k
2 - 12
GRUNDIG Service
Descriptions
GDV 100 D
GB
Description
Power Supply
Function description
MOSFET T7105 is used as a power switch controlled by the current
mode controller IC7115. 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 UC3842 is an integrated pulse width modulator.
A clock signal initiates power pulses at a fixed frequency. The termina-
tion 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.
6
8
2.5V
ERROR
AMP
I
SENSE
COMP
S
1
R
7
7105
UC3842
5
+16V
4
OSC
Rt/ct
GND
Vcc
2
Vfb
COMP
OUTPUT
V
ref
LATCH
I
sense
1
3
2R
R
1V
-
+
-
6V
+
BIAS
S/R
5V
2.5V
REF
+
-
Description of UC3842
The input voltage V
cc
(pin 7) is monitored by a comparator with
hysteresis, enabling the circuit at 16V and disabling the circuit below
10V. The error amplifier compares a voltage Vfb (pin 2) 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 3) 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 low-resistance push-pull stage,
capable of driving a MOSFET directly.
Start-up sequence
t1: Charging the capacitor at V
cc
C2121 connected to the V
cc
pin 7 will be charged exponentially via a
bleeder resistor R3123. The output is switched off during t1.
t2: Charging of output capacitors
When the input voltage of the IC exceeds 16V, the circuit is enabled
and starts to produce output pulses. The current consumption of the
circuit increases to about 20mA, depending on the external loads of the
IC. At first, the capacitor at the V
cc
pin will discharge because the output
capacitors of the power supply are still below the reflected V
cc
voltage.
At some moment during
t2, V
cc
reaches a minimum
voltage, determined by
the value of C2121.
t3: regulation
The output voltage of the
power supply is in regula-
tion
t4: overload
When the output is short-
ened, the supply voltage
of the circuit will decrease
and after some time drop
below the lower thresh-
old voltage. At that mo-
ment, the output will be
disabled and the process
of charging the V
cc
ca-
pacitor starts again. If the
output is still shorted at
the next t2 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.
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
on
phase: A drain current will flow from the positive supply at pin
9 through the transformer’s primary winding, the MOSFET and R
sense
R3110, R3111 to ground. As the positive voltage at pin 9 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
diode
phase: When
the MOSFET is switched
off, energy is no longer
supplied to the trans-
former. The inductance
of the transformer now
tries to maintain the cur-
rent which has been
flowing through it at a
constant level. The po-
larity of the voltage from
the transformer there-
fore becomes reversed.
This results in a current
flow through the trans-
former’s secondary
winding via the diodes,
electrolytic capacitors
and the load. This cur-
rent is also ramp shaped
but decreasing.
3. Time
dead
phase: when
the stored energy has
been supplied to the
load, the voltage from
the secondary windings
falls below the output
voltage (held constant by the electrolytic capacitors) plus the thresh-
old 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 a certain charge is present between drain and source. This
charge will start a sine-shaped ringing together with the transform-
er’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
dead
is maximum at an input of 400V
DC
and minimum load, it will be
zero at an input of 100V
DC
and overload.
Circuit description
Input circuit
The input circuit consists of an EMI filter.
The EMI filter is formed by L5101, C2101, C2102 and C2103. It
prevents inflow of noises into/from the mains.
Primary rectifier/smoothing circuit
The AC input is rectified by rectifier bridge D7101 and smoothed into
C2105. The voltage over C2105 is approximately 300V. It can vary
from 100V to 390V.
Start circuit and V
cc
supply
This circuit is formed by R3105, R3106, D6120, R3123, C2121,
D6122, C2125, R3125, and D6125.
When the power plug is connected to the mains voltage, the stabilised
voltage over D6120 (20V) will charge C2121 via R3123. When the
voltage reaches 16V across C2121, the control circuit of IC7115 is
turned on and the regulation starts. During regulation, V
cc
of IC7115 will
be supplied by the rectified voltage from winding 3-4 (D6122, C2121).
Control circuit
The control circuit exists of IC7115, and its peripheral components.
The frequency of the oscillator (~30kHz) is defined by R3119 and
C2117.
0V
1mA
20mA
t1
t2
t3
t4
short
0
10V
U
t
Vcc
Icc
t
OUTPUT
t
Vo
t
T
on
T
dead
T
diode
V
2
V
1
0V
t
V
o
V
comp
V
sense
t
V
gate
t
V
drain
t
I
drain
t
t
I
diodes
