Faultfinding Guide
GB 57CDR775 8.
Circuit description of PSU 20PS317
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 C2126. It prevents inflow of noise into the mains.
Primary rectifier/smoothing
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 up 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.
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
Figure 8-17 ‘TL 431’
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.
Demagnetisation
The auxiliary winding (7-9) voltage is used to detect magnetic
saturation of the transformer core and connected via R3101 to
pin 8 of IC7110. During the demagnetisation phase, the output
will be disabled.
Overvoltage protection circuit
This circuit consist of D6114, C2114, R3115 and 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 at the auxiliary
winding 7-9.
When an overvoltage longer than 2.0 (s is 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. IC7110 is switched off and thus the output
supply voltages.
A
2.5V
R
K
CL 96532076_036.eps
290799
To Next Page
Loading...
