High voltage power supply device for lighting discharge tube...

Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific voltage responsive fault sensor

Reexamination Certificate

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Details

C361S091100

Reexamination Certificate

active

06654221

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high voltage power supply device for lighting a discharge tube, and more particularly, to a high voltage power supply devices for lighting discharge tubes having a fault protection circuit such as inverter power sources for liquid crystal panel back lights in portable information devices.
2. Description of the Related Art
FIG. 7
is a circuit diagram showing an example of a cold cathode tube lighting inverter using a conventional fault protection circuit. In
FIG. 7
, the cold cathode tube lighting inverter contains an inverter unit
10
and a fault protection circuit
20
. To light a cold cathode tube
2
, the inverter unit
10
comprises a step-up transformer
1
, a tube current control circuit
3
, a drive circuit
4
, and a resistor
5
as a current-voltage converter, and a rectifier circuit
9
. The drive circuit
4
produces an AC signal for driving the step-up transformer
1
, corresponding to an input voltage, and feeds the signal to the step-up transformer
1
. The step-up transformer
1
increases the voltage of the AC signal, and provides the signal to one of the electrodes of the cold cathode tube
2
to light the cold cathode tube
2
.
The resistor
5
is connected between the other electrode of the cold cathode tube
2
and ground. A tube current flowing into the resistor
5
causes a voltage. The voltage is rectified in a rectifier circuit
9
comprising a diode
6
, a resistor
7
, and a capacitor
8
. The rectification voltage Vrct is fed to the tube current control circuit
3
. The tube current control circuit
3
controls the drive circuit
4
so that the rectification voltage Vrct becomes substantially equal to a desired constant value. In this way, the tube current is controlled to be substantially constant, due to the operation of the respective parts of the inverter unit
10
. As a result, the brightness (brilliance) is also controlled to be substantially constant.
The fault protection circuit
20
comprises a resistor
21
, a transistor
22
, a capacitor
23
, a constant-current source
24
, and a thyristor
25
. A remote signal is provided to the On-Off terminal of the tube current control circuit
3
via a resistor
26
, and also, to the anode of the thyristor
25
. The cathode of the thyristor is grounded. The rectification voltage Vrct output from the rectifier circuit
9
is given to the base of the transistor
22
via the resistor
21
. The emitter of the transistor
22
is grounded. The gate terminal of the thyristor
25
and the constant-current source
24
are connected to the collector. The fault protection capacitor
23
is connected between the collector of the transistor
22
and ground.
Next, operation of the cold cathode tube lighting inverter shown in
FIG. 7
will be described. When the cold cathode tube
2
lights normally, the tube current flows through the resistor
5
, and the rectification voltage Vrct is thereby fed to the base of the transistor
22
via the resistor
21
of the fault protection circuit
20
. Thus, the transistor
22
is able to conduct, and a charging current, caused by the constant-current source
24
, bypasses the fault protection capacitor
23
. Thus, no voltage is stored in the fault protection capacitor
23
. As a result, the voltage at the gate terminal of the thyristor
25
is not increased, so that the thyristor
25
remains off, and the On-Off terminal of the inverter unit
10
, maintained at the H level, continues to operate normally.
If the cold cathode tube
2
is not connected or malfunctions, no tube current flows in the resistor
5
. Thus, the rectification voltage Vrct of the rectifier circuit
9
becomes zero, and the transistor
22
becomes unable to conduct. Thereby, a charging current from the constant-current source
24
flows into the fault protection capacitor
23
. The gate voltage of the thyristor
25
is increased by a time constant value determined by the amount of the existing charging current and the electrostatic capacitance of the fault protection capacitor
23
. When the gate voltage exceeds the constant value, the thyristor
25
is turned on, the on-off terminal of the inverter unit
10
reaches the L level, and the operation of the inverter unit
10
is stopped. That is, the circuit configuration is such that protection is provided if the cold cathode tube
2
is not connected or malfunctions.
FIG. 8
is a circuit diagram showing another example of a conventional cold cathode tube lighting inverter.
In
FIG. 8
, an inverter unit
30
includes a tube current control circuit
33
in addition to the step-up transformer
1
and the drive circuit
4
shown in FIG.
7
. In this example, characteristically, the tube current control circuit
33
is AC-coupled by capacitor
34
to the cold cathode tube
2
. The other electrode of the cold cathode tube
2
is grounded via a resistor
35
. One terminal of a capacitor
34
is connected to the node of the cathode and resistor
35
. The other terminal of the capacitor
34
is connected to the base of a transistor
38
. The base of the transistor
38
is the input terminal of the tube current control circuit
33
. A constant-current source
36
and a diode
51
are connected in series with each other. The voltage at the node is fed as a bias voltage Vf to the input terminal via a resistor
37
.
This bias voltage Vf is cancelled out by the base-emitter voltage Vbe of the transistor
38
. If at that time, the diode
51
and the transistor
38
are in the same chip, the temperature characteristic of the bias voltage Vf and that of the base-emitter voltage Vbe can be completely cancelled out. That is, it is assumed that the capacitor
34
, the constant current source
36
, the diode
51
, the resistor
37
, and the transistor
38
constitute an ideal diode which eliminates the bias voltage Vf. Then, the peak voltage of the voltage Vfb obtained by voltage-conversion of the tube current and the peak voltage of the rectification voltage Vrct, which is the emitter voltage of the transistor
38
, are equal to each other.
A resistor
39
and a capacitor
40
are connected in parallel to each other between the emitter of the transistor
38
and ground. The rectification voltage Vrct is fed to the comparison input terminal of a comparator
41
. A target voltage Vcnt is applied to the standard input terminal of the comparator
41
. The rectification voltage Vrct is compared with the target voltage Vcnt by the comparator
41
. The output is fed to an integration circuit
51
and integrated therein, and is input to an on-duty modulation circuit
42
. The on-duty modulation circuit
42
controls the on-duty of the drive circuit
4
so that the average rectification voltage Vrct and the target voltage Vcnt become equal to each other. Thus, the tube current of the cold cathode tube
2
, and moreover, the brilliance of the cold cathode tube
2
are controlled to have a constant value.
The conversion voltage Vfb, obtained by converting the tube current to a voltage by means of the resistor
35
, is also input to the comparison input terminal of a comparator
43
in an fault protection circuit
50
. A reference voltage Vudr is applied to the reference input terminal of the comparator
43
. The output from the comparator
43
is fed to the base of a transistor
45
via a resistor
44
. The emitter of the transistor
45
is grounded, and the collector is connected to the gate terminal of a thyristor
48
. A constant-current source
47
is connected to the collector of the transistor
45
, and a fault protection capacitor
46
is connected between the collector and ground. When the conversion voltage Vfb exceeds the reference voltage Vudr, the comparator
43
outputs an H level signal, causing the transistor
45
to be turned on, so that the charge stored in the fault protection capacitor
46
is discharged.
If the cold cathode tube
2
is broken, is not connected, or the like, resulting in no tube current, output from the comparator
43
is maintained at the L level. Thus, the v

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