Apparatus for operating a discharge lamp

Abstract: An apparatus for operating a discharge lamp. The apparatus includes a source of electrical power (1, 2, 3), a switching circuit (5, 6) for switching the electrical power, a control circuit (8) for controlling the switching circuit (5, 6), a transformer (11) having a primary winding (11a) for receiving the electrical power switched by the switching circuit (5, 6) and a secondary winding (11b) for supplying power to the lighting apparatus, a resonant circuit including an inductor (7) and a capacitor (9) for regulating the frequency of the electrical power and a circuit for powering the control circuit (8). The powering circuit includes a secondary winding (7b) on the inductor (7) and an additional winding (11c) on the transformer (11) connected in a differential manner.

Description: The present invention relates generally to an apparatus for operating a discharge lamp, and more particularly, to an apparatus for operating a discharge lamp with a high frequency power source. An apparatus for operating a discharge lamp known to the applicant is shown in FIGURE 1. This apparatus has a rectifier 2, a smoothing circuit 3, an inverter circuit 4, and an output transformer 11. The discharge lamp is referenced 12. The rectifier 2 is adapted to rectify the AC frequency output, e.g. a 50/60 Hz, 100/200 V output, from a commercial power suppply 1. The rectified signal from the rectifier 2 is supplied to the inverter circuit 4 through the smoothing circuit 3. The smoothing circuit 3 removes undesired ripples from the rectified signal. The rectifier 2 is a full-wave rectifier. The smoothing circuit 3 is a voltage-split type smoothing circuit. Full-wave rectifiers and voltage-split type smoothing circuits are well-known in the art, so that detailed descriptions thereof are omitted herefrom. The output transformer 11 includes a primary winding 11a and a secondary winding 11b. The inverter circuit 4 includes a series circuit of FETs 5 and 6, a choke 7, a control circuit 8, a capacitor 9, a diode 10 and a series circuit of coupling capacitors 17a and 17b. The series circuit of FETs 5 and 6 provides a switching operation for the rectified voltage from the smoothing circuit 3. The choke 7 has a primary winding 7a and a secondary winding 7b. The series circuit of the FETs 5, 6 and the series circuit of the coupling capacitors 17a, 17b are respectively connected across the rectifier 3. The gates of the FETs 5, 6 are connected to outputs of the control circuit 8 so that they are alternately turned ON or OFF at a high frequency under the control of the control circuit 8. The primary winding 11a of the ouput transformer 11 is coupled between the coupling node of the FETs 5, 6 and the coupling node of the coupling capacitors 17a and 17b through the choke 7. The secondary winding 11b is connected to the dischage lamp 12. The capacitor 9 is coupled across the primary winding 11a of the output transformer 11. Thus a high frequency AC current flows through a resonant circuit consisting of the primary winding 7a of the choke 7 and the capacitor 9. An output voltage corresponding to the high frequency AC current is produced across the primary winding 11a of the ouput transformer 11. The secondary winding 7b of the choke 7 is coupled to inputs of the control circuit 8 through the diode 10. Thus an induced voltage related to the high frequency AC current appears on the secondary winding 7b of the choke 7 and is rectified by the diode 10. The rectified voltage is input to the control circuit 8 as a DC power source. The regulated high frequency AC current output is fed to the discharge lamp 12 through the output transformer 11. In the output transformer 11, the primary winding 11a is connected in parallel with the capacitor 9 so that a boosted high frequency AC voltage is generated on the secondary winding 11b for lighting the discharge lamp 12. In the above example of a known apparatus for operating a discharge lamp, the DC power source required for driving the control circuit 8 of the inverter circuit 4 is generated by the secondary winding 7b of the choke 7. Further, FIGURE 2 is a block diagram showing another example, known to the applicants, of an apparatus for operating a discharge lamp. This example differs form the above in that an additional winding 11c is coupled to the output transformer 11 in place of the secondary winding 7b of the choke 7. In this latter example, the DC power, required for the control circuit 8 is generated by the additional winding 11c of the output transformer 11. In these examples of known apparatus for operating a discharge lamp, the DC power source for the control circuit 8 is either obtained from the secondary windings 7b of the choke 7 or an additional winding 11c of the output transformer 11. Therefore, if the high frequency AC current flowing through the choke 7 and the transformer 11 changes, the DC power source output to the control circuit 8 also changes in response to the AC current change and interferes with the stable operation of the control circuit 8. A somewhat similar apparatus is disclosed in JP-A-1-071095. Furthermore, an apparatus for operating a discharge lamp is disclosed in WO 83/00271, the operating apparatus having a source of electrical power, switching means, a control means, a transformer with secondary winding connected to a discharge lamp, a resonant circuit, and means for powering the control means. In this apparatus the primary winding of a current transformer is connected in series with the primary winding of the transformer, and two secondary windings of the current transformer are connected to the bases of the switching transistors, so that one of the transistors is switched on, and the other is switched off, when the current flowing through the primary winding of the current transformer passes through zero. The present invention therefore seeks to provide an apparatus for operating a discharge lamp which is able stably to light a discharge lamp. The present invention also seeks to provide an apparatus for operating a discharge lamp which is able to generate a stable DC power source for a control circuit of an inverter circuit. An apparatus for operating a discharge lamp according to one aspect of the present invention includes a source of electrical power, a switching device for switching the electrical power, a control circuit for controlling the switching device, a transformer having a primary winding for receiving the electrical power switched by the switching device and a secondary winding for supplying power to the lighting apparatus, a resonant circuit including an inductor and a capacitor for regulating the frequency of the electrical power and a circuit for powering the control circuit, the powering circuit including a secondary winding on the inductor and an additional winding on the transformer connected in a differential manner. For a better understanding of the present invention and many of the attendant advantages thereof reference will be now made by way of example to the accompanying drawings, wherein: FIGURES 1 and 2 are schematic diagrams showing examples of apparatus for operating discharge lamps known to the applicant; FIGURE 3 is a block diagram showing an embodiment of the apparatus for operating discharge lamp according to the present invention; FIGURE 4 is an explanatory diagram showing voltage by the secondary windings and their differential voltage; FIGURE 5 is the block diagram showing a first modification of this embodiment; and FIGURE 6 is the block diagram showing a second modification of this embodiment. The present invention will be described in detail with reference to the FIGURES 3 through 6. Throughout the drawings, reference numerals or letters used in FIGURES 1 and 2 will be used to designate like or equivalent elements for simplicity of explanation. Referring now to FIGURE 3, a first embodiment of an apparatus for operating a discharge lamp according to the present invention will be described in detail. As shown in FIGURE 3, the apparatus for operating a discharge lamp has a rectifier 2, a smoothing circuit 3, an inverter circuit 4, an output transformer 11 and a discharge lamp 12. The rectifier 2 is adapted to rectify the AC frequency output of, e.g. 50/60 Hz, 100/200 V, from a commercial power supply 1. The rectified voltage from the rectifier 2 is supplied to the inverter circuit 4 through the smoothing circuit 3. The smoothing circuit 3 removes undesired ripples in the rectified voltage. The rectifier 2 and the smoothing circuit 3 are well-known and correspond to those in the known apparatus of FIGURES 1 and 2 so that detailed descriptions thereof are also omitted herein. The output transformer 11 includes a primary windings 11a, a secondary winding 11b and an additional winding 11c. The inverter circuit 4 includes a series circuit of FETs 5 and 6, a choke 7, a control circuit 8, a capacitor 9, a diode 10 and a series circuit of coupling capacitors 17a and 17b. The series circuit of FETs 5 and 6 enables a switching operation on the rectified voltage from the smoothing circuit 3. The choke 7 has a primary winding 7a and a secondary winding 7b. The series circuit of the FETs 5, 6 and the series circuit of the coupling capacitors 17a, 17b are respectively connected across the rectifier 3. The gates of the FETs 5, 6 are connected to outputs of the control circuit 8 so that they are alternately turned ON or OFF at a high frequency under control of the control circuit 8. The primary winding 11a of the output transformer 11 is coupled between the coupling node of the FETs 5, 6 and the coupling node of the coupling capacitors 17a and 17b through the choke 7. The secondary winding 11b is connected to the discharge lamp 12. The capacitor 9 is coupled across the primary winding 11a of the output transformer 11. Thus a high frequency AC current flows through a resonant circuit consisting of the primary winding 7a of the choke 7 and the capacitor 9. An output voltage corresponding to the high frequency AC current is generated on the primary winding 11a of the output transformer 11. The secondary winding 7b of the choke 7 is coupled to inputs of the control circuit 8 through the diode 10 and the additional winding 11c of the output transformer 11. Thus an induced voltage responsive to the high frequency AC current appears on the secondary winding 7b. Another induced voltage also responsive to the high frequency AC current appears on the additional winding 11c of the output transformer 11. These two induced voltages are rectified by the diode 10, and then input to the control circuit 8 as a DC power source. The regulated high frequency AC current is output to the discharge lamp 12 through the output transformer 11. In the output transformer 11, the primary winding 11a is connected in parallel with the capacitor 9 so that a boosted high frequency AC voltage is generated on the secondary winding 11b for lighting the discharge lamp 12. In this embodiment of the apparatus for operating a discharge lamp, the secondary winding 7b of the choke 7 and the additional winding 11c of the output transformer 11 are connected together in series. Thus the DC power source required for driving the control circuit 8 of the inverter circuit 4 is generated by both the secondary winding 7b of the choke 7 and the additional winding 11c of the output transformer 11. However, these windings 7b and 11c are coupled differentially in terms of winding polarity relation so that their induced voltages are differentially combined in this series circuit. Referring now to FIGURE 4, the induced voltage V1 appearing on the secondary winding 7b of the choke 7, the induced voltage V2 appearing on the additional winding 11c of the output transformer 11 and the DC voltage Vdc of the DC power source to the control circuit 8 will be described in detail. The DC voltage Vdc of the DC power source results from the differential combination of the induced voltages V1 and V2. When the discharge lamp 12 is not a load on the output transformer 11, a large amount of current flows through the primary winding 7a of the choke 7 and the primary winding 11a of the output transformer 11. Accordingly, the induced voltage V1, e.g. of 32 V, and the induced voltage V2, of e.g., 20 V, are combined and correspondingly a DC power source, of e.g. 12 V, is obtained, as shown in FIGURE 4(A). When the discharge lamp 12 is a load on the output transformer 11, the high frequency AC current flowing through the primary winding 7a of the choke 7 and the primary winding 11a of the output transformer 11 decreases. Accordingly, the induced voltages V1 and V2 and the voltage Vdc of the DC power source decrease to 16 V, 6 V, and 10 V, respectively, as shown in FIGURE 4(B). Further, if both ends of the discharge lamp 12 are shorted, the induced voltages V1 and V2 decrease to 10V and 0V, respectively, while the voltage Vdc of the DC power source remains at voltage 10 V, the same as the load condition, as shown in FIGURE 4(C). As may be easily understood from the above description, even when the induced voltages V1 and V2 appearing on the windings 7a and 11c vary in response to load conditions, the DC power source obtained by differential combination of the induced voltages V1 and V2 stays almost constant, or within a relatively short voltage range, e.g., the range from 10 V to 12V. Therefore, it becomes possible to prevent a large fluctuation of the DC power source to the control circuit 8 so that the FETs 5 and 6 can be stably operated. In the above embodiment, the smoothing circuit 3 is a voltage-split type smoothing circuit, similar to the examples of the known discharge lamp lighting apparatus shown in FIGURES 1 and 2. However, the smoothing circuit 3 is not limited to the voltage-split type smoothing circuit For example, the smoothing circuit 3 can be replaced by a single smoothing capacitor 15, as shown in FIGURE 5. FIGURE 5 shows a block diagram of a first modification of this embodiment. Further, a dropping resistor 19 can be inserted in series with the diode 10, or a zener diode 16 can be coupled across the inputs of the control circuit 8, for further stabilizing the DC power source to be applied to the control circuit 8. Referring now to FIGURE 6, a second modification of the apparatus for operating discharge lamp will be briefly described below. As shown in FIGURE 6, this second modification differs from the above embodiment, e.g., the apparatus of FIGURE 3 as follows. That is, an additional winding 11c is provided by using a part of the primary winding 11a of the transformer 11. Thus, a tap 11d provided on the primary winding 11a is coupled to the secondary winding 7b of the choke 7. Further, one end of the primary winding 11a of the transformer 11 is coupled with the connecting node of the FETs 5 and 6 through a coupling capacitor 17c as well as the choke 7, while another end of the primary winding 11a is directly coupled with only the other end of the FET 6. This modification of the apparatus for operating a discharge lamp is advantageous for simplifying the circuit construction by comparison with the embodiment of FIGURE 3. As described above, the present invention provides an advantageous form of apparatus for operating a discharge lamp. While there have been illustrated and described what are at present considered to be preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the appended claims.

Claims: An apparatus for operating a discharge lamp, comprising: a source of electrical power (1, 2, 3); switching means (5, 6) for switching the electrical power; control means (8) for controlling the switching means (5, 6); transformer means (11) having a primary winding (11a) for receiving the electrical power switched by the switching means (5, 6) and a secondary winding (11b) for supplying power to the discharge lamp; a resonant circuit including an inductor (7) and a capacitor (9) for regulating the frequency of the electrical power; and a circuit for powering the control means (8); CHARACTERISED IN THAT the control means (8) powering circuit includes a secondary winding (7b) on the inductor (7) and an additional winding (11c) on the transformer means (11) connected in a differential manner. An apparatus for operating a discharge lamp as in claim 1, wherein the additional winding (11c) on the transformer means (11) comprises an additional tap (11d) onto the primary winding (11a) of the transformer means (11). An apparatus for operating a discharge lamp as in claim 1, further including coupling means (17a, 17b) for coupling the electrical power from the switching means (5, 6) to the primary winding (lla) of the transformer means (11). An apparatus for operating a discharge lamp as in claim 3, wherein the coupling means includes at least one capacitor (17c) connected between the switching means (5, 6) and the transformer means (11). An apparatus for operating a discharge lamp as in claim 1, wherein the switching means comprises at least two switching elements connected in series, with their control terminals connected to the control means. An apparatus for operating a discharge lamp as in claim 5, wherein the switching elements (5, 6) comprise transistors. An apparatus for operating a discharge lamp as in claim 5, wherein the switching elements (5, 6) comprise FETs. An apparatus for operating a discharge lamp as in claim 1, wherein the circuit for powering the control means also includes means (19, 16) for stabilizing the voltage supplied to the control means (8). An apparatus for operating a discharge lamp as in claim 8, wherein the means for stabilizing means includes a zener diode (16).

Inventors: INUI KENICHI, KAKITANI TSUTOMU, INUI, KENICHI, KAKITANI, TSUTOMU
Applicants: TOSHIBA LIGHTING & TECHNOLOGY, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1991
Main classification: H05B41
IPC codes: H05B41, H02M7