Low pressure discharge lamp lighting device and illumination fixture using the same

Abstract

 [Object] To provide a low pressure discharge lamp lighting device which can stably performs dimmed lighting of the lamp to a lower luminous flux level as compared to a control to make a lamp current or a lamp power constant. [Means for Settlement] In a low pressure discharge lamp lighting device including an inverter circuit 4 including one or more switching elements Q1, Q2 for switching a DC voltage outputted from a DC voltage source Vdc, a driving circuit 8 for turning on/off the switching elements Q1, Q2 with a high frequency and a resonance circuit to which a voltage switched by the switching elements Q1, Q2 is applied and supplies a resonance voltage to a discharge lamp LA, wherein feedback means 5 adapted to adjust the DC voltage outputted from the DC voltage source Vdc or turning-on/off of the switching elements Q1, Q2 so that an output of the calculating means 10 which is obtained by dividing the voltage V1a of the discharge lamp LA detected by the lamp voltage detecting means 11 by a current I1a of the discharge lamp detected by the lamp current detecting means 13 becomes substantially equal to a reference voltage Vdim is provided.

Description

[Field of the Invention]

[0001] The present invention relates to a low pressure discharge lamp lighting device for lighting a low-pressure discharge lamp such as a fluorescent lamp at a high frequency by an inverter and an illumination fixture using the same.

[Background Art]

[0002] A general discharge lamp lighting device for dimming a low-pressure discharge lamp typified by a fluorescent lamp, for example, dims a discharge lamp by converting AC power of low frequencies from a commercial AC power source into AC power of high frequency by use of an inverter circuit, supplying the inverted power to the discharge lamp and adjusting the power supplied from the inverter circuit to the discharge lamp. In such a discharge lamp lighting device, when the power supplied to the discharge lamp is lowered, a lighting state of the discharge lamp becomes unstable, possibly leading to phenomena such as flickering and going-out. Therefore, there is a demand to obtain a stable dimming performance which does not cause the above-mentioned phenomena even during dimming lighting at a low luminous flux level.

[0003] As conventional examples of the discharge lamp lighting device which can obtain a stable dimming performance during dimming lighting at a low luminous flux level, there are a technique of making a lamp current constant as disclosed in Patent document 1, a technique of making lamp power constant as disclosed in Patent document 2 and a technique of superimposing a DC voltage and a pulse voltage on a high-frequency voltage applied to the discharge lamp as disclosed in Patent document 3.

(First conventional example)

[0004] Fig. 19 is a circuit diagram of a first conventional example disclosed in Patent document 1 (Patent Publication No. 2727476). A variable AC voltage source 25 outputs an AC voltage to a resonance LC circuit 27 with frequencies which is equal to or close to a resonance frequency thereof. The resonance LC circuit 27 supplies a current to the fluorescent lamp LA. A current sensor 29 senses an amount of a current flowing to a lamp and supplies a detecting signal to an adding part 30. The adding part 30 compares this signal with a signal from a reference signal source 31 and supplies an error signal in proportion to a difference between these signals to an amplifier 32. The amplifier 32 adjusts the variable AC voltage source 25 so as to decrease a difference between the signal from the current sensor 29 and the signal from the reference signal source 31, thereby decreasing magnitude of variation in the current in the fluorescent lamp LA and increasing an output impedance of the circuit. Thus, the equivalent output impedance of this circuit is very high and actually much higher than the impedance of the resonance LC circuit 27 alone. It is assumed that the dimming circuit including a ballast 23 in this mode, which has an equivalent output impedance of about 35 kΩ can stably operate the compact fluorescent lamp at the luminous level less than about 1% of the level during rated lighting.

[0005] Fig. 20 is a V-I characteristic diagram (voltage-current characteristic diagram) of the fluorescent lamp LA used in the first conventional example. In this figure, a maximum negative resistance point A is an operating point at which the lamp is in the most unstable state and there is a very high possibility that an arc current and an optical output vary. This document teaches that the output impedance of the ballast is measured at the maximum negative resistance point of the lamp and is preferably set to be 5kΩ or more especially in the case of dimming of 40% or less.

[0006] Fig. 21 shows a V-I characteristic of the lamp (solid line) and a V-I characteristic of the ballast (dotted line, chain line) in the first conventional example and an intersection point of the lamp characteristic and the ballast characteristic represents the operating point during lighting of the lamp. The ballast characteristic in the first conventional example is represented by the dotted line (a) in the case of high impedance (for example, 5kΩ) and by the chain line (b) in the ideal case where the lamp current is made completely constant.

(Second conventional example)

[0007] Fig. 22 is a circuit diagram of a second conventional example disclosed in Patent document 2 (Unexamined Patent Publication No. 2005-339972). In this circuit, an AC power source Vs is inputted to a rectifying circuit 2 via a low-pass filter circuit 1 for rectification, a predetermined DC voltage is further obtained at a chopper circuit 3 and a smoothing capacitor C0 and a voltage between both ends of the smoothing capacitor C0 as a DC power source is supplied to a half-bridge inverter circuit 4. A series circuit including switching elements Q1, Q2 formed of FETs and a resistor Rs is connected to both ends of the smoothing capacitor C0 and a resonance circuit including a resonance inductor L1, a resonance capacitor C1 and a discharge lamp LA connected to the resonance capacitor C1 in parallel is connected to a series circuit formed of the low-side switching element Q2 and the resistor Rs to apply a voltage between both ends of the resonance capacitor C1 to the discharge lamp LA. Then, output power of the discharge lamp LA is detected based on a voltage between both ends of the resistor Rs, the voltage between both ends is compared with a variable reference voltage Vref which determines a dimming level by an error amplifier EA forming a comparator of feedback means 5 and a voltage signal based on the difference is outputted as a feedback signal. The frequency of a pulse signal from an oscillator 8a is modulated based on a voltage signal obtained by adding a control signal formed of a voltage signal from a pulse signal generator 7 to the feedback signal by an adding device 6.

[0008] A driver circuit 8b generates a gate signal for alternately turning on/off the switching elements Q1, Q2 based on an oscillating signal from the oscillator 8a. A chopper control circuit 3a controls the chopper circuit 3 so that an output voltage of the chopper circuit 3 becomes a predetermined voltage. A capacitor Cfb and a resistor Rfi constitute a delay element of the error amplifier EA.

[0009] Fig. 23 shows a V-I characteristic of the lamp (solid line) and a V-I characteristic of the ballast (dotted line) in the second conventional example and an intersection point of the lamp characteristic and the ballast characteristic represents the operating point during lighting of the lamp. According to the V-I characteristic of the ballast (dotted line) in the second conventional example, a lamp power W1a is controlled to be constant. For this reason, given that a lamp power factor is 1, the following relation: W1a = V1a·I1a (= constant) holds for the lamp power W1a, a lamp voltage V1a and a lamp current I1a and V1a is proportional to an inverse number of I1a (1/I1a).

(Third conventional example)

[0010] Fig. 24 is a circuit diagram of a third conventional example disclosed in Patent document 3 (Unexamined Patent Publication No. 2005-339976). This discharge lamp lighting device includes an inverter circuit 4a formed of two switching elements (field effect transistors) Q1, Q2 serially connected between both electrodes of a DC power source E, a resonance circuit 4b including an inductor L1 and a capacitor C1 which are serially connected between both ends (drain-source) of the high-side switching element Q1 through a DC cutting capacitor C2 and a preheating circuit 4c for preheating each filament of a discharge lamp (fluorescent lamp) LA, one end of the filament (not shown in the drawing) being connected to the resonance capacitor C1.

[0011] The preheating circuit 4c includes secondary windings L2, L3 which are provided at the resonance inductor L1 and serially connected both ends of the filament of the discharge lamp 3 and DC cutting capacitors Cf1, Cf2 inserted between the secondary windings L2, L3 and one ends of the filaments on the non-power source side, respectively. One ends of two filaments on the non-power source side are connected to each other through a resistor Rdc1, and an output detecting part 16 including of a series circuit formed of voltage-dividing resistors Rdc2, Rdc3 and a smoothing capacitor Cdc connected to the voltage-dividing resistor Rdc3 in parallel is provided on the power source side of the two filaments. A DC voltage is applied from the DC power source E to the discharge lamp LA through the output detecting part 16 and the resistor Rdc1 inserted between the filaments.

[0012] In the so-called half-bridge type inverter circuit 4a, the switching elements Q1, Q2 are alternately turned on/off (switched) with a high frequency by feeding of a driving signal outputted from a driving circuit 8 to a gate through gate resistors Rg1, Rg2 and the DC voltage supplied from the DC power source E is inverted into a high-frequency voltage. The driving circuit 8 is formed of a, for example, general-purpose timer IC and is oscillated with a frequency set by a frequency setting circuit 15 formed of an external capacitor CX and three resistors Rx1, Rx2, Rx3. The oscillating frequency corresponds to a driving frequency of the inverter circuit 4a, that is, a switching frequency of the switching elements Q1, Q2.

[0013] As a composite current value of a current flowing to the capacitor CX and the resistors Rx1 to Rx3 in the frequency setting circuit 15 increases, the oscillating frequency of the driving circuit 8 becomes higher, and as the oscillating frequency of the driving circuit 8, that is, the driving frequency of the inverter circuit 4a becomes higher, a resonance voltage of the resonance circuit 4b lowers and high-frequency power supplied to the discharge lamp LA also decreases. A lighting control signal Vref is inputted from a dimmer not shown in the drawing to the frequency setting circuit 15 through the resistor Rx2. The lighting control signal Vref is obtained from the DC voltage which is proportional to a dimming ratio (illuminance ratio assuming that the illumination intensity during the rated lighting is 100%). Thus, since a value of the current flowing to the resistor Rx2 increases as the DC voltage level lowers with lowering of the dimming ratio, the composite current value of the frequency setting circuit 5 increases and the driving frequency of the inverter circuit 4a becomes higher to dim the discharge lamp LA.

[0014] The output detecting part 16 equivalently detects the DC voltage applied to the discharge lamp LA as voltage drop of the voltage-dividing resistor Rdc3 and the detecting signal is smoothed by the smoothing capacitor Cdc connected to the voltage-dividing resistor Rdc3 in parallel and at the same time, high-frequency noise is removed. The detecting signal of the output detecting part 16 together with the lighting control signal Vref are inputted to the feedback means 5.

[0015] The feedback means 5 includes an operational amplifier OP1 to which the detecting signal is inputted through an input resistor Rin at its inverting input terminal and the lighting control signal Vref is inputted at its non-inverting input terminal, and a feedback resistor Rfb1 and a capacitor Cfb which are connected between an output terminal and the inverting input terminal of the operational amplifier OP1 in parallel, and performs control (feedback control) to adjust the oscillating frequency of the driving circuit 8 by increasing/decreasing the current value of the frequency setting circuit 15 so that the voltage level of the detecting signal is equal to that of the lighting control signal Vref at all times.

[0016] Furthermore, by applying a periodic pulse voltage from a pulse voltage superimposing circuit 18 to the resistor Rx3 of the frequency setting circuit 15, the driving frequency of the inverter circuit 4a is periodically changed, thereby superimposing the high pulsing voltage (pulse voltage) on the high-frequency voltage (lamp voltage) applied to the discharge lamp LA.

[0017] The pulse voltage superimposing circuit 18 includes buffers B1 to B3 formed of operational amplifiers and diodes D1, D2 for selecting a higher voltage out of DC voltage signal Vdc and a triangular wave pulse signal Vp1, and a pulse voltage command signal Vpc as a signal obtained by combining the two types of signals is fed to the resistor Rx3 of the frequency setting circuit 15. The pulse voltage command signal Vpc outputted from the pulse voltage superimposing circuit 18 becomes a triangular wave pulse signal whose amplitude becomes smaller as the DC voltage level of the DC voltage signal Vdc is larger.

[0018] The frequency setting circuit 15 sets the oscillating frequency of the driving circuit 8 to a value corresponding to a dimming ratio of the lighting control signal Vref according to the output voltage level of the feedback means 5 and periodically changes the oscillating frequency to a lower value by the frequency corresponding to the pulse amplitude of the pulse voltage command signal Vpc from the pulse voltage superimposing circuit 18. As a result, when the oscillating frequency is periodically changed to a lower value, the pulse voltage is superimposed on the lamp voltage V1a applied to the discharge lamp LA from the inverter circuit 4a through the resonance circuit 4b.

[0019] The lamp voltage V1a of the discharge lamp LA exhibits a waveform as shown in Fig. 25. By increasing a peak value of the pulse voltage in the case of the dimming ratio which easily causes unstable discharging and applying the pulse voltage required to sufficiently maintain lighting of the discharge lamp LA, the discharge lamp LA can be stably lighted even in the case of the dimming ratio which easily causes the most unstable discharging.

[Conventional Technique Document]

[Patent Document]

[0020] 

[Patent document1] Patent Publication No. 2727476

[Patent document2] Japanese Unexamined Patent Publication No. 2005-339972

[Patent document3] Japanese Unexamined Patent Publication No. 2005-339976

[Disclosure of the Invention]

[Problem to be Solved by the Invention]

[0021] In the first conventional example and the second conventional example, since the lamp current I1a is fed back so as to be made constant or the lamp power W1a is fed back so as to be made constant respectively, relatively stable lighting can be achieved even at a point of low dimming level. However, when the dimming level is further lowered to a lower luminous flux level below a lower limit thereof, a divergence between the V-I characteristic of the lamp and the V-I characteristic of the ballast occurs, resulting in undesired phenomena such as going-out, flickering of the lamp and jump of operating point in the lamp.

[0022] In the third conventional example, although stable lighting can be achieved at the low luminous flux level by superimposition of a DC voltage component and application of the pulse voltage, the control circuit becomes complicated. In addition, disadvantages are caused such as the superimposition of the DC voltage component easily generates Cataphoresis phenomenon of the lamp.

[0023] In consideration of these matters, an object of the present invention is to provide a low pressure discharge lamp lighting device which can stably dim the lamp to a further low luminous flux level without superimposing the DC voltage or the pulse voltage as compared to conventional control which makes the lamp current or the lamp power constant.

[Means Adapted to Solve the Problems]

[0024] According to a first aspect of the present invention, to attain the above-mentioned object, as shown in Fig. 1, in a discharge lamp lighting device including an inverter circuit 4 including one or more switching elements Q1, Q2 for switching a DC voltage outputted from a DC voltage source Vdc, a driving circuit 8 for turning on/off the switching elements Q1, Q2 with a high frequency and a resonance circuit (inductor L1, capacitor C1) to which a voltage switched by the switching elements Q1, Q2 is applied, the resonance circuit supplying a resonance voltage to a discharge lamp LA, means adapted to perform feedback control to make a lamp impedance of the discharge lamp LA substantially constant is provided.

[0025] According to a second aspect of the present invention, in the first aspect of the present invention, as the means adapted to perform feedback control to make the lamp impedance of the discharge lamp LA substantially constant, lamp voltage detecting means 11 adapted to detect a voltage V1a of the discharge lamp LA, lamp current detecting means 13 adapted to detect a current I1a of the discharge lamp LA, calculating means 10 adapted to drive the voltage V1a of the discharge lamp LA detected by the lamp voltage detecting means 11 by the current I1a of the discharge lamp detected by the lamp current detecting means 13 and feedback means 5 adapted to adjust the DC voltage outputted from the DC voltage source Vdc or turning-on/off of the switching elements Q1, Q2 so that an output of the calculating means 10 becomes substantially equal to a reference voltage Vdim are provided.

[0026] According to a third aspect of the present invention, in the second aspect of the present invention, the reference voltage Vdim can be adjusted by a dimmer 9.
According to a fourth aspect of the present invention, in the third aspect of the present invention, the reference voltage Vdim adjusted by the dimmer 9 is continuously variable or a set value during dimming is an integral multiple of a set value during rated lighting.

[0027] According to a fifth aspect of the present invention, in any of the second to fourth aspects of the present invention, as shown in Figs. 5 to 7, a gain of the feedback means 5 is varied according to the dimming level.

[0028] According to a sixth aspect of the present invention, in any of the second to fifth aspects of the present invention, as shown in Figs. 8 and 9, the function of the feedback means 5 is deactivated before start of the discharge lamp LA and activated after start of the discharge lamp LA.

[0029] According to a seventh aspect of the present invention, in any of the second to sixth aspects of the present invention, as shown in Figs. 10 and 11, the feedback means 5 can change over between a first feedback function to adjust the DC voltage outputted from the DC voltage source Vdc or turning-on/off of the switching elements Q1, Q2 so that the output of the calculating means 10 becomes substantially equal to the reference voltage Vdim and a second feedback function to adjust the DC voltage outputted from the DC voltage source Vdc or turning-on/off of the switching elements Q1, Q2 so that the output of the lamp current detecting means 13 becomes substantially equal to the reference voltage Vdim.

[0030] According to an eighth aspect of the present invention, in any of the second to seventh aspects of the present invention, as shown in Fig. 12, an abnormal detecting circuit 51 for suppressing or stopping the output of the inverter circuit 4 when the output of any one of the lamp voltage detecting means 11 or the lamp current detecting means 13 becomes equal to or larger than a predetermined threshold value is provided.

[0031] According to a ninth aspect of the present invention, in any of the second to eighth aspects of the present invention, as shown in Fig. 13, an upper limiter 52 for limiting the output of the calculating means 10 to be equal to or smaller than a predetermined upper limit value is provided.

[0032] According to a tenth aspect of the present invention, in any of the second to eighth aspects of the present invention, as shown in Fig. 15, a lower limiter 53 for limiting the output of the lamp current detecting means 13 to be equal to or larger than a predetermined lower limit value is provided.

[0033] According to an eleventh aspect of the present invention, in any of the second to tenth aspects of the present invention, the calculating means 10 is any one of a dividing analog IC, a microcomputer or a digital signal processor.

[0034] According to a twelfth aspect of the present invention, in any of the first to eleventh aspects of the present invention, as shown in Fig. 16, a sensor 17 is provided and the dimming level is varied according to an output of the sensor 17.

[0035] A thirteenth aspect of the present invention is an illumination fixture incorporating the low pressure discharge lamp lighting device according to any of the first to twelfth aspects of the present invention therein (Fig. 18).

[Effect of the Invention]

[0036] According to the first and second aspects of the present invention, since the feedback means adapted to perform control to make a calculation result obtained by dividing the lamp voltage by the lamp current, that is, a value corresponding to the lamp impedance constant is provided, as compared to the conventional control such as the lamp current feedback and the lamp power feedback, an intersection point of the voltage-current characteristic of the discharge lamp and the voltage-current characteristic of the inverter circuit can be formed more stably and hence, dimming to the lower luminous flux level can be achieved.

[0037] According to the third or fourth aspect of the present invention, the optical output at the lower limit of dimming can be finely adjusted.
According to the fifth aspect of the present invention, since the gain need not be forcedly increased in the vicinity of a rating, divergence in the feedback means is hard to occur. The lamp is more tolerant of going-out by increasing the gain as dimming is deeper.
According to the sixth aspect of the present invention, control according to the present invention to be tolerant of going-out during dimming can be started after the lamp reliably starts.

[0038] According to the seventh aspect of the present invention, there is an advantageous effect that lamps with a same rated current and different rated power can be lighted by using the same inverter circuit and are hard to go out during dimming.
According to the eighth aspect of the present invention, in a no-load state or an abnormal state due to divergence in the feedback means, oscillation of the circuit can be safely weakened or stopped. By using the lamp voltage detecting means or the lamp current detecting means also as the abnormal detecting circuit, a circuit configuration can be simplified.

[0039] According to the ninth or tenth aspect of the present invention, going-out of the lamp out of feedback control can be prevented.
According to the twelfth aspect of the present invention, dimming to the low luminous flux level can be achieved while making illumination intensity constant according to correction by the sensor.

[Brief Description of the Drawings]

[0040] 

[Fig. 1] Fig. 1 is a circuit diagram of a first embodiment according to the present invention.

[Fig. 2] Fig. 2 is a diagram for describing operation of the first embodiment according to the present invention.

[Fig. 3] Fig. 3 is a diagram for describing operation of the first embodiment according to the present invention.

[Fig. 4] Fig. 4 is a diagram for describing operation of the first embodiment according to the present invention.

[Fig. 5] Fig. 5 is a circuit diagram of a second embodiment according to the present invention.

[Fig. 6] Fig. 6 is a diagram for describing operation of the second embodiment according to the present invention.

[Fig. 7] Fig. 7 is a diagram for describing operation of a modification of the second embodiment according to the present invention.

[Fig. 8] Fig. 8 is a circuit diagram of a third embodiment according to the present invention.

[Fig. 9] Fig. 9 is a diagram for describing operation of the third embodiment according to the present invention.

[Fig. 10] Fig. 10 is a circuit diagram of a fourth embodiment according to the present invention.

[Fig. 11] Fig. 11 is a diagram for describing operation of the fourth embodiment according to the present invention.

[Fig. 12] Fig. 12 is a circuit diagram of a fifth embodiment according to the present invention.

[Fig. 13] Fig. 13 is a circuit diagram of a sixth embodiment according to the present invention.

[Fig. 14] Fig. 14 is a diagram for describing operation of the sixth embodiment according to the present invention.

[Fig. 15] Fig. 15 is a circuit diagram of a modification of the sixth embodiment according to the present invention.

[Fig. 16] Fig. 16 is a circuit diagram of a seventh embodiment according to the present invention.

[Fig. 17] Fig. 17 is a diagram for describing operation of the seventh embodiment according to the present invention.

[Fig. 18] Fig. 18 is a perspective view showing appearance of an illumination fixture in an eighth embodiment according to the present invention.

[Fig. 19] Fig. 19 is a block diagram showing configuration of a first conventional example.

[Fig. 20] Fig. 20 is a diagram for describing a characteristic of a fluorescent lamp in the first conventional example.

[Fig. 21] Fig. 21 is a diagram for describing operation of the first conventional example.

[Fig. 22] Fig. 22 is a circuit diagram of a second conventional example according to the present invention.

[Fig. 23] Fig. 23 is a diagram for describing operation of the second conventional example.

[Fig. 24] Fig. 24 is a circuit diagram of a third conventional example according to the present invention.

[Fig. 25] Fig. 25 is a diagram for describing operation of the third conventional example.

[Best Mode for Carrying Out the Invention]

(First embodiment)

[0041] Fig. 1 is a circuit diagram showing a basic configuration of a first embodiment according to the present invention. A series circuit formed of switching elements Q1, Q2 is connected to a DC voltage source Vdc. The switching elements Q1, Q2 are turned on/off at high frequency by a driving circuit 8. A series resonance circuit formed of an inductor L1 and a capacitor C1 is connected between a connection point of the switching elements Q1, Q2 and one end (here, a negative electrode) of the DC voltage source Vdc. One end of the capacitor C2 is connected to one end of the capacitor C1. The above-mentioned circuits constitute an inverter circuit 4 as an electronic ballast (ballast).

[0042] A fluorescent lamp LA is connected between the other end of the capacitor C2 and a terminal on the source side of the switching element Q2. A V1a detecting circuit 11 for detecting a lamp voltage V1a is connected to the fluorescent lamp LA. An I1a detecting circuit 13 for detecting a lamp current I1a is connected to a lamp current path of the fluorescent lamp LA. An output of the V1a detecting circuit 11 and an output of the I1a detecting circuit 13 are inputted to the calculating means 10. The calculating means 10 is calculating means adapted to divide a detected value of the lamp voltage V1a by a detected value of the lamp current I1a.

[0043] An output signal of the calculating means 10 is inputted to feedback means 5 and applied to an input terminal on the negative side of an operational amplifier OP through a resistor R. An adjustable reference voltage Vdim is applied to an input terminal on the positive side of the operational amplifier OP. A capacitor C3 is connected between the input terminal on the negative side and an output terminal of the operational amplifier OP to constitute a so-called integrated circuit. An output of the operational amplifier OP is inputted to the driving circuit 8 of the inverter circuit 4 to constitute a feedback system so that V1a/I1a agrees with a target value corresponding to the reference voltage Vdim. V1a/I1a represents a lamp impedance R1a and is feedback controlled by the operational amplifier OP so that the value becomes equal to a reference value. Although a switching frequency of the inverter circuit 4 is controlled by the driving circuit 8 herein, ON duration (duty) of the switching elements Q1, Q2 may be controlled or a voltage of the DC voltage source Vdc may be controlled.

[0044] The DC voltage source Vdc can be obtained from a commercial power source through a PFC circuit (power factor improving control circuit) such as a step-up chopper circuit, for example, and the voltage may be adjusted by means such as changing frequency or duty of a switching element of the PFC circuit. A value of the reference voltage Vdim as a target value for dimming can be varied by a dimmer 9 installed outside of the inverter circuit 4. An output signal of the dimmer 9 may be a DC voltage, a PWM signal or a digital signal such as a binary code.

[0045] Operation of the present embodiment will be described hereinafter. At lighting of the fluorescent lamp LA, the V1a detecting circuit 11 detects a voltage corresponding to an effective value of the lamp voltage V1a and the I1a detecting circuit 13 detects a voltage corresponding to an effective value of the lamp current I1a. Then, the calculating means 10 calculates V1a/I1a. An operation of the integrated circuit formed of the operational amplifier OP, the resistor R and the capacitor C3 feedback controls the output V1a/I1a of the calculating means 10 so as to become substantially equal to the reference voltage Vdim.

[0046] The above-mentioned operation will be specifically described referring to Fig. 2. Fig. 2 shows the V-I characteristic when a horizontal axis represents the lamp current I1a and a vertical axis represents the lamp voltage V1a, (a) and (b) in this figure show the V-I characteristic of the ballast and (c) in this figure the V-I characteristic of the lamp. A chain line in this figure shows the V-I characteristic of the ballast in a so-called open state in which the feedback means 5 does not operate (conditions when the switching frequency is constant).

[0047] The above-mentioned V1a/I1a of the calculating means 10 means merely a calculation of the lamp impedance R1a. Accordingly, when the above-mentioned V1a/I1a (= R1a) is controlled to be constant, the V-I characteristic of the ballast exhibits a straight line which passes the origin and has a constant gradient as shown in (a) and (b) in Fig. 2. Since an operating point during lighting of the lamp is an intersection point of the V-I characteristic of the lamp and the V-I characteristic of the ballast, the intersection point of (a) and (c) and the intersection point of (b) and (c) in this figure become the operating points during lighting.

[0048] By varying the reference voltage Vdim by the dimmer 9, the V-I characteristic of the ballast exhibits a straight line which passes the origin and has a variable gradient as shown in (a) and (b) in Fig. 2, and thus, arbitrary dimming can be achieved. Since the reference voltage Vdim represents the lamp impedance R1a, the value during dimming becomes larger that during rated lighting. The value of the lamp impedance R1a varies depending on the type of the lamp and becomes about a few hundreds of (Ω) during rated lighting and becomes a few k(Ω) to a few dozens of k(Ω) during dimming.

[0049] Fig. 3 is an enlarged view of the vicinity of the intersection point in Fig. 2. In this figure, a solid line represents the V-I characteristic of the lamp, a dotted line represents the V-I characteristic of the ballast and chain lines represent the V-I characteristic of the ballast in the open state under a condition of constant frequency. In the vicinity of an intersection point of the chain line (B) and the V-I characteristic of the lamp as the operating point, the point repeatedly reciprocates between (a) and (b) on the V-I characteristic of the ballast (dotted line) and finally converges to a point (B). As a gain of a feedback system is larger, the reciprocating width between (a) and (b) becomes smaller and the point converges more quickly. However, when the gain is too large, the system diverges and the point has a tendency to easily deviate from the intersection point.

[0050] When the point deviates from the intersection point, the point does not move to unlimitedly increase the value along the dotted line of the V-I characteristic of the ballast, but transit on a track of a resonance system (the chain line in this figure) in the open state at the time when it deviates. For example, when the point deviates at a point (b), the point moves upward along the line (C). Since this state is the same as a non-load state in the case where the feedback system is not used, it can be addressed by a conventional non-load detecting circuit such as a circuit detecting the lamp voltage and stopping oscillation.

[0051] As apparent from the above-mentioned description, as compared to the control to make the lamp current constant (the first conventional example) and make the lamp power constant (the second conventional example), since the V-I characteristic of the lamp intersects the V-I characteristic of the ballast more perpendicularly especially in the vicinity of the lower limit of dimming, the operating point is stably formed and the dimming lower limit can be set deeper.

[0052] An analog divider may be used as the calculating means 10. Alternatively, a microcomputer or DSP (digital signal processor) may be used as a digital divider. Since the feedback means 5 as means adapted to make V1a/I1a constant can perform digital processing, assuming that all inputs from the dimmer 9 are unified to digital signals and the whole feedback circuit can perform digital processing. Thus, all processing can be performed by using the microcomputer, resulting in simplification and size reduction of the circuit.

[0053] It is preferred that outputs of the detecting circuits 11, 13 are, for example, set to DC values corresponding to the effective value. By setting the outputs to such values, calculation can be made in instantaneous values, which leads to faster convergence. However, in the case of the instantaneous values, since the lamp current becomes 0 at a point when the polarity of the AC current is inverted, the output at division tends to diverge easily and hence, a circuit for correcting the divergence separately becomes necessary.

[0054] Although continuous lighting is described in this embodiment, for example, the present invention can be also applied to so-called burst dimming of intermittently turning on/off the lamp and control during lighting in an ON period is performed in the same way. Alternatively, the present invention can be also applied to control which intermittently repeats rated lighting and dimming lighting at the low luminous flux level. Since the dimming state at the low luminous flux level can be darker according to the control of the present invention, the control of the present invention is effective to increase the dimming ratio.

[0055] When the load LA is a hot cathode fluorescent lamp, by combining the control to increase a preheating current corresponding to generally used dimming, lighting stability at the low luminous flux level is further improved.

[0056] Fig. 4 is a diagram for describing a dimming method using the dimmer 9. The gradient (set value of R1a) of the ballast characteristic with respect to the origin is continuously varied by continuously varying the reference voltage Vdim in Fig. 1 by the dimmer 9 according to the above-mentioned control, resulting in that the dimming operation is also continuously performed. This method is suitable especially to the case of varying the dimming level in an analog fashion. However, for example, in the case such as staged dimming and digital control using the microcomputer or the like, a command value of the dimming level becomes discrete.

[0057] In Fig. 4, it is assumed that the operating point during rated lighting is (a) and the reference voltage at this time is N(V). In Fig. 4, in the dimming from this point, the command value is varied at regular intervals: 2·N, 3·N, 4·N, .... As the operating point starting from (a) during rated lighting changes to (b), (c), (d), (e), ..., the dimming level becomes deeper. The gradient of the ballast characteristic with respect to the origin increase by twice, three times, ..., while the V-I characteristic of the lamp, as shown in Fig. 4, is represented by a monotonically decreasing convex downward curve. Therefore, the dimming width in the vicinity of the dimming lower limit becomes smaller than the change width in the vicinity of the rating. For this reason, dimming in the vicinity of the dimming lower limit can be automatically adjusted with small intervals and going-out and jump of the lamp due to sudden change of the operating point hardly occurs.

[0058] As described above, in this embodiment, since the intersection point of the V-I characteristic of the lamp and the V-I characteristic of the ballast perpendicularly approaches, dimming to the lower luminous flux level can be achieved. Especially in the low luminous flux region, two intersection points of the V-I characteristic of the lamp and the V-I characteristic of the ballast never exist as in open control and thus, the operating point hardly jumps. When feedback control is disabled (closed loop diverges) due to no-load and the like, the operating point moves on the LC resonance system, and therefore, security can be ensured by stopping the operation point by means of a conventional no-load detecting circuit (for example, detection of V1a).

[0059] The above-mentioned operation can be achieved by a simple feedback circuit and a dividing function can be inexpensively performed by a microcomputer and the like, enabling reduction of costs. Further, when the load is a hot cathode fluorescent lamp, the lamp can be combined with the preheating current control such as increasing the preheating current in the vicinity of the dimming lower limit, thereby further improving stability of lighting at the low luminous flux level.

(Second embodiment)

[0060] Fig. 5 is a circuit diagram of a second embodiment according to the present invention. This configuration is different from the configuration shown in Fig. 1 in that a resistor R' is serially connected to the resistor R and a switch SW1 is parallelly connected to the resistor R'. The switch SW1 can change over on/off thereof according to a signal of the dimmer 9. Since the other configuration is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0061] This embodiment is an example in which the gain of feedback is changed in the midst of the dimming operation. Since the resistor R' is short-circuited when the switch SW1 is turned on, the circuit operation is the same as that shown in Fig. 1. At this time, an integrated gain G1 of the feedback means 5 is calculated by the following equation: G1 = 1/(2π f·C3·R). Here, f represents frequency.

[0062] Since the resistor R' is opened when the switch SW1 is turned off, an integrated gain G2 of the feedback means 5 is calculated by the following equation: G2 = 1/(2 π f·C3·(R+R')). Therefore, the integrated gain at the time when the switch SW1 is turned off is smaller than the integrated gain at the time when the switch SW1 is turned on.

[0063] A specific circuit operation will be described referring to Fig. 6. On/off of the switch SW1 is set to change over according to the dimming level. It is controlled so that the switch SW1 is turned on, on a left side from a chain line and turned off on a right side from the chain line. According to such control, the circuit is controlled by a relatively moderate feedback gain in the vicinity of rated lighting and is controlled by a relatively strong feedback gain in the deep dimming region. The magnitude of the feedback gain is conceptually represented by the length of arrows in Fig. 6. The figure shows that as the arrows are longer, it takes a longer time to converge the feedback gain.

[0064] Fig. 7 shows relationship between the lamp current I1a and the gain G in another example in which the gain is changed. A chain line (b) in this figure represents lighting during the rated lighting and a chain line (a) represents a turning point of increase/decrease of the gain. When the lamp is gradually dimmed from the operating point during the rated lighting, the angle of the intersection point (of tangent line) of the V-I characteristic of the ballast and the V-I characteristic of the lamp gradually changes from a blunt angle (acute angle) to a right angle and then, to an acute angle (blunt angle) after passing the turning point. For this reason, the gain is set to be a minimum value at the point where the angle is the closest to the right angle as shown by the point (a) and the gain is continuously increased according to the degree of the intersection point moving away from the point (a). Although a circuit for performing the operation shown in Fig. 7 is not shown in the drawing, the resistor R in place of the switch SW1 may be variably controlled continuously or the capacity of the capacitor C3 may be variably controlled continuously.

[0065] In this embodiment, since it is no need to forcedly increase the gain of the feedback means in the vicinity of the rating, divergence in the feedback means hardly occurs. The lamp becomes more tolerant of going-out by increasing the gain as dimming is deeper. Furthermore, by setting a gain minimum point to a point where the V-I characteristic of the ballast and the V-I characteristic of the lamp (tangent line thereof) intersect at right angle and increasing the gain as the intersection point moves away from the point, the lamp becomes tolerant of going out over all segments of dimming.

(Third embodiment)

[0066] Fig. 8 is a circuit diagram of a third embodiment according to the present invention. This configuration is different from the configuration shown in Fig. 1 in that a switch SW2 is inserted between the output of the feedback means 5 and the input of the driving circuit 8 and a circuit for changing over on/off of the switch SW2 according to the detection output of the I1a detecting circuit 13 is added. Since the other configuration is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0067] This embodiment is an example showing specific control of starting of the hot cathode fluorescent lamp which includes sequence of preheating, starting and lighting. The above-mentioned operation is the operation during lighting and preferably, is not performed prior to starting of the lamp. For this reason, before the lamp LA is started by application of a high voltage, the switch SW2 is turned off, thereby deactivating the feedback control is ensured, and after starting the lamp, the above-mentioned feedback operation is started by turning on the switch SW2 after detecting the lamp current is flowing.

[0068] Operation in this embodiment will be described referring to Fig. 9. In this figure, (a) represents the on/off state of the switch SW2 and (b) represents the voltage applied to the lamp terminal. During preheating, although the voltage is applied to the lamp terminal and a preheating current flows to the filament of the lamp, no lamp current flows. At starting, the high voltage is applied to the lamp terminal discharge breakdown occurs in the lamp and the lamp current starts to flow. The I1a detecting circuit 13 detects that the lamp current starts to flow, allowing the switch SW2 to be turned on. Operation after lighting is the same as described above.

[0069] In this embodiment, the feedback control according to the present invention which reliably starts the lamp and is tolerant of going-out during dimming can be started.

(Fourth embodiment)

[0070] Fig. 10 is a circuit diagram of a fourth embodiment according to the present invention. This configuration is different from the configuration shown in Fig. 1 in that a switch SW3 for selectively changing over between the output of the I1a detecting circuit 13 and the output of the calculating means 10 as the input of the feedback means 5 is provided and a circuit for changing over the switch SW3 according to the output of the dimmer 9 is added. Since the other portion is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0071] This embodiment is an example in which a target for the feedback control is changed over according to the dimming level. In other words, by changing over the switch SW3 according to the dimming level, the lamp current I1a is controlled to be constant when the switch SW3 is connected to the output of the I1a detecting circuit 13 and the lamp impedance R1a (= V1a/I1a) is controlled to be constant when the switch SW3 is connected to the output of the calculating means 10.

[0072] Fig. 11 is a diagram for describing operation in the case where the control of this embodiment is applied. By changing over the switch SW3 to the side of the I1a detecting circuit 13 on a right side from a chain line and to the side of the calculating means 10 on a left side from the chain line according to the dimming level, the lamp current I1a is feedback controlled to be constant at a point (A) in the vicinity of the rating. Further, the lamp impedance R1a (= V1a/I1a) is controlled to be constant at points (B) and (C) during deep dimming.

[0073] According to such control, for example, the lamps with the same rated current and different power can be lighted by the same inverter circuit and the above-mentioned circuit which is tolerant of going-out during dimming can be constituted.

[0074] Although the feedback control to make the lamp impedance R1a (= V1a/I1a) constant is changed over to the feedback control to make the lamp current I1a constant in this embodiment, the target for changing over is not limited to the feedback control to make the lamp current I1a constant and for example, may be changed over to the conventional feedback control to make the lamp power W1a constant. In this case, it is needed to separately add a circuit for calculating the lamp power W1a or a value proportional to the lamp power W1a.

[0075] In this embodiment, the lamps with the same rated current and different power can be lighted by the same inverter circuit and the lamp hardly goes out during dimming.

(Fifth embodiment)

[0076] Fig. 12 is a circuit diagram of a fifth embodiment according to the present invention. This configuration is different from the configuration shown in Fig. 1 in that an abnormal detecting circuit 51 for receiving an output of the V1a detecting circuit 11 and detecting abnormality is added. The output of the abnormal detecting circuit 51 is inputted to the driving circuit 8. Since the other portion is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0077] In this embodiment, the output of V1a detecting circuit 11 is also used to detect abnormality. In other words, by using the output of the V1a detecting circuit 11 not only for the feedback control but also for abnormality detection, the circuit configuration can be simplified. When the detected value of the lamp voltage V1a exceeds a predetermined threshold value, the abnormal detecting circuit 51 may weaken or stop an oscillating output of the inverter circuit 4.

[0078] Although the output of the V1a detecting circuit 11 is also used for abnormality detection in this embodiment, for example, abnormality may be detected based on the output of I1a detecting circuit 13 or on the values of both detecting circuits.

[0079] In this embodiment, by also using the detecting circuit for lamp voltage or the lamp current for abnormality detection, the oscillating output of the inverter circuit can be safely weakened or stopped in a no-load state or abnormal state due to divergence in the feedback means, while simplifying the circuit.

(Sixth embodiment)

[0080] Fig. 13 is a circuit diagram of a sixth embodiment according to the present invention. This embodiment is an example that going-out of the lamp caused by deviation of the operating point of the feedback control from the V-I characteristic of the lamp can be prevented in advance. This configuration is different from the configuration shown in Fig. 1 in that an upper limiter circuit 52 is added to the output of the V1a/I1a calculating means 10. Since the other configuration is the same as the configuration shown in Fig. 1, redundant description thereof is omitted. The upper limiter circuit 52 has a function to limit the value of R1a (= V1a/I1a) inputted to the feedback means 5 so that the value becomes equal to or larger than a predetermined upper limit value.

[0081] Operation in this embodiment will be described referring to Fig. 14. As shown in Fig. 14, when the value of R1a(= V1a/I1a) inputted to the feedback means 5 is limited to be below a dotted line (a), the scope of the feedback control becomes a diagonally shaded area including a dotted line (b) and therefore, the V-I characteristic of the lamp and the V-I characteristic of the ballast reliably have the intersection point thereof, thereby preventing going-out. In other words, the V-I characteristic of the ballast and a negative characteristic area of the V-I characteristic of the lamp reliably have the intersection point thereof.

[0082] Similarly, for example, as shown in Fig. 15, a lower limiter circuit 53 may be provided at the output of the I1a detecting circuit 13. The lower limiter circuit 53 has a function to limit the detected value of the Ila detecting circuit 13 so that the value becomes equal to or smaller than a predetermined lower limit value. Since the other configuration is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0083] Since a circuit shown in Fig. 15 can prevent a value of a denominator for calculating V1a/I1a from being close to 0, resulting in being equivalent to setting an upper limit value in the calculation result of R1a (= V1a/I1a) and therefore, operation can be limited in the diagonally shaded area shown in Fig. 14.

[0084] In this embodiment, the V-I characteristic of the ballast and the V-I characteristic of the lamp reliably have the intersection point thereof, thereby preventing going-out due to deactivation of from the feedback control.

(Seventh embodiment)

[0085] Fig. 16 is a circuit diagram of a seventh embodiment according to the present invention. This configuration is different from the configuration shown in Fig. 1 in that an output of an illuminance sensor 17 is superimposed on the output of the dimmer 9. Since the other configuration is the same as the configuration shown in Fig. 1, redundant description thereof is omitted.

[0086] In this embodiment, the optical output of the illuminance sensor 17 is corrected. According to the control explained in the first embodiment, when the V-I characteristic of the lamp changes due to, for example, ambient temperature characteristic of the lamp LA, the operating point consistently moves on the line of the V-I characteristic of the ballast.

[0087] Then, for example, in the case of the operation at the intersection point on (a) in Fig. 2, due to change of the V-I characteristic of the lamp, the operating point moves on the line (a) in Fig. 2 in the upper right or lower left direction. There is no problem merely in terms of prevention of going-out. However, the movement of the intersection point in the upper right or lower left direction means the lamp power increases or decreases, respectively, resulting in that visual illumination intensity increases/decreases.

[0088] Thus, as shown in Fig. 16, the illumination intensity is corrected to be constant by inputting the output of the illuminance sensor 17 to an adder 19 and adding the output to the lighting control signal from the dimmer 9.

[0089] Fig. 17(A) shows a relationship between the actual illumination intensity (horizontal axis) and the output of the illuminance sensor 17. As the illumination intensity is higher, the output of the illuminance sensor 17 becomes larger. On the contrary, as the illumination intensity is lower, the output of the illuminance sensor 17 becomes smaller.

[0090] Fig. 17(B) shows a relationship between the output of the dimmer 9 and the reference voltage Vdim. A solid line represents characteristic in the case where the output of the illuminance sensor 17 is not superimposed, a dotted line (a) represents a corrected value in the case where the output of the illuminance sensor 17 is larger than a threshold value L and a dotted line (b) represents a corrected value in the case where the output of the illuminance sensor 17 is smaller than the threshold value L.

[0091] Thereby, a correcting signal of the illuminance sensor 17 is superimposed on the output of the dimmer 9. Since the signal superimposed on the output of the dimmer 9 becomes larger as the output of the illuminance sensor 17 is larger, control is made in the direction of further deepening dimming. According to this control, the illumination intensity is made constant. On the contrary, since the signal superimposed on the output of the dimmer 9 becomes smaller as the output of the illuminance sensor 17 is smaller and hence, the dimming level becomes shallower. According to this control, the illumination intensity is made constant.

[0092] In this embodiment, the lamp LA can be stably lighted during dimming at the low luminous flux level dimming while maintaining the illumination intensity constant by correction by the illuminance sensor 17.

[0093] Although use of the illuminance sensor 17 is described in this embodiment as an example, it is needless to say that use of a sensor is not limited to the illumination sensor and an output signal of the other sensor (for example, an ambient temperature sensor and the like) may be superimposed on the output of the dimmer 9.

(Eighth embodiment)

[0094] Fig. 18 shows appearance of an illumination fixture which mounts the low pressure discharge lamp lighting device according to any of the first to the seventh embodiments thereon. The illumination fixture 40 includes a fixture main body 41 which incorporates the discharge lamp lighting device as stated in any of the first to the seventh embodiments therein and a pair of sockets 42 for electrically connecting the discharge lamp lighting device to the fluorescent lamp LA, wherein a filament electrodes of the fluorescent lamp LA are detachably attached to the sockets 42. Although the straight-tube fluorescent lamp is used as the load herein, the present invention may be applied to a lighting device for a circular fluorescent lamp, a compact fluorescent lamp or a double annular fluorescent lamp. In this embodiment, the illumination fixture capable of stably performing dimming lighting to the low luminous flux level can be realized.

[Description of Reference Numerals]

[0095] 

Vdc

DC voltage source

Q1

Switching element

Q2

Switching element

4

Inverter circuit

5

Feedback means

8

Driving circuit

LA

Discharge lamp

Claims

1. A discharge lamp lighting device comprising:

an inverter circuit formed of one or more switching elements for switching a DC voltage outputted from a DC voltage source;

a driving circuit for turning on/off the switching elements with high frequency and a resonance circuit to which a voltage switched by the switching elements is applied;

the resonance circuit supplying a resonance voltage to a discharge lamp, wherein:

means adapted to perform feedback control to make a lamp impedance of the discharge lamp substantially constant is provided.

2. The low pressure discharge lamp lighting device according to claim 1, wherein:

as the means adapted to perform feedback control to make the lamp impedance of the discharge lamp substantially constant;

lamp voltage detecting means adapted to detect a voltage of the discharge lamp;

lamp current detecting means adapted to detect a current of the discharge lamp;

calculating means adapted to divide the voltage of the discharge lamp detected by the lamp voltage detecting means by the current of the discharge lamp detected by the lamp current detecting means; and

feedback means adapted to adjust the DC voltage outputted from the DC voltage source or turning-on/off of the switching elements so that an output of the calculating means becomes substantially equal to a reference voltage are provided.

3. The low pressure discharge lamp lighting device according to claim 2, wherein:

the reference voltage can be adjusted by a dimmer.

4. The low pressure discharge lamp lighting device according to claim 3, wherein:

the reference voltage adjusted by the dimmer is continuously variable or a set value during dimming is an integral multiple of a set value during rated lighting.

5. The low pressure discharge lamp lighting device according to any of claims 2 to 4, wherein:

a gain of the feedback means is varied according to the dimming level.

6. The low pressure discharge lamp lighting device according to any of claims 2 to 5, wherein:

a function of the feedback means is deactivated before start of the discharge lamp and activated after start of the discharge lamp.

7. The low pressure discharge lamp lighting device according to any of claims 2 to 6, wherein:

the feedback means can change over between a first feedback function to adjust the DC voltage outputted from the DC voltage source or turning-on/off of the switching elements so that the output of the calculating means becomes substantially equal to the reference voltage;

and a second feedback function to adjust the DC voltage outputted from the DC voltage source or turning-on/off of the switching elements so that the output of the lamp current detecting means becomes substantially equal to the reference voltage.

8. The low pressure discharge lamp lighting device according to any of claims 2 to 7, wherein:

the low-pressure discharge lamp further includes an abnormal detecting circuit for suppressing or stopping the output of the inverter circuit upon the output of any one of the lamp voltage detecting means or the lamp current detecting means becoming equal to or larger than a predetermined threshold value.

9. The low pressure discharge lamp lighting device according to any of claims 2 to 8, wherein:

the low-pressure discharge lamp further includes an upper limiter for limiting the output of the calculating means to be equal to or smaller than a predetermined upper limit value.

10. The low pressure discharge lamp lighting device according to any of claims 2 to 8, wherein:

the low-pressure discharge lamp further includes a lower limiter for limiting the output of the lamp current detecting means to be equal to or larger than a predetermined lower limit value.

11. The low pressure discharge lamp lighting device according to any of claims 2 to 10, wherein:

the calculating means is any one of a dividing analog IC, a microcomputer or a digital signal processor.

12. The low pressure discharge lamp lighting device according to any of claims 1 to 11, wherein:

the low-pressure discharge lamp further includes a sensor; and

the dimming level is varied according to an output of the sensor.

13. An illumination fixture incorporating the low pressure discharge lamp lighting device according to any of claims 1 to 12 therein.

Drawing