Fluorescent lamp lighting apparatus

Abstract: A fluorescent lamp lighting apparatus comprising a DC power source (10), a pair of field-effect transistors (32, 34) which are connected in series to each other, in which drains and sources of these transistors are connected in series to each other, whereas gates are connected to an oscillator (36), a pair of voltage-­ dividing capacitors (38, 40) which are connected in parallel to the DC power source (10), and an inverter circuit (30) having an insulative leakage transformer (42) whose primary coil (42₁) is connected to a contact between these field-effect transistors (32, 34) and also to the other contact between those capacitors (38, 40). One-ends of filaments (46, 48) of a fluorescent lamp (44) are connected to both ends of the secondary coil (42₂) of the leakage transformer (42), whereas a startup capacitor (50) is connected between the other ends of the filaments (46, 48). By provision of a series resonant circuit composed of the secondary coil (42₂) and the startup capacitor (50), part of the series resonant circuit is released while the fluorescent lamp is off from the lighting apparatus, thus minimizing the resonant output.

Description: The present invention relates to a fluorescent lamp lighting apparatus, and more particularly, to a fluorescent lamp lighting apparatus using an inverter circuit. Generally, any conventional apparatus for lighting an electric discharge lamp like a fluorescent lamp for example uses an inverter circuit. For example, a typical conventional fluorescent lamp lighting apparatus has the structure described below. The positive electrode of DC power source is connected to the drain of the first field-effect transistor, whereas the negative electrode of this DC power source is connected to the source of the second field-effect transistor. The source of the first field-effect transistor is connected to the drain of the second field-effect transistor and also to an end of the primary coil of a leakage transformer. The other end of this primary coil is connected to a contact between the first and second capacitors which are connected in series between both ends of the DC power source. Furthermore, one ends of filaments on both sides of a fluorescent lamp are connected to both ends of the secondary coil of this leakage transformer. The other ends of these filaments are connected to such portions slightly inside of the both ends of the secondary coil. A startup capacitor is connected in parallel between the other sides of these filaments. Assume that the first and second field-effect transistors of the lighting apparatus having the above structure are alternately turned ON and OFF. Then, DC voltage delivered from the DC power source is converted into AC voltage to induce alternate current on the part of the secondary coil of the leakage transformer before eventually lighting up the fluorescent lamp. Nevertheless, this conventional fluorescent lamp lighting apparatus magnifies resonant current when no load is present in the apparatus. This in turn causes the first and second field-effect transistors to incur unwanted destruction in some cases. To prevent this, all the conventional fluorescent lamp light apparatuses need to install an independent safety circuit. To prevent these field-effect transistors from incurring unwanted destruction, there is such a conventional electric-discharge lamp lighting apparatus having a typical structure described below. The positive electrode of the DC power source is connected to the drain side of the first field-effect transistor having the source connected to the drain of the second field-effect transistor. The negative electrode of this DC power source is connected to the source of the second field-effect transistor. The first and second capacitors are connected to each other in series, which are respectively connected between both ends of the DC power source. An end of one of filaments of a fluorescent lamp is connected to the contact between the first and second field-effect transistors via a reactor. An end of the other filament of this fluorescent lamp is connected to the contact between the first and second capacitors. A startup capacitor is connected between the other end of one of these filaments and the other end of the other filaments. The first and second field-effect transistors of the fluorescent lamp lighting apparatus having the above structure are alternately turned ON and OFF to convert DC voltage into the predetermined AC voltage so that the fluorescent lamp can be lit up. While the fluorescent lamp is not loaded in the lighting apparatus, circuits of this lighting apparatus remain open and inoperative so that the first and second field-effect transistors can be prevented from incurring unwanted destruction while no load is present. Nevertheless, there is no means of insulating the fluorescent lamp itself from the DC power supply source, and thus, there is potential fear.to incur electric shock while loading and unloading the fluorescent lamp. Prior art document DE-A-30 31 322 discloses a fluorescent lamp lighting apparatus having features similar to those included in the precharacterizing part of claim 1. Further, prior art document EP-A-0 178 852 describes an electronic ballast circuit for fluorescent or other gaseous discharge lamps which includes a resonant half-bridge inverter circuit. The source voltage to the inverter is a full-wave rectified line voltage together with a DC carry-over voltage for supplying power in the inter-cusp period of the rectified line voltage. A negative feedback circuit is responsive to lamp current to vary the inverter drive frequency and thereby regulate lamp current. The frequency response of the feedback loop is high enough and the gain-versus-frequency response of the inverter is such that lamp current and voltage are regulated to reduce the crest factor of lamp current to compensate for variation in the amplitude of the voltage across the semiconductor switches. I is an object of the present invention to provide a novel apparatus for lighting up a fluorescent lamp, which securely prevents an inverter circuit from incurring destruction without the need of independently providing a safety circuit and also electric shock from occurrence while loading and unloading a fluorescent lamp. To solve this object the present invention provides a fluorescent lamp lighting apparatus as specified in claim 1. A fluorescent lamp lighting apparatus comprises especially a DC power source, an inverter means including a pair of switching elements serially connected to each other for converting into AC DC delivered from the DC power source, and a series resonant circuit having inductive elements and a capacitance element, at least one of the inductive elements having an insulative transformer, and a fluorescent lamp means, including a pair of filaments, to be lit up on receipt of AC output converted by the inverter means, each filament having one-end and the other end, wherein the series resonant circuit is formed by connecting at least the insulative transformer between one ends of the pair of filaments, and connecting the capacitance element between the other ends of the pair of filaments. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic circuit block diagram of the fluorescent lamp lighting apparatus according to an embodiment of the invention; Fig. 2 is a schematic circuit block diagram of the fluorescent lamp lighting apparatus using a choke coil on the part of the secondary coil of an insulative transformer according to a second embodiment of the invention; Fig. 3 is a schematic circuit block diagram of the fluorescent lamp lighting apparatus according to a third embodiment of the invention, in which a plurality of bipolar transistors compose switching elements of the inverter circuit; Fig. 4 is a schematic circuit block diagram of the self-exciting inverter-type fluorescent lamp lighting apparatus according to a fourth embodiment of the invention; Fig. 5 is a schematic circuit block diagram of the fluorescent lamp lighting apparatus loaded with a pair of fluorescent lamps according to a fifth embodiment of the invention; and Fig. 6 is a concrete circuit block diagram of the fluorescent lamp lighting apparatus shown in Fig. 1 reflecting a sixth embodiment of the invention. Referring now more particularly to the accompanying drawings, embodiments of the invention are described below. Fig. 1 illustrates the schematic circuit block diagram of the fluorescent lamp lighting apparatus reflecting an embodiment of the invention. The reference numeral 10 shown in Fig. 1 designates a DC power source. AC input terminals of rectifying circuit 14 of the DC power source 10 are connected to both terminals of a commercially available AC power source 12, whereas AC output terminals of the rectifying circuit 14 are connected to both terminals of a voltage-smoothing electrolytic capacitor 16. One of the terminals of the electrolytic capacitor 16 makes up the positive output terminal of the DC power source 10, whereas the other terminal makes up the negative terminal of the DC power source 10. A half-bridge type inverter 30 is connected to both terminals of the electrolytic capacitor 16 of the DC power source 10. The inverter circuit 30 incorporates a pair of switching elements 32 and 34 which are connected to each other in series. Concretely, drain of a field-effect transistor functioning as the switching element 32 is connected to the positive output terminal of the DC power source 10. Source of the other field-effect transistor functioning as the switching element 34 is connected to the negative output terminal of the DC power source 10. Source of the field-effect transistor 32 and drain of the field-effect transistor 34 are connected to each other. An oscillator 36 is connected to gate sides of these field-effect transistors 32 and 34. Capacitors 38 and 40 are connected to each other in series between the drain of the field-effect transistor 32 and the source of the field-effect transistor 34. The primary coil 42₁ of a transformer 42 (this will be described later on) is connected to the contact between the capacitors 38 and 40 and the other contact between the source side of the field-effect transistor 32 and the drain side of the field-effect transistor 34. The transformer 42 is insulative, which is of the leakage type for example. The secondary coil 42₂ of the transformer 42 makes up the AC output terminal of the inverter circuit 30. An end of the secondary coil 42₂ is connected to an end of a filament 46 of a fluorescent lamp 44. Likewise, the other end of the secondary coil 42₂ is connected to an end of the other filament 48 of the fluorescent lamp 44. A startup capacitor 50 is connected between the other ends of the filaments 46 and 48. The startup capacitor 50 and leakage inductance of the insulative transformer 42 jointly compose a series resonant circuit in this embodiment. Next, functional operation of the fluorescent lamp lighting apparatus of the above embodiment is described below. First, when the commercial AC power source 12 is turned ON, the AC output voltage is rectified by a rectifying circuit 14. Next, the rectified AC voltage is smoothed by a field capacitor 16, and then the smoothed output voltage is converted into a DC voltage before being output from the DC power source 10. Next, the rectified and smoothed DC voltage is transmitted to the field-effect transistors 32 and 34. Simultaneously, these field-effect transistors 32 and 34 are alternately turned ON and OFF by high-frequency signal delivered from the oscillator 36 inside of the inverter circuit 30. Then, voltage of high-frequency power source is transmitted to the primary coil 42₁ of the insulative leakage transformer 42. As a result, as is conventionally known, due to function of the series resonant circuit composed of the startup capacitor 50 and the leakage inductance, the fluorescent lamp 44 is preheated. Next, as soon as the voltage between electrodes of the fluorescent lamp 44 exceeds the startup voltage, the fluorescent lamp 44 illuminates itself. Now, when the fluorescent lamp 44 is off from the lighting apparatus, some portions of the filaments 46 and 48 remain open. In other words, both ends of the secondary coil 42₂ of the insulative leakage transformer 42 are held open to inactivate the operation of the series resonant circuit. In consequence, this results in the elimination of unwanted destruction of the field-effect transistors 32 and 34 in the inverter circuit 30. Concretely, when no load is present in the lighting apparatus, only the excited inductance component remains in the leakage transformer 42 internally holding leakage inductance. Nevertheless, generally, the excited inductance contains a greater amount of inductance that that of the leakage inductance. As a result, only a negligible amount of current flows through the lighting apparatus while no load is present. Furthermore, even when the fluorescent lamp 44 is off from the lighting apparatus, the inverter circuit 30 is still operative. As a result, when the fluorescent lamp 44 is loaded into the lighting apparatus, the fluorescent lamp 44 instantly illuminates itself. In this way, the secondary coil 42₂ of the leakage transformer 42 of the inverter circuit 30 energizes the fluorescent lamp 44. One of those elements composing the series resonant circuit is connected between filaments of the fluorescent lamp 44 on the side opposite from the power source. As a result, while the fluorescent lamp 44 is off from the lighting apparatus, the series resonant circuit is open. Accordingly, even when no load is present, there is no fear of destroying the inverter circuit 30. Furthermore, owing to the presence of the insulative leakage transformer 42, the fluorescent lamp 44 is insulated from the DC power source. This in turn securely prevents electric shock from occurrence otherwise likely to take place in the course of loading and unloading the fluorescent lamp 44. The above embodiment respectively refers to the use of the insulative leakage transformer 42. Nevertheless, the invention does not confine the scope of available transformers merely to this insulative leakage transformer 42. In this case, as shown in Fig. 2, a choke coil 58 concurrently serving as ballast is connected between the output terminal of a secondary coil 56₂ inside of an inverter circuit 30₁ and filament 46 of the fluorescent lamp 44. The series resonant circuit can be opened by the above structure when the fluorescent lamp 44 is not loaded in the lighting apparatus. Consequently, even when no load is present, there is no fear of causing the inverter circuit 30₁ to be destroyed. Other aspects of the structure and operation of those circuits are identical to those of the preceding embodiments, and thus, description of these is deleted. Figs. 3 and 4 respectively illustrate other embodiments of the circuit structure of the inverter circuit 30 shown in Fig. 1. Regarding the circuit diagrams of the following embodiments, description shall merely refer to those components different from those which are shown in the preceding embodiments, where identical reference numerals will be given to those corresponding components. Since other aspects of the structure and operation of those circuits in the following embodiments are identical to those of the preceding embodiments, description of these is deleted. Fig. 3 illustrates an embodiment in which a pair of bipolar transistors are available for composing switching elements inside of the inverter 30 of the fluorescent lamp lighting apparatus shown in Fig. 1. Diodes 60 and 62 which are connected to each other in series in the inverter circuit 302 according to the illustrated polarity are respectively connected between the output terminals of the DC power source 10. Collectors and emitters of bipolar transistors 64 and 66 are connected to both terminals of the diodes 60 and 62. Bases of these bipolar transistors 64 and 66 are respectively connected to an oscillator 36, and thus, in response to the operation of the oscillator 36, alternate switching operations are executed between these bipolar transistors 64 and 66. Fig. 4 illustrates an embodiment in which the inverter circuit 30 shown in Fig. 1 is replaced by a self-exciting inverter circuit 30₃. Diodes 60 and 62 are connected to each other in series according to the illustrated polarity. These diodes 60 and 62 are also connected to a series circuit composed of a resistor 68 and a capacitor 70, while these diodes 60 and 62 are connected in parallel between output terminals of the DC power source 10. Another diode 72 is connected to the contact between the series circuit composed of the resistor 68 and the capacitor 70 and the contact between the diodes 60 and 62. Collector and emitter of a transistor 74 functioning as a switching element are respectively connected to the cathode and anode between both terminals of the diode 60. Like-wise, collector and emitter of a transistor 76 are respectively connected to the cathode and anode between both terminals of the diode 62. Along with the resistor 68 and the capacitor 70, a trigger diode 78 composing an activating circuit of the inverter circuit 303 is connected to the contact between the series circuit composed of the resistor 68 and the capacitor 70 and the base of the transistor 76. Along which capacitor 80, a resistor 82 and a series circuit of one of the secondary coil 84₂₁ of a drive transformer 84 are also connected between the base and the emitter of the transistor 74. Likewise, along with a capacitor 86, a resistor 88 and a series circuit of the other secondary coil 84₂₂ of the drive transformer 84 are also connected between the base and the emitter of the transistor 76. Diodes 90 and 92 are respectively connected to the resistors 82 and 88 in parallel. An end of the primary coil 84₁ of the drive transformer 84 is connected to the contact between the transistors 74 and 76, whereas the other end is connected to the primary coil 42₁ of the insulative leakage transformer 42. When the commercial AC power source 12 is turned ON in the self-exciting inverter circuit 30₃, the transistor 76 is turned ON via the trigger diode 78 composing the activating circuit. Simultaneously, AC current flows through a closed circuit composed of the transistor 76, capacitor 40, the primary coil 42₁ of the insulative leakage transformer 42, and the primary coil 84₁ of the drive transformer 84. When the AC current flows through the primary coil 84₁ of the drive transformer 84, current is generated in the secondary coils 84₂₁ and 84₂₂ in response to it. As a result, the transistor 76 is turned OFF, whereas the transistor 74 is turned ON. This causes the AC current to flow through another closed circuit composed of the transistor 74, capacitor 38, the primary coil 42₁ of the insulative leakage transformer 42, and the primary coil 84₁ of the drive transformer 84 in the direction inverse from the last flow. As a result, the transistor 74 is turned OFF and the transistor 76 ON. In this way, by causing transistors 74 and 76 to be turned ON and OFF and vice versa, in other words, by alternately switching these transistors 74 and 76, voltage from high-frequency power source is delivered to the primary coil 42₁ of the insulative leakage transformer 42 before eventually lighting up the fluorescent lamp 44 as was done for the preceding embodiments. Fig. 5 illustrates an embodiment in which a plurality (like a pair) of fluorescent lamp circuits are provided on the part of the secondary coil of the transformer shown in Fig. 2. Concretely, a choke coil 58₁ concurrently functioning as ballast is connected between the output terminal of a secondary coil 56₂₁ of a transformer 56′ inside of an inverter circuit 30₄ and filament 46₁ of a fluorescent lamp 44₁. A capacitor 50₁ is connected between the other ends of the filament 46₁ and a filament 48₁. Likewise, a choke coil 58₂ concurrently functioning as ballast is connected between the output terminal of the other secondary coil 56₂₂ of the transformer 56′ and a filament 46₂ of fluorescent lamp 44₂. A capacitor 50₂ is connected between the other ends of the filament 46₂ and a filament 48₂. The above structure providing a plurality of fluorescent lamps on the part of the secondary coil of an insulative transformer can also achieve the identical effect to that is achieved by the preceding embodiments. Fig. 6 illustrates the concrete circuit block diagram of the fluorescent lamp lighting apparatus according to the invention. DC power source 10 shown in Fig. 6 incorporates a rectifying circuit 14 which is connected between both terminals of AC power source 12 (having a semiconductor switch 94 connected in parallel) via a transformer 96 and a choke coil 98. The DC power source 10 also incorporates capacitors 100, 102 and 104 which are respectively connected to the rectifying circuit 14 in parallel. In addition to these, in order to smoothen current output from a rectifying circuit 52, the DC power source 10 also incorporates diodes 106, 108, and 110 each having the illustrated polarity, field capacitors 112 and 114, and resistors 116 and 118, respectively. In addition to those transistors 32 and 34, the capacitors 38 and 40, and the insulative leakage transformer 42 containing the primary coil 42₁ and the secondary coil 42₂ shown in Fig. 1, the inverter circuit 30 further incorporates those components including the following; resistors 120 and 122, 126 and 128, and transformers 124 and 130 are respectively connected between the gates and sources of the field-effect transistors 32 and 34. A resistor 132, a transistor 134, and a diode 136, are respectively connected between the drain of the field-effect transistor 32 and the oscillator 36. A field capacitor 138 is connected to the diode 136. A diode 140, a current transformer CT, and a Zener diode 142, are respectively connected to the emitter of the transistor 134. A resistor 144 is connected between the base of the transistor 134 and a diode 110. The oscillator 36 is connected to the transistor 134 via the diode 136, a resistor 146, and a transformer 148. As shown in Fig. 6, the oscillator 36 incorporates a V/F converter 150 converting voltage into frequency for example, a transistor 152, resistors 154 and 156, and capacitors 158 and 160. The oscillator 36 is connected to a differential amplifier 162 which is composed of transistors 100 and 104, a diode 166, resistors 170, 172, 174, 176, 178, 180 and 182, and a capacitor 184, as shown in Fig. 6. The reference numeral 186 designates a voltage detection circuit. The voltage detection circuit 186 incorporates a voltage-detecting coil PT of the insulative leakage transformer 42, a rectifying circuit 188 whose input terminal is connected to the voltage-detecting coil PT, and a voltage-smoothing capacitor 190 which is connected to the output terminal of the rectifying circuit 188. The voltage detection circuit 186 is connected to a soft-start circuit 202 which is composed of a transistor 192, a variable resistor 194, and resistors 196, 198 and 200. A serial circuit composed of a Zener diode 204 and a field capacitor 206, the other serial circuit composed of a diode 208 and a resistor 210, and a parallel circuit composed of a diode 202 and a resistor 214, are respectively connected between the soft-start circuit 202 and the oscillator 36. As far as a fluorescent lamp is energized by the secondary coil of a transformer of the inverter circuit and one of elements composing a series resonant circuit is connected between filaments on the side opposite from the power source of the fluorescent lamp, the scope of the invention is not solely confined to those embodiments described above, but the invention also provides a variety of applicable fields as well.

Claims: A fluorescent lamp lighting apparatus comprising a DC power source (10), an inverter means (30, 30₁ - 30₄) including a pair of switching elements (32, 34, 64, 66, 74, 76) serially connected to each other for converting DC delivered from said DC power source (10) into AC, and a series resonant circuit having inductive elements (42₂, 58, 58₁, 58₂) and a capacitance element (50, 50₁, 50₂), at least one of said inductive elements having an insulative transformer (42), and a fluorescent lamp means (44), including a pair of filaments (46, 48), to be lit up on receipt of AC output converted by said inverter means (30, 30₁ - 30₄), each filament (46, 48) having one end and an other end, wherein: said series resonant circuit is formed by connecting at least said insulative transformer (42) between the one ends of said pair of filaments (46, 48, 46₁, 48₁, 46₂, 48₂), characterized in that said capacitance element (50, 50₁, 50₂) is connected between the other ends of said filaments (46, 48) in said series resonant circuit. An apparatus according to claim 1, characterized in that said insulative transformer (56) and another of said inductive elements (58) are connected in series between the one ends of said pair of filaments (46, 48) whereas said capacitance element (50) is connected between the other ends of said pair of filaments (46, 48) in said series resonant circuit. An apparatus according to claim 1, characterized in that said insulative transformer (42) includes a leakage transformer. An apparatus according to claim 1, characterized in that said inverter means (30, 30₁, 30₂, 30₃, 30₄) comprises an oscillation means for driving said switching elements (32, 34, 64, 66, 74, 76). An apparatus according to claim 1, characterized in that said inverter means comprises an oscillator (36), a differential amplifier (162), a voltage-detection circuit (186), and a soft-start circuit (202). An apparatus according to claim 4, characterized in that the one ends and the other ends of the current route of said pair of switching elements (32, 34, 64, 66) are connected in series to said DC power source (10), whereas control electrodes thereof are respectively connected to said oscillation means (36), a pair of voltage-dividing capacitors are connected in parallel to said DC power source (10), and a primary coil (42₁) of said transformer (42) is connected to a contact between said pair of switching elements (32, 34, 64, 66) and also to the other contact between said pair of voltage-dividing capacitors (38, 40), whereas the secondary coil (42₂) is connected between the one ends of said pair of filaments (46, 48) of said fluorescent lamp means (44). An apparatus according to claim 4, characterized in that the one ends and the other ends of the current route of said switching elements (32, 34) are connected in series to said DC power source (10), whereas control electrodes thereof are connected to said oscillation means (36), a pair of voltage-dividing capacitors (38, 40) are connected in parallel to said DC power source (10), and primary coils (51) of said insulative transformer (56, 56′) are connected to a contact between said pair of switching elements (32, 34) and also to the other contact between said pair of voltage-dividing capacitors (38, 40), whereas the secondary coils (56₂, 56₂₁, 56₂₂) and choke coils (58, 58₁, 58₂) are connected to the one ends of said pair of filaments (46, 48, 46₁, 48₁, 46₂, 48₂) of said fluorescent lamp means (44, 44₁, 44₂).

Inventors: KAKITANI TSUTOMU, KAKITANI, TSUTOMU
Applicants: TOSHIBA LIGHTING & TECHNOLOGY, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1990
Main classification: H05B41
IPC codes: H05B41