Grundig STR-622 TWIN Service Manual Download Page 16

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Schaltungsbeschreibung / Circuit Description

STR 622 TWIN

2 - 4

GRUNDIG Service

Circuit Descriptions

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 fre-
quency of 100kHz. Control and error signal identification is exclusively
effected with a pulse width modulator (PWM).

IC1725 pin configuration
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 operating voltage input,
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
D1701…1704 via winding 3, 1 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. 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.

1.3 Control
An increasing load in the transformer causes all voltages to decrease
and with them also the voltage at the control amplifier TL431-(3) via the
voltage divider CR1762, CR1763. Consequently, less current will flow
through the cathode of TL431 so that the LED in optocoupler OK1736
becomes darker. 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: TV/VCR LNC supply voltage via diode CD1732.
+12V:

supply voltage for signal processing, via diode CD1735.

+5V/D: supply voltage via diode CD1736, for

P CIC1401, SAT Tuner

and OSD processor CIC1331.

1.5 LNC Switching Voltage 14/18V
The switching command U

14/18V A

or U

14/18V B

is used to switch over the

LNC polarity via transistor CT1675 or CT1645 and the linear control
IC1655 or IC1650.

1.6 LNC Voltage Switch-off
For switching off the LNC supply, the switching voltage U

LNCOFF

con-

nects the switch control IC1655-(1) and IC1650-(1) via CT1665 and
CT1640 to ground.

1.7 Stand by Mode
In Standby mode, the two MOSFET switches CT1733 turn off the +12V
and +5V loads with the switching voltage U

Standby

via CT1737, CT1736

thus reducing the current consumption.

1.8 The 22kHz Switching Frequency
The switching voltage U

22kHz

applies the 22kHz switching frequency,

IC1100-(29), via CT1650, CT1715 or CT1710 to the linear control
CT1650-(1) or IC1655-(1). Consequently, the LNB voltage is superim-
posed with a 22kHz signal at about 600mVpp. This 22kHz output signal
can optionally also be pulse-modulated with a DiSEqC Protocol.

2. System Control

This system contains an OTP or a mask-programmed microcomputer
(CIC1401) and the NVM-type EPROM CIC1140 for the programme
data like channel table, station ident, deviation, and so on. The
exchange of data between the

P and control unit, IC1100 or IC2100,

CIC1331 and the SAT Tuner is carried out on the I

C-bus and CS

OSD

2.1 Pin Configuration of Processor CIC1401
Pin 30:
Power-on reset for the processor, generated by IC1402. The pro-
gramme sequence will not start before all supplies of the receiver have
built up.
Pin 28, 29:
The system clock is generated by an oscillator whose frequency is
determined by Q1402 (4MHz). A proportion of this signal is fed through
CC1025 and CR1025 and is used to provide clock for IC1100.
Pin 59:
The IR input is active LOW.
Pin 60:
Infrared output to Remote Cinch socket

Externally

Triggered

Shutdown

Thermal

Shutdown

Power-up

Reset

5.7V
4.7V

Shutdown/

Auto-Restart

Internal

Supply