Discharge lamp lighting device to light a plurality of...

Electric lamp and discharge devices: systems – Current and/or voltage regulation

Reexamination Certificate

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C315S277000, C315S312000, C315S224000

Reexamination Certificate

active

06784627

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge lamp lighting device which, as a backlight source for a large liquid crystal display device, lights a plurality of discharge lamps.
2. Description of the Related Art
A discharge lamp lighting device with a high-frequency lighting circuit has been proposed which lights cold cathode discharge lamps as a backlight source for a large liquid crystal display device.
FIG. 6
shows such a discharge lamp lighting device, in which light rays emitted from a plurality (six in the figure) of cold cathode discharge lamps L
1
to L
6
are adapted to illuminate a liquid crystal display device by means of reflectors R and a light guiding plate PL disposed between the reflectors R.
FIGS. 7 and 8
are block diagrams of conventional discharge lamp lighting devices described with reference to FIG.
6
. Referring to
FIG. 7
, each lighting circuit LC comprises: a control circuit CT; a driving circuit D driven by the control circuit CT; a leakage transformer T; a discharge lamp L; and a resistor R
1
connected in series to the discharge lamp L, and one lighting circuit LC is provided with each discharge lamp. Referring to
FIG. 8
, a lighting circuit LC comprises: a control circuit CT; a driving circuit D driven by the control circuit CT; a leakage transformer T; three ballast capacitors CB connected in parallel with one another; three discharge lamps L connected in parallel with one another and in series to respective ballast capacitors CB; and a resistor R
1
connected in series to the three discharge lamps L.
The control circuit CT receives a DC power supply V, outputs a predetermined AC signal, detects a tube current flowing from the resistor R
1
to the discharge lamps L, and controls the oscillation amplitude of the driving circuit D. In the discharge lamp lighting device shown in
FIG. 7
, one discharge lamp L requires one high-voltage and high-frequency transformer therefore requiring a plurality of transformers, and the plurality of transformers must be regulated so that tube currents in respective discharge lamps L are equal to one another. In the discharge lamp lighting device shown in
FIG. 8
, high-voltage and high-current capacitors are required, and the discharge lamps L have their lighting frequencies increased to, for example, 50 kHz for stable lighting operation. As a result, stray capacitances CS present between the reflectors R and the cold cathode discharge lamps L
1
to L
6
and present between the cold cathode discharge lamps L
1
to L
6
(see
FIG. 6
) make an impact, whereby tube currents in the discharge lamps change thus generating variance in illuminance.
To overcome the above problem, a discharge lamp lighting device shown in
FIG. 9
for lighting a plurality of discharge lamps was disclosed in Japanese Patent Application Laid-open No. Hei 11-260580. The discharge lamp lighting device comprises: a DC power supply
31
; and first and second switching elements (FETs)
12
and
13
which are connected in series to each other and which are connected respectively to one end and the other end of the DC power supply
31
. A first series resonant circuit
15
consisting of an inductor
15
a
and a first capacitor
17
is connected to the connection between the first and second switching elements
12
and
13
and to the other end of the DC power supply
31
. And a second capacitor
15
b
is connected to the connection between the inductor
15
a
and the first capacitor
17
and to the other end of the DC power supply
31
. The discharge lamp lighting device further comprises: a second series circuit consisting of a first discharge lamp
18
and a first resistor
19
; and a first control circuit
14
to control the switching frequencies of the first and second switching elements
12
and
13
in order to equalize the current in the first discharge lamp
18
to a predetermined value.
A second series resonant circuit
20
consisting of a variable inductor
21
a
and a third capacitor
22
is connected to the connection between the first and second switching elements
12
and
13
and to the other end of the DC power supply
31
. A series circuit consisting of a fourth capacitor
10
, a second discharge lamp
11
, and a second resistor
23
a
to detect current is connected to the connection between the variable inductor
21
a
and the third capacitor
22
and to the other end of the DC power supply
31
. A second control circuit
23
is provided which controls the inductance of the variable inductor
21
a
thereby equalizing the current in the second discharge lamp
11
to a predetermined value. For lighting a plurality of discharge lamps, there are provided a plurality of second series resonant circuits
20
each consisting of the variable inductor
21
a
and the third capacitor
22
, a plurality of series circuits each consisting of the fourth capacitor
10
, the second discharge lamp
11
and the second resistor
23
a
to detect current, and plurality of second control circuits
23
.
The FETs
12
and
13
as switching elements are alternately switched on and off by respective control signals supplied from the first control circuit
14
comprising a microcomputer, and so on to respective gates of the FETs. The first control circuit
14
is capable of controlling the frequency of the control signal across a predetermined range. The connection between a source S of the FET
12
and a drain D of the FET
13
is connected to a cathode of the DC power supply
31
via the series circuit consisting of the inductor
15
a
constituting the first series resonant circuit
15
and the second capacitor
15
b
, and the inductance of the inductor
15
a
and the capacitance of the capacitor
15
b
are set to respective predetermined values so as to set a resonant frequency f0 of the first series resonant circuit
15
to a predetermined frequency.
The above discharge lamp lighting devices have the following problem. Since the inductance value of the variable capacitor
21
a
is controlled so that the current of the second discharge lamp
11
is equal to a predetermined value, the second control circuit
23
for controlling the inductance value is required. Further, for lighting a plurality of discharge lamps, there must be provided a plurality of second series resonant circuits
20
each consisting of the variable inductor
21
a
and the third capacitor
22
, a plurality of series circuits each consisting of the fourth capacitor
10
, the second discharge lamp
11
and the second resistor
23
a
to detect current, and plurality of second control circuits
23
. Accordingly, for example, if six discharge lamps are lighted as shown in
FIG. 6
, its circuit has to be complicated and the number of the components is inevitably increased, thereby making it difficult to realize cost reduction. Also, the increased number of the components tends to degrade the reliability of the device.
SUMMARY OF THE INVENTION
The present invention has been made in light of the above problem, and it is an object of the present invention to provide a reliable discharge lamp lighting device, which uses a limited number of components, and which is capable of lighting a plurality of discharge lamps without suffering the effects of stray capacitances present between and around the discharge lamps.
In order to achieve the above object, according to a first aspect of the present invention, a discharge lamp lighting device comprises a plurality of discharge lamps, at least one reflector to reflect light rays emitted from the discharge lamps, and at least one leakage transformer, and each leakage transformer is adapted to light three discharge lamps, and comprises: a first leakage transformer, which has two primary windings and two secondary windings structurally independent of the two primary windings, and is adapted to light two discharge lamps of the three; and a second leakage transformer, which has a primary winding and a secondary winding structurally independent of the primary winding, and is adapted to light remaining one

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