GB2274220A - Electronic ballast for fluorescent lamps - Google Patents

Abstract

The ballast is constructed on a plurality of interconnected boards. On a first board, which is standard to a range of ballast circuits, there are mounted long-lived components C1 to C4, C6, C7, L1, L2, R3, TH1, BR1 common to a range of ballasts while on a second board there are mounted components C5 D1 to D6, Q1, Q2, F1, R1, R2, T1 which are short-lived or specific to a particular ballast within a range. Typically, the first board is a mother board while the second is a daughter board. The ballast may comprise a series-resonant oscillator having two series-connected transistors Q1, Q2 driven by a base-drive transformer T1, connection between the transformer T1 and the bases of the transistors being through respective parallel-connected ultra-high-speed diodes and resistors D5, R1 and D6, R2. Use of diodes D5, D6 give enhanced temperature control of transistors Q1, Q2 by improving their switching and storage timer, so minimising the time during which both transistors are simultaneously conductive. Ultra-high-speed diodes D3, D4 are also connected in parallel with transistors Q1, Q2. <IMAGE>

Description

ELECTRONIC BALLAST FOR FLUORESCENT LAMPS The present invention relates to electronic ballasts for fluorescent lamps.

A fluorescent lamp operates by passing a current between two electrodes through an ionised gas contained within a sealed enclosure. In practice, most commonly the enclosure comprises a glass tube, a filament constituting an electrode being disposed near each end of the tube, such an enclosure assembly being thus generally referred to as a fluorescent tube.

A particular problem with fluorescent lighting arises from the electrical properties of the gas within the tube. When the tube is first energised, the gas is in a non-ionised state and has a high resistance. To initiate conduction, the tube is struck - that is, a high voltage is applied across the electrodes to ionise the gas. Once the gas is ionised, its resistance drops so allowing a steady current to flow through it.

However, the voltage required to maintain this current is much less than is required to strike the tube.

Therefore, some means must be provided within the lamp to arrange for the generation of the striking voltage and subsequently regulate the current flowing in the tube. Such means are generally referred to as "ballast" for the tube.

The well-known, traditional ballast using a large inductance controlled by a glow starter is gradually being superceded by electronic ballast which operates the tube at high frequency (typically 20-60 kHz).

These electronic ballasts offer improved efficiency and the high frequency of operation means that flickering of the tube (which is virtually an inevitable consequence of driving a tube with an alternating current) occurs at a frequency too great to be perceived by humans. An explanation of construction of high-frequency electronic ballast circuits and of their operation can be found in, for example, Motorola Semiconductor Technical Data Sheet No. AN 1049/D - The Electronic Control of Fluorescent Lamps, and in U.K.

Patent Application GB-A-2 106 339.

The electronic ballast of the prior art has several disadvantages.

Firstly, certain components of the ballast circuit must be selected to correspond to the characteristics of the fluorescent tube that it is to operate. This is particularly inconvenient where a range of similar lamps are to be manufactured for use with tubes of a range of lengths. In this situation, a range of ballast units of known type must be manufactured to suit the range of lengths.

Secondly, it has been found that certain of the components of an electronic ballast are, by their nature, typically much shorter-lived than others. It is generally not practical to replace individual components of a ballast unit so units with one defective component are regularly disposed of so wasting the unexpired life of the remaining components. This is particularly inconvenient since it has been found that often the longest-lived components are the most expensive.

It is an aim of the present invention to provide an electronic ballast which overcomes, or at least, ameliorates the disadvantages of the prior art.

The present invention provides in a first of its aspects a ballast for a fluorescent lamp comprising a ballast circuit having a plurality of electronic components, the components being mounted on a plurality of readily-separable interconnected circuit boards.

The components mounted on a particular board may be selected on the basis of the particular advantage sought. For instance, longer-lived components may be mounted on a first board while shorter-lived components may be mounted on a second board. Alternatively, components general to a range of lamps may be mounted on a first board while components specific to a particular lamp within the range may be mounted on a second board.

The present invention has proved to be more advantageous than might be expected because it has been found in practice that the range of components specific to a particular light fitting is often the same or very similar to the range of components which are comparatively short-lived.

Preferably, at least one circuit board is a mother board fixed within the lamp while at least one other circuit board is a daughter board, releasably connected to the mother board. By arranging the shorter-lived components on one or more daughter boards, a component failure is likely to necessitate only the change of a daughter board. Similarly, by arranging the components specific to a particular lamp on one or more daughter boards, a general purpose ballast unit may be configured for operation in any one of a range of lamp units simply by selecting the appropriate one or more daughter boards.

In a particular embodiment, advantage has been found to arise from placing on a mother board components required to filter, transform, rectify and smooth the mains supply to the lamp, together with the ballast choke, and on a daughter board the remaining semiconductor and other components required to make up the oscillator circuit to drive the tube.

In a second aspect the invention provides a fluorescent lamp incorporating a ballast according to the first aspect of the invention.

Increasingly, users of conventional fluorescent lamps with a glow-starter are wishing to take advantage of the benefits provided by electronic ballast. In order to avoid the costs of replacing whole lamp units, there is a desire to replace only the ballast units and retain other, common components. Typically, a number of lamps will be converted in a single operation, for instance in a factory or suite of offices, including lamps of a variety of lengths within a common range.

The present invention offers a significant advantage in such conversion operations since a ballast may be constructed from a mother board common to all lamps to be converted together with a daughter board to configure the ballast to a particular lamp.

In this respect, from a third aspect, the invention provides a method of converting a set of fluorescent lamps to operate using electronic ballast comprising selecting components of ballast circuits for each lamp in the set which are common to all such circuits and providing each lamp of the set with a mother board on which the selected components are mounted and, for each lamp in the set, connecting to the mother board a daughter board on which is mounted the remaining components of the ballast circuit for that lamp.

This method has the advantage that a constructor may stock a large number of mother boards which can be used for substantially any lamp in the range to be converted, and daughter boards in numbers corresponding to typical demand for each particular board. This significantly increases the flexibility with which a stock of boards may be used.

In any of the above-defined embodiments, interconnection between the boards may be achieved by means of an edge connector or any other suitable circuit board connector of which there are many types well-known to those skilled in the art.

Commonly, electronic ballasts comprise a ballast circuit incorporating a voltage-fed resonant circuit to generate a high-frequency alternating voltage for application to the tube. The resonant circuit, in a common form, incorporates a pair of series-connected bipolar transistors controlled by a base drive transformer arranged to switch on one respective transistor in each of the positive-going and negative-going parts of the a.c. cycle. Ballast circuits such as this will be referred to herein, for convenience, as "series-resonant ballast circuits".

In practice, difficulties have been encountered in the design of satisfactory series-resonant ballast circuits. A particular problem has been elevated temperatures generated in the bipolar transistors and other components associated therewith. One particular reason of this is that in many series-resonant ballast circuits a state is encountered within a significant part of the a.c. cycle, around its zero-volt-crossing point, in which both transistors are switched on.

A further aim of the present invention is to provide a series-resonant ballast circuit having improved temperative control of, in particular, its bipolar transistors.

According to a fourth aspect of the invention there is provided a series-resonant ballast circuit having a base-drive transformer and a pair of series-connected bipolar transistors the base of each transistor being connected to the base-drive transformer through a respective diode.

This diode has the effect of improving the switching and storage times of the transistors so allowing the time of during which both transistors are conducting to be minimised.

The diodes are preferably ultra-high speed diodes, these resulting in yet greater improvements to the performance of the circuit.

Each diode may be connected in parallel with a resistor. An additional diode may be connected between the collector and the emitter of each transistor.

Embodiments of the fourth aspect of the invention may be constructed in accordance with the first aspect.

Embodiments of the invention will now be described in detail by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a ballast circuit for a fluorescent tube; Figures 2 and 3 are circuit diagrams of the circuit of Figure 1 built on two separable boards in accordance with the invention, the Figures representing mother and daughter boards respectively; and Figure 4 is a circuit diagram of a series-resonant ballast circuit being a second embodiment of the invention.

The components shown in Figures 2 and 3 are given the same references as the corresponding components in Figure 1.

Mains input to the circuit is received through connector CNl. A protective fuse F1 is connected in series with the live mains line. The fused mains supply provides the input to a mains processing stage MP in which the mains supply is filtered by common-cored, symmetrical inductors L2 and capacitors C1 and C2, the inductors L2 being connected in series with each mains supply line and the capacitors C1, C2 being connected between the supply lines respectively on opposite sides of the inductors L2. A thermistor TH1 is also connected in series with the live mains line within the mains processing stage MD.

The output from the mains processing stage MP supplies a bridge rectifier BR1, the outputs from which form the positive and negative supply rails +V,OV for the remainder of the circuit, the output from the rectifier BR1 being first smoothed by a capacitor C1 connected between the supply rails +V,OV.

The supply rails provide power to an oscillator stage OSC. This is, in this embodiment, a voltage feed serial oscillator the function and construction of which are well-known to those skilled in the art and will therefore not be described in detail here. It will be appreciated that other embodiments may incorporate different types of oscillator.

The output from the oscillator group OSC is fed to a first side of one electrode of a fluorescent tube FT through a ballast choke L1. The other electrode of the tube FT has a first side connected to a pair of capacitors C6,C7 which are connected in series between the supply rails +V,OV and from resonant capacitors for the oscillator. The second sides of two electrodes of the tube are interconnected by a capacitor C5.

For convenience of assembly, a connector C2 is disposed to form a releasable connection between the four connections to the electrodes of the tube FT and the ballast circuit.

The physical arrangement of the ballast circuit is illustrated by Figures 2 and 3. In this embodiment, the circuit is built upon one mother board and one daughter board. It has been found in practice that the mains processing stage MP, the bridge rectifier BR1 and the smoothing capacitor C3 are all long-lived components. Accordingly, these are placed on the mother board. However, it will be appreciated that the fuse F1 may require replacement from time to time so this is placed on the daughter board. It will also be appreciated that the mains processing stage MP is virtually standard in construction while the fuse may need to be varied in dependence upon the power of the tube to be driven.

Similarly, the ballast choke L1 and the resonant capacitors C6,C7 are robust components which can be made to a standard specification so these are mounted on a mother board.

Within the oscillator stage OSC there are two transistors Ql,Q2, a diac D2 and several diodes D1, D3, D4,D5,D6. These have all been found to be susceptible to failure and so are mounted on the daughter board.

Associated resistors R1,R2 are mounted on the daughter board for convenience of circuit layout. This places the larger part of the oscillator stage OSC on the daughter board. Also on the daughter board is the capacitor C5 which interconnects the electrodes of the tube FT since this capacitor must be selected to correspond with the particular tube being used. A resistor R1 and a capacitor C2 are provided for the sole purpose of creating a pulse to start the oscillator when the circuit is switched on so they need not be specific to the particular tube in use nor are they subject to severe loading and are therefore mounted on the mother board.

To interconnect the mother and daughter boards, an edge connector is used. In this embodiment a connector block is secured to the mother board and the daughter board has co-operating connector strips formed at one of its edge portions.

With reference to Figure 4, a series-resonant ballast circuit comprises a power supply stage 110, the details of which will not be described herein. The power supply stage 110 receives main supply power at its input 112 and provides at its output 114 a DC supply relative to a negative supply line -V.

The ballast circuit further comprises an oscillator stage 120. The oscillator stage 120 has a positive supply line +V connected to the output 114 of the power supply stage 110, and has a negative supply line -V in common with that of the power supply stage 110. The oscillator stage provides at its output 122 a high-frequency oscillating signal which is fed to a lamp stage 130.

The lamp stage 130 receives the high-frequency signal from the oscillator stage 120 and power from the supply lines +V, -V, and provides in a known manner, an output suitable for application to (in this embodiment) two fluorescent tubes 132.

The oscillator stage 120, will now be described in greater detail.

The oscillator stage 120 comprises first and second series-connected npn bipolar transistors Q101,Q102. The collector of the first transistor Q101 is connected to the positive supply line +V and the emitter of the second transistor Q102 is connected to the negative supply line -V. The emitter of the first transistor Q101 and the collector of the second transistor Q102 are interconnected through a common point 124.

The cathode of a diode D101 is connected to the common point 124, its anode being connected to the positive supply line +V through a resistor R101 and to the negative supply line -V through a capacitor C102.

A diac D102 is connected between the base ofthe second transistor Q102 and the anode of the diode 0101. This group of components constitutes a starting circuit for the oscillator stage and is conventional in arrangement.

The oscillator stage 120 further comprises a base-drive transformer TR101. The transformer TR101 has a primary winding TRlOla connected between the common point 124 and the output 122 of the oscillator stage 120. The transformer TR101 has a first secondary winding TRlOlbl connected between the common point 124 and, as will be described, indirectly, the base of the first transistor Q101. The transformer TR101 also has a second secondary winding TRlOlb2 connected between the negative supply line -V and, indirectly as above, the base of the second transistor Q102.The secondary windings TRlOlbl, TRlOlb2, are connected such that the signal supplied by each winding of the base of its respective transistor Q101,Q102 are in antiphase, so ensuring that each transistor Q101,Q102 is switched during alternate half cycles of the input signal to the primary winding TRlOla.

Each secondary winding TRlOlbl, TRlOlb2 is connected to the base of the respective transistor Q101, Q102 through a respective ultra-high-speed diode D103, D104 connected in parallel with a resister R102, R103, the anode of each diode D103, D104 being connected to the base of the corresponding transistor Q101, Q102.

Each transistor Q101, Q102 has connected between its collector and its emitter of respective further ultra-high-speed diode D105, D106, the anode of each diode being connected to the emitter of the corresponding transistor Q101, Q102.

It has been found that the following components give particularly satisfactory operation in this circuit: Transistors Q101, Q102 - Motorola BUL 45 Diodes D103, D104 - Motorola MUR 110 D105, D106 - Motorola MUR 160 Resistors R102, R103 - 47Q It has been found that certain of these components in particular the diodes, may have a performance better than components stated to be the of equivalents in published literature.

Claims (14)

1. A series-resonant ballast circuit having a base drive transformer and a pair of series-connected bipolar transistors, the base of each transistor being connected to the base-drive transformer through a respective diode.

2. A series-resonant ballast circuit according to claim 1 in which each diode is an ultra-high-speed device.

3. A series-resonant ballast circuit according to claim 1 or claim 2 in which there is a resister connected in parallel with each diode.

4. A series-resonant ballast circuit according to any proceeding claim in which each transistor has connected between its collector and its emitter a respective further diode.

5. A series-resonant ballast circuit according to claim 4 in which each further diode is an ultra-high-speed device.

6. A series-resonant ballast circuit according having a base drive transformer and a pair of series connected Motorola BUL 45 transistors the base of each transistor being connected through a respective Motorola MUR 110 diode connected in parallel with a 47Q register.

7. A series-resonant ballast circuit according to claim 6 having a Motorola MUR160 diode connected between the collector and the emitter of each transistor.

8. A series-resonant ballast circuit according to any preceding claim the components of which are mounted upon a plurality of readily-separable, interconnected circuit boards.

9. A ballast circuit having a plurality of electronic components, the components being mounted on a plurality of readily-separable interconnected circuit boards.

10. A ballast circuit according to claim 9 in which longer-lived components may be mounted on a first board while shorter-lived components may be mounted on a second board.

11. A ballast circuit according to claim 9 or claim 10 in which components general to a range of lamps may be mounted on a first board while components specific to a particular lamp within the range may be mounted on a second board.

12. A ballast circuit according to any one of claims 9 to 11 in which one board is a mother board and at least one other board is a daughter board releasably connected to the mother board.

13. A fluorescent lamp incorporating a ballast according to any preceding claim.

14. A series-resonant ballast circuit substantially as described herein with reference to Figures 1 to 3 or Figure 4 of the accompanying drawings.