Grundig GV 6400 HiFi Скачать руководство пользователя страница 24

Страница: 24 / 82

2 - 8

GRUNDIG Service

Descriptions

GV 6400 …, GV 6401 NIC

When the whole amount of energy stored in the transformer has been
transferred to the load and no magnetic field is left in the transformer,
the voltages at the secondary windings fall below 0V. The transistor
T7040 is switched on again and the next cycle is started.
Control of the switched mode power supply is effected by varying the
conducting phase of the switching transistor to the effect that the energy
transferred from the mains to the transformer is increased or reduced.
The control information is obtained from IC7074 which monitors the
output voltage of the switched mode power supply. IC7074 is used as
a reference element with an internal 2.5V reference voltage and a
comparator stage. This control information is taken via the optocoupler
OK7070 (electrical isolation) to Pin 14 of IC7020. This IC7020 compares
this voltage with an internal reference. By means of this comparison the
level of the voltage used to be compared with the voltage at Pin 7 of
IC7020 (representing the primary current) is changed.
During the period T7040 is switched off, D6042…C2042 limit the
voltage peaks in the primary side.
To avoid static charges the gate of the switching transistor T7040 is
provided with the pull down resistor R3040. The voltage at Pin 5 of
IC7020 is used for stepping down the current and the voltage in short-
circuit conditions (FOLD BACK).
The maximum power possible to be taken from the secondary side is
determined by R3046 / R3048. At 1V (typically) on IC7020-(7) the
power supply unit reaches the reversal point.
The external circuit at Pin 11 is an option of IC7020. By means of
C2014 the pulses in the start-up phase are shorter so that the switching
frequency is outside the audible range.
From the secondary side, voltages are available which are rectified
and filtered by the respective components (diodes / capacitors / chokes).

The Start-up Phase
When connecting the video recorder to the mains the following
voltages at the pins of IC7020 start to rise at time t

(see fig.):

– The voltage V

, IC7020-(1), increases according to the half-wave

charge via R3050 / R3052 until it reaches the voltage level V

cc Start

at the typical current
consumption (I

) of

0.3mA. The internal ref-
erence voltage V

ref

IC7020 is switched on
as soon as V

cc Start

reached and the current
consumption increases
to 17mA.

– The voltage at Pin 11 of

IC7020 rises linearly up
to 2.4V. During this pe-
riod, IC7020 drives the
Power-MOS transistor
T7040 by shortened
pulses.

– If the voltage V

IC7020-(1), falls below
the limit value V

dis2

be-

fore the reversal point is
reached the start-up is
stopped. For this, the
drive to T7040 is
stopped and IC7020
switches off the internal
V

ref

= 0,3mA). The

voltage V

increases

according to a half-wave
charge via R3050 /
R3052. The next start-
up cycle commences.

Normal Operation, Overload and Standby Operation
As soon as the power supply stage is working, IC7020 operates in the
normal mode (control range). The voltage at IC7020-(14) is 2.5V
(typically). If the load in the secondary side increases, the switch-on
period is increased. As a result the peak voltage value at IC7020-(7)
"representation of drain current" increases.
If the load continues to increase, that is also the voltage at IC7020-(7),
the overload amplifier of the IC starts to reduce the pulse width of the
T7040 driving voltage at IC7020-(3). This point is the so-called reversal
point. The IC supply voltage V

behaves in the same way as do the

secondary voltages. This voltage decreases also along with the
increasing load.

Descriptions

1. Power Supply (PSM…)

Typical Data:

Mains voltage:

187…264V~

Mains frequency:

45…65Hz

Maximum power:

50W

Switching frequency: 100kHz
Efficiency:

78% at maximum load

Short-circuit protection provided for all outputs

Principle of the Blocking-Oscillator Type Transformer
During the conducting phase of the switching transistor energy is
transferred from the mains to the transformer. This energy is fed out to
the load during the phase the transistor is switched off. By means of the
switch-on period and the frequency the energy transfer during each
cycle is so controlled that the output voltages are independent of
changes in the load or the input voltage. Controlling and driving the
power transistor is effected by IC7020.

The Different Load Conditions
– No-load operation (Standby):

In recorders with a standby power consumption of >1W, the
switched mode power supply operates at a controlled low frequency
(approx. 50kHz) to minimize the switching losses at the Power-MOS
transistor T7040 and in the transformer 5050.
In models with a standby power consumption of <1W, the switched
mode power supply operates in burst mode where only the standby
operating voltage 5VSTBY is present.

– Normal operation (control range):

The duty cycle is mainly controlled by the mains voltage and the load.
The output voltages are influenced by the load to a minor extent.

– Reversal point:

At this point of the output characteristic the maximum power is
transferred.

– Overload:

The power supply operates in Burst Mode (polling operation mode),
i.e. the energy of each cycle is limited so that the output power is low.

Circuit Description
The mains voltage is rectified by the bridge rectifier D6050 and filtered
by C2070. L5010 is provided to protect the power supply from interfer-
ing pulses. During the starting phase the power for IC7020 is supplied
to Pin 1 via R3050 and R3052. After the starting phase the power is
obtained from the transformer
winding 4/3 and D6036. The
inductance of the primary
windings 6/9 determines the
natural frequency of the
switched mode power supply in
normal operating mode. The
frequency is determined by
C2128 at IC7020-(10).
During the switch-on period of
the switching transistor T7040
the current of the rectified mains
voltage flows through the pri-
mary winding of the transformer
(contacts 9/6), T7040 and
R3046 / R3048 to ground (in
the primary side). Since the volt-
age at contact 9 of the trans-
former is almost constant the
current rises linearly. The intensity of this current depends on the mains
voltage and the inductance of the primary winding. In the transformer, a
magnetic field develops which corresponds to a certain amount of
energy. During this phase, the diodes are cut off due to the polarity of the
secondary voltages. Via the resistor R3026 a voltage which represents
the primary current is fed to Pin 7 of IC7020. If this voltage exceeds a
certain level depending on the control voltage at IC7020-(14), the
switching transistor T7040 is switched off. This process is repeated
whenever the switching transistor T7040 is switched on.
As soon as the switching transistor T7040 is switched off the energy
transfer to the transformer is stopped. The energy accumulated in the
transformer is now transferred to the secondary windings. Due to the
fact that the polarities of the voltages are reversed by the transformer
current flows through the secondary windings of the transformer,
through the diodes, electrolytic capacitors into the load.