Philips CDR600/00/17 Service Manual page 51

Faultfinding Guide
Regulation
Figure 4 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.
1. Time phase : A drain current will flow from the
ON
positive supply at pin 1 through the transformer's
primary winding, the MOSFET and Rsense to
ground. As the positive voltage at pin 1 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 secundary windings is such that the
diodes are non-conducting.
2. Time phase : When the MOSFET is switched
DIODE
off, energy is no longer supplied to the tranformer.
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 becomes reversed. This
results in a current flow through the tranformer's
secondary winding via the diodes, electrolytic
capacitors and the load. This current is also ramp
shaped but decreasing.
3. Time phase : when the stored energy has been
DEAD
supplied to the load, the voltage from the secondary
windings falls below the output voltage(held constant
by the electrolytic capacitors) plus the threshold
voltage of the diodes. The current in the secondary
winding stops flowing. At this point, the drain
voltage of the MOSFET is not yet zero because
C2609 between drain and source contains a certain
charge. This charge will start a sine-shaped ringing
together with the transformer's self-induction.
The oscillator will start a next cyclus which consists of
the described three phases.
The time of the different phases depends on the mains
voltage and the load.
Time is maximum at an input of 400V
DEAD
minimum load, it will be zero at an input of 100V
overload.
Vosc
Vcomp
Vsense
Vgate
Vdrain
Idrain
Idiodes
Figure D
and
DC
and
DC
CDR600
V2
V1
0
Ton Tdiode Tdead
8. GB 47