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Description
Description
GB
Power Supply
Function description
MOSFET T7125 is used as a power switch controlled by the current
mode controller IC7110. When the switch is closed, energy is trans-
ferred from the mains into the transformer. This energy is then supplied
to the load when the switch is opened. By control of the switched-on
time, the energy which is transferred in each cycle is regulated so that
the output voltages are independent of load or input voltage variations.
The controlling device MC44603P is an integrated pulse width modu-
lator. A clock signal initiates power pulses at a fixed frequency. The
termination of each output pulse occurs when an analogue of the
inductor current reaches a threshold established by the error signal. In
this way the error signal actually controls the peak inductor current.
16
Vref iref
DEMAGNETIZATION
DETECT
8
reference
DEMAGNETISATION
MANAGEMENT
block
iref
9
SYNC INPUT
VOSC PROT
OSCILLATOR
VOSC
10
CT
RF
15
STANDBY
=1
Vstby
RP
12
STANDBY
STANDBY
(reduced frequency)
2.5V
VSB OUT
ERROR
14
AMP
CURRENT
SENSE
VOLTAGE
FEEDBACK
Dmax &
13
SOFT-START
E/A OUT
CONTROL
FOLDBACK
FOLDBACK
CURRENT
INPUT
SENSE INPUT
5
7
Description of MC44603P
The input voltage V
(pin 1) is monitored by a comparator with
cc
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 I
(pin 7) of the power supply. The output of
sense
the current sense comparator resets a latch, which is set every cycle
by the oscillator. The output stage is a low-resistance push-pull stage,
capable of driving a MOSFET directly.
Start-up sequence
t1: Charging the capacitor at V
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
U
Vcc
10V
0V
Icc
20mA
1mA
OUTPUT
Vo
0
t1
t2
2 - 8
1
supply
UVL01
Vref enable
initialization block
VC
Set
OUT
BUFFER
LATCH
Q
Reset
GND
THERMAL
SHUTDOWN
Vref Voc
OVER
OVER
VOLTAGE
VOLTAGE
iref
MANAGEMENT
PROTECT
UVL01
Vref
MC44603P
SOFT START
& DMAX
11
cc
t
t
t
short
t
t3
t4
circuit increases to about 17mA, depending on the external loads of the
IC. At first, the capacitor at the V
primary auxiliary voltage, comin from winding 7-9 is below the V
voltage. At some moment during t2, the primary auxiliary voltage
reaches the same level as V
this primary auxiliary voltage.
t3: regulation
The output voltage of the power supply is in regulation.
t4: overload
When the output is shortened, 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 V
capacitor starts again. If the output is still shorted at the next t2
cc
phase, the complete start and stop sequence will repeat. The power
supply comes in a hiccup mode.
Regulation
The Figures show the most relevant signals during the regulation
phase of the power supply.
V
2
V
1
0V
2
V
3
o
V
4
comp
V
sense
V
gate
6
V
drain
I
drain
I
diodes
T
T
on
diode
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 V
current sense level exceeds the level at the output of the error amplifier.
1. Time
phase: A drain current will flow from the positive supply at
on
pin 1 through the transformer's primary winding, the MOSFET and
R
R3126, R3127 and R3128 to ground. As the positive voltage
sense
at pin 91 of the transformer is constant, the current will increase
linearly 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 off, energy is no
diode
longer supplied to the transformer. The inductance of the trans-
former 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 transformer'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 supplied to the
dead
load, the voltage from 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 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
zero at an input of 100V
GDV 100 D/002
pin will discharge because the
cc
.The V
voltage is now determinated by
cc
cc
T
dead
. The output is switched off when the
comp
and minimum load, it will be
DC
and overload.
DC
GRUNDIG Service
cc
t
t
t
t
t
t
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