WO1996022007A1

WO1996022007A1 - Electronic starter for fluorescent lamp

Info

Publication number: WO1996022007A1
Application number: PCT/KR1995/000035
Authority: WO
Inventors: Yeong Choon Chung
Original assignee: Yeong Choon Chung
Priority date: 1995-01-13
Filing date: 1995-04-07
Publication date: 1996-07-18

Abstract

An electronic starter for a fluorescent lamp (4) comprising a rectifying circuit (5) for rectifying an AC input voltage (1), a switching device connected to an output of the rectifying circuit, a drive voltage supply circuit (7, 8, 9) for supplying a drive voltage to the switching device, a drive voltage interrupt circuit (10-14) for interrupting the drive voltage to the switching device, and an overheat protecting device (16, 161) for protecting the switching device from an overheat. The switching device may include a field effect transistor (6) or an insulated gate bipolar transistor. According to the present invention, an ignition switching operation is always made at a peak point of the AC input voltage (1). Therefore, a single-shot lighting operation of the fluorescent lamp (4) can be performed.

Description

ELECTRONIC STARTER FOR FLUORESCENT LAMP

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates in general to a lighting circuit for a fluorescent lamp having a magnetic coil type- ballast and a starter, and more particularly to an electronic starter for a fluorescent lamp in which a filament is preheated for the lighting of the fluorescent lamp, a filament preheating time is adjustable according to temperature and voltage variations, a filament preheating voltage is gradually increased to protect the filament from a damage during the preheating procedure, so as to assure the thermionic emission life of the filament, and an ignition switching operation is performed at a peak point of an alternating current (referred to hereinafter as AC) input voltage to generate a highest strike voltage, so that a single-shot lighting operation of the fluorescent lamp can be carried out.

Description of the Prior Art

Generally, a starter for a fluorescent lamp comprises a CR resonance circuit or RC time constant circuit for generating a strike voltage. Also, the starter comprises a transistor, a silicon controlled rectifier (referred to hereinafter as SCR) or a triac as a switching power device.

Such conventional starters are disclosed in U.S. Patent No.

4,513,227, U.S. Patent No. 4,749,909, Japanese Patent Publication No. Sho. 52-4672, Japanese Patent Publication

No. Sho. 52-18078, Japanese Patent Publication No. Sho. 52-

19481, Japanese Patent Publication No. Sho. 54-0584 and

U.K. Patent No. 159225.

However, the above-mentioned conventional starters have a disadvantage in that they have no means for generating the strike voltage at a peak point of an AC voltage being supplied to a filament of the fluorescent lamp, thereby causing an ignition switching operation to be performed at a phase other than the peak point. For this reason, the conventional starters cannot perform a single- shot lighting operation of the fluorescent lamp. Also, a filament preheating time cannot be adjusted properly according to temperature and voltage variations, resulting in a reduction in the life of the fluorescent lamp and an early blackening of the fluorescent lamp. Also, in the conventional starter utilizing the transistor or the SCR, the lighting operation is unstably performed because a half-wave rectifying manner is employed. Similarly, in the conventional starter utilizing the triac, the lighting operation is unstably performed because the triac has nonuniform bidirectional rectifying and electrical characteristics. Further, in the conventional starters utilizing such semiconductor devices, a switching time is long and a switching impedance is low because a gate or a base is of the current ON/OFF type. In this connection, the conventional starters may reduce the strike voltage in a lighting circuit.

In result, the above-mentioned conventional starters have a narrower adaptive width to temperature and voltage variations as compared with a metal contact type-starter. In this connection, the above-mentioned conventional starters have a limited application depending on the type of the fluorescent lamp. Further, the above-mentioned conventional starters have no means for protecting the circuit from an overheat due to a faulty operation of the circuit.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an electronic starter for a fluorescent lamp in which an ignition switching operation performed at a peak point of an AC input voltage so that a single-shot lighting operation of the fluorescent lamp can be carried out, resulting in an increase in the life of the fluorescent lamp, and a filament preheating time is adjustable according to temperature and voltage variations.

In accordance with the present invention, the above and other objects can be accomplished by a provision of an electronic starter for a fluorescent lamp comprising rectifying means for rectifying an AC input voltage; switching means connected to an output of the rectifying means; drive voltage supply means for supplying a drive voltage to the switching means; drive voltage interrupt means for interrupting the drive voltage to the switching means; and overheat protecting means for protecting the switching means from an overheat.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of an electronic starter for a fluorescent lamp in accordance with an embodiment of the present invention; and

Fig. 2 is a view illustrating operating characteristics of the electronic starter in Fig. 1 in accordance with the embodiment of the present invention, wherein:

Fig. 2a is a waveform diagram illustrating a filament voltage characteristic;

Fig. 2b is a waveform diagram illustrating a rectified voltage characteristic; and Fig. 2c is a waveform diagram illustrating a timing condenser voltage characteristic. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1, there is shown a circuit diagram of an electronic starter for a fluorescent lamp in accordance with an embodiment of the present invention. As shown in this drawing, an AC input voltage 1 is connected through a ballast 2 to one terminal of one side 3 of a filament and directly to one terminal of the other side 31 of the filament, and a rectifying circuit 5 is connected between the other terminals of both the sides 3 and 31 of the filament, the ballast 2 being of the magnetic coil type.

The electronic starter comprises a field effect transistor (FET) 6 connected between positive and negative direct current (referred to hereinafter as DC) output terminals (+) and (-) of the rectifying circuit 5. The field effect transistor 6 acts as a switching device.

The electronic starter further comprises a gate voltage supply circuit for supplying a voltage to a gate of the field effect transistor 6, a gate voltage interrupt circuit for interrupting the voltage to the gate of the field effect transistor 6, and an overheat protecting device for protecting the field effect transistor 6 from an overheat. The gate voltage supply circuit includes a resistor 7, a zener diode 8 and a smoothing condenser 9. The gate voltage interrupt circuit includes an SCR 10 connected between the gate of the field effect transistor 6 and the negative DC output terminal (-) of the rectifying circuit 5, and a trigger circuit for triggering the SCR 10. The trigger circuit includes a resistor 13, a thermistor 12 and a timing (time constant) circuit. The timing circuit is provided with a timing condenser 11 and a resistor 14. The overheat protecting device includes a thermistor 16 connected between the gate of the field effect transistor 6 and the negative DC output terminal (-) of the rectifying circuit 5, and a thermal plate 161 disposed between the field effect transistor 6 and the thermistor 16.

In detail, in Fig. 1, the rectifying circuit 5 has AC input terminals connected to the other terminals of both the sides 3 and 31 of the filament, respectively. The field effect transistor 6 is connected in parallel between the positive and negative DC output terminals (+) and (-) of the rectifying circuit 5. The field effect transistor 6 includes a source connected to the positive DC output terminal (+) of the rectifying circuit 5, and a drain connected to the negative DC output terminal (-) of the rectifying circuit 5. In the gate voltage supply circuit, the resistor 7 is connected between the source and the gate of the field effect transistor 6. The zener diode 8 and the smoothing condenser 9 are connected in parallel between the gate and the drain of the field effect transistor 6.

In the overheat protecting device, the thermistor 16 is connected in parallel to the smoothing condenser 9 and the zener diode 8 in the gate voltage supply circuit. In the gate voltage interrupt circuit, the SCR 10 is connected in parallel to the thermistor 16 in the overheat protecting device and the smoothing condenser 9 and the zener diode 8 in the gate voltage supply circuit. The SCR 10 has an anode connected to the gate of the field effect transistor 6 and a cathode connected to the negative DC output terminal (-) of the rectifying circuit 5. The SCR 10 also has a gate connected to one side of the thermistor 12, the other side of which is connected in series to the resistor 13. The timing condenser 11 and the resistor 14 are connected in parallel between the gate and the cathode of the SCR 10.

The operation of the electronic starter for the fluorescent lamp with the above-mentioned construction in accordance with the embodiment of the present invention will hereinafter be described in detail with reference to Figs. 2a to 2c. Fig. 2a is a waveform diagram illustrating a filament voltage characteristic, Fig. 2b is a waveform diagram illustrating a rectified voltage characteristic and Fig. 2c is a waveform diagram illustrating a timing condenser voltage characteristic. First, the AC input voltage 1 is applied across the rectifying circuit 5 through the ballast 2 and the filament. Then, a rectified pulsating voltage from the rectifying circuit 5 is applied to the gate of the field effect transistor 6 through the resistor 7. At this time, the gate voltage of the field effect transistor 6 is substantially 10-15V, which is determined by the zener diode 8. The pulsating voltage being applied to the gate of the field effect transistor 6 is smoothed into a DC voltage by the smoothing condenser 9. In result, the field effect transistor 6 is turned on by the DC voltage smoothed by the smoothing condenser 9. As the field effect transistor 6 is turned on, the positive DC output terminal (+) of the rectifying circuit 5 is connected to the negative DC output terminal (-) thereof, resulting in the formation of a closed circuit. As a result, an AC voltage 15 as shown in Fig. 2a is applied across the filament for a preheating time Hf as shown in Fig. 2a, so that the filament can be preheated for the preheating time Hf. At this time, because a high semiconductor junction resistance of about 1.5 to 2.5Ω is generated between the source and the drain of the field effect transistor 6, the gate voltage of the field effect transistor 6 is continuously maintained by the resistor 7 and the zener diode 8 in such a manner that it can turn on the field effect transistor 6, although the gate voltage interrupt circuit of the resistor 13, the thermistor 12, the timing condenser 11 and the resistor 14 is connected in parallel between the source and the drain of the field effect transistor 6. As a result, the field effect transistor 6 is continuously maintained at its ON state for the preheating time Hf.

While the field effect transistor 6 is turned on in response to the voltage being applied to the gate thereof, a pulsating voltage as shown in Fig. 2b is supplied to the timing condenser 11 through the resistor 13 and the thermistor 12. The timing condenser 11 is charged for a time period which is determined by a combined resistance of the resistor 13 and the thermistor 12 and a time constant of the timing condenser 11. A charged voltage as shown in Fig. 2c appears at the output of the timing condenser 11 under the effect of the discharging resistor 14. The charged voltage at the output of the timing condenser 11 includes a pulsating ripple component as shown in Fig. 2c. At that time the charged voltage from the timing condenser 11 reaches a predetermined level Tv capable of triggering the SCR 10, the SCR 10 is turned on to discharge the voltage being applied to the gate of the field effect transistor 6. As a result, the field effect transistor 6 is turned off, resulting in a strike voltage Vst as shown in Fig. 2a being generated across the filament. Then, the fluorescent lamp is turned on by the strike voltage Vst. At this time, a tube current 4 flows through the fluorescent lamp as shown in Fig. 1 and the voltage across the filament falls below 1/2.

Noticeably, the strike voltage Vst is generated at the moment that a trigger voltage Tv arrival point of the timing condenser 11 is synchronized with a peak point of the AC input voltage. This makes it possible to induce the most powerful and effective magnetic induction voltage from the ballast 2. Therefore, a single-shot lighting operation of the fluorescent lamp can be performed after the lapse of the sufficient preheating period.

Once being triggered, the SCR 10 remains at its ON state as long as it is supplied with a DC current, resulting in the continuous OFF state of the field effect transistor 6. Although the preheating time may be somewhat different according to the type of the fluorescent lamp, it is typically 0.8 to 2.4 sec at a normal temperature (25^*C) under the condition that a rated voltage is applied. In the case where the fluorescent lamp is designed according to a standard requirement, the optimum preheating time can preferably be obtained under the condition of the normal temperature and the rated voltage. In accordance with the preferred embodiment of the present invention, in the case where an ambient temperature falls, the resistance of the thermistor 12 is increased because of its negative characteristic. As a result, the charging time constant of the timing condenser 11 becomes larger, resulting in the preheating period being lengthened. On the contrary, in the case where the ambient temperature rises, the resistance of the thermistor 12 is reduced, thereby causing the charging time constant of the timing condenser 11 to become smaller. As a result, the preheating period is shortened. Such an optimum preheating time can provide a very ideal lighting condition because it acts to compensate for a thermionic emission of the filament of the fluorescent lamp according to the temperature. On the other hand, if the voltage is higher, the preheating period must become shorter because the filament current is thus increased. On the contrary, if the voltage is lower, the preheating period must become longer. In the case where the voltage is higher, the charging time of the timing condenser 11 becomes shorter, resulting in the preheating period becoming shorter. In the case where the voltage is lower, the charging time of the timing condenser 11 becomes longer, resulting in the preheating period becoming longer.

On the other hand, the thermistor 16 has a negative temperature resistance characteristic (NTC). Namely, the resistance of the thermistor 16 is maintained at its high state up to a predetermined value above the normal temperature, but abruptly lowered because of the heat transfer through the thermal plate 161 thereto when the field effect transistor 6 is overheated above about 60"C, thereby causing the field effect transistor 6 to be turned off. As a result, no further current flows through the field effect transistor 6. Therefore, in accordance with the preferred embodiment of the present invention, no overcurrent or a little current flows through the field effect transistor 6. In this connection, the present invention provides more effective means which can protect the circuit from a damage, as compared with a conventional art in which a fuse is broken upon the generation of overheat. Also, the electronic starter of the present invention can automatically be restored to the normal state after a fault in a lighting circuit is corrected. Therefore, the electronic starter of the present invention is economical and reliable.

Alternatively, the electronic starter of the present invention may employ an insulated gate bipolar transistor (IGBT) as the switching device instead of the field effect transistor.

As apparent from the above description, the present invention provides the function of adjusting automatically the preheating time according to the temperature and voltage variations. Also, the ON/OFF switching device is comprised of the field effect transistor 6 which is triggered by the resistor 13, the thermistor 12, the timing condenser 11 and the resistor 14. The timing condenser 11 is supplied with the pulsating voltage full-wave rectified by the rectifying circuit 5. Therefore, an ignition switching operation is always made at the peak point of the AC input voltage as shown in Fig. 2c so that the single- shot lighting operation of the fluorescent lamp can be performed. On the other hand, the single-shot lighting operation of the fluorescent lamp cannot be performed in the case where the ignition switching operation is made at a zero cross point of the AC input voltage as in the conventional art. Therefore, the present invention provides the low cost contactless electronic starter which can perform the ideal lighting operation of the fluorescent lamp by comprising the field effect transistor and the associated components. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. An electronic starter for a fluorescent lamp comprising: rectifying means for rectifying an AC input voltage; switching means connected to an output of said rectifying means; drive voltage supply means for supplying a drive voltage to said switching means; drive voltage interrupt means for interrupting the drive voltage to said switching means; and overheat protecting means for protecting said switching means from an overheat.

2. An electronic starter for a fluorescent lamp as set forth in Claim 1, wherein said switching means includes a field effect transistor, said field effect transistor having a source connected to a positive DC output terminal of said rectifying means, and a drain connected to a negative DC output terminal of said rectifying means.

3. An electronic starter for a fluorescent lamp as set forth in Claim 1, wherein said switching means includes an insulated gate bipolar transistor, said insulated gate bipolar transistor having a collector connected to a positive DC output terminal of said rectifying means, and an emitter connected to a negative DC output terminal of said rectifying means.

4. An electronic starter for a fluorescent lamp as set forth in Claim 2 or 3, wherein said drive voltage supply means includes: a resistor connected between the source (collector) and a gate (base) of said switching means; and a zener diode and a smoothing condenser connected in parallel between the gate (base) and the drain (emitter) of said switching means.

5. An electronic starter for a fluorescent lamp as set forth in Claim 2 or Claim 3, wherein said drive voltage interrupt means includes: a silicon controlled rectifier connected between a gate (base) of said switching means and the negative DC output terminal of said rectifying means; and a trigger circuit for triggering said switching means, said trigger circuit including a first resistor and a thermistor connected in series between the source (collector) and the drain (emitter) of said switching means, and a timing circuit connected between said thermistor and the drain (emitter) of said switching means, said timing circuit including a timing condenser and a second resistor connected in parallel between said thermistor and the drain (emitter) of said switching means.

6. An electronic starter for a fluorescent lamp as set forth in Claim 2 or Claim 3, wherein said overheat protecting means includes: a thermistor connected between a gate (base) of said switching means and the negative DC output terminal of said rectifying means; and a thermal plate disposed between said switching means and said thermistor.