Method and device for driving a gas discharge lamp

FIELD OF THE INVENTION

The present invention relates in general to a method and device for driving a gas discharge lamp. Especially, but not exclusively, the present invention relates in general to a method and device for driving a compact fluorescent lamp in a dimmed mode.

BACKGROUND OF THE INVENTION

In general, gas discharge lamps are known in the art, so an elaborate explanation of gas discharge lamps is not needed here. Suffice it to say that a gas discharge lamp comprises a closed vessel filled with an ionizable gas or vapour. The lamp comprises two electrodes at a certain mutual distance, for coupling energy into the gas filling. The lamp is operated by applying electric power to the electrodes, resulting in a discharge current in the lamp, which current results in UV light being generated. To produce visible light, the inner surface of the tube, typically glass, is coated with a fluorescent material that converts UV light into visible wavelength. Since gas discharge lamps are known per se, a further explanation is not needed; by way of example, the well-known TL lamp and the compact fluorescent lamp (CFL) are mentioned.

Gas discharge lamps can not be supplied from mains directly. As will be known to persons skilled in the art, a gas discharge lamp is driven by a driver which at its input receives mains voltage (in Europe, mains voltage has a typical rating of 230 V AC at 50 Hz, but in different countries the circumstances may be different), and which at its output provides suitable lamp voltage and lamp current.

Electronic drivers are commonly known to persons skilled in this art, so an elaborate discussion of the design of electronic drivers is not needed here. Although it is possible, in principle, to drive a gas discharge lamp with constant current, it is typically preferred that a gas discharge lamp is driven with an alternating current. Electronic drivers may be of a resonant type, in which case the current frequency is determined by a resonance circuit in the driver, or of a frequency-controlled type, in which case the current direction, and the frequency at which the current direction is reversed, is determined by a controller that

controls one or more switches. Typical examples of such frequency-controlled drivers typically have a bridge topology, as should be known to persons skilled in the art.

Gas discharge lamps have a nominal rating, which means that they are designed to be driven with a certain nominal lamp voltage and nominal lamp current, in which case such lamp produces a nominal light output (nominal light intensity). However, for certain applications there is a desire to be able to dim a lamp, i.e. to reduce the light output as compared to nominal. This can be performed by reducing the lamp current, since the light output is generally proportional to the lamp current. However, reducing the lamp current has some disadvantages, including the fact that the color of the generated light changes with changing current magnitude, and this mechanism can only be used for dimming down to a certain level.

Another, more sophisticated way of dimming is achieved by duty cycle control. In this case, the lamp is alternatively switched ON and OFF, at a frequency that is typically in the range of 100 - 200 Hz so as not to be visible to the human eye. The repetition period of this switching is indicated as lamp current period. Because of the ON/OFF switching, the lamp current period is divided into two portions, i.e. an ON portion in which the nominal current is actually flowing and an OFF period in which no current is actually flowing. The ratio of the duration of the ON portion to the duration of the entire current period is indicated as duty cycle; by varying the duty cycle, the light output of the lamp can be varied (variable dimming). The present invention relates particularly to a driver applying duty cycle current.

Lamps being driven by duty cycle current may be used for illumination. However, gas discharge lamps driven by duty cycle current may also be applied as backlighting for LCD panels, such as for instance used in televisions and monitors. In principle, by varying the duty cycle between 0% and 100%, it is possible to vary the dimming level between 0% and 100% of the nominal light output. However, a problem is that, in practice, it has been found that a gas discharge lamp driven at a duty cycle below 100% may generate undesirable peaks in the infrared region of the spectrum. While this is not visible to the human eye, it may interfere with the correct functioning of infrared- controlled devices, such as for instance the remote control of a television set. This problem appears to become worse as the lamp is dimmed to a lower light output level.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome or at least reduce this problem.

To this end, the present invention proposes that, in the OFF portions of the current period, the lamp current is not reduced to zero but to a value higher than zero.

Depending on the desired dimming level, this value can be close to zero, so that virtually no light output is produced, but high enough to ensure the presence of free electrons in the vessel, which facilitate the re-ignition of the lamp. Preferably, the lamp current in the OFF portions is chosen above the light-generating threshold of the lamp, in order to assure that the lamp continues to generate a low level of light output, because it has been found that the presence of photons contributes to facilitating the re-ignition of the lamp.

This invention is based on the understanding that the infrared problem as mentioned is mainly caused by the re-ignition process. Particularly, when a gas discharge lamp is ignited, the buffer gas will become ionized first to produce free electrons, resulting in a discharge avalanche and the ionization of the mercury atoms. The infrared radiation appears to be generated in the initial phase of the ignition, when the mercury has not ignited yet. Once the mercury has ignited, the amount of infrared radiation will reduce substantially. The smaller the duty cycle (more specifically: the longer the duration of the OFF portions), the more free electrons will have disappeared and the more difficult it becomes for the mercury to ignite, hence the more infrared is produced. Based on this understanding, by ensuring the presence of a certain level of free electrons in the vessel, the present invention facilitates the ignition of the mercury and hence reduces the amount of infrared light produced.

Further advantageous elaborations are mentioned in the dependent claims. It is noted that US-6.828.740 discloses a lamp driver for an inductively coupled lamp, where the driver is alternatively operating with a high output current and a low output current. However, in this case the output current is the current in the output coil, which is not identical to lamp current. The document specifically states that in the periods of low output current, the lamp is not operated, i.e. no light is generated, which implies that no lamp current is flowing. The document is silent about any infrared problems. The document does contend (in column 13 lines 39-43) that an ionized gas exists during the non-operation time, but this is on the one hand contradictory with earlier information (compare "ionized gas light emitting gas" in column 13 lines 39-40 with "insufficient amount discharge for emitting light" in column 13 lines 19-20), and on the other hand, if an ionized gas does exist, it can only be for an insignificant amount.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1 is a block diagram schematically illustrating a general design of a lamp driver; figure 2 is a circuit diagram schematically illustrating an inverter for use in the driver of figure 1 ; figure 3 is a graph illustrating duty cycle dimming according to the state of the art; figure 4 is a graph illustrating duty cycle dimming according to the state of the art, in a case of very low duty cycle; figure 5 is a graph comparable to figure 3, illustrating duty cycle dimming according to the present invention; figure 6 is a block diagram schematically illustrating a first embodiment of a lamp driver according to the present invention; figure 7 is a block diagram schematically illustrating a second embodiment of a lamp driver according to the present invention; figure 8 is a graph illustrating a current envelope.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically shows a general design of a lamp driver 1 for driving a gas discharge lamp 2. The driver 1 has input terminals 3 for connecting to mains, for instance 23 OVAC @ 50Hz, and output terminals 4 for connecting to the lamp electrodes. The driver 1 comprises an ACDC converter 5, rectifying the AC mains voltage and transforming the rectified input voltage to a higher voltage level. The driver 1 further comprises an inverter 6, controlled by a controller 7, receiving the output voltage from the converter 5 and designed for generating a high-frequency commutating output current to the lamp 2. The commutation frequency of the output current may for instance be in the order of about 100 kHz.

ACDC converters, suitable for use as the converter 5, are known per se, and a more detailed description of a possible embodiment of such converter will be omitted here.

Inverters, suitable for use as the inverter 6, are known per se. By way of example, a possible embodiment of an inverter 6 is illustrated in the schematic diagram of figure 2. The inverter 6 has a high voltage line 11 and a low voltage line 12, connected to the output of converter 5 (not shown in this figure). The low voltage line 12 in this example is shown connected to earth, so its voltage is zero here. Two controllable switches 13 and 14, typically implemented as MOSFET transistors, are connected in series between said two voltage lines 11 and 12; a node between said switches is indicated at A. A series arrangement of a capacitor 15, a primary winding 21 of a transformer 20, and an inductor 16 is connected between said node A and the zero voltage line 12. A secondary winding 22 of the transformer 20 is coupled to the driver output terminals 4.

The controller 7 has control output terminals coupled to control input terminals of the switches 13 and 14, respectively.

The controller 7 can operate in two different operative states. In a first operative state, indicated as high power state H in figure 3, the controller 7 drives the switches 13 and 14 alternatively ON and OFF at a commutation frequency. More specifically, when the upper switch 13 is ON the lower switch 14 is OFF, and vice versa. The duty cycle of the high frequency commutation is typically equal to 50%, the frequency is typically in the order of 100 kHz. As a result, an alternating current is generated in the primary transformer winding 21, which is transformed by the transformer 20 and supplied to the lamp 2, to result in a nominal lamp current having nominal lamp current magnitude Im, associated with nominal lamp output light.

In a second operative state, indicated as low power state L in figure 3, the controller 7 drives the switches 13 and 14 both OFF (i.e. non-conductive). As a result, no current is generated in the primary transformer winding 21, hence no light is produced by the lamp 2.

The controller 7 alternates between its first operative state and its second operative state, thus defining a lamp current period T, which may for instance be in the order of 10 ms. A duty cycle δ is defined as the ratio WT, in which tπ indicates the duration of the first operative state H, indicated as ON-duration. It should be clear that the average current lav can be written as lav = δTm. Thus, considering that the average light intensity is proportional to the average current, dimming can be achieved by reducing the duty cycle. However, at very low dimming levels of for instance 5% of the nominal light intensity, and less, the ON-duration tH becomes very short and the time between successive current pulses becomes very long, as illustrated in figure 4, it becomes difficult for the lamp to re-ignite in

every current period, the lamp behavior becomes irregular, and the infrared problem as mentioned occurs.

Figure 5 is a graph comparable to figure 3, illustrating the behavior of a driver according to the present invention. Again, the controller 7 can operate in two different operative states, i.e. a high power state H and a low power state L. In the high power state H, the controller 7 drives the switches 13 and 14 alternatively ON and OFF at a first commutation frequency; this can be identical to the high power state H of figure 3 of the prior art. In the low power state L, the controller 7 also drives the switches 13 and 14 alternatively ON and OFF, at a second commutation frequency which may differ from the first commutation frequency. Again, an alternating lamp current is generated, but now having a current magnitude Ix lower than the nominal lamp current magnitude Im, so that a reduced light output lower than the nominal lamp output results.

The controller 7 alternates between its two operative states H and L, at a relatively low mode switching frequency f ^* _M in the order of about 100 Hz. If tπ indicates the duration of the high power state H and t _L indicates the duration of the low power state L, it should be clear that the current period T is given by T = l/fjvi = tπ + t _L , and that a duty cycle δ can again be defined as δ = WT. It should further be clear that the average current IAV can be written as l _AV = (t _H -Im + t _L -Ix)/T which is a function of the duty cycle:

Uv = δ-Im + (l-δ) Ix = Ix + (Im-Ix) δ

Varying the duty cycle from 100% to 0% will vary the average current IAV from nominal current magnitude Im to Ix. Since, at any moment in time, the momentary current magnitude is at least Ix, while Ix is larger than zero, a relatively large amount of free electrons are present in the lamp allowing a quick restart, and the amount of infrared generated will be substantially reduced.

In a suitable example, the reduced current magnitude Ix is equal to 0.05 times the nominal current magnitude Im, although Ix can be as low as 0.01 times the nominal current magnitude Im or lower or Ix can be as high as 0.1 times the nominal current magnitude Im or higher.

Figure 6 is a block diagram schematically illustrating a first embodiment of a lamp driver according to the present invention, indicated by reference numeral 101. In this embodiment, the single inverter 6 of figure 1 is replaced by two separate inverters 116

and 126, having their inputs connected in parallel to the output of up-converter 5, and having their respective outputs connected to different inputs of a controllable mode selection switch device 108, whose output is coupled to the lamp output terminals 4. Each inverter 116 and 126 may individually be implemented as a prior art inverter, for instance of a design as illustrated in figure 2, designed to generate lamp current at a predetermined current magnitudes Im and Ix, respectively. This difference may for instance be embodied in different winding ratios for the different transformers 20 in the different inverters, in different ballast inductances 16 in the different inverters, in different commutation frequencies in the different inverters, or by any combination of these measures. The controller 7 is replaced by a controller 107 that controls the operation of the individual inverters 116 and 126 (the series arrangement of switches 13 and 14 in each individual inverter 116, 126, for instance) and that further controls the position of the mode selection switch 108.

In the high power state H, the controller 107 drives the switches of the first inverter 116 alternatively ON and OFF at a first commutation frequency. The switches of the second inverter 126 may be maintained in a non-conductive state (OFF). The controller 107 controls the mode selection switch 108 such as to be in a first state in which the output of the first inverter 116 is connected to the output of the mode selection switch 108. Thus, the lamp 2 receives nominal magnitude current Im from the first inverter 116.

In the low power state L, the controller 107 drives the switches of the second inverter 126 alternatively ON and OFF, at a second commutation frequency which may be equal to the first commutation frequency. The switches of the first inverter 116 may be maintained in a non-conductive state (OFF). The controller 107 controls the mode selection switch 108 such as to be in a second state in which the output of the second inverter 126 is connected to the output of the mode selection switch 108. Thus, the lamp 2 receives low magnitude current Ix from the second inverter 126.

An advantage of this embodiment is that use can be made of fixed, relatively simple and low-cost inverters of a standard design.

In a possible variation, the output of the second inverter 126 is connected to the output of mode selection switch 108 in stead of to its second input. The switch 108 can be replaced by a simple switch having one input and one output, for instance a MOSFET. The second inverter 126 can be operating continuously, so that the controller 107 does not need to switch OFF the second inverter 126 in the low power state L. In the high power state H, the lamp receives current from both inverters 116 and 126.

In another variation, the mode selection switch 108 is controlled by a second controller separate from the controller 107.

Figure 7 is a block diagram schematically illustrating a second embodiment of a lamp driver according to the present invention, indicated by reference numeral 201. In this embodiment, the driver 201 comprises a single inverter 206, comparable to the design as shown in figure 1, which single inverter 206 basically is implemented as the inverter 6 of figure 2, yet the single inductor 16 of figure 2 is replaced by a series arrangement of two inductors 216, 316 having a common node C. A first of said inductors 216 is arranged between said common node C and the primary transformer winding 21, while a second of said inductors 316 is arranged between said common node C and the ground line 12. A controllable mode selection switch 208, which may suitably be implemented as a MOSFET, is connected in parallel to said second inductor 316. A mode controller 207 controls to operation of the mode selection switch 208. The commutation controller 7 continuously drives the switches 13 and 14 of the inverter 6 alternatively ON and OFF at a predetermined commutation frequency. In the high power state H, the mode controller 207 controls the mode selection switch 208 such as to be closed (conductive) such as to short-circuit the second inductor 316 and reduce the inductance in series with the lamp; the lamp 2 now receives nominal magnitude current Im from the inverter 206. In the low power state L, the controller 207 controls the mode selection switch 208 such as to be open (non-conductive) and increase the inductance in series with the lamp; the lamp 2 now receives low magnitude current Ix from the inverter 206.

In a possible variation, the commutation controller 7 and the mode controller 207 are combined as one integrated controller.

In the above, the short-circuiting element 208 is described as an ideal switch, which can either be closed or open, and which may be considered as switching infinitely fast between these two states. In a further elaboration of this embodiment, the short-circuiting element 208 is embodied as a controllable resistor, having the property that its resistance can be controlled to be set anywhere in a range from zero to infinity, or at least from a sufficiently low value close to zero to a sufficiently high value representing infinity (for instance 100 Mω). A suitable example of an implementation of such element is a FET.

In this further elaboration, it is possible for the mode controller 207 to switch the short-circuiting element 208 from its high resistance state to its low resistance state within a finite switching time τs larger than zero. As a result, as illustrated in figure 8, the resistance of the short-circuiting element 208 gradually drops from high to low within the finite

switching time τs, hence the impedance in series with the lamp gradually drops within the finite switching time τs, hence the lamp current gradually rises from Ix to Im within the finite switching time τs. This offers an improvement in the reduction of the IR radiation, and further offers an improvement reducing possible EMC/EMI problems. In the context of reducing possible EMC/EMI problems, it is further preferred to also switch the short-circuiting element 208 from its low resistance state to its high resistance state within a finite switching time larger than zero, so that the lamp current gradually drops from Im to Ix within a finite switching time. Both switching times may be equal (and in the following it will be assumed that it is), but this is not essential. The duration of the finite switching time τs can now be considered to constitute a further parameter. In principle, its value is not essential, although higher values are associated with higher EMC/EMI reductions, while the value should preferably be set to be larger than the commutation period of the switches 13, 14. For many implementations, a value in the order of about 0.5 ms seems satisfying. At low duty cycles, τs should preferably be chosen less than τπ, unless it is desirable to reduce the maximum current level.

Summarizing, the present invention provides a method for reducing infrared emission from a gas discharge lamp 2 driven in a dimmed condition, the lamp having a nominal rating corresponding to a nominal lamp current magnitude Im and a nominal light output. The method comprises the step of driving the lamp at a dimmed light output by alternatively operating the lamp in a high power state H and in a low power state L.

During the high power state H, the lamp is supplied with alternating lamp current having the nominal lamp current magnitude Im and having a predetermined commutation frequency. During the low power state L, the lamp is supplied with reduced alternating lamp current having the same predetermined commutation frequency and having a reduced current magnitude Ix less than the nominal lamp current magnitude Im but higher than zero.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, in the above explanation it is assumed that, within one current period T, the system is in a high power state H once and the system is in a low power state L

once. However, it is also possible that, within one current period T, the system is in a high power state H two or more times and/or is in a low power state L two or more times. In that case, the duty cycle is determined by the total duration of all high power states and the total duration of all low power states, as should be clear to a person skilled in the art. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.