AU732605B1 - Control circuits for fluorescent tubes - Google Patents

Description

S&F Ref: 467434

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Brenex Electrics Pty Limited 1st Floor Spencer Street Fairfield NSW 2165 Australia Vitaliano Masciari Spruson Ferguson St Martins Tower 31 Market Street Sydney NSW 2000 Control Circuits for Fluorescent Tubes Invention Title: The following statement is a full description of this invention, including the best method of performing it known to me/us:- S4 3 [R1 i c~iOr. L

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5815c BW]3 1472.doc:vsg CONTROL CIRCUITS FOR FLUORESCENT TUBES Field of the Invention The present invention relates to generally to fluorescent tubes and, in particular, to control circuitry for operating fluorescent tubes.

Background Art To turn on a preheated internal-discharge fluorescent lamp it is necessary to supply an a.c. current at a predetermined frequency to a resonating circuit applied to the lamp, such that the voltage at the terminals of the lamp builds up gradually and a current begins to flow. The energy issued by the ions of the low-pressure gas within the tube causes fluorescence, either of the atoms of vapour within the tube or of phosphor coating the inner surface of the tube.

Fig. 1 illustrates how such an oscillating current is produced by a conventional starting circuit reactor. The power supply, which is rectified and filtered upstream of the illustrated circuit, provides a d.c. voltage of 310 V. The capacitor 10 is charged through the resistor 20 until a point that the diac 30 conducts, discharging all the energy stored in the capacitor 10 onto the base of the transistor T2, turning the transistor T2 on. A current thus passes through inductors 40 and 50 (flowing from right to left in Fig. 1) and is grounded by transistor T2.

When the current passes through the inductor 50, a mutual inductance with inductors 60 and 70 is created, the three inductances beingarranged so as to form a three coil transformer. As a result, by virtue of the suitable orientation of the primary inductor and of the secondary inductors 60 and 70, transistor T2 is cut off and transistor T1 is activated in its saturation region.

When transistor T 1 starts conducting, the current passing through the inductor changes direction since the current now comes from the emitter of transistor T1 and flows towards the load (from left to right in Fig. By driving the two transistors T1 and T2 in this way, i.e. by causing their alternating conduction, a square wave current is created R:\LIB00\4835.doc:GMM which causes resonance of the circuit formed by the exit inductance 50, capacitor 80 and the internal connections of the lamp 90, thereby provoking the ignition of the lamp The inductor 40 performs a current limiting function during running of the lamp A reactor as described above is able to turn on only a specific kind bf lamp, having a predetermined power (for example 18 watts, 36 watts, etc.); it cannot be used for lamps having different power ratings or characteristics (for example when different gases are contained in the lamp).

Conventional reactors have the same life as that of the lamp. When the lamp no longer operates (for example when the low-pressure gases no longer react with the phosphor coating) but remains connected to the power supply, a high energy dissipation is generated due to the repeated turning-on of the cathode.

Summary of the Invention It is an object of the present invention to substantially overcome, or at least S 20 ameliorate, one or more disadvantages of existing arrangements.

The invention provides an electronic control circuit for one or more fluorescent lamps, said control circuit comprising: a power buffer responsive to an a.c. voltage supply to produce a first d.c.

voltage; an oscillator producing a square-wave signal; and a final power stage receiving said first d.c. voltage and said square-wave signal, and providing said square-wave signal to a switching circuit that switches said d.c. power supply into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit comprising a push-pull transistor circuit in which only one transistor has a-shunt diode and wherein said switching circuit supplies the full current demanded by said one or more ST J .,fluorescent lamps.

O R:\LIBOO\4835.doc:GMM The invention further provides a method of controlling the starting operation of one or more fluorescent lamps, said method comprising the steps of: producing a first d.c. voltage from an a.c. voltage supply; producing a square-wave signal; and providing said first d.c. voltage and said square-wave signal to a switching circuit that switches said first d.c. voltage into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit supplying the full current demanded by said one or more fluorescent lamps, and wherein said switching circuit comprises a push-pull transistor circuit in which only one said transistor has a shunt diode.

The circuit can further comprise a low-voltage buffer to produce a second d.c.

voltage to drive the oscillator, the second voltage being lower than the first.

Advantageously, the ignition current is stopped when a voltage derived from the switching circuit, which is proportional to the duration of the ignition period of the lamp(s) exceeds a predefined limit. The protection can act to cut-off the second d.c.

voltage from the oscillator.

20 The invention further provides a fluorescent light assembly comprising one or more fluorescent lamps and having connection with an electronic control circuit as described above.

Brief Description of the Drawings A preferred embodiment of the present invention will now be described with reference to the drawings, in which: Fig. 1 shows a conventional reactor starting circuit connected to a fluorescent lamp; Fig. 2 is a schematic block diagram of a control circuit for a fluorescent lamp <z,according to an embodiment of the present invention; and Fig. 3 shows a detailed circuit diagram of the circuit shown in Fig. 2.

R:IBOO83.doc:GMM R:\LIBOO\4835.doc:GMM -4- Detailed Description Including Best Mode In a preferred embodiment, a fluorescent lamp reactor circuit comprises a series of main blocks as illustrated in Fig. 2.

A low-pass filter 100 is formed by a double-phase coil and a capacitor, and serves to filter the a.c. power supply, which can vary from a minimum of 220 V to a maximum of 280 V. It should be appreciated that by adding a suitable power-factor preregulator upstream of the illustrated circuit, it is also possible to operate the circuit at a 1o mains voltage of between 110 V and 125 V.

The power buffer 110 is formed by a diode bridge between the ends of which an electrolytic polarised capacitor is located. The low-voltage buffer 120 is connected in parallel to the low-pass filter 100 and comprises a polyester capacitor, an electrolytic capacitor and a diode bridge. The stabiliser circuit 130 exploits the amplification of a BJT (Bipolar Junction Transistor) to provide a suitable input voltage to the downstream components. The oscillator 140 is implemented by an integrated CMOS circuit, and the driver stage 150 comprises high-frequency transformers and components adapted to the control and protection of the apparatus. The final high-frequency power stage 160 uses a pair of N-channel MOSFET transistors. The circuit also includes protective circuitry 170.

The oscillator 140 and the driver stage 150 can each be substituted by corresponding programmable logic units. The protection circuit 170 can also be replaced by a programmable logic unit programmed so that the circuit achieves the same results and is still capable of being connected to any kind of lamp associated with the reactor.

A more detailed description will now be given with reference to Fig. 3.

The mains voltage is supplied through the fusible resistor R1 and is fed to the low-pass filter 100, comprising the capacitor C 1 and two coils 200, 202 which are pushpull wound on a toroidal core. The mains voltage, rectified by the diode bridge PD1 and filtered by the electrolytic capacitor C2, is applied between the drain of the transistor MS 1 and the source of the transistor MS2, with a value of about 320 V.

.IROO\4R3 The oscillator 140 and the driver 150 are powered by voltages lower than that required by the final stage 160. Instead of using resistors or transformers, which create heating problems in traditional ballasts, the required low-voltage power supply is obtained from the mains supply using the diode bridge PD2 and the capacitor connected between the mains and the low-voltage stages. The capacitor C10, in particular, provides the necessary voltage drop, furnishing a suitable impedance (about 4,7000) at a frequency of 50 Hz without an undue increase in temperature. The output voltage provided by the capacitor C10 and diode bridge PD2 is limited to 15 V by the zener diode DZ3. This voltage is further reduced to a value of about 12 V by the stabiliser circuit 130 which makes use of the bipolar junction transistor TR3. This reduced voltage feeds the oscillator circuit 140 and the stabiliser circuit 130. The oscillator 140 comprises a base oscillator, whose operational frequency of 65 kHz is fixed by the resistor R8 and the capacitor C7, and by the two pairs of inverters, AB and CD, that are used by bridgeconnected followers. The square wave (with a duty-cycle of 50%) produced by the base oscillator is applied to the transistor TRI and, in a push-pull way, to the transistor TR2.

The two transistors TR1 and TR2 in turn transfer the signal to the final power stage 160 through two high-frequency transformers Ti and T2.

The two final N-channel MOSFET transistors MS1 and MS2 are brought into conduction alternately, forming a switching circuit which creates a square wave at the input of the coil L1 having an amplitude of 150 V and an initial current of 0.3 A, as required for the preheating of the lamp 90. The capacitor C5, connected in parallel with the lamp 90, begins to resonate with the exit coil L1, thus increasing the voltage to about 500 V. This leads, after about 0.7 seconds, to the ignition of the lamp Inside the final stage 160, the conduction of the MOSFET MS2 is limited by the zener diode DZ1, since it is necessary to limit the current returning to the negative end of the diode bridge PD1. The MOSFET MS1, in contrast, does not have a diode across its gate-source junction. This arrangement allows the reactor to turn on lamps of different power and/or kind without any modification to the circuit. Since the conduction of the transistor MS 1 is not limited, its output adjusts as a function of the load features of the lamp 90. Substantially, transistor MSI1 works as a potentiometer and since it outputs the exact current needed by the lamp, it avoids undue dissipation and unwanted power wastage. The usual lighting period of a lamp is about 0.7 seconds, as mentioned above.

After this period, the circuit stops operating.

R:\LIROO\4835.doc:GMM It should be appreciated that in the event the lamp 90 is broken or has finished its normal life, the circuit will continue to apply an ignition current to the end terminals of the lamp. This may provoke accidents caused by any direct contact with the electrodes, and is in addition a waste of energy. The protection circuit 170 is connected to the final stage 160 in order to overcome this problem. The protection circuit 170 measures the ignition time with a current which is proportional to the ignition current. This proportional current is obtained from the final stage 160 by means of the transformer T3, and is rectified by means of the diode D3. This current charges the capacitor C6 such that the voltage measured across the capacitor C6 is substantially proportional to the time passed since the reactor was connected to the mains supply. When this voltage exceeds a predetermined value (calculated as a function of the correct ignition time), the thyristor SCR1 starts conducting and cuts off the oscillator power supply.

is Since the ignition time and the ignition current (about 300 mA) must be the same for any kind of lamp, the protection circuit 170 is able to detect breakdowns related to any kind of load. If for any reason the ignition phase is prolonged, the charge accumulated on capacitor C6 causes the voltage at the ends of capacitor C6 to exceed 0.8 V. The thyristor SCR1 then begins conducting and, as it is connected to the base of transistor TR3, sends the latter to ground, thus preventing the stabiliser circuit 130 from powering the oscillator 140. The control square-wave on the final components is thus eliminated and the output current is stopped.

When the protection circuit 170 is activated, the diode bridge PD2 is closed on the diode DZ2 and on the resistor R13, resulting in a current of only 77 mA. Further, the capacitor C9 is protected by the zener diode DZ3, which prevents an excessive voltage drop at the ends of capacitor C9.

In order to substitute the broken lamp with another having different characteristics, it is necessary to turn off the circuit for a few seconds, to permit capacitor C6 to discharge to ground. On the other hand, a lamp, identical to the one broken or exhausted, can be connected without adopting any particular change-over arrangement.

R:\.IR0OO\4835.doc:GMM Advantages The control circuitry disclosed above can operate with lamps of different kinds containing different gases) and different powers (from 18 watts to 56 watts) even at the same time. No modification to the circuit or manual intervention is required when changing to a different type of lamp.

When the apparatus is controlling different lamps, and one of these lamps fails or is exhausted, all the other lamps continue to work correctly.

The light emitted by the lamp is safe, clear and cold-coloured, thus avoiding any visual disturbance.

The temperature of the lamp is lower by 25% than that reached using conventional reactors, and in consequence the lamp can be affixed to a double ceiling, since the temperature is kept sufficiently low and the equipment is sufficiently light. The ceiling light fixture does not need internal wiring clamps.

The circuitry is free of vibration and does not generate background noise (as occurs in the prior art). In addition, it does not induce noise on radios, computers or any other apparatus connected to the main supply.

The preferred embodiment of the invention ensures a low energy consumption with savings of up to 50% with respect to conventional reactors. No power-factor correctors are necessary, since the power factor is sufficiently high (about 0.96).

The life of the lamp is prolonged by 50% with respect to lamps using conventional reactors.

In the context of this specification, the word "comprising" must be understood not to mean "consisting only or solely of'.

The foregoing describes only one embodiment of the present invention, and modifications or changes can be made thereto without departing from the scope and spirit of the invention.

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Claims (9)

1. An electronic control circuit for one or more fluorescent lamps, said control circuit comprising: a power buffer responsive to an a.c. voltage supply to produce a first d.c. voltage; an oscillator producing a square-wave signal; and a final power stage receiving said first d.c. voltage and said square-wave signal, and providing said square-wave signal to a switching circuit that switches said d.c. power supply into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit comprising a push-pull transistor circuit in which only one said transistor has a shunt diode and wherein said switching circuit supplies the full current demanded by said one or more fluorescent lamps.

2. An electronic control circuit as claimed in claim 1, further comprising a protection circuit which is triggered to stop an ignition current when a voltage derived from said switching circuit and proportional to the duration of the ignition period of said one or more fluorescent lamps exceeds a predefined limit.

3. An electronic control circuit as claimed in claim 3, wherein said protection circuit acts to cut-off said second d.c. voltage from said oscillator.

4. An electronic circuit as claimed in any one of claims 1 to 3, further comprising a low-voltage buffer to produce a second d.c. voltage to drive said oscillator, said second d.c. voltage being lower than said first d.c. voltage. K 1

>5 C 2:' A method of controlling the starting operation of one or more fluorescent lamps, said method comprising the steps of: producing a first d.c. voltage from an a.c. voltage supply; producing a square-wave signal; and providing said first d.c. voltage and said square-wave signal to a switching circuit that switches said first d.c. voltage into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit supplying the full current demanded by said one R:\LIBOO\4835.doc:GMM or more fluorescent lamps, and wherein said switching circuit comprises a push-pull transistor circuit in which only one said transistor has a shunt diode.

6. A method as claimed in claim 5, comprising the further step of halting said amplified alternating output when the duration of the ignition period of said one or more fluorescent lamps exceeds a pre-defined limit.

7. A fluorescent light assembly comprising one or more fluorescent lamps having connection with an electronic control circuit as claimed in any one of claims 1 to 4.

8. An electronic control circuit for one or more fluorescent lamps substantially as described herein with reference to Figs. 2-3 of the accompanying drawings.

9. A method of controlling one or more fluorescent lamps substantially as described herein with reference to Figs. 2-3 of the accompanying drawings. DATED this fifth Day of February, 2001 Brenex Electrics Pty Limited Patent Attorneys for the Applicant SPRUSON FERGUSON nn R:\LIBOO\835.doc:GMM