation starts.
During regulation, Vcc of IC7110 will be supplied by the
rectified voltage from winding 7-9 via L5132, D6132 and
C2133.
Control circuit
The control circuit exists of IC7110, C2102, C2104, C2107,
C2109, C2110, R3102, R3103, R3104, R3107, R3108, R3109
and R3110. C2102 and R3110 define the frequency of the
oscillator.
Power switch circuit
This circuit comprises MOSFET 7125, Rsense 3126, 3127 and
3128, R3125, C2127, L5125, R3112 and R3113. R3125 is a
pull-down resistor to remove static charges from the gate of the
MOSFET.
CDR600
Regulation circuit
The regulation circuit comprises opto-coupler 7200 which
isolates the error signal from the control IC on the primary side
and a reference component 7201.
The TL431(7201) can be represented by two components:
•
a very stable and accurate reference diode
•
a high gain amplifier
R
2.5V
TL431 will conduct from cathode to anode when the reference
is higher than the internal reference voltage of about 2.5V. If
the reference voltage is lower, the cathode current is almost
zero.
The cathode current flows through the LED of the opto-
coupler. The collector current of the opto-coupler flows
through R3106, producing an error voltage, connected to
voltage feedback pin 14 of IC7110.
Overvoltage protection circuit
This circuit consist of D6114, C2114, R3115and R3116.
When the regulation circuit is interrupted due to an error in the
control loop, the regulated output voltage will increase
(overvoltage). This overvoltage is sensed on the primary
winding 7-9. When an RYHUYROWDJH ORQJHU WKDQ  V LV
detected, the output is disabled until Vcc is removed and then
re-applied. The power supply will come in a hiccup mode as
long as the error in the control loop is present.
Secondary rectifier/smoothing circuit
There are 5 rectifier/smoothing circuits on the secondary side.
Each voltage depends on the number of windings of the
transformer.
The –8V supply is regulated by voltage regulator 7249.
On/off circuit
In off mode pin 1 and pin 2 of connector 0206 are connected.
The high voltage (-8V, +12V) over opto coupler 7200 forces
this one to conduct . IC 7110 is switched off
8. GB 49
K
A