Descriptions; Power Supply - Grundig CUC 7305 Service Manual

Allgemeiner Teil / General Section
Description

1. Power Supply

1.1 12V Power Supply 29305 007 3100 (29305 007 3300)
General:
The control stage MC34060 is a pulse-width modulator working at a
fixed frequency (typical 40kHz) which is determined by CR60041
(CR60441) and CC60028 (CC60428). The power supply unit is
configured as a blocking oscillator converter. The transformer needs
not to be separated from the mains according to VDE regulations.
On battery operation, the mains socket is covered by a slider so that
it is no longer accessible.
The diode D60004 (D60404) and fuse SI60001 (SI60401) are used to
avoid wrong polarity conditions.
The "Standby" function is not possible in battery mode. The "Power-off"
button on the remote control does not operate.
at the ST-BAT1 connector the µC obtains
Via the signal lead U
Batt
information that the TV receiver is powered from the battery instead of
the mains.
U = "High" = battery operation.
Batt
On battery operation, the TV receiver is switched off by the low-voltage
contact (no wiper) of the mains switch. When the mains switch is
depressed the line U is connected to chassis. In this condition the
poweroff
power supply unit does not operate, the supply voltage to the control
IC drops below 3V. The power consumption is less than 300mW in this
condition.
Start-up:
To start up the power supply unit a voltage is applied to the control IC
via T60001, CT60002, CT60003 (T60401, CT60402, CT60403). With
the mains switch depressed the U
chassis. The base potential of CT60031 (CT60431) increases, CT60031
(CT60431) switches off and via CT60027 (CT60427) the potential at
Pin 4 decreases.
The switched-on period (of the two PowerMOSFET's) t
up power supply is no longer limited. For the power supply unit to start
up, T60001 (T60401) needs to be conductive so that current begins to
flow through R60001 (R60401) and D60003 (D60403). The IC is
supplied with voltage, the power supply unit starts operating now
drawing its supply current from the supply winding of the transformer.
The +M voltage, as soon as it is present, switches CT60003 (CT60403)
on causing CT60002 (CT60402) in turn to switch off T60001 (T60401)
so that no energy is transferred on this path to the IC. The IC supply
voltage is typ. 12V over the complete input voltage range. T60001
(T60401) and CT60002 (CT60402) together with the feedback resistor
CR60008 (CR60408) serve for stabilising the IC supply voltage during
the start-up phase.
Current Rise Protection and Drain Current Limiting
The diodes CD60038 (CD60438) and CR60038 (CR60438) are re-
sponsible for current rise protection in the case of a VDE failure. They
act on the circuit CT60032/CT60035 (CT60432/CT60435) whose
main task it is to limit the drain current. On the four sense resistors
R60051...R60054 (R60451...R60454) which are connected in paral-
lel a voltage develops proportional to the current. If the level of this
voltage is too high, the conducting phase of CT60027 becomes longer.
As a result, the voltage at Pin 4 increases and the IC reduces t
emitter of CT60027 (CT60427) is at U
Driver PowerMOSFET
Two FETs T60047/60048 (T60447/60448) in parallel connection are
provided because of the high current levels within the switch element.
A "High" level at Pin 8 of the control IC causes the two FETs (self-
blocking, n-channel) via CD60043 (CD60443) to switch on. For
blocking the FETs quickly (= discharge of the capacitances C
CT60044 (CT60444) pulls the gates to chassis when a "Low" level is
applied to Pin 8.
The Control
Control of the power supply unit is realised via Pin 1 and Pin 4 of the
control IC. The measured values are taken via a combined control (+A
and +M) to Pin 1. This is the non-inverting input of an error amplifier.
If one of these two voltages increases, the PWM-comparator inte-
grated in the chip reduces the switch-on period t
until the voltages have reached their desired level again. U
is used as a reference. It is 5V.
2 - 8
line is no longer connected to
poweroff
of the starting
ON
. The
ON
.
Ref
GS
of the power supply
ON
at Pin 12
REF
Switching-off and Undervoltage Detection
These functions are activated via CT60031 (CT60431). If the base
voltage falls below a certain level, the transistor switches on causing
in turn CT60027 (CT60427) to increase the potential at Pin 4 to a level
so that t is reduced to zero and the power supply unit stops operating.
ON
Demagnetization:
Demagnetization is effected with every "Power-on" in mains as well as
in battery mode and is triggered by the µC.
The capacitor C60076 (C60476) is charged to about 360V by means
of the voltages +A, negative line (ca. -110V) and positive line (ca. +150V).
When the relay closes, an aperiodic oscillation develops
C=2.2µF,
L = inductance of the demagnetization coil,
R= ohmic resistance of the coil.
Block diagram MC 34060
6
Oscillator
5
R T
C T
~0.12V
4
Dead-Time
~0.7V
Control
~0.7mA
+
1
-
1 2
Error Amp Feedback/P.W.M.
1 Comparator Input
Timing diagram
Capacitor C T
Feedback/P.W.M.
Comparator Input
Dead-Time Control
Output Q1
Emitter
),
Reference
Regulator
Dead-Time
Comparator
-
+
-
+
P.W.M.
Q1
Comparator
+
2
Gnd
~
-
3 13 14 7
Error Amp
2
GRUNDIG Service
CUC 7305
12
Ref Out
10
Vcc
9
8
t on
t off