Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-02-01
2001-08-28
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S2090SC, C315SDIG004, C315SDIG007
Reexamination Certificate
active
06281639
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lighting circuit of a cold cathode discharge lamp, and more specifically, to a circuit for adjusting luminance of the cold cathode discharge lamp by a duty light adjusting system.
2. Description of the Related Art
FIG. 5
is a schematic constructional view showing one mode of a lighting circuit of a cold cathode discharge lamp in which luminance is controlled by a duty light adjusting system.
As shown in this figure, a high frequency voltage (about 60 kHz) generated by a ROYER oscillating circuit
12
is increased by a transformer
13
and the cold cathode discharge lamp
11
is lighted by this increased high voltage (from 600 to 1600 V). In the lighting circuit of the cold cathode discharge lamp having such a circuit construction, an oscillating operation of the ROYER oscillating circuit is normally turned on/off every constant period and a ratio of a turning-on period to the constant period, i.e., a duty ratio of the oscillating operation of the ROYER oscillating circuit is changed by a PWM (Pulse Width Modulation) circuit
17
so that luminance of the cold cathode discharge lamp is adjusted. In the mode shown in
FIG. 5
, a luminance adjusting waveform
16
of H/L (High state/Low state) outputted from an IC (comparator) X
4
of the PWM circuit
17
is inputted to a switching circuit
14
and the operation of the ROYER oscillating circuit
12
is controlled by an output signal from this switching circuit
14
. The duty ratio of the luminance adjusting waveform
16
of the cold cathode discharge lamp
11
is controlled by the magnitude of a light adjusting signal voltage inputted to an inversion input terminal of the comparator X
4
.
FIG. 6
shows a voltage waveform inputted to an input terminal of the comparator X
4
and a voltage waveform outputted from an output terminal of this comparator.
A triangular wave voltage Va is inputted to a non-inversion input terminal of the comparator X
4
and light adjusting signal voltages
18
(Vb
1
, Vb
2
, Vb
3
) are inputted to the inversion input terminal of the comparator X
4
. Rectangular wave voltages Vo
1
, Vo
2
, Vo
3
corresponding to these light adjusting signal voltages Vb
1
, Vb
2
, Vb
3
are respectively outputted from the output terminal of the comparator. For example, when the light adjusting signal voltage Vb
1
is inputted to the inversion input terminal, this light adjusting signal voltage Vb
1
and the triangular wave voltage Va inputted to the non-inversion input terminal are compared with each other. A voltage level of the output terminal is set to a high (H) state in a period in which the triangular wave voltage Va is higher than the light adjusting signal voltage Vb
1
. In contrast to this, the voltage level of the output terminal is set to a low (L) state in a period in which the triangular wave voltage Va is lower than the light adjusting signal voltage Vb
1
. Namely, the rectangular wave voltage Vo
1
is outputted from the output terminal. Similarly, when light adjusting signal voltages Vb
2
, Vb
3
are inputted to the inversion input terminal, light adjusting signal voltages Vb
2
, Vb
3
are respectively compared with the triangular wave voltage Va, and rectangular wave voltages Vo
2
, Vo
3
are respectively outputted from the output terminal.
Thus, a PWM signal varying a ratio of the high (H) period to the low (L) period in one cycle depending on the magnitude of a light adjusting signal voltage is outputted, and the high (H) period is lengthened as the light adjusting signal voltage is reduced. This output voltage is inputted to a transistor Q
1
of the switching circuit
14
, turns on transistors Q
1
and Q
2
during the high (H) period and makes the ROYER oscillating circuit
12
start oscillating operation, so that a high frequency voltage is increased by the transformer
13
and is applied to the cold cathode discharge lamp
11
. Luminance of the cold cathode discharge lamp
11
is increased as the light adjusting signal voltage is reduced, i.e., as a duty ratio of output waveforms is increased. The duty ratio of the PWM signal is normally set such that this duty ratio varies in a range from 10 to 100% when the light adjusting signal voltage varies from 0 to 5 V.
FIG. 7
is a constructional view showing a conventional example of the lighting circuit of the cold cathode discharge lamp using the duty light adjusting system.
A ROYER oscillating circuit
12
is a voltage resonance type circuit constructed by transistors Q
3
, Q
4
, a capacitor C
2
and a transformer (T
1
)
13
. As mentioned above, when the transistors Q
1
, Q
2
of the switching circuit
14
are turned on, a direct current bias is applied to the transistors Q
3
, Q
4
of the ROYER oscillating circuit
12
from a DC power source (12 V) through the transistor Q
2
and a resistor R
8
so that the ROYER oscillating circuit
12
is oscillated. In this example, an oscillating frequency of the ROYER oscillating circuit
12
is set to 60 kHz and a secondary voltage of the transformer
13
is increased such that an alternating voltage from about 600 to 1600 V
P-P
is generated on a secondary side of the transformer
13
.
The triangular wave voltage Va inputted to the non-inversion input terminal of the comparator X
4
that constitutes the PWM circuit
17
is generated by operational amplifiers X
2
, X
3
. A rectangular wave voltage is first generated by positively feeding an output voltage of the operational amplifier X
2
back to a non-inversion input terminal through a resistor R
14
. Zener diodes ZD
2
, ZD
3
between the output terminal and an inversion input terminal of the operational amplifier X
2
are connected to set a wave height value of the rectangular wave voltage to a constant value. The rectangular wave voltage, that is, the output voltage of the operational amplifier X
2
is inputted to an inversion input terminal of the operational amplifier X
3
. The operational amplifier X
3
forms an integrator and is fed back from an output terminal to an inversion input terminal through a capacitor C
6
. Thus, the inputted rectangular wave voltage is integrated and is outputted from the output terminal of the operational amplifier X
3
as a triangular wave voltage of the same frequency as the rectangular wave voltage. A frequency of the triangular wave voltage is normally set to from 100 to 600 Hz. A three-terminal regulator X
1
is used as a power source for supplying a power voltage to the above operational amplifiers X
2
, X
3
and the comparator X
4
. The power source voltage can be stably supplied irrespective of a change in the power source voltage (12 V) by using the three-terminal regulator X
1
, so that a change in the PWM signal voltage can be reduced.
However, due to its limited performance, the power source voltage would vary by about ±10%. Accordingly, when the power source voltage (12 V) varies by ±10% in the above conventional example, a primary voltage of the transformer
13
is also varied by ±10%. As a result, the increased secondary voltage of the transformer
13
is varied (by ±10%), so that an electric current flowing through the cold cathode discharge lamp
11
is also changed and so is the luminance.
Therefore, a circuit of the following system has been used to prevent this change in luminance.
FIG. 8
is a view showing the construction of a circuit using a DC/DC converter
20
to reduce the change in luminance caused by the change in the power source voltage.
A PWM signal voltage outputted from a comparator X
4
is transmitted to a transistor Q
1
. Thus, transistors Q
1
, Q
2
are turned on and a power source voltage is supplied to operational amplifiers X
5
, X
6
. A voltage proportional to an electric current flowing through the cold cathode discharge lamp is applied to both ends of a resistor R
1
. This voltage is rectified and smoothed by a diode D
1
and a capacitor C
9
and is applied to an inversion input terminal of the operational amplifier X
6
. The applied voltage is compared with a reference volt
Ota Masayuki
Suzuki Koichiro
Suzuki Shin'ichi
Minebea Co. Ltd.
Oliff & Berridg,e PLC
Philogene Haissa
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