GB2388973A

GB2388973A - Dimmable control means for discharge lamp

Info

Publication number: GB2388973A
Application number: GB0211942A
Authority: GB
Inventors: Lawrence Samuel Tyson
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-05-23
Filing date: 2002-05-23
Publication date: 2003-11-26
Also published as: GB0211942D0; WO2003101159A1; AU2003236889A1

Abstract

A control means (600) for a discharge lamp, has a power input (604) arranged to be connected to an alternating current electrical power supply, and an output arranged to be connected to a discharge lamp (601). The control means is arranged to control the electrical power supplied to a discharge lamp connected to the output. The control comprising a first inductor (611) connected in series between the input and the output, and a second inductor (603) connected in parallel with the first inductor, and a switch means (606), which may be a MOSFET device, in series with the second inductor, the inductors being arranged so that when the switch means is in an on state a lamp connected to the output may be operated at a first power level, and when the switch means is in an off state the lamp may be operated at a second power level, the first power level being higher than the second power level. The control means may also have a controllable trigger means arranged to operate the switch means, using phase angle control to vary the power supplied to the lamp.

Description

improvements in and relating to controls for electrical lighting The

present invention relates to controls for electrical discharge lamps.

It is known to have electrical lighting systems utilising discharge lamps, and examples of these include lamp fittings which use low pressure discharge iamps7 5 such as fluorescent tubes and lamp fittings which use high pressure discharge lamps such as metal halide lamps, mercury lamps or sodium lamps.

Luminaires utiiising discharge lamps are in widespread use. A disadvantage of these lamps is that additional components are required for their proper operation. In particular a means of limiting or controlling the current drawn by the lamp from the 10 power supply is required, together with a means of starting an electrical discharge in the lamp. The components associated with the control of lamp current when the lamp is lit are known as the ballast, whilst those components performing the function of starting the lamp are known as igniter or starter circuits. Such a combination of a ballast circuit and a starting circuit is known as the lamp control gear.

15 Discharge lamps are characterized in that they have a high impedance before they are lit, and a low or negative impedance while they are lit.

This characteristic means that in order to use such lamps powered by an ordinary power source, for example the mains electricity supply, it is necessary to combine the lamp with electrical control gear precisely matched to the lamp characteristic.

20 Historically, for fluorescent luminaires, operated from an alternating current supply, the control gear comprises a wound choke or ballast, which has a sufficiently high reactive impedance at the supply frequency so as to limit the current flow through the lamp to a safe value while the lamp is lit. To enable the lamp to become lit a starter is provided which comprises a switch which intermittently shorts out the lamp until the 25 lamp has become lit and is conducting electricity.

A known arrangement of this kind is shown in Figure 1. The means by which this arrangement causes an unlit lamp 101, which has a very high impedance, to enter a conductive or low impedance state and to draw current from the alternating current supply 104, is that when the starter switch 102 opens and breaks the conductive path 30 in parallel with the lamp the collapsing magnetic field in the wound choke 103

produces a very high voltage which is normally sufficient to break down the open circuit lamp 101 and cause it to enter a conductive state. Such starting

- 2 arrangements may comprise magnetically, thermally or electronically operated switches. For luminaires utilising high pressure discharge lamps (HID lamps), operating from an alternating current supply, a similar arrangement of control gear comprising a 5 wound choke may be provided, however some HID lamps require a higher ignition or starting voltage than conventional fluorescent lamps and for such lamps an improved means of generating the increased ignition or starting voltage is required.

Typically the starting arrangement for an HID lamp will comprise a small step up transformer operating in conjunction with a suddenly discharged capacitor. A known 10 arrangement of this kind is shown in Figure 2. The means by which this arrangement causes an unlit lamp 201, which has a very high impedance, to enter a conductive or low impedance state is by the production of a train of high voltage pulses superimposed onto the supply terminals of the lamp 201. Each breakdown of the gas discharge switch 202 causes an abrupt discharge of the capacitor 205 which 15 is charged to a voltage by way of a current flowing through resistor 209.

Each such discharge of capacitor 205 causes a high voltage to be produced across the secondary terminals 206 and 207 of the step up transformer 208. Such a starting arrangement is known as a pulse igniter circuit.

Although control gear for both low and high pressure discharge lamps operating from 20 alternating current supplies which utilise wound chokes and pulse igniters is both reliable and of low cost, a number of important limitations are known for this arrangement. In particular the power output of the lamp cannot readily be controlled, that is to say the lamp operates at a particular power level primarily determined by the reactive 25 impedance of the choke at the power supply frequency, the voltage of the power supply and the voltage drop across the discharge lamp. This limitation has as one consequence the result that the power at which the lamp operates will vary with fluctuations of the supply voltage and with variations of the voltage drop across the lamp such as occur naturally during the lifetime of the lamp and between different 30 lamps as a result of ordinary tolerances in the manufacture of lamps. Production variations in the manufacture of the choke will also introduce further variation in the operating power of the lamp. These unwanted power variations may lead to

- 3 substantial reductions in lamp life and / or unwanted variations in the colour rendering of the lamp A further limitation of this simple ballast arrangement is that discharge lamps of the same nominal power rating made by different lamp manufacturers, or discharge 5 lamps of the same nominal power rating but of different types, fluorescent, metal halide, mercury or sodium lamps for example, have different voltage drops during operation so that control gear intended for one specific lamp type may be unsuitable or unsafe when used with a lamp of the same nominal power rating, but of a different type or manufacture, because the actual power at which the lamp is operated may be 10 far from the correct value for the particular lamp used.

This particular limitation means that end users of the iuminaire must exercise caution in the selection of replacement lamps, such caution may require that the end user possess considerable and specialized knowledge. If inappropriate lamp substitutions are made by end users who do not possess the appropriate expertise catastrophic 15 failures of the control gear or lamp may ensue. Additional difficulties exist for the luminaire manufacturer, in that a wide range of different control gear types must be held in stock in order that the differing electrical requirements of the various lamp types may be met. This increased inventory overhead adds considerably to the manufactured cost of luminaires.

20 A still further limitation of the above-mentioned simple control gear arrangement is that it is not readily possible to reduce the operating power of the discharge lamp so as to bring about the dimming of the discharge lamp.

The ability to dim lamps confers many advantages in service, for example particular lighting schemes may call for a given luminaire to produce different amounts of light, 25 perhaps according to the variable level of ambient illumination arising from natural light or in accordance with a wanted variation in lighting levels between daytime and night time.

A significant benefit arises in that considerable amounts of energy may be saved if the luminaire does not produce more light than is strictly necessary for the given 30 application. One example of this is in street or amenity lighting where high levels of illumination may be required at certain times, whereas substantially reduced levels of illumination suffice at other times, thereby giving rise to the possibility of very substantial energy savings.

( - 4 Recently, electronic control gear for the operation of discharge lamps has become available. These electronic controls overcome some of the above-mentioned limitations of conventional wound control gear in that they eliminate unwanted variations in the operating power of the discharge lamp and make possible the 5 dimming of discharge lamps, so as to meet the above-mentioned objectives of controlled levels of illumination and the benefits of energy saving arising from such control. The electronic control means employed to date have therefore been successful in overcoming the above-mentioned disadvantages, but as a result of their complexity, 10 new disadvantages of cost and reliability have precluded their widespread use in certain applications.

For example in street and amenity lighting applications, it is expected that control gear lifetimes of greater than 25 years will be achieved, but certain of the components used in electronic control gear do not readily meet these lifetime 5 requirements Conventional wound choke control gear, however, is well proven in these and other demanding applications and it is desirable therefore to retain the well proven reliability of conventional control gear for such applications.

An intention of the present invention is to overcome a disadvantage of cost, complexity and reliability associated with modern electronic control gear technology, 20 so that the low cost and high reliability advantages of conventional wound control gear may be retained, but the limitations of wound control gear to precisely regulate lamp power, to cope with different discharge lamp types and to dim lamps may be overcome. Historically the power supplied to electric light sources has been controlled by various 25 means. Such means have included rheostats, variacs, and more recently phase angle control means. Phase angle control is appropriate only for lamps operating from alternating current power sources. Phase angle control offers substantial advantages in the control of power to conventional filament lamps. The technique is of low cost, high reliability, and high electrical efficiency.

30 Phase angle control of the AC power delivered to a load such as a filament lamp is a technique whereby a latching semiconductor switch, for example a silicon controlled rectifier, is utilised in such a way as to connect the load to the AC supply for a controlled fraction of each power supply cycle. In this way the average power

- s - delivered to the load may be varied by adjusting the point in time (or phase angle) in each power supply cycle at which the latching semiconductor switch is triggered.

Once triggered the semiconductor switch will continue to conduct until the current through the device falls below some value, known as the holding current, at or near a 5 zero crossing point of the power supply waveform.

Figure 3 shows an arrangement comprising an alternating current supply 304, a latching semiconductor switch 306, a trigger circuits means 307 and a filament lamp 301. It is readily apparent that current from the supply is only able to flow in the filament lamp when the semiconductor switch is closed. A characteristic of the 10 semiconductor switch 306 is that once the switch is closed, by means of a trigger current pulse derived from the trigger circuit means 307, the switch 306 remains in the conducting state until the current flowing through the switch 306 is reduced to some low holding value, usually in the 5 - 10 milliampere range. This commutation of the semiconductor switch 306 current occurs naturally when the polarity of the AC 15 supply 304 reverses, that is to say when the supply voltage traverses zero level. if a suitable arrangement is made whereby a trigger pulse is repetitively applied to the semiconductor switch 306 and this pulse is synchronous with the supply frequency it will be seen that only a portion of the supply half cycle is applied to the lamp 301.

Figure 4 shows an example of a more detailed circuit which shows the use of a 20 silicon controlled rectifier 406 in conjunction with diode 402, diode 403, diode 405 and diode 408. This arrangement of components forms a bi-directional latching semiconductor switch. Other arrangements of components may, of course, be used to perform the required function, for example a pair of SCRs connected in an inverse parallel arrangement or the use of a triac or other semiconductor switching device 25 are known in the art.

The trigger circuit means 407 provides a suitable train of trigger pulses to the gate of the SCR 406 so as to initiate the above-mentioned switch conduction. Commutation of the switch 406 occurs, as before, when the current through the SCR 406 falls to zero. In this way the average power supplied to the lamp 401 is made variable by 30 altering the phase relationship of the trigger pulses derived from the trigger circuit means 407 to the AC supply voltage 404. It will be seen that, dependent upon the phase angle of the trigger pulses derived from the trigger circuit means 407, the current in the lamp 401 is discontinuous to a greater or lesser extent.

( - 6 Figure 5 shows the typical waveforms of voltage present in the circuit of Figure 4.

The waveform labelled 501depicts the ac power supply voltage 404 of Figure 4, the waveform 502 depicts the output voltage of the trigger circuit means 407 of 5 Figure 4, The shaded areas of waveform 503 depict the conduction periods of SCR 406 of Figure 4 and the shaded areas of waveform 504 depict the voltage applied to the filament lamp 401 of Figure 4.

Unfortunately, an inherent property of phase angle control is that the current supplied to the lamp is discontinuous, i.e. there are substantial periods of time when no 10 current flows in the lamp. This is of no consequence to a conventional filament lamp, but precludes the use of phase angle control techniques for the control of power to a discharge lamp. During the above-mentioned periods when no current flows to the lamp a discharge lamp will extinguish. An aim of the present invention is to overcome this difficulty by arranging for a continuous minimum current to flow in the 15 lamp so as to avoid lamp extinction.

According to the present invention there is provided a control means for a discharge lamp, the control means having a power input arranged to be connected to an 20 alternating current electrical power supply, and an output arranged to be connected to a discharge lamp, the control means arranged to control the electrical power supplied to a discharge lamp connected to the output, the control comprising a first inductor connected in series between the input and the output, and a second inductor connected in parallel with the first inductor, and a switch means in series with the 25 second inductor, the inductors being arranged so that when the switch means is in an on state a lamp connected to the output may be operated at a first power level, and when the switch means is in an off state the lamp may be operated at a second power level, the first power level being higher than the second power level.

30 A benefit of the invention is that the power supply to the lamp through the second inductor may be disconnected from the lamp, while the lamp is maintained in a lit state by the power supplied through the first inductor. Hence a power level supplied to the lamp may be reduced, without the lamp being extinguished.

- 7 Preferably the control means further comprises a controllable trigger means that is arranged to operate the switching means. i A benefit of the controllable trigger is that the power supplied to the lamp may be: closely controlled.

Preferably the trigger means is further arranged to operate the switching means from an on state to the off state at a controllable point in a supply cycle waveform every half cycle of the supply cycle waveform of the alternating current electrical power.

A benefit of the operation of the trigger being synchronized with the supply waveform 10 is that switching losses may be minimised.

Preferably the trigger means is further arranged to operate the switching means from the on state to the off state only while the supply voltage is near to a zero voltage level. 15 A benefit of switching at or near a zero voltage level is that stress on the switch means may be minimised.

Preferably the switch elements are mosfet solid state devices.

Preferably an ignition means is provided to generate a very high output voltage to 20 start a discharge lamp connected to the output.

Preferably a discharge lamp connected to the output is operated at a controlled power level when the first switch element is operated at the controllable point in the supply cycle waveform every half cycle.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a known arrangement of a discharge lamp, such as a fluorescent lamp;

( - 8 Figure 2 is a known arrangement of a high intensity discharge lamp and a known control; i Figure 3 is a known arrangement of a filament lamp and a semi-conductor switch: arranged to controllably dim the filament lamp; 5 Figure 4 is a known arrangement of a particular semi-conductor switch for the circuit shown in Figure 3; Figure 5 shows the typical waveforms of voltage present in the circuit of Figure 4; Figure 6 shows a discharge lamp and a control according to an embodiment of the present invention; 10 Figure 7 shows a more detailed circuit for the embodiment shown in Figure 6; and Figure 8 shows the typical waveforms of voltage present in the circuit of Figure 7.

15 In the embodiment 600 of the present invention shown in Figure 6, a minimum current is supplied by an auxiliary inductor 611 to avoid lamp extinction while an! alternating current power supply 604 is supplied to the control means 600. The action of this circuit is as follows.

20 Figure 6 shows an arrangement comprising an alternating current power supply 604, a latching semiconductor switch 606, being a switch means, a trigger circuit means: 607, a first inductor 611, a second inductor 603, a discharge lamp 601 and a lamp ignition means comprising resistor 609, capacitor 605, gas discharge switch 602 and a step up transformer 608. The action of the ignition circuit means may be 25 understood by reference to Figure 2 and the associated description.

It is readily apparent that current from the supply 604 is able to flow in the discharge lamp 601 through two separate paths. The first path for lamp current is by way of the first inductor 611. This current is unaffected by the operation of the semiconductor switch 606 and provides some minimum current through the lamp 601 such that the

( - 9 - lamp will not extinguish. A typical value for such a current is 50% of the normal operating current of the lamp.

A second path for current flow to the lamp 601 exists via the semiconductor switch 606 and the second inductor 603 when the semiconductor switch 606 is closed. A 5 characteristic of the semiconductor switch 606 is that once the switch is closed, by means of trigger current pulses derived from the trigger circuit means 607, the switch remains in the conducting state until the current flowing through the switch is reduced to some low holding value, usually in the 5 milliampere range. This commutation of the semiconductor switch 606 current occurs naturally when the polarity of the AC 10 supply 604 reverses, that is to say when the supply voltage traverses a zero level.

If a suitable arrangement is made whereby a trigger pulse is repetitively applied to the semiconductor switch 606 by the trigger circuit means 607 and this pulse is made synchronous with the supply voltage 604, it will be seen that only a portion of the supply half cycle is applied to the lamp 601 via the semiconductor switch 606 and the 15 second inductor 603.

In this way the total current flowing in the discharge lamp 601 is the sum of the fixed current flowing via the first inductor 611 and the variable average current flowing through the semiconductor switch 606 and the second inductor 603. It will be seen that by varying the phase angle of the trigger pulses supplied by the trigger circuit 20 means 607 the lamp power may be smoothly varied between a minimum power established by the current flowing through the first inductor 611 and a maximum value established by the sum of the currents flowing through the first inductor 611 and the second inductor 603 when the semiconductor switch 606 is in a state of continuous conduction.

Figure 7 shows one example of a more detailed circuit and discloses the use of a silicon controlled rectifier 706 in conjunction with diode 710, diode 712, diode 713, diode 714. This arrangement of components forms a bi-directional latching semiconductor switch 750. Other arrangements of components may, of course, be 30 used to perform the required function, for example a pair of SCRs connected in an inverse parallel arrangement or the use of a triac or other semiconductor switching device are known in the art.

- 1o -

Figure 7 also shows an alternating current supply 704, a trigger circuit means 707, a first inductor 711, a second inductor 703, a discharge lamp 701 and a lamp ignition means comprising resistor 709, capacitor 705, gas discharge switch 702 and a step up transformer 708. The action of the ignition circuit means may be understood by 5 reference to Figure 2 and the associated description.

The trigger circuit means 707 provides a suitable train of trigger pulses to the gate of the SCR 706 so as to initiate the above-mentioned switch conduction. Commutation of the switch occurs, as before when the current flowing through SCR 706 falls to 10 zero. In the embodiment 700 of the present invention shown in Figure 7, a minimum current is supplied by an auxiliary inductor 711 to avoid lamp extinction. Hence the lamp 701 is operated at a low power level, the second power level, when the switch 15 means, switch 750 is in an off state.

The lamp 701 is operated a first power level when the switch 750 is in a continuously on state.

The lamp 701 is operated at an intermediate power level when the switch 750 is off for a part of every half cycle of the power supply waveform.

Figure 8 shows the typical waveforms of voltage present in the circuit of Figure 7.

The waveform labeiled 810 shows the supply voltage waveform.

The waveform labelled 820 shows the output voltage pulses of the trigger circuit means. 25 The waveform labelled 830 shows the conduction periods of SCR 706.

The waveform labelled 840 shows the voltage present across the second inductor 703. The waveform labelled 850 shows the current flowing in lamp 701

- 11 Waveform 810 depicts the ac power supply voltage 704 of Figure 7, the supply voltage waveform having positive half cycles 811, negative half cycles 812 and zero crossing points 813 and 814.

Waveform 820 depicts the output voltage of the trigger circuit means 707 of 5 Figure 7, The trigger waveform has a zero level 823, a positive going pulse 821, used to trigger the SCR 706 into a state of conduction during a negative half cycle of the supply waveform and a positive going pulse 822, used to trigger the SCR 706 into a state of conduction during a positive half cycle of the supply waveform. The trigger circuit means therefore produces output pulses at twice the power supply 10 frequency. If the trigger circuit means 707 produces output pulses that are coincident with the start of a power supply half cycle, for example at point 813 of waveform 810, then the SCR 706 will be in a state of conduction for that entire half cycle. If, however the 15 trigger circuit means produces output pulses that are coincident with the end of a power supply half cycle for example at point 814 of waveform 810, then the SCR 706 will be in a non conducting state for that entire half cycle.

It will be seen therefore that by varying the phase relationship between the zero crossing points 813 and 814 of the power supply voltage waveform 810 and the 20 generation of a trigger pulses 821 and 822 the conduction angle of the SCR 706 may be varied between O and 180 degrees.

Waveform 830 depicts the relationship between the trigger pulses 821 and 822 and the conducting state of SCR 706 represented by the shaded areas 831 and 833 of waveform 830. For example when the trigger circuit means 707 produces a trigger 25 pulse 821 the SCR 706 is triggered into a state of conduction corresponding to the shaded area 831 of waveform 830.

Waveform 840 depicts the voltage waveform applied to the second inductor 703 of figure 7. Shaded areas 841 and 842 of waveform 840 correspond to the conduction of SCR 706 represented by the shaded areas 831 and 833 of waveform 830.

30 Waveform 850 depicts the current flowing in the lamp 701 of Figure 7. The unshaded region 853 of waveform 850 represents a period during a half cycle when current to the lamp 701 is provided only by the first inductor 711 of figure 7. When

- 12 SCR 706 is triggered into conduction by trigger pulse 821 of waveform 820 a step increase in lamp current occurs at point 854 of the waveform 850. During the shaded region 855 of waveform 850 current to the lamp 701 is provided by both the first inductor 711 and the second inductor 703 of figure 7.

The SCR 706 commutates between a conducting and a non conducting state at point 852 of the waveform 850. During the unshaded region 856 of waveform 850 current to the lamp 701 is again provided only by the first inductor 711 of figure 7. When SCR 706 is triggered into conduction by trigger pulse 822 of waveform 820 a step 10 increase in lamp current occurs at point 857 of waveform 850. During the shaded region 858 of waveform 850 current to the lamp 701 is provided by both the first inductor 711 and the second inductor 703 of figure 7. The SCR 706 commutates between a conducting and a non conducting state at point 859 of waveform 850. It will be readily seen that by varying the phase angle of the trigger pulses 821 and 822 15 with respect to the zero crossing points 813 and 814 of waveform 810 the lamp power may be continuously varied between a first low power established by the current provided only by the first inductor 711 of figure 7 and a second maximum power established by the sum of the currents provided by the first inductor 711 and the second inductor 703 of figure 7.

The control means thus far described may be improved by using a microprocessor having at least two analogue to digital input pins which may be made sensitive to the lamp current and voltage. The microprocessor may perform true RMS calculations of the lamp voltage and the lamp current so that the true RMS lamp power may be 25 known. Any desired set point of lamp power can therefore be maintained by means of the microprocessor adjusting the trigger point phase angle of the semiconductor switches. In this way the closed loop system comprising a microprocessor and associated semiconductor switches can eliminate those undesired lamp power variations such as might be caused by fluctuations in the supply voltage or 30 frequency. Lamp power variations caused by changes in the lamp arc voltage such as may occur through the life of a particular lamp or as a result of lamp substitution are also eliminated.

Claims

- 13 CLAIMS

1. A control means for a discharge lamp, the control means having a power input arranged to be connected to an alternating current electrical power supply, and 5 an output arranged to be connected to a discharge lamp, the control means arranged to control the electrical power supplied to a discharge lamp connected to the output, the control comprising a first inductor connected in series between the input and the output, and a second inductor connected in parallel with the first inductor, and a switch means in series with the second inductor, the inductors being arranged so that 10 when the switch means is in an on state a lamp connected to the output may be operated at a first power level, and when the switch means is in an off state the lamp may be operated at a second power level, the first power level being higher than the second power level.

15

2. A control means for a discharge lamp as claimed in claim 1, wherein the control means further comprises a controllable trigger means that is arranged to operate the switching means

3. A control means for a discharge lamp as claimed in claim 2, wherein the 20 trigger means is further arranged to operate the switching means from an off state to the on state at a controllable point in a supply cycle waveform every half cycle of the supply cycle waveform of the alternating current electrical power.

4 A control means for a discharge lamp as claimed in claim 3, wherein the 25 trigger means is further arranged to operate the switching means from the on state to the off state only while the supply voltage is near to a zero voltage level.

5. A control means for a discharge lamp as claimed in any of the preceding claims, wherein the switch elements are mosfet solid state devices.

- 14

6. A control means for a discharge lamp as claimed in any of the preceding claims, wherein the control means further comprises an ignition means to generate a very high output voltage to start a discharge lamp connected to the output.

5

7. A control means for a discharge lamp as claimed in any of the preceding claims when dependent on claim 3, wherein a discharge lamp connected to the output is operated at a reduced power level when the first switch element is operated at the controllable point in the supply cycle waveform every half cycle.

10

8. A control means for a discharge lamp as described with reference to Figures 6, 7 and 8.