CUC 7303 FR
GRUNDIG Service
2 - 7
Description des circuits / Circuit Description
Description
1. Power Supply
1.1 Basic Circuit
Current mode converters can exhibit subharmonic oscillations when
operating at a duty cycle greater than 50% with continuous inductor
current. This instability is independent of the regulators closed loop
characteristics and is caused by the simultaneous operating condi-
tions of fixed frequency and peak current detecting.
Figure 1 shows the phenomenon graphically. At t
0
, switch conduction
begins, causing the inductor current to rise at a slope of m
1
.This slope
is a function of the input voltage divided by the inductance. At t
1
, the
Current Sense Input reaches the threshold established by the control
voltage. This causes the switch to turn off and the current to decay at
a slope of m
2
, until the next oscillator cycle. The unstable condition can
be shown if a pertubation is added to the control voltage, resulting in
a small
∆
l (dashed line). With a fixed oscillator period, the current decay
time is reduced, and the minimum current at switch turn-on (t
2
) is
increased by
∆
l +
∆
l m
2
/m
1
. The minimum current at the next cycle (t
3
)
decreases to (
∆
l +
∆
l m
2
/m
1
) (m
2
/m
1
). This pertubation is multiplied by
m
2
/m
1
on each succeeding cycle, alternately increasing and decreas-
ing the inductor current at switch turn-on. Several oscillator cycles may
be required before the inductor current reaches zero causing the
process to commence again. If m
2
/m
1
is greater than 1, the converter
will be unstable. Figure 1 shows that by adding an artificial ramp that
is synchronized with the PWM clock to the control voltage, the
∆
l
pertubation will decrease to zero on succeeding cycles. This compen-
sating ramp (m
3
) must have a slope equal to or slightly greater than
m
2
/2 for stability. With m
2
/2 slope compensation, the average inductor
current follows the control voltage yielding true current mode opera-
tion. The compensating ramp can be derived from the oscillator and
added to either the Voltage Feedback or Current Sense inputs (Figure 2).
Fig. 1
Fig. 2
1.2 Normal / Controlled Operation
For the power supply of this TV receiver a blocking oscillator-type
converter power supply with a switching frequency of 50kHz approxi-
mately is used (at normal operation and a mains voltage of 230V).
The collector contact of the power transistor T665 is connected via the
primary winding 3/1 of the blocking oscillator-type transformer TR601
to the rectified mains voltage, D621…D624. At a mains voltage of 230V
the voltage level present at the charging electrolytic capacitor C626 is
approx. +320V.
∆
l +
∆
l
m2
m
m
2
1
∆
l +
∆
l m
m
2
1
m
m
2
1
(
) )
(
m1
Inductor
Current
Control Voltage
∆
l
t0
t1
t2
t3
Oscillator Period
(A)
t4
5
t
t6
∆
l
m1
m2
m3
Inductor
Current
Oscillator Period
Control Voltage
(B)
The IC630 is responsible for driving, controlling and monitoring the
bipolar power transistor T665. The supply for the control-IC is 12V and
is present on Pin 7. As soon as the switch-on threshold is reached on
Pin 7 via the resistor R633 and the capacitor C667, the IC feeds out a
positive start pulse (1
µ
s) of 10V pp at Pin 6. After start-up of the IC, the
supply voltage is obtained via the diode D667 from the winding 5/7 of
the transformer. During the conducting phase of the transistor, energy
is stored in the transformer and this is transferred into the secondary
winding when the transistor is switched off. The IC630 controls by the
period during which the transistor T665 is switched on, the transfer of
energy at Pin 6 so that the secondary voltages are stable and are
largely not affected by variations of the mains supply, mains frequency
and the load.
The power transistor T665 is driven by a pulse-width modulator which
is triggered by an oscillator integrated in the IC. The frequency of the
oscillator is determined by the components C652 and R652. For
stabilisation, the feedback voltage which is rectified by D654 is
compared in IC630 with the 5V reference voltage provided at IC630-(8).
If the feedback voltage decreases by a small amout due to a heavier
load the drive pulse to the transistor T665 is prolonged. As a result, the
conducting period of T665 will be longer so that additional energy
transfer will be provided to compensate for the load. Pin 3 of IC630 is
a current sense input and will stop the drive to T665 at IC630-(6) in the
event of excessive current drain from a heavy secondary load.
If there was a short circuit condition at the transistor T665, the circuit
UC3842 would be destroyed. Therefore, the diodes D666 and D664
are provided to avoid the voltage at pin 3 exceeding 1.2V. The
components D668, C669, and R669 work as a snap stage.
The components CD654, C656, CD656, and CR656 delay the rise of
the pulse start duration (soft start).
The adjustment control R654 is used to set the secondary voltages by
regulating the +A voltage at minimum brightness and contrast.
1.3 Standby Mode
In normal operating mode, a voltage of approx. 10.5V is present on
IC676-(1) (LM317). If the TV receiver is to be switched to standby, the
µ
P switches U
Standby
to "High" level so that the level on IC676-(1) is
< 0.7 V. As a result, the vB is switched off and the TV receiver
goes to standby.
1.4 Secondary Voltages
+A:
Supply for the horizontal output stage from the winding
2/10 and D682. The power supply unit is set to this level.
+33V:
The upper tuning voltage limit for the tuner is produced at
the Z-diode D683 and the resistor R681 from the winding
2/10 via D682.
+M =16.5V Supply for the sound output stage from the winding 6/10
and the diode D671.
+B = 12V Power supply for the Tuner and the horizontal driver T501.
This voltage is supplied from the winding 6/10 via the diode
D671 and is stabilised by the adjustment control IC676.
Switching off of the +12V supply, see "Standby Opera-
tion".
+E = 8V
Power supply for the Video Processor IC150. In Standby
mode it is switched off.
+H = 5V
Power supply for the
µ
P IC850, the infrared amplifier
IR810, Tuner, and CIC105.
This voltage is also present in Standby mode.
Additionally necessary voltages
+D: +25V Power supply for the vertical output stage from the line
transformer winding B/H via D444.
+C: 125V The power supply for the picture tube panel is obtained
190V from the line transformer winding G/H via R543 and the
diode D543. 125V/14" CRT; 190V/15…21" CRT.
