GB2421645A

GB2421645A - Gas discharge lamp drive circuitry

Info

Publication number: GB2421645A
Application number: GB0604590A
Authority: GB
Inventors: David John Aarons
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-12-31
Filing date: 2002-12-31
Publication date: 2006-06-28
Anticipated expiration: 2022-12-31
Also published as: GB2421645B; GB2397182A; GB0230314D0; GB0604590D0; GB2397182B

Abstract

An apparatus and method for controlling the operation of a gas discharge lamp, for example a fluorescent lamp, metal halide (HID) lamp, low or high pressure sodium vapour lamp, comprises an electronic circuit 1 for controlling the gas discharge lamp 40. The electronic circuit includes a pulse generator 18 for generating a high frequency pulse train that is applied to the electrodes 1L-4L of the lamp to light the lamp, and a controller 17 for controlling the applied frequency of the pulse train. The controller varies the frequency of the pulse train within an upper frequency band during a first mode of operation for striking the lamp, and within a lower frequency band during a second mode of operation for running the lamp once struck. In the first mode the frequency of the pulse train is scanned downwards until the lamp strikes. In the second mode the frequency of the pulse train is scanned downwards until the lamp provides a desired output.

Description

-1- 2421645 Gas Discharge Lamp Drive Circuitry The present invention

relates to an apparatus and method for controlling a gas discharge lamp, and particularly for avoiding acoustic resonances within a gas discharge lamp, and for striking and running a gas discharge lamp, for example fluorescent lamps, metal halide (HID) and low and high pressure sodium vapour lamps.

Conventional gas discharge lamps require a ballast to operate. The ballast provides a high initial voltage to initiate the discharge, then rapidly limits the lamp current to safely sustain the discharge. The ballast is most usually a capacitor in series with a coil, or other such resonant circuit, through which the gas discharge drive current must pass.

Preheat operation lamp electrodes are heated prior to initiating the discharge. A starter switch closes, permitting a current to flow through each electrode. The starter switch rapidly cools down, opening the switch, and triggering the supply voltage across the arc tube, initiating the discharge. No auxiliary power is applied across the electrodes during operation.

Rapid start operation lamp electrodes are heated prior to and during operation. A transformer has two special secondary windings to provide the proper low voltage to heat the electrodes.

Instant start operation lamp electrodes are not heated prior to operation, as so are referred to as "cold cathode" lamps. Ballasts for instant start lamps are designed to provide a relatively high starting voltage, as compared with preheat and rapid start lamps, to initiate the discharge across the unheated electrodes.

A common problem with gas discharge lamp drive circuitry is that the electrical characteristics to strike and run a lamp change over time, mainly due to aging of the lamp as it is used. In particular, the minimum voltage needed to strike a lamp increases with age owing to a number of effects, in particular erosion of the lamp electrodes.

Therefore, discharge lamp drive circuitry is normally designed to provide a strike voltage sufficient to strike the lamp at the end of its expected lifetime, which will be significantly higher than the minimum strike voltage needed to strike a new lamp. Although in principle it might be possible to vary the voltage with lifetime, drive circuitry is normally fixed in place with the lamp and it may be impossible to predict accurately in advance how this voltage should vary for a particular lamp. In many applications, the cost of drive circuitry needs to be as low as possible, and varying the drive voltage with time in a predetermined manner may add unacceptable cost to a lamp unit.

Although cold cathode lamps generally have a superior lifetime to heated electrode lamps, the higher voltage needed to strike such a lamp would increases the difficulty of matching drive voltage to the lamp characteristics over the lifetime of lamp.

In order to increase the efficiency of gas discharge lamps, it is now common to drive such lamps at a frequency that is well above mains frequency. However, another problem has been noted with such "high frequency" lamps, namely a pressure or "acoustic" resonance in the gas discharge medium. Acoustic resonance can occur when the drive frequency of the lamp excites an acoustic mode within the lamp. Essentially, there are three different types of acoustic resonance: longitudinal resonance, in which an acoustic wave travels along the length of the gas discharge; radial resonance in which an acoustic wave moves radially with respect to a longitudinal direction of the lamp; and azimuthal resonance, in which an acoustic wave travels around a longitudinal axis of the lamp. The latter type of resonance can be the most damaging - if the gas discharge touches a glass envelope, the glass may shatter.

One solution to this problem is to modulate or pulse the applied high frequency drive within low frequency envelope, for example 50 Hz. The periodic interruption of the high frequency drive then tends to disrupt any tendency towards acoustic resonance. This approach is not always totally effective, and presents its own problems, particularly the need to apply a higher than average voltage during the phases when the high frequency drive is present in order to achieve the same average light output.

The higher peak drive current will then decrease lamp lifetime.

It is an object of the current invention to provide a more convenient apparatus and method of driving a gas discharge lamp.

Accordingly, the invention provides an electronic circuit for controlling a gas discharge lamp, comprising a pulse generator for generating a high frequency pulse train that may be applied to the electrodes of the lamp to light the lamp, and a controller for controlling at least one characteristic parameter of the applied pulse train, and a power measurement circuit for monitoring the electrical power drawn by the lamp, wherein the controller is arranged to control the characteristic parameter of the applied pulse train in response to the monitored electrical power.

In one preferred embodiment of the invention, the characteristic parameter is a frequency of the applied pulse train. In another embodiment, the characteristic parameter may be a pulse width or a pulse height of the applied pulse train. It may, however, be desirable to control any combination of frequency, pulse width or pulse height.

The power measurement circuit may be an integrated circuit may consist of discrete electrical components, or be a combination of discrete and integrated electrical components. In particular, the power measurement circuit may be either fully or partially integrated within the controller, for example as part of an integrated microprocessor device.

In one embodiment of the invention, the controller during steady state operation of the lamp is arranged to monitor the electrical power drawn by the lamp and to detect from a change in said drawn electrical power the onset of acoustic resonance of the gas discharge within the lamp, and vary the frequency to stop said acoustic resonance.

The change may be a drop in power, a rise in power, or other such fluctuation in power.

The different types of acoustic resonance all share the characteristic that the length of the discharge path through the lamp will vary from that when there is no acoustic resonance In another embodiment of the invention, the circuit has a first mode of operation for striking a gas discharge lamp and a second mode of operation for running said lamp once struck, wherein the controller is arranged in response to the monitored power to vary the frequency of the pulse train within an upper frequency band during the first mode of operation and within a lower frequency band during the second mode of operation.

The invention further provides an electronic circuit for controlling a gas discharge lamp, comprising a pulse generator for generating a high frequency pulse train that may be applied to the electrodes of the lamp to light the lamp, and a controller for controlling the frequency of the applied pulse train, the circuit having a first mode of operation for striking a gas discharge lamp and a second mode of operation for running said lamp once struck, wherein the controller is arranged to vary the frequency of the pulse train within an upper frequency band during the first mode of operation and within a lower frequency band during the second mode of operation.

The frequency bands may be distinct, or there may be some overlap between the bands, with an upper end of the low frequency band overlapping with a low end of the upper frequency band.

Also according to the invention, there is provided a method of using an electronic circuit to control a gas discharge lamp, comprising the steps of using the electronic circuit to: I) generate a high frequency electronic pulse train; ii) apply the electronic pulse train across electrodes of the lamp; wherein iii) the circuit monitors the electrical power drawn by the lamp; and iv) controls at least one characteristic parameter of the applied pulse train in response to the monitored electrical power.

Also according to the invention, there is provided a method of using an electronic circuit to control a gas discharge lamp, comprising the steps of using the electronic circuit to: i) generate a high frequency electronic pulse train; ii) apply the electronic pulse train across electrodes of the lamp; wherein iii) the circuit operates in a first mode of operation in which the pulse train is generated within an upper frequency band in order to strike the lamp; and iv) once the lamp is struck, the circuit operates in a second mode of operation in which the pulse train is generated within a lower frequency band in order to run the lamp.

In the context of the present invention, the term "high frequency" means a frequency above approximately 2 kHz and below approximately 400 kHz.

In a preferred embodiment of the invention, the circuit is adapted to scan the frequency of the pulse train downwards in the first mode of operation until the lamp strikes.

Therefore the method may comprise during the first mode of operation the step of decreasing the frequency of the pulse train within the upper frequency band until the lamp strikes, and then operating the circuit in the second mode of operation.

Also in a preferred embodiment, the circuit is adapted to scan the frequency of the pulse train downwards in the second mode of operation until the lamp provides a desired output.

Therefore, the method may include dropping the frequency in the first frequency band until the lamp strikes, and then continuing to drop the frequency in the second frequency band until the output of the lamp reaches a desired level.

To help control the rate at which the frequency is varied, the circuit may include feedback by which the output of the lamp can be determined, for example a circuit by which a measure of the current and/or voltage supplied to the lamp may be had.

In the second mode of operation, the frequency of the pulse train may also be decreased as the lamp warms up in order to steady the light output of the lamp.

Preferably, the circuit includes a choke to limit the current drawn by the lamp. The choke may be an electronic coil or other device that serves to limit the current through the lamp, including solid state devices. The impedance of such a choke may vary inversely with frequency of the applied pulse train, so that as the frequency increases the choke limits the current drawn by the lamp from the high frequency pulse train.

The circuit may include a power supply stage with a power factor correction circuit, the controller being adapted to monitor and correct the power factor of electrical current drawn by the power supply stage.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a first embodiment of the invention, having a controller that that controls a "fly-back" power supply section and a lamp driver section with series resonant circuit for diving a fluorescent lamp; Figure 2 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a second embodiment of the invention, similar to the first embodiment but having a "buck converter" power supply; Figure 3 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a third embodiment of the invention, similar to the first embodiment but having a power supply with "boost topology" for power factor control; Figure 4 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a fourth embodiment of the invention, with the same power supply section as the third embodiment and having a lamp driver section that uses a phase shifting technique to control the light power output of the lamp; Figure 5 is a simplified schematic drawing of an - 10 - electronic circuit for driving a gas discharge lamp, according to a fifth embodiment of the invention, with the same power supply section as the second embodiment but with the lamp driver section of the fourth embodiment; Figure 6 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a sixth embodiment of the invention, with the same power supply section as the first embodiment but with the lamp driver section of the fourth embodiment; Figure 7 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a seventh embodiment of the invention, with the same power supply section as the second embodiment, having a lamp driver section suitable for driving a High Intensity Discharge (HID) lamp, such as a street lamp; Figure 8 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to an eighth embodiment of the invention, with the same power supply section as the third embodiment, but with the lamp driver section of the seventh embodiment; Figure 9 is a simplified schematic drawing of an electronic circuit for driving a gas discharge lamp, according to a ninth embodiment of the invention, with the same power supply section as the first - 11 - embodiment, but with the lamp driver section of the seventh embodiment; Figure 10 is a simplified schematic drawing of a lamp driver section that uses a phase shifting technique to control the light power output of the lamp, as used in the embodiments shown in Figures 4, 5 and 6; Figure 11 is a plot of Voltage and Lamp Power against Time and drive Frequency for an electronic circuit according to the invention when used to strike and run a gas discharge lamp; Figure 12 is a plot of Lamp Electrical Power and drive Frequency against Time for an electronic circuit according to the invention when used to avoid acoustic resonances in a gas discharge lamp.

Figure 1 shows an electronic circuit 1 for driving a gas discharge lamp, according to a first embodiment of the invention. The circuit has a power supply section (PS1) 11 that is energised when this is connected to mains power 30. The power supply 11 feeds stabilised current to a lamp driver section (LD11) 14, which in turn is connected to the electrodes (lL, 2L, 3L, 4L) of a lamp 40, here a fluorescent lamp.

Both the power supply section 11 and lamp driver section 14 are connected to and controlled by a microprocessor controller (pP) 17. The microprocessor 17 includes two oscillators (01 and 02) 18,19, one of which 18 provides opposite polarity drive signals to two MOSFET N power - 12 transistors (TR21 and TR31) . The lamp driver section 14 is a series resonant circuit, with a resonant frequency of between 95 to 97 kHz.

Reference is now made also to Figure 11. When the lamp 40 is to be run, the microprocessor controller 17 first generates a high frequency pulse train at about 150 kHz.

This frequency is too high to strike the lamp, but the frequency is progressively decreased at a rate of between 10 kHz/s to 20 kHz/s towards the resonant frequency. As this happens, the voltage V across the lamp rises. At a certain voltage Vo the lamp will strike.

The strike voltage for a new fluorescent lamp is about 600 V. The strike voltage of a new cold cathode lamp of the type used in street lighting will be about 900 V. Once the lamp 40 is struck, the voltage across the lamp will drop to the rated lamp voltage, for example about 100 V. A lamp which has been aged in use will have a higher strike voltage V1. The characteristics of the resonant circuit can be chosen so that the maximum voltage VM from the resonant circuit are above the maximum expected voltage for a lamp within its lifetime.

Because the voltage drops once the lamp 40 is struck, the lamp is not exposed to voltages in excess of that needed to strike the lamp. This helps to extend the lifetime of the lamp.

The typical time taken from the circuit turn on at a time to to the striking of the lamp at time tj. or t2 will be between 1 s and 3 s. In the case of a fluorescent lamp - 13 - with heated cathodes, the downward sweep in frequency may be paused at a time t3 for between 1 s and 2 s before the lamp is struck to allow time for the cathodes to become sufficiently warm.

The lamp initially lights with a relatively low power Po, with the current being limited by the choke (Lii) 25. As the frequency continues to drop, more current passes through the choke 25, resulting in an increase in the lamp power. The frequency then drops steadily until it reaches the region of 30 kHz to 50 kHz. The final running frequency R can be chosen according to the desired optical output of the lamp 40.

In the case of an aged lamp, the running frequency R may be chosen to be lower than for a new lamp in order to achieve the same light output from the lamp. The microprocessor 17 may therefore include a lookup table (not shown) with typical expected lamp aging characteristics. These may be related to the strike voltage.

Both the lamp voltage and current are monitored by the microprocessor along feedback line Iload and Vload. Both of these can be used as a measure of the lamp performance, both during strike of the lamp and during running. The microprocessor 17 then adjusts the applied frequency or rate of change of the applied frequency, according to these measured parameters.

The lamp driver circuit 14 when controlled by the microprocessor 17 can therefore be used to vary or shift - 14 - the frequency of the pulse train applied to the electrodes of the lamp 40.

Optionally, the second oscillator 19 of the microprocessor 17 may be used with a second lamp driver circuit (LD21) 114 similar to the first lamp driver circuit 14 to drive another fluorescent lamp (not shown) The microprocessor 17 also controls the power supply section 11 in the following manner. The microprocessor 17 monitors the input voltage and current on lines Vin and un. By comparing both input and output voltages and currents, the microprocessor calculates a desired power factor correction, to keep the power factor of input current and voltage as close to unity as possible. In particular, the microprocessor can be programmed to control the power factor of any of the various embodiments of the power supply sections described herein by applying suitable control pulses on a power factor correction output line PFC OUT.

The power supply 11 of Figure 1 is a so-called a "fly-back" power supply. This provides significant advantages in many applications. First, it is possible to get good electrical isolation between the power supply section 11 and lamp driver section 14, by omitting to connect the primary and secondary windings of transformer Til along line 24 and by providing a separate ground connection 23 for the lamp driver section 14. Such "fly-back topology also provides for efficient Power Factor Correction. Additionally, the "fly-back" circuit can be designed for universal mains connection, for - 15 - example between about 70 VRNS to 300 VRMS.

The power supply 11 has a main drive MOSFET N transistor (TR11) . The microprocessor applies a pulse width modulated or frequency modulated signal to this transistor, in order to set the level of electrical power supplied to the lamp drive section 14. Current drawn by the transistor TR11 is pulled through the primary winding of a transformer (Til) Current induced in the secondary winding of transformer Til is then rectified by a diode (D21), and collected by a capacitor (C41) . The PFC modulation provided by the microprocessor 17 on line PFC OUT therefore controls the output voltage across C41, and it is this voltage which is supplied to the lamp driver section 14. The PFC modulation can be either "continuous" (for example pulse width or frequency modulation or a continuous pulse train) or "discontinuous" (for example a series of separate pulses), depending on the programming of the microprocessor 17, depending on the required power output of the power supply section 11. Continuous mode PFC modulation with the circuit 11 is capable of achieving higher power levels than discontinuous mode PFC modulation.

A number of control inputs are also provided by which the operation of the microprocessor 17 can be set or controlled, including: - a digital input/output (D I/O) 20, which is a serial digital data interface that can be connected to an RS232 or RS485 compliant interface, or any other type of digital serial network; - 16 - - a general purpose input/output (GPIO) interface 21 connected to eight general purpose input/output lines that may be used to control other devices connected to the microprocessor, for example indicator LEDs or a display (not shown); and - and an analogue control input (Al) 22 which may be used to control the output power or to dim the lamp 40.

The circuit 1 can therefore be controlled either manually at a dimmer switch or electronically in order to control operation of the lamp 40.

Figure 2 shows a second embodiment of the invention, with a circuit 2 that is similar in operation to the first embodiment 1, but which has as the power supply (PS2) 12 a circuit is known as a "buck converter". This is driven in a similar manner to that described above, although the circuit 21 leads to a greater inrush of current when first switched on. The "buck converter" 21 topology is suitable only for continuous mode power factor correction.

Figure 3 shows a third embodiment of the invention, with a circuit 3 that again is similar in operation to the first embodiment 1, but which has as the power supply section (2S3) a circuit 13 referred to herein as a "boost topology" circuit. This is a particularly versatile circuit for power factor control (PFC) . Again, the microprocessor 17 sends a modulation signal, which can be either continuous or discontinuous mode modulation, to a MOSFET N transistor (TR13) . Here discontinuous mode - 17 modulation would be used at power levels of less than W, and continuous mode for power levels above 100 W. Figure 4 shows a fourth embodiment of the invention, with a circuit 4 that is similar in operation to the third embodiment 3, in that the circuit has a power supply section (PS4) with "boost topology" for improved power factor control, but which uses in the lamp driver section (LD4) 15 a phase shifting circuit 10 (shown in detail in Figure 10) to control the light power output of the fluorescent lamp 40. The phase shifting circuit 10 of Figure 10 takes two square wave pulse trains as inputs P0 and P1. When the phase difference between the pulse trains is shifted, the effect is to modulate the width of a combined pulse train applied across the electrodes 1L and 4L of the lamp 40. The operation of the circuit of Figure is described more fully in patent document WO 98/51134.

An advantage of this circuit is that by shifting the phase difference between pulse trains P0 and P1, it is possible to dim the light output from the lamp 40.

Because the circuit 10 requires as an input two independent pulse trains P0 and P1, both of the microprocessor oscillators 18,19 are used to supply two square wave pulse trains to the circuit 10.

Optionally, the circuit 10 may be used to drive a HID or cold cathode lamp 140, in which case there is only one connection (1L and 4L) between the circuit 10 and each end of the lamp 140.

- 18 - Figure 5 shows a fifth embodiment of the invention, with a circuit 5 with similar lamp driving characteristics as the third embodiment 3, but which uses in the power supply (PS5) section a "buck converter" power supply 12, having the same advantages as those described above for the second embodiment 2.

Figure 6 shows a sixth embodiment of the invention, with a circuit 6 that has similar lamp driving characteristics as the third embodiment 3, but which uses in the power supply section (PS6) section a "fly-back" power supply 11, having the same advantages as those described above for the first embodiment 1.

Figure 7 shows a seventh embodiment of the invention, with a circuit 7 that is similar in operation to the second embodiment 2, in that the circuit has a power supply section (PS7) with a "buck converter" circuit 12, but having a lamp driver section (LD71) 16 suitable for driving a HID or a cold cathode type lamp 140. This type of lamp is used for street light and shop font/windows and the like. Because such lamps do not have heated electrodes, only two connections 1L and 4L are made between the lamp driver section (LD71) 16 and the lamp 140. Compared with a fluorescent lamp 40 with heated electrodes, a HID or cold cathode lamps 140 needs a higher starting voltage ionize the gas inside the lamp.

Figure 8 shows an eighth embodiment of the invention, with a circuit 8 that has similar lamp driving characteristics as the seventh embodiment 7, but which uses in the power supply section (PS7) the boost topology circuit 13 similar - 19 - of the third embodiment 3 for improved power factor control. Power factor control is of particular importance in Street lighting applications, where the power utilities desire a base load at night to have a power factor as close to unity as possible.

Figure 9 shows a ninth embodiment of the invention, with a circuit 9 that that has similar lamp driving characteristics as the seventh embodiment 7, but which uses in the power supply section (PS7) the "fly-back" circuit 11 as used in the first embodiment 1.

As can be seen from Figures 1 to 9, each embodiment of the lamp driver section 14,15,16 includes a choke in the form of a coil 25,26,27 for which the impedance varies inversely with frequency. In each embodiment 1- 9, the coil limits the current drawn by the lamp 40,140 from the high frequency pulse train applied to the lamp by the lamp driver section 14, 15,16.

All of the embodiments described above have essentially the same microprocessor 17, although this may be loaded with different firmware depending on the electrical characteristics of the particular embodiment, and the expected strike and run characteristics of the lamp.

Although the schematic drawings of Figures 1 to 9 all show a connection between power ground and output ground, such a connection can be omitted to give isolation between the power supply section and lamp driver section.

The use of the microprocessor 17 to control both the power - 20 - supply section and the lamp driver section provides a number of benefits. First the lamp can be struck and run in a way that places a reduced operating stress on the lamp, mainly from reduced strike and operating voltages, and from a controlled warm up sequence and time scale. As described above, the microprocessor 17 is programmed to operate the lamp 40,140 in one of two modes of operation: a strike mode in which the gas in the lamp is excited at a relatively high voltage and low current; and a run mode, in which the lamp once struck is made to run and emit a useful amount of light at a relatively low voltage and high current. During the first mode of operation, a pulse train is initially generated with a relatively high frequency, until the lamp strikes. Then the pulse train is switched in the second mode of operation to a relatively low frequency. The frequency of the pulse train can be decreased as the lamp warms up in order to steady the light output of the lamp.

At the same time, the microprocessor 17 can monitor and control the operation of the power supply section. The power factor can be controlled in any of several ways by one type of microprocessor, suitably programmed. This provides economies in manufacture as one model of microprocessor can be used in different applications with different types of power supply section and lamp driver section.

The range of the upper and lower frequency bands will depend on the type of lamp and choice of choke, but will in most cases be respectively between about 100 kHz to 350 kHz and between about 20 kHz and 100 kHz.

- 21 - In all the embodiments 1-9 described above, the controller 17 alsomonitors both the lamp current on signal line Iload, and the lamp voltage on signal line Vload. From this, the controller can calculate both an "average" lamp power and an "instantaneous" lamp power. The average lamp power may be a moving averaged over between 10 to 600 seconds, and preferably 60 seconds, while the instantaneous lamp power is averaged over a much shorter period, for example 0.1 to 2 seconds, and preferably 1 second. In most cases, the gas discharge through the lamp in the absence of acoustic resonance will be an essentially straight line path. When acoustic resonance begins, the gas discharge path then increase, leading to increased impedance through the gas discharge, and a reduced lamp power at a given drive voltage. The controller then shifts the drive frequency either up or down while monitoring the instantaneous power. If the power continues to drop, then the controller immediately begins to shift the drive frequency in the opposite direction, while if the instantaneous power begins to rise, the controller continues to shift the drive frequency until the instantaneous power shows that acoustic resonance has ceased. In doing this, the controller would be programmed to accommodate any baseline shift in average power owing to changes in drive frequency, for example owing to the resonant character of the ballast inductor.

Reference is now made to Figure 12, which is an example of the degree to which the controller 17 may shift the drive frequency to avoid acoustic resonance in a typical street - 22 - lamp, such as the Sylvania (reg. TM) SON 70 W high pressure Na2 lamp, sold as part number SHP-TS. This has a gas discharge tube about 5 cm long, and which can exhibit acoustic resonance at roughly 20 kHz, 40 kHz, 80 kHz, etc. The resonant frequencies cannot be predicted exactly and will vary with individual lamp characteristics including the age of the lamp, the drive power, and the temperature of the Na2 vapour and surrounding glass envelope. Each resonance is about 1 kHz wide, and so the controller may at times have to shift the drive frequency, either up or down, by at least 500 Hz to avoid the onset of resonance.

As shown in Figure 12, initially the lamp power is about W, and the drive frequency is about 120 kHz. At a time t0 of 4 seconds, the drawn electrical power, as monitored by the controller 17, begins to drop, as a result of acoustic resonance in the lamp. It is not possible at this stage to tell if the drive frequency should be raised or lowered to avoid the resonance. At time t1 of 4.7 s, the controller reacts to the sudden drop in power by raising the drive frequency, but this causes a sharper fall in power. The controller interprets this is indicating that the drive frequency needs to be shifted in the opposite direction. Therefore, at time t2 the controller reacts by lowering the drive frequency until at a time t3 of 7.5 s, the power has returned to 70 W. As a failsafe, in order to ensure that the lamp during steady state operation is not in acoustic resonance, the controller may be arranged periodically to raise and/or lower temporarily the drive frequency. The controller then - 23 - detects if the power changes in an expected way over this frequency range for a non-resonant lamp.

It may sometimes be desirable to run a lamp at a lower frequency, for example if frequency control is used to dim a lamp. Resonant acoustic frequencies are closer together at such lower frequencies. The invention makes it easier to operate at such frequencies while avoiding acoustic resonances, and therefore provides the additional benefit of allowing a wider potential range of drive frequencies.

The invention also makes it easier to design a generic controller that may be used with a wide range of different types of lamps, since it is not necessary to predict in advance the particular strike characteristics or acoustic resonance characteristics. Essentially, the controller can be programmed to recognize from monitored current and voltage optimum strike and running conditions for different types of lamp. This then provides additional benefits in terms of manufacturing cost for the electronic circuit, owing to increased manufacturing efficiencies of scale.

The invention described above therefore provides a number of convenient solutions to problems of achieving good power factor control, and matching the electrical output of gas discharge lamp drive circuitry to the characteristics of a lamp, both when the lamp is new, and as it ages with use.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of - 24 - separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.

It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention, as defined by the appended claims.

Claims

- 25 - Claims 1. An electronic circuit for controlling a gas discharge

lamp, comprising a pulse generator for generating a high frequency pulse train that may be applied to the electrodes of the lamp to light the lamp, and a controller for controlling the frequency of the applied pulse train, the circuit having a first mode of operation for striking a gas discharge lamp and a second mode of operation for running said lamp once struck, the controller being arranged to vary the frequency of the pulse train within an upper frequency band during the first mode of operation and within a lower frequency band during the second mode of operation, wherein the circuit is adapted to scan the frequency of the pulse train downwards in the first mode of operation until the lamp strikes.
2. An electronic circuit as claimed in Claim 1, in which the circuit is adapted to scan the frequency of the pulse train downwards in the second mode of operation until the lamp provides a desired output.
3. An electronic circuit as claimed in Claim 1 or Claim 2, in which the upper frequency band extends between 100 kHz and 350 kHz, and the lower frequency band extends between 20 kHz and about 100 kHz.
4. An electronic circuit as claimed in any preceding claim, in which the circuit includes a resonant circuit with a characteristic resonant frequency, the upper frequency band extending across a frequency range above the characteristic frequency, and the lower frequency band - 26 extending across a frequency range below the characteristic frequency.
5. An electronic circuit as claimed in any preceding claim, in which the circuit includes feedback by which the output of the lamp can be determined by the controller.
6. An electronic circuit as claimed in Claim 5, in which the feedback includes a measure of the current and/or voltage supplied to the lamp.
7. An electronic circuit as claimed in any preceding claim, in which the circuit includes a choke to limit the current drawn by the lamp.
8. An electronic circuit as claimed in any preceding claim, in which the circuit includes a power supply stage with a power factor correction circuit, the controller being adapted to monitor and correct the power factor of electrical current drawn by the power supply stage.
9. A method of using an electronic circuit to control a gas discharge lamp, comprising the steps of using the electronic circuit to: i) generate a high frequency electronic pulse train; ii) apply the electronic pulse train across electrodes of the lamp; wherein - 27 - iii) the circuit operates in a first mode of operation in which the pulse train is generated within an upper frequency band in order to strike the lamp; and iv) once the lamp is struck, the circuit operates in a second mode of operation in which the pulse train is generated within a lower frequency band in order to run the lamp.
10. A method as claimed in Claim 9, in which during the first mode of operation the method comprises the step decreasing the frequency of the pulse train within the upper frequency band until the lamp strikes, and then operating the circuit in the second mode of operation.
11. A method as claimed in Claim 9, in which the method includes dropping the frequency in the first frequency band until the lamp strikes, and then continuing to drop the frequency in the second frequency band until the output of the lamp reaches a desired level.
12. A method as claimed in any of Claims 9 to 11, in which the circuit includes means by which the optical power of the lamp can be determined.
13. A method as claimed in any of Claims 9 to 12, in which the electronic circuit includes an electronic choke in which impedance varies inversely with frequency, and the method includes the step of using the choke to limit the current drawn by the lamp from the high frequency pulse train.

- 28 -
14. A method as claimed in any of Claims 9 to 12, in which during the second mode of operation, the frequency of the pulse train is decreased as the lamp warms up in order to steady the light output of the lamp.