Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch in the primary circuit of the supply...
Reexamination Certificate
2001-01-12
2002-11-19
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch in the primary circuit of the supply...
C315S291000, C331S047000, C331S048000
Reexamination Certificate
active
06483253
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains generally to a light source device including a dielectric barrier discharge fluorescent lamp that fires utilizing ultraviolet light generated by dielectric barrier discharge. More specifically, the invention is directed to a light source device for use in image readout devices capable of light emission in which a dielectric barrier discharge fluorescent lamp is synchronized with an external synchronization signal without attendant fluctuation in optical power. A phase comparator compares the oscillation signal phase of a variable frequency oscillator divided by a frequency divider with an external synchronization signal. The phase comparator controls the oscillation frequency of a variable frequency oscillator as a function of the phase difference. Consequently, the oscillation phase of the variable frequency oscillator is phase locked by the external synchronization signal. The oscillation signal of the variable frequency oscillator is input to the gate signal generation circuit, and the switch devices of an inverter circuit are opened and closed by the output of a gate signal generation circuit. The direct current voltage output by a DC power source is converted into alternating current voltage and is output by an inverter circuit and is applied to a lamp through a boosting transformer in order to light the lamp.
2. Description of the Related Art
Light source devices including dielectric barrier discharge fluorescent lamps are conventionally used as light sources for image readout devices.
FIG. 10
is a diagram showing a constituent example of the lighting circuit of a dielectric barrier discharge fluorescent lamp
1
utilizing a push-pull inverter circuit while
FIG. 11
is a diagram showing the operation of the inverter circuit presented in FIG.
10
. Dielectric barrier discharge fluorescent lamp
1
includes a boosting transformer
2
, and an inverter circuit
3
comprising switch devices Q
1
, Q
2
connected on the primary winding of boosting transformer
2
. Alternating current voltage is applied to the primary side of boosting transformer
2
by alternately turning on switch devices Q
1
, Q
2
, and lamp
1
is lit.
The sawtooth wave shown in FIG.
11
(
a
) is output by sawtooth wave oscillator
11
and is input to comparator Cmp which compares the sawtooth wave with the voltage Vs at a fixed level, and generates output when the sawtooth wave exceeds a fixed level signal Vs. For this reason, a pulse signal having a prescribed cycle is output as shown in FIG.
11
(
b
) from comparator Cmp when a sawtooth wave is input. This pulse signal is input to clock terminal CLK of flipflop FF of gate signal generation circuit
4
and flipflop FF is inverted as shown in FIG.
11
(
c
) by this pulse signal. The output Q of flipflop FF and its inverted output Q′ (a horizontal line is appended over Q in the diagram, the same hereinafter) are input to the input terminals of gate circuits G
1
, G
2
, and the pulse signal output by the comparator Cmp is input to the other input terminal of aforementioned gate circuits G
1
, G
2
.
Accordingly, a 2-phase pulse signal is output from gate circuits G
1
, G
2
, as shown in FIGS.
11
(
d
) and
11
(
e
), and switch devices Q
3
, Q
4
are alternately turned on by the 2-phase pulse signal. The output of switch devices Q
3
, Q
4
is applied to the gate terminals of the switch devices Q
1
, Q
2
via resistors R
1
, R
2
as gate signals GU, GL for inverter circuits. In so doing, switch devices Q
1
, Q
2
are alternately turned on, voltage is applied to lamp
1
as shown in FIG.
11
(
f
) to light the lamp
1
. Since light emission of the dielectric barrier discharge fluorescent lamp is pulse light emission rather than continuous light, the following problems arise when it is used as the light source of image readout devices.
Specifically, if the processing cycle of an image input means such as a CCD is not synchronized with the inverter oscillation of a power supply device that supplies power to a dielectric barrier discharge fluorescent lamp, the light emission pulse number participating in image readout would not be constant per processing cycle of an image input means such as a CCD, and the brightness of the image that is read per processing cycle of an image input means such as a CCD would change. The mode of this change would be periodic based on the beat of the processing cycle of an image input means such as a CCD and of inverter oscillation. Accordingly, the image that is read would contain strip-like unevenness.
When a power supply device using a flyback inverter circuit is used, synchronization of the two is easily completed by initializing oscillation of an inverter oscillator via an external synchronization signal that exhibits a specific phase (for example, initial timing of the readout cycle) in the processing cycle of an image input means such as a CCD. However, various problems arise following the initialization of an oscillator by an external synchronization signal and synchronization of the two in full-bridge, half-bridge and push-pull inverter circuits.
The lighting circuit shown in
FIG. 10
is explained here. The oscillation phase of inverter oscillators and the synchronization signal Sync fluctuate is based upon various factors. If the two do not overlap, the amount of light would fluctuate based solely on the fluctuation of the light emission cycle, but if the oscillation phases of the oscillator overlap the external synchronization signal due to fluctuation of the oscillation phase, the amount of light from the lamp would change. For example, an external synchronization signal is input, as shown by the broken line in
FIG. 10
, when synchronizing the processing cycle of an image input means such as a CCD with lamp light emission in the lighting circuit, and oscillation of a sawtooth wave oscillator must be initialized by an external synchronization signal.
As shown in FIG.
12
(
a
), sawtooth wave oscillator Ocs is initialized by synchronization signal Sync and oscillation commences after a prescribed period of time when the external synchronization signal Sync is input at timing (following output of a pulse signal that inverts flipflop FF from comparator Cmp). Accordingly, and as shown in FIG.
12
(
c
), flipflop FF is inverted, and as a result, a gate signal as shown in FIGS.
12
(
d
) and
12
(
e
) is input to the gate terminal of switch devices Q
1
, Q
2
of an inverter circuit and the lamp
1
is lit. In contrast, and as shown in FIG.
13
(
a
), sawtooth wave oscillator Ocs is initialized before the sawtooth wave reaches the fixed level of voltage Vs when external synchronization signal Sync is input at the timing, and pulse signal A that inverts the flipflop FF is lost. Thus, the output of flipflop FF becomes the output, as shown in FIG.
13
(
c
). The timing at which switch devices Q
1
, Q
2
turn on adopts the shape shown in FIGS.
13
(
d
) and
13
(
e
). Consequently, the optical power of the lamp decreases.
Usually, the frequency of external synchronization signal Sync and the oscillation frequency of the sawtooth wave oscillator are adjusted since the pulse oscillation phase/frequency fluctuate due to various factors. Even if the sawtooth wave oscillator is synchronized to a certain extent by external synchronization signal Sync, the oscillation phase of the oscillator will occasionally overlap external synchronization signal Sync. The light emission of the lamp would fluctuate if the oscillation phase of the oscillator and external synchronization signal Sync alternate between overlapping and not overlapping due to fluctuation of the oscillation phase of the oscillator and of external synchronization signal Sync.
Furthermore, the ON duration of switch devices Q
1
(or Q
2
) would contract, as shown in FIGS.
14
(
a
)-(
d
), when external synchronization signal Sync and the oscillation phase of the sawtooth wave oscillator overlap, and a large surge voltage would be applied to switch devices Q
1
(or Q
2
). This is because t
Asahina Takashi
Okamoto Masashi
Nixon & Peabody LLP
Safran David S.
Tran Thuy Vinh
Ushiodenki Kabushiki Kaisha
Wong Don
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