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Schaltungsbeschreibung / Circuit Description
Description
1. 12V Power Supply
General:
The control stage MC34060 is a pulse-width modulator working at a
fixed frequency (typical 40kHz) which is determined by CR60041 and
CC60028. 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 and fuse SI60001 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. With the mains switch depressed the
U
line is no longer connected to chassis. The base potential of
poweroff
CT60031 increases, CT60031 switches off and via CT60027 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 needs to be conductive so that current begins to flow
through R60001 and D60003. 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 on causing
CT60002 in turn to switch off T60001 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 and CT60002 together with the
feedback resistor CR60008 serve for stabilising the IC supply voltage
during the start-up phase.
Current Rise Protection and Drain Current Limiting
The diodes CD60038 and CR60038 are responsible for current rise
protection in the case of a VDE failure. They act on the circuit CT60032/
CT60035 whose main task it is to limit the drain current. On the four
sense resistors R60051...R60054 which are connected in parallel 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
of CT60027 is at U
.
Ref
Driver PowerMOSFET
Two FETs (T60047/60048) in parallel connection are provided be-
cause 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 to switch on. For blocking the FETs quickly
(= discharge of the capacitances C
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.
Switching-off and Undervoltage Detection
These functions are activated via CT60031. If the base voltage falls
below a certain level, the transistor switches on causing in turn
CT60027 to increase the potential at Pin 4 to a level so that t
reduced to zero and the power supply unit stops operating.
2 - 8
of the starting
ON
. The emitter
ON
), CT60044 pulls the gates to
GS
of the power supply
ON
at Pin 12
REF
is
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 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
Dead-Time
C T
Comparator
-
~0.12V
4
+
Dead-Time
~0.7V
Control
-
+
P.W.M.
Comparator
~0.7mA
+
1
-
1
2
3
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
CUC 7305
Reference
12
Ref Out
Regulator
10
9
Q1
8
+
2
Gnd
~
-
13
14
7
Error Amp
2
t on
t off
GRUNDIG Service
Vcc
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