MIVAR 14M3 CHASSIS CS1001 POWER SUPPLY UNIT CS1004

The power supply unit is separated from the receiver chassis and it's based on the TEA2261 THOMSON

TEA2261 SWITCH MODE POWER SUPPLY CONTROLLERDESCRIPTION
The TEA2260/61 is a monolithic integrated circuit
for the use in primary part of an off-line switching
mode power supply.
All functions required for SMPS control under normal
operating, transient or abnormal conditions
are provided.
The capability of working according to the “master-
slave” concept, or according to the “primary
regulation” mode makes the TEA2260/61 very
flexible and easy to use. This is particularly true for
TV receivers where the IC provides an attractive
and low cost solution (no need of stand-by auxiliary
power supply).


GENERAL DESCRIPTION
The TEA2260/61 is an off-line switch mode power
supply controller. The synchronization function
and the specific operation in stand-by mode make
it well adapted to video applications such as TV
sets, VCRs, monitors, etc..
The TEA2260/61 can be used in two types of architectures:
– Master/Slave architecture. In this case, the
TEA2260/61 drives the power transistor according
to the pulse width modulated signals generated
by the secondary located master circuit. A
pulse transformer provides the feedback (see
Figure 1).
– Conventional architecture with linear feedback
signal (feedback sources: optocoupler or transformer
winding) (see Figure 2).
Using the TEA2260/61, the stand-by auxiliary
power supply, often realized with a small but costly
50Hz transformer, is no longer necessary. The
burst mode operation of the TEA2260/61 makes
possible the control of very low output power
(down to less than 1W) with the main power transformer.
When used in a master/slave architecture, the
TEA2260/61 and also the power transistor turn-off
can be easily synchronized with the line transformer.
The switching noise cannot disturb the picture
in this case.
As an S.M.P.S. controller, the TEA2260/61 features
the following functions:
– Power supply start-up (with soft-start)
– PWM generator
– Direct power transistor drive (+1.2A, -2.0A)
– Safety functions: pulse by pulse current limitation,
output power limitation, over and under voltage
lock-out.
S.M.P.S. OPERATING DESCRIPTION
Starting Mode - Stand By Mode
Power for circuit supply is taken from the mains
through a high value resistor before starting. As
long as VCC of the TEA2260/61 is below VCC start,
the quiescent current is very low (typically 0.7mA)
and the electrolytic capacitor across VCC is linearly
charged. When VCC reaches VCC start (typically
10.3V), the circuit starts, generating output pulses
with a soft-starting. Then the SMPS goes into the
stand-by mode and the output voltage is a percentage
of the nominal output voltage (e.g. 80%).
To do this, the TEA2260/61 contains all the functions
required for primary mode regulation: a fixed
frequency oscillator, a voltage reference, an error
amplifier and a pulse width modulator (PWM).
For transmission of low power with a good efficiency
in stand-by, an automatic burst generation system
is used, in order to avoid audible noise.
Normal Mode (Secondary Regulation)
The normal operating of the TV set is obtained by
sending to the TEA2260/61 regulation pulses generated
by a regulator located in the secondary side
of the power supply.
This architecture uses the “Master/Slave Concept”,
advantages of which are now well-known
especially the very high efficiency in Stand-by
mode, and the accurate regulation in Normal
mode.
Stand-by mode or normal mode are obtained by
supplying or not the secondary regulator. This can
be ordonnered for example by a microprocessor in
relation with the remote control unit.
Regulation pulses are applied to the TEA2260/61
through a small pulse-transformer to the IN input
(Pin 2). This input is sensitive to positive square
pulses. The typical threshold of this input is 0.85V.
The frequency of pulses coming from the secondary
regulator can be lower or higher than the frequency
of the starting oscillator.
The TEA2260/61 has no soft-starting system
when it receives pulses from the secondary. The
soft-start must be located in the secondary regulator.
Due to the principle of the primary regulation, pulses
generated by the starting system automatically
disappear when the voltage delivered by the
SMPS increases.
Stand-by Mode - Normal Mode Transition
During the transition there are simultaneously
pulses coming from the primary and secondary
regulators.
These signals are not synchronized and some
care has to be taken to ensure the safety of the
switching power transistor.
A very sure and simple way consist in checking the
transformer demagnetization state.
– A primary pulse is taken in account only if the
transformer is demagnetized after a conduction
of the power transistor required by the secondary
regulator.
– A secondary pulse is taken in account only if the
transformer is demagnetized after a conduction
of the power transistor required by the primary
regulator.
With this arrangement the switching safety area of
the power transistor is respected and there is no
risk of transformer magnetization.

The magnetization state of the transformer is
checked by sensing the voltage across a winding
of the transformer (generally the same which supplies
the TEA2261). This is made by connecting a
resistor between this winding and the demagnetization
sensing input of the circuit (Pin 1).

SECURITY FUNCTIONS OF THE TEA2261
Undervoltage Detection. This protection works in
association with the starting device “VCC switch”
(see paragraph Starting-mode - stand-by mode). If
VCC is lower than VCCstop (typically 7.4V) output
pulses are inhibited, in order to avoid wrong operation
of the power supply or bad power transistor
drive.
Overvoltage Detection. If VCC exceeds VCCmax
(typically 15.7V) output pulses are inhibited and
the external capacitor C2 is charged as long as
VCC is higher than VCC stop. Restarting of the
power supply is obtained by reducing VCC below
VCCstop except if the voltage across C2 reaches
VC2 (typically 2.55V) (refer to “Restart of the power
supply” paragraph).In this last case, the circuit is
definitively stopped.
Current Limitation of the Power Transistor. The
current is measured by a shunt resistor. A double
threshold system is used:
– When the first threshold (VIM1) is reached, the
conduction of the power transistor is stopped until
the end of the period: a new conduction signal
is needed to obtain conduction again.
– Furthermore as long as the first threshold is
reached (it means during several periods), an external
capacitor C2 is charged. When the voltage
across the capacitor reaches VC2 (typically
2.55V) the output is inhibited. This is called the
“repetitive overload protection”. If the overload
disappears before VC2 is reached, C2 is discharged,
so transient overloads are tolerated.
– Second current limitation threshold (VIM2). When
this threshold is reached the output of the circuit
is immediately inhibited. This protection is helpful
in case of hard overload for example to avoid the
magnetization of the transformer.
Restart of the Power Supply. After stopping due
to VIM2, VCCMax or VCCstop triggering, restart of
the power supply can be obtained by the normal
operating of the “VCC switch” VCC switch sequency
from VCCstop to VCCstart. After stopping due to
VC2 threshold reaching, the circuit is definitively
stopped. In this case it is necessary to reduce VCC
below approximately 5V to reset the circuit. From a
practical point of view, it means that the power
supply has to be temporarily disconnected from
any power source to get the restart.