Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-06-27
2001-05-22
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021030, C363S070000, C363S097000
Reexamination Certificate
active
06236577
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a power circuit for use in a television (TV) receiver.
BACKGROUND OF THE INVENTION
A power circuit for use in a TV receiver having a satellite broadcast receiving function or a teletext receiving function frequently comprises:
a main power source for supplying a large power to the main load, i.e., the signal-reception, signal-processing, and display circuits of the TV receiver (hereinafter, called the TV circuit); and
a sub-power source supplying power to small-power drawing circuits such as a satellite broadcast receiving tuner, a UHF/VHF (U/V) tuner, a teletext receiving circuit, a microprocessor, and a remote control receiving circuit.
FIG. 5
shows a circuit of the switching power source of a TV receiver having such a sub-power source as described above. In
FIG. 5
, AC source terminal
1
is connected to rectifier circuit
3
for converting AC to DC power source through power switch
2
. A pair of DC output terminals
4
and
5
of rectifier circuit
3
have transistor
8
as the switching element connected therebetween through primary winding
7
of transformer
6
.
Transformer
6
has secondary winding
9
, tertiary winding
10
, and fourth winding
11
magnetically coupled with primary winding
7
. Fourth winding
11
, connected between the base and emitter of transistor
8
through a parallel circuit of resistor
12
and capacitor
13
, is a drive winding allowing transistor
8
to turn ON/OFF in a self-excited manner by means of positive feedback. Starting resistor
14
is connected between DC terminal
4
of rectifier circuit
3
and the base of transistor
8
. Phototransistor
16
, connected between the base and emitter of transistor
8
, is a constant-voltage control element. Phototransistor
16
serves as a bypass for allowing the base current of transistor
8
to pass therethrough so that constant-voltage control of the output voltage of transistor
8
is achieved.
Secondary winding
9
is connected with first rectifier-and-smoothing circuit
19
formed of first rectifier diode
17
and first smoothing capacitor
18
. The input line to first smoothing diode
17
is connected with switch
20
. Output line
21
of first rectifier-and-smoothing circuit
19
is connected to TV circuit
22
as the main load.
Tertiary winding
10
is connected to second rectifier-and-smoothing circuit
25
formed of second rectifier diode
23
and second smoothing capacitor
24
. Output line
26
of second rectifier-and-smoothing circuit
25
is connected to satellite tuner
27
, U/V tuner
28
, microprocessor
50
, and remote control receiving circuit
29
as the sub-load.
To detect the voltage on output line
26
for achieving constant-voltage control while power is being supplied only to the sub-load, second output line
26
is connected with second output-voltage-detection and error-amplifier circuit
39
, which is formed of voltage detecting resistor
41
, error amplifying transistor
42
, and zener diode
43
as a second reference voltage source. Resistor
41
is connected between second output line
26
and the base of transistor
42
.
Zener diode
43
is inserted between the collector of transistor
42
and the cathode of light emitting diode
37
, with its anode connected to the collector of transistor
42
and its cathode connected to the cathode of light emitting diode
37
. The anode of light emitting diode
37
is connected to second output line
26
through a resistor
36
. Light emitting diode
37
is in common use for first and second output-voltage-detection and error-amplifier circuits
30
and
39
.
Switch circuit
40
is constituted of transistor
45
. Transistor
45
has its collector connected to the base of second error amplifying transistor
42
and its emitter connected to the ground.
When power is supplied to TV circuit
22
as the main load, microprocessor
50
applies a high-level output of TV-circuit-ON/OFF-control signal
46
to the base of transistor
45
of switch circuit
40
. Thereby, while switch
20
is turned ON, transistor
45
of output switch circuit
40
is turned ON and second error amplifying transistor
42
is turned OFF.
Upon turning ON of switch
20
, the voltage on output line
21
from first-rectifier-and-smoothing circuit
19
rises. To detect the voltage on output line
21
and perform constant-voltage control, there is output-voltage-detection and error-amplifier circuit
30
connected between first output line
21
and the ground. Circuit
30
is made up of resistors
31
and
32
connected between output line
21
and the ground, error amplifying transistor
33
, zener diode
34
as a reference voltage source, and a resistor
35
.
The base of transistor
33
is connected to the voltage dividing point of resistors
31
and
32
. Zener diode
34
is connected to the emitter of transistor
33
. Resistor
35
is inserted between output line
21
and the cathode of zener diode
34
. Accordingly, a collector current corresponding to the difference between the voltage on output line
21
and the reference voltage given by zener diode
34
is allowed to flow. At this time, second output-voltage-detection and error-amplifier circuit
39
is inoperative because error amplifying transistor
42
is OFF.
Light emitting diode
37
, connected between output line
21
and the collector of transistor
33
through resistor
36
, emits a light quantity corresponding to the flow of collector current of transistor
33
to provide an optical input to phototransistor
16
optically coupled with light emitting diode
37
.
Operations of transistor
8
as the switching element and the energy transferring operation performed through transformer
6
will be described.
When power switch
2
is turned ON, a flow of base current is produced in transistor
8
through starting resistor
14
and hence transistor
8
is turned ON. Upon turning ON of transistor
8
, a positive feedback voltage is produced in fourth winding
11
so that transistor
8
is held ON. Since primary winding
7
of transformer
6
has an inductance, the collector current of transistor
8
increases with time.
The largest value of the collector current of transistor
8
is determined by the base current and the current amplification factor. When the collector current reaches the value equivalent to the product of the base current and the current amplification factor, transistor
8
is turned into its unsaturated state so that the positive feedback voltage in fourth winding
11
decreases. As a result, transistor
8
is turned into its OFF state. First and second rectifier diodes
17
and
23
are held OFF while transistor
8
is ON, and held ON while transistor
8
is OFF.
Upon turning OFF of transistor
8
, the energy stored in the inductance of transistor
6
causes a flyback voltage to be generated. When switch
20
is OFF so that power is supplied only to the sub-load, only second smoothing capacitor
24
is charged with the generated flyback voltage. When switch
20
is ON so that power is also supplied to the TV circuit as the main load, both first and second smoothing capacitors
18
and
24
are charged.
When the main load, i.e., TV circuit
22
, is started to operate, switch
20
and switch circuit
40
are turned ON with the sub-load, i.e., satellite tuner
27
, U/V tuner
28
, and the like, held operative. In this state, switch
20
and switch circuit
40
are turned ON by a signal from microprocessor
50
. At this time, the main load drawing a much greater power than that drawn by the sub-load is suddenly put into connection with output line
21
. Therefore, a phenomenon of voltage drop as shown in
FIG. 6
occurs on output line
26
.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid occurrence of the phenomenon as described above and make the sub-source voltage stabler.
In order to achieve the object as described above, the power circuit of the present invention comprises:
a transformer including a secondary winding and a tertiary winding, magnetically coupled with a primary winding, and a switching
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
Sterrett Jeffrey
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