Rare gas fluorescent lamp lighting apparatus

There is provided a rare gas fluorescent lamp lighting apparatus including: an input power source; a transformer having a rare gas fluorescent lamp connected to the secondary side thereof; a switching element connected in series to the primary side of the transformer; and a drive block to drive the switching element. The drive block includes: a constant voltage circuit to convert the voltage of the input power source into a constant voltage as an output; a constant current circuit to output a charging current according to the voltage of the input power source; a triangular waveform oscillation circuit to increase and decrease oscillation frequency according respectively to an increase and a decrease in a voltage of the input power source; and a comparison circuit to transform a triangular waveform outputted from the triangular waveform oscillation circuit into a rectangular waveform with a predetermined duty ratio as an output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rare gas fluorescent lamp lighting apparatus, and more particularly to a rare gas fluorescent lighting apparatus to light a rare gas fluorescent lamp as a light source in a document reading device, such as a facsimile machine, an image scanner, and a copying machine.

2. Description of the Related Art

Recently, a rare gas fluorescent lamp, which emits light through rare gas discharging, is increasingly used as a light source for illumination in a document reading device, such as a copying machine, and an image scanner. It is well known that a rare gas fluorescent lamp is lighted with a high-intensity luminance when a high-frequency pulse voltage is applied, and a lighting apparatus with an inverter circuit to generate such a high-frequency pulse voltage is preferably employed for lighting a rare gas fluorescent lamp.

On the other hand, there is a problem with a rare gas fluorescent lamp that when a lamp current is varied due to the fluctuation of an input voltage, or the like during the operation of the aforementioned document reading device, the light amount of a lamp is varied thereby deteriorating accuracy of reading a document, which results in impairing image reproduction quality. In order to deal with this problem, a rare gas fluorescent lamp lighting apparatus is proposed, in which a DC-DC converter is put at the input side of the circuit to generate a high-frequency voltage thereby stabilizing electric power at the input side so that the light mount of a rare gas fluorescent lamp is stabilized without suffering the influence of the fluctuation of the input voltage (refer to, for example, Japanese Patent Application Laid-Open No. 2001-15284).

FIG. 18is a block diagram of such a rare gas fluorescent lamp lighting apparatus as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2001-15284, in which a step-up DC-DC converter CV is provided at the input side of a high-frequency voltage generating circuit HC. A second driving circuit CT in the DC-DC converter CV is adapted to feed a PWM modulated driving signal to a second switching element S2based on a signal fed back from a current detecting circuit R so that a current detected at the current detecting circuit R has a constant peak value, whereby the output is controlled at a constant electric power without receiving the influence of voltage fluctuation of a DC power supply E1thus stabilizing the light amount of a rare gas fluorescent lamp DL.

The rare gas fluorescent lamp lighting apparatus shown inFIG. 18, which includes the step-up DC-DC converter CV disposed at the input side of the high-frequency voltage generating circuit HC as described above, suffers deterioration in efficiency due to the switching loss of the switching element S2, the copper loss and the iron loss of a coil L1, and the loss at a diode D1, and also requires a large space for mounting components thus preventing downsizing of the apparatus. And, the second driving circuit CT is usually constituted as an IC, and therefore the entire cost is pushed up.

SUMMARY OF THE INVENTION

The present invention has been made in light of the above problems and therefore has as an object to provide a rare gas fluorescent lamp lighting apparatus, in which the amount of light is stabilized without suffering the influence of the voltage fluctuation of the input power supply and without deterioration of efficiency, and which is inexpensively fabricated with a smaller dimension.

In order to achieve the object described above, according to an aspect of the present invention, a rare gas fluorescent lamp lighting apparatus is provided, which includes: an input power source; a transformer having a rare gas fluorescent lamp connected to a secondary side thereof; a switching element connected in series to a primary side of the transformer; and a drive block to drive the switching element. The drive block includes: a triangular waveform oscillation circuit to increase and decrease oscillation frequency according respectively to an increase and a decrease in a voltage of the input power source; and a comparison circuit to transform a triangular waveform outputted from the triangular waveform oscillation circuit into a rectangular waveform with a predetermined duty ratio. Since the drive block, which drives the switching element connected to the primary side of the transformer, includes the oscillation circuit adapted to vary the frequency according to the variation of the input voltage, the voltage inputted to the primary side of the transformer can be maintained substantially constant, whereby the light amount of the rare gas fluorescent lamp is stabilized.

In the aspect of the present invention, the drive block may further include: a constant voltage circuit to convert the voltage of the input power source into a constant voltage as an output; and a constant current circuit to output a charging current according to the voltage of the input power source, and the triangular waveform oscillation circuit may include: an oscillator capacitor to be charged by the charging current of the constant current circuit and to be discharged at a predetermined time constant; and a comparator which switches between a charging mode and a discharging mode for the oscillator capacitor, and to which a threshold voltage resulting from division of the constant voltage from the constant voltage circuit, and a terminal-based voltage across both terminals of the oscillator capacitor are inputted, whereby the terminal-based voltage is outputted as a triangular waveform voltage. Since the oscillation circuit is simply structured with the comparator and the capacitor as main components without employing circuit components for electric power control to stabilize the voltage of the input power source, the light amount of the rare gas fluorescent lamp can be stabilized without deterioration in efficiency, thus enabling downsizing and cost reduction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1, a rare gas fluorescent lamp lighting apparatus50according the embodiment of the present invention generally includes an inverter circuit6connected to an input power source Vin, a drive block10to drive the inverter circuit6, and a protection circuit7provided between the inverter circuit6and the drive block10. A rare gas fluorescent lamp DL containing a rare gas consisting mainly of, for example, xenon (Xe) is connected to the output side of the inverter circuit6. The inverter circuit6includes a transformer TR and a switching element Q1connected in series to the primary side TRp of the transformer TR, and the switching element Q1is constituted by, for example, an n-channel MOSFET. The drive block10includes a constant voltage circuit1and a constant current circuit2, which are connected to the input power source Vin, and further includes a triangular waveform oscillation circuit3, a comparison circuit4, and a drive circuit5, which receive a voltage from the constant voltage circuit1, and the output of the constant current circuit2is connected to the triangular waveform oscillation circuit3. The output of the drive circuit5is connected to the switching element Q1.

Referring toFIG. 2, the structures and operations of the circuits in the rare gas fluorescent lamp lighting apparatus50will be discussed.

The constant voltage circuit1includes resistors R1and R2, a zener diode ZD, and a transistor Tr1. The connection portion of the resistor R1and the zener diode ZD is connected to the base terminal of the transistor Tr1, and a constant voltage V set by a zener voltage of the zener diode ZD is supplied to the triangular waveform oscillation circuit3, the comparison circuit4, and the drive circuit5.

The constant current circuit2includes variable resistors R3, R4, R5, R6and R7, a diode D1, and a transistor Tr2. The connection portion of the resistor R6and the diode D1is connected to the base terminal of the transistor Tr2, and a charging current Ic set by a voltage of the input power source Vin and resistance values of the variable resistors R3, R4, R5, R6and R7is supplied to the triangular waveform oscillation circuit3. In the constant current circuit2, the variable resistors R3, R4, R5, R6and R7are set at respective predetermined values thereby supplying a constant current independent of a variation of a load connected to the collector terminal of the transistor Tr2, but when the voltage of the input power source Vin undergoes a variation, a voltage applied across both terminals of a series circuit consisting of the resistor R5and the diode D1is caused to vary, specifically increase and decrease according respectively to the increase and decrease of the voltage of the input power source Vin, which causes a current value set by the voltage and the resistor R7to increase and decrease. Thus, the constant current circuit2in the present embodiment is adapted to supply to the triangular waveform oscillation circuit3the charging current Ic which varies according to the variation of the voltage of the input power source Vin. The constant current2may alternatively be constituted by a well known current mirror circuit.

The triangular waveform oscillation circuit3includes a comparator COMP1and an oscillator capacitor C1as main components. A non-inverting input terminal COMP(+) of the comparator COMP1is connected to the connection portion of resistors R8and R9which constitute a series circuit between an output voltage line11of the constant voltage circuit1and the ground and is connected also to an output terminal COMP1o of the comparator COMP1via a feedback resistor R11. The oscillator capacitor C1has its one terminal connected to the inverting input terminal COMP(−) of the comparator COMP1and has the other terminal grounded. The one terminal of the oscillator capacitor C1is also connected to the constant current circuit2via a diode2and to the output terminal COMP1o of comparator COMP1via a diode D3and a resistor R12. The triangular waveform oscillation circuit3in the present embodiment is adapted to output to the comparison circuit4a triangular waveform voltage generated across both terminals of the oscillator capacitor C1, and the operation of the triangular waveform oscillation circuit3will be detailed later.

The comparison circuit4includes a comparator COMP2and resistors R13and14. The inverting input terminal of the comparator COMP2is connected to the connection portion of the resistors R13and R14which constitute a series circuit between the output voltage line11of the constant voltage circuit1and the ground, and the aforementioned triangular waveform voltage as the output from the triangular waveform oscillation circuit3is inputted to the non-inverting terminal of the comparator COMP2. The comparison circuit4in the present embodiment is adapted to compare the triangular waveform voltage with a threshold voltage resulting from the constant voltage V divided by the resistors R13and R14, whereby a rectangular waveform voltage having a prescribed duty ratio is generated at an output terminal COMP2o of the comparator COMP2and outputted to the drive circuit5.

The drive circuit5is a push-pull current amplification circuit including transistors Tr3and Tr4. The drive circuit5is driven by the rectangular waveform voltage from the comparison circuit4so as to rapidly charge and discharge a capacitance between the gate and source of the switching element Q1of the inverter circuit6, thereby supplying a driving signal Vgs having a high-frequency rectangular waveform to the gate terminal of the switching element Q1.

The inverter circuit6includes, as described above, the transformer TR and the switching element Q1connected in series to the primary side TRp of the transformer TR. The switching element Q1receives the high-frequency driving signal Vgs supplied from the drive circuit5and is thereby turned on and off. When the switching element Q1is turned on, a current Id, which increases linearly, is caused to flow at the primary side TRp of the transformer TR, and energy is stored at the transformer TR. Then, when the switching element Q1is turned off, the current Id is cut off, and the energy stored is discharged to the secondary side TRs of the transformer TR, whereby an output voltage is induced at the secondary side TRs of the transformer TR and applied to the rage gas fluorescent lamp DL, and the rare gas fluorescent lamp DL is lighted. During this process, a pulse voltage Vds according to the energy stored in the transformer TR is generated across the drain and source of the switching element Q1.

The protection circuit7is adapted to protect circuit elements against stresses generated during no-load discharge at the secondary side TRs of the transformer TR. The protection circuit7detects a current flowing at the secondary side TRs of the transformer TR, whereby the drive circuit5is deactivated, for example, when the rare gas fluorescent lamp DL is not connected. The operation of the protection circuit7will be briefly explained. When the rare gas fluorescent lamp DL is connected, the current flowing at the secondary side TRs of the transformer TR is rectified by a diode D4so as to charge a capacitor C4thereby turning on a transistor Tr6, which causes electric charge stored at a capacitor C5to be discharged, consequently turning off a transistor Tr5. Accordingly, the aforementioned rectangular wave voltage is generated at the output terminal COMP2o of the comparator COMP2of the comparison circuit4connected to the collector terminal of the transistor Tr5, and the drive circuit5is duly activated. On the other hand, when the rare gas fluorescent lamp DL is not connected, the capacitor C4is discharged thereby turning off the transistor Tr6, which causes the capacitor C5to be charged, consequently turning on the transistor Tr5. Accordingly, the output of the comparator COMP2of the comparison circuit4is fixedly maintained substantially at the ground potential, and the drive circuit5is deactivated.

The operations of the triangular waveform oscillation circuit3and the comparison circuit4will be described with reference toFIGS. 3A to 3C, andFIGS. 4A and 4B. In the description below, the output voltage from the constant voltage circuit1is defined as V, and R8=R9=R11=R where R is substantially larger than R10.

Description will first be made, with reference toFIGS. 3A to 3C, on how the triangular waveform oscillation circuit3operates when the voltage of the input power source Vin is constant.

Referring toFIG. 3A, it is assumed that an output voltage at the output terminal COMP1o of the comparator COMP1stays at the high level (i.e., voltage V) during time period TC. Referring toFIG. 3B, a threshold voltage VTH(VTH=R/(R+R/2)V≈0.66 V) is inputted to the non-inverting input terminal COMP(+) of the comparator COMP1during period TC. The oscillator capacitor C1is charged by the charging current Ic supplied from the constant current circuit2, and the voltage across both terminals of the oscillator capacitor C1increases almost linearly during time period TCas shown inFIG. 3C. This voltage is inputted to the inverting input terminal COMP(−) of the comparator COMP1, and when the input voltage at the inverting terminal COMP(−) increases and reaches the threshold voltage VTHat time point t1, the output voltage at the output terminal COMP1o of the comparator COMP1is switched to the low level as shown inFIG. 3A.

Referring again toFIG. 3A, the output at the output terminal COMP1o of the comparator COMP1stays at the low level during time period TD, and a threshold voltage VTL(VTL=(R/2)/(R+R/2)V≈0.33 V<VTH) is inputted to the non-inverting input terminal COMP(+) of the comparator COMP1during time period TDas shown inFIG. 3B. The oscillator capacitor C1is discharged via the diode D3and the resistor R12, and consequently the voltage across both terminals of the oscillator capacitor C1decreases by a time constant determined by the oscillator capacitor C1and the resistor R12(the time constant in the present embodiment is adjusted so that the voltage decreases almost linearly as shown inFIG. 3C). This voltage is inputted to the inverting input terminal COMP(−) of the comparator COMP1, and when the input voltage at the inverting input terminal COMP(−) decreases and reaches the threshold voltage VTLat time point t2, the output voltage at the output terminal COMP1o of the comparator COMP1is switched back to the high level.

The above-described operations during time periods TCand TDare repeated, and the triangular waveform voltage shown inFIG. 3Cis generated across both terminals of the oscillator capacitor C1. The triangular waveform oscillation circuit3in the present embodiment is adapted to output this triangular waveform voltage to the comparison circuit4.

The impact of variation of the voltage of the input power source Vin on the operation of the triangular waveform oscillation circuit3will now be described with reference toFIG. 4A. When the voltage of the input power source Vin increases while the triangular waveform oscillation circuit3outputs the triangular waveform voltage having a waveform indicated by a solid line shown inFIG. 4A, the waveform is caused to change into a waveform indicated by a dashed line shown therein. That is to say, since the charging current Ic from the constant circuit2increases according to the increase of the voltage of the input power source Vin as described above, the speed of charging the oscillator capacitor C1increases. Consequently, the gradient of solid straight line VC1appearing during charge period T1Cchanges into the gradient of dashed straight line VC2appearing during charge period T2C, thus meaning an increase in the gradient. Meanwhile, since the threshold voltages VTHand VTLwhich are generated on the basis of the input voltage V from the constant voltage circuit1are maintained substantially constant, charge time T2Cafter the increase of the voltage is decreased compared with charge time T1Cbefore the increase of the voltage. And, since the processes of discharging the oscillator capacitor1during discharge periods T1Dand T2Care identical with each other, the gradients of solid straight line VD1and dashed straight line VD2are almost identical with each other, and accordingly discharge periods T1Dand T2Dare also almost identical with each other. Consequently, cycle time T2of the triangular waveform voltage after the increase of the voltage of the input power source Vin is decreased compared with cycle time T1of the triangular waveform voltage before the increase of the voltage of the input power source Vin, thus a triangular waveform voltage with a higher frequency is outputted.

The triangular waveform voltage outputted from the triangular waveform oscillation circuit3is inputted to the non-inverting input terminal of the comparator COMP2of the comparison circuit4, and the threshold voltage Vthresulting from the output voltage V of the constant voltage circuit1divided by the resistors R13and R14is inputted to the inverting input terminal of the comparator COMP2. Consequently, the rectangular waveform generated at the output terminal COMP2o of the comparator COMP2is at the high level when the triangular waveform voltage is higher than the threshold voltage Vth, and is at the low level when the triangular waveform voltage is lower than the threshold voltage Vth, thus a rectangular waveform voltage is generated. In the present invention, the threshold voltage Vthof the comparator COMP2is set to stay substantially at the halfway level between the two threshold voltages VTHand VTLof the comparator COMP1, and the duty ratio of the rectangular waveform voltage outputted from the output terminal COMP2o of the comparator COMP2is set about at 50% regardless of frequency.

FIG. 4Bshows rectangular waveform voltages which correspond respectively to the triangular waveform voltages indicated by solid and dashed lines, and which are transformed at the threshold voltage Vth. Since the rectangular waveform voltage has its frequency increased with its duty ratio kept substantially constant while the voltage of the input power source Vin increases, the time periods at the high level and the low level in one cycle time are shortened. As described above, the rectangular waveform voltage is adapted to drive, via the drive circuit5, the switching element Q1of the inverter circuit6, for example, such that the rectangular waveform voltage is at the high level when the switching element Q1is turned on. Consequently, it will be understood that the on-time of the on/off operation of the switching element Q1is caused to decrease with an increase in the voltage of the input power source Vin.

The effect and advantage of the rare gas fluorescent lamp lighting apparatus50according to the present embodiment will be described with reference toFIGS. 5 to 17.

Referring first toFIGS. 5 to 9, it is understood that the frequency of the voltage at the inverting input terminal COMP(−) of the comparator COMP1, that is to say, the frequency of the driving signal Vgs is increased with an increase in the voltage of the input power source Vin, which is shown inFIG. 10.

Referring then toFIGS. 11 to 15, it is understood that the peak value of the current Id increasing during one on-operation of the switching element Q1, and the peak value of the pulse voltage Vds generated at the primary side TRp of the transformer TR at the off-time of the switching element Q1are decreased with an increase in the voltage of the input power source Vin, which means that the energy stored in the transformer TR during one on-operation of the switching element Q1is reduced. The above-described variation of the peak value of the voltage Vds corresponding to the variation of the voltage at the input power source Vin is shown inFIG. 16.

Thus, in the rare gas fluorescent lamp lighting apparatus50according to the present embodiment, the operating frequency of the switching element Q1is varied with the duty ratio of the input power source Vin maintained substantially constant, whereby the energy stored at the transformer TR during one on-operation of the switching element Q1is caused to vary so as to cancel out the variation of the voltage of the input power source Vin, and consequently the electric power applied to the transformer TR is maintained substantially constant. As a result, the light amount of the rare gas fluorescent lamp DL connected to the secondary side TRs of the transformer TR is maintained substantially constant, even if the voltage of the input power source Vin varies, which is evidenced by the graph ofFIG. 17.