GB 608.CDR779Faultfinding Guide
capacitor at the Vcc pin will discharge because the primairy becomes reversed. This results in a current flow through the
auxiliary voltage, coming from winding7-9 is below the Vcc tranformer?s secondary winding via the diodes, electrolytic
voltage. At some moment during t2, the primary auxiliary capacitors and the load. This current is also ramp shaped but
voltages reaches the same level as Vcc.decreasing.
This primary auxiliary voltage now determines the Vcc TimeDEAD phase : when the stored energy has been
voltagesupplied to the load, the voltage from the secondary windings
t3: regulationfalls below the output voltage(held constant by the
The output voltage of the power supply is in regulationelectrolytic capacitors) plus the threshold voltage of the
t4: overloaddiodes. The current in the secondary winding stops flowing.
When the output is shortened, the supply voltage of the At this point, the drain voltage of the MOSFET is not yet zero
circuit will decrease and after some time drop below the because C2609 between drain and source contains a certain
lower threshold voltage. At that moment, the output will be charge. This charge will start a sine-shaped ringing together
disabled and the process of charging the Vcc capacitor starts with the transformer?s self-induction.
again. If the output is still shorted at the next t2 phase, the The oscillator will start a next cyclus which consists of the
complete start-and stop sequence will repeat. The power described three phases. The time of the different phases
supply comes in a hiccup mode.depends on the mains voltage and the load.
TimeDEAD is maximum at an input of 400VDC and minimum
load, it will be zero at an input of 100VDC and overload.
16V
VccV2
12V
10VVosc
Vc2134
V1
0V
0
Vcomp
20mA
Vsense
Icc
1mA
Vgate
OUTPUT
Vdrain
short
VoIdrain
0
t1t2t3t4Idiodes
Figure C : Start-up sequence
CL 06532151_020.eps
271100Ton Tdiode TdeadCL 06532151_021.eps
271100
Figure 8-12 Figure 8-13
8.2.5 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.
TimeON phase : A drain current will flow from the 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.
TimeDIODE phase : When the MOSFET is switched 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 |
ACEC
