High-frequency power unit for neon tubes

Abstract: DC power is converted by an inverter (20) to high-frequency power, which is supplied to the primary winding (Wp) of a neon transformer (17). One or more neon tubes are connected in series across the secondary winding (Ws) of the neon transformer. A saturable reactor (30) is connected across the secondary winding of the neon transformer. The saturable reactor has a characteristic that its magnetic flux is saturated when the output voltage from the secondary winding of the neon transformer increases 1.1 to 2.0 times the rated voltage.

Description: BACKGROUND OF THE INVENTION The present invention relates to a power unit for neon sign and particularly to a high-frequency power unit which boosts high-frequency power by a transformer to light neon or argon tubes connected to the secondary side thereof. Conventionally, the commercial line power is boosted prior to its application to neon or argon tubes to light them, but this method necessitates the use of a large or bulky boosting transformer. In view of this, it has been proposed to utilize high-frequency power of, say, 20 or 30 kHz for lighting neon or argon tubes (hereinafter referred to simply as neon tubes) so as to permit the use of a small boosting transformer. A power unit accordina to the precharacterizing portion of claim 1 is disclosed in US-A-4,414,491. In a display of the type employing high-frequency driven neon tubes, though dependent on their diameters or gas pressures, stripe patterns commonly referred to as "jelly beans" may sometimes appear on the neon tubes lengthwise thereof during their ON state. With the prior art, it is impossible to prevent the jelly beans from occurrence. It is an object of the present invention to provide a neon tube lighting high-frequency power unit which eliminates the possibility of stripe patterns appearing on the neon tubes lengthwise thereof. This object is achieved with a power unit as claimed in claim 1. Preferred embodiments are subject-matter of the dependent claims. According to the present invention, the inverter, which is used to convert DC power to high-frequency square-wave power for supply to the neon transformer, is designed so that the duty ratio of the high-frequency power is off 50%. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a block diagram illustrating an embodiment according to the invention; Fig.2 is a plan view of the neon transformer; DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.1 illustrates an embodiment of the neon tube lighting high-frequency power unit according to the present invention, which is adapted to preclude the possibility of introducing the so-called jelly beans in the light of the neon tube. For example, the commercial AC power supply 5 is connected between the input terminals 11 and 12, and the output AC current from the AC power supply 5 is provided, if necessary, via a switch 14, to the full-wave rectifier 15, by which it is rectified. The rectified output is smoothed by the smoothing circuit 16. That is, DC power is obtained in the smoothing circuit 16. The capacitors C1 and C2 are connected in series between the both ends of the smoothing circuit 16. Further, the switching elements SW1 and SW2, each formed by an FET, are connected in series between the both ends of the smoothing circuit 16. The primary winding Wp of the neon transformer 17 is connected between the connection point of the capacitors C1 and C2 and the connection point of the switching elements SW1 and SW2. The neon tube 4 is connected across the secondary winding Ws of the neon transformer 17 that is to be lighted or energized. The neon tube 4 may also be a series connection of a plurality of neon tubes of a number within a rated value. One end of the positive side of the smoothing circuit 16 is connected to the negative side thereof via the resistor 21 and the capacitor 23. The Zener diode 24 and the switching regulator 22 for generating the rectangular high-frequency wave are connected across the capacitor 23. The switching regulator 22 may be an IC M51996 by Mitsubishi Denki K.K. of Japan. A variable resistor 28R is connected between an 11th terminal of the switching regulator 22 and the negative side (hereinafter referred to as a negative terminal) of the smoothing circuit 16, and a capacitor 28C is connected between a 10th terminal of the switching regulator 22 and the negative terminal. Moreover, the winding 26P is connected between a second terminal of the switching regulator 22 and the negative terminal via a resistor 27 and a capacitor 25. The winding 26P is coupled to the windings 26S1 and 26S2 to form the pulse transformer 26. The winding Wt is provided which is coupled to the neon transformer 17, and both ends of the winding Wt are connected to both ends of the capacitor 23 via the diode 29D and a resistor 29R. The windings 26S1 and 26S2 are connected between sources and gates of the FETs that form the switching elements SW1 and SW2, respectively. Upon turning ON the switch 14, the DC current from the smoothing circuit 16 flows via the resistor 21 to the capacitor 23 to charge it. When the voltage across the capacitor 23 exceeds a certain value, the switching regulator 22 starts oscillation and its oscillation output is applied to the winding 26P. In consequence, the switching elements SW1 and SW2 are alternately turned ON and OFF by the rectangular pulses of the oscillation output. When the switching element SW1 is turned ON, charges stored in the capacitor C1 are discharged via the switching element SW1 and the winding Wp. When the switching element SW2 is turned ON, charges in the capacitor C2 are discharged via the winding Wp and the switching element SW2. In other words, current flows in the winding Wp alternately in opposite directions and a rectangular current flows therein. As the oscillation is thus started, the voltage induced in the winding Wt is rectified by the diode 29D and is charged in the capacitor 23 via the resistor 29R, by which the power voltage for the switching regulator 22 is maintained. Thus, the resistor 21 needs only to supply the capacitor 23 with only a small initial charging current for starting the switching regulator 22; therefore, the resistor 21 can be made high in resistance but small in capacity. The oscillation frequency of the switching regulator 22 is set in the range of 20 to 30 kHz, for instance. The OFF period of the output rectangular wave depends on the product of the resistance value of the variable resistor 28R and the capacitance value of the capacitor 28C of the switching regulator 22. According to the invention, the OFF period is adjusted by the resistor 28R and the duty ratio of the rectangular output is shifted off 50%. The duty ratio is set chosen in the range of 45 to 48% or 52 to 55%. With the duty ratio of the rectangular wave or the ON-OFF operation of the switching elements SW1 and SW2 thus shifted off 50%, the amount of harmonic components contained in the high-frequency power to be applied to the neon tube 4 increases. This prevents generation of the jelly beans in the neon tube 4 connected across the secondary winding Ws of the neon transformer 17. In this case, since the neon tube 4 is lighted via the neon transformer 17, a sine-wave voltage, not a rectangular one, is provided to the neon tube 4. When the duty ratio is 50%, the amount of harmonic components in the high-frequency power that is applied to the neon tube 4 is so small that a standing wave is liable to be induced in the lighted neon tube 4, producing regularly-spaced-apart stripe patterns called "jelly beans" in the luminous state along the tube envelope. The resistor 28R may also be a fixed resistor. Alternatively, it is possible to produce special lighting effects or neon display by preventing the generation of "jelly beans" or positively generating them through control of the variable resistor 28R. The resistance value of the resistor 28R need not always be continuously varied but may also be switched between two or more values. Instead of varying the resistance of the resistor 28R, the capacitance of the capacitor 28C may be switched between two capacitance values. Since the high-frequency rectangular power, whose duty ratio is shifted off 50%, is applied to the neon transformer 17, its magnetic core (or iron core) may sometimes be nonuniformly magnetized. In such an instance, the driving of the switching elements SW1 and SW2 is made unbalanced by the nonuniform magnetization, that is, only one of the switching element is turned ON and OFF and the other left uncontrolled. Accordingly, there is the possibility of the switching elements SW1 and SW2 being broken down by a large current flowing therein which is caused by saturation. A possible solution to this problem is such a transformer structure as shown in Fig.2, in which the neon transformer 17 has a pair of opposed E-shaped magnetic cores 17Ca and 17Cb with their legs on one side spaced a very small gap 17G apart to form a closed magnetic path  and the primary and secondary windings Wp and Ws are wound over the legs of the both magnetic cores 17Ca and 17Cb on one and the other sides thereof, respectively. With the provision of the gap 17G in the magnetic path , the magnetic cores 17Ca and 17Cb are prevented from magnetic saturation. Incidentally, the transformer 17 in this example is what is called a leakage transformer in which the leakage iron cores 17Y and 17Y are extended toward each other from the intermediate portions of the magnetic cores 17Ca and 17Cb between the primary and secondary windings Wp and Ws. As described above, according to the present invention, generation of the "jelly beans" in the neon tube connected to the neon transformer can be avoided by shifting the duty ratio of the high-frequency rectangular power off 50% and supplying the power to the neon transformer.

Claims: A power unit for generating high-frequency power for lighting neon tubes or argon tubes (4), comprising: inverter means (10, 20) for converting commercial AC power to rectangular high-frequency power, and transformer means (17) having a magnetic core forming a closed magnetic path and primary and secondary windings (Wp, Ws) wound on said magnetic core, the primary winding (Wp) being supplied with said high-frequency power from said inverter means (10, 20) and outputs high-voltage, high-frequency power to said secondary winding, characterized in that said inverter means has its output duty ratio shifted off 50%. The power unit of claim 1, further comprising means for changing the duty ratio of the output from said inverter means (10, 20). The power unit of claim 1 or 2, wherein a magnetic gap (17G) is provided in the magnetic flux path of said transformer means (17). The power unit of any one of the preceding claims, wherein said duty ratio is selected within a range of 45% to 48% or within a range of 52% to 55%.

Inventors: ICHIMIYA FUMIO, NODA MAKOTO, UDA RYOICHI, ICHIMIYA, FUMIO, NODA, MAKOTO, UDA, RYOICHI
Applicants: SANYO ELECTRIC WORKS, KABUSHIKI KAISHA SANYO DENKI SEISAKUSHO
Application year: 1993
Main classification: H05B41
IPC codes: G05F1, H05B41, H01F38