Power Supply; Standby Mode - Grundig STR 110 microSAT Bedienungsanleitung

Schaltungsbeschreibung / Circuit Description
Circuit Descriptions
GB

1. Power Supply

1.1 General
The TOPSwitch family of the switched-mode power supply units
distinguishes itself by a minimum number of peripheral components
connected to the PowerMOSFET-IC1725 and a fixed switching frequency
of 100kHz. Control and error signal identification is exclusively effected
with a pulse width modulator (PWM).
IC1725 pin configuation
Drain Pin:
Drain output of the internal MOSFET transistor T1. Supplies the
internal current during the start-up phase via an integrated high-
voltage switch and is the current sense point via Z
Control Pin:
Error signal identification and control input for the internal shunt
regulator during normal operation. It is also the trigger input for the
protection and auto-restart stages.
Source Pin:
Source output of the internal MOSFET transistor T1. Primary-side
chassis return for the power supply unit and reference point.
1.2 Function
The supply voltage for the Drain of IC1725 is delivered by the rectifier
D1712 via winding 5, 4 of the converter transformer. The electrolytic
capacitor C1717 charges to about 320V from a 230V mains supply.
On application of the operating voltage the internal oscillator starts to
oscillate and feeds out positive pulses from the Drain Pin. During the
conducting phase of the IC, engery is stored in the transformer which
is fed to the secondary winding during the cut-off phase. To hold the
secondary voltages at a constant level, the Control input of IC1725
(typ. 5,7V) is controlled by means of a reference voltage via the
optocoupler OK1736.
Pin Configuration IC1725
TOP201
Block Diagram IC1725
V C
Control
Z C
Shunt Regulator/
Error Amplifier
-
T2
+
I FB
Oscillator
D
Max
Clock
SAW
R E
2 - 4
.
C
DRAIN
SOURCE (TAB)
CONTROL
0
1
Shutdown/
Auto-Restart
+
5.7V
-
4.7V
5.7V
Externally
Triggered
Shutdown
Thermal
Shutdown
Power-up
Reset
-
+
PWM
Comparator
1.3 Control
When the load connected to the transformer increases, all voltages
decrease and with them the voltage on pin 2 of OK1736 via the control
amplifier TL 431 and diode D1736. The voltage at the gate of TL431 is
typically 2.5V so that less light is emitted from the LED in the optocoupler.
The transistor between pin 4 and pin 5 becomes high resistance. In
proportion to the sawtooth voltage on pin 2 the voltage decreases and
consequently the conducting phase of the high-voltage transistor in
IC1725 becomes longer. The voltage will increase.
With a short circuit in the secondary side, the current flowing through
the MOSFET rises and the built in protection circuit triggers the power
supply unit at a cycle of 1-2 seconds. When the load is removed, the
power supply is started automatically by the Auto-Restart stage in the
IC.
Diodes D1723, D1722 are provided to limit the voltage peaks of
overshoots at the Drain output of IC1725.
1.4 Secondary Voltages
+33V: tuning voltage for SAT Tuner via diode D1731.
+19V: LNC supply voltage via diode CD1732.
+14V: LNC supply voltage via diode CD1733.
+12V: supply voltge for signal processing, via diode CD1735.
+5V/D: supply voltage via diode CD1736, for µP CIC1400, SAT Tuner
and OSD processor IC1330.
1.5 LNC Supply
The separate 14/18V LNC Power supply allows to reduce power
consumption. If 14V supply is requested for the LNC, the transistor
CT1746 is activated by command U
CT1732 stops the 19V voltage via CD1732. For 18V "LNC Power",
CT1746 is switched off and CT1732 is activated.
The current limiting resistor CR1760, IC1760-(2) is so designed that
the Low-Drop control-IC1760 switches off "LNC Power" if the LNC-
current exceeds 350mA. The LNC_PG lead transfers this overload
message to µP CIC1400-(60) and switches off the "LNC Power" with
the command U at a scanning cycle of 4-5 seconds accompanied
LNC OFF
by the message "LNC ?" indicated on the screen.
1.6 Stand by Mode
In Stand by mode, the MOSFET switches CT1733 turn off the +12V
and +5V loads with the switching voltage U
thus reducing the current consumption.
Internal
Supply
8
S Q
R Q
Controlled Turn-on
S Q
Leading
Edge
R Q
Blanking
STR 110 microSAT
and the MOSFET-switch
14/18V
via CT1737, CT1736
Standby
Drain
+
V I
-
Limit
Gate Driver
T1
Minimum
On-Time
Delay
Source
GRUNDIG Service