Philips CDR570 Service Manual page 49

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Faultfinding Guide
Operating description of MC44603
The input voltage Vcc (pin 1) is monitored by a comparator with
hysteresis, enabling the circuit at 14.5V and disabling the
circuit below 7.5V. The error amplifier compares a voltage Vfb
(pin 14) 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 7) 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 of PSU 20PS317
t1: Charging the capacitors at Vcc
C2129 will 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 capacitors at the Vcc
pin will discharge because the primary auxiliary voltage,
coming from winding 7-9 is below the Vcc voltage. At some
moment during t2, the primary auxiliary voltage 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 shorted, 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 capacitors starts again. If the
output is still shorted at the next t2 phase, the complete start-
and stop sequence will repeat. The power supply goes in a
hiccup mode.
14.5V
Vcc
10V
7.5V
p.a.v.
0V
17mA
Icc
1mA
OUTPUT
Vo
0
t1
Figure 8-22 'Start-up sequence'
short
t2 t3 t4
CL 96532076_035.eps
Regulation of PSU 20PS317
Figure B shows the most relevant signals during the regulation
phase of the power supply.
The oscillator voltage ramps up and down between V1 and V2.
The voltage at the current sense terminal is compared every
cycle with the output of the error amplifier Vcomp. The output
is switched off when the current sense level exceeds the level
at the output of the error amplifier.
TimeON phase : A drain current will flow from the positive
supply at pin 2 of the transformer through the transformer's
primary winding, the MOSFET and Rsense to ground. As the
positive voltage at pin 2 of the transformer is constant, the
current will increase linearly and create a ramp dependent on
the mains voltage and the inductance of the primary winding. A
certain amount of energy is stored in the transformer in the form
of a magnetic field. The polarity of the voltages at the
secondary windings is opposite to the primary winding so that
the diodes are non-conducting in this phase.
TimeDIODE phase : When the MOSFET is switched off,
energy is no longer supplied to the transformer. The inductance
of the tranformer now tries to maintain the current which has
been flowing through it at a constant level. The polarity of the
voltage from the transformer therefore reverses. This results in
a current flow through the transformer's secondary winding via
the now conducting diodes, electrolytic capacitors and the load.
This current is also ramp shaped but decreasing.
TimeDEAD phase : when the stored energy has been supplied
to the load, the current in the secondary windings stops flowing.
At this point, the drain voltage of the MOSFET will drop to the
voltage of C2121 with a ringing caused by the drain-source
capacitance with the primary inductance.
The oscillator will start a next cycle which consists of the above
described three phases. The time of the different phases
depends on the mains voltage and the load.
TimeDEAD is maximum with an input of 400VDC and a
minimum load. It will be zero with an input of 100VDC and an
overload.
V2
Vosc
V1
0
Vcomp
Vsense
Vgate
Vdrain
Idrain
Idiodes
Ton Tdiode Tdead
290799
CDR570/930 8.
CL 96532076_034.eps
Figure 8-23 'Regulation'
GB 49
290799