WO2008132662A2

WO2008132662A2 - Method and ballast for operating a gas discharge lamp

Info

Publication number: WO2008132662A2
Application number: PCT/IB2008/051556
Authority: WO
Inventors: Johannis A. A. Toonen; Mario G. M. Flos; Roy H. A. M. Van Zundert
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-04-27
Filing date: 2008-04-23
Publication date: 2008-11-06
Also published as: WO2008132662A3

Abstract

A ballast for operating a gas discharge lamp is controlled to re-ignite the lamp within a short time period after extinguishing due to unforeseen circumstances. After ignition of the lamp, an extinguishing of the lamp is monitored. After detecting an extinguishing of the lamp, a high frequency voltage is generated across the lamp, in particular immediately after detecting an extinguishing of the lamp, or after a brief delay time period following detecting an extinguishing of the lamp. This method can be used both in run-up and in steady state operation mode. Detection of an extinguishing lamp may be performed by sensing a lamp current (CH4) or a lamp voltage (CH1).

Description

Method and ballast for operating a gas discharge lamp

FIELD OF THE INVENTION

The present invention relates to a method and corresponding ballast for operating a gas discharge lamp.

BACKGROUND OF THE INVENTION

In the control of a gas discharge lamp, such as a High Intensity Discharge (HID) lamp, different modes of operation may be distinguished. After an idle mode, in which the lamp is off, an ignition mode may be entered, in which the lamp is ignited by generating a voltage of several kilo volts across the lamp terminals. The ignition mode may be preceded by a calibration mode, in which the static and dynamic zero current level of a lamp current sensor circuit are recorded. The ignition mode is optionally followed by a run-up mode, in which the voltage across the ignited lamp is slowly rising due to a limited current sent through the lamp, and a down-converter feeding the lamp is switching with a very low duty cycle. In the run-up mode, the electrodes of the lamp are heated to obtain a sufficient emission of electrons. Also the lamp itself heats up. After the lamp has reached a certain lamp voltage during run-up mode, a normal operation mode is entered in which a power control algorithm controls the lamp power. In normal operation mode, the electrodes are hot. An idle mode is re-entered if a command to switch off the lamp is issued to the lamp control, if too high a lamp voltage is detected, or if too low a lamp current is detected. Commonly, a full bridge commutating forward (FBCF) circuit topology is used as a lamp driving circuit (or ballast) for gas discharge lamps. In this topology, the lamp is connected between a common node of a first pair of series-connected switches, and a common node of a second pair of series-connected switches. Each pair of switches is connected between power supply lines. The switches on one side of the FBCF (e.g. the left side) can be used for controlling the lamp power during stationary operation, i.e. in the normal operation mode, and the switches on the other side (e.g. the right side) can be used for ignition, i.e. in the ignition mode of the lamp control.

In the ignition of the gas discharge lamp, an ignition voltage generally is a high-frequency (HF) voltage, and can be generated rendering the right side switches alternately conducting and non-conducting at a high frequency. After breakdown of the gas in the gas discharge lamp, and possibly a short period of HF run-up, the bridge circuit can adopt a 'commutating forward operation', in which the left side switches are controlled at a high frequency and the right side switches at a low frequency. The resulting lamp current is a substantially square wave shaped low frequency current. In the remainder of the run-up mode and the normal operation mode, the left side switches are controlled, e.g. to obtain a power control in the normal operation mode.

It is observed that as an alternative for 'commutating forward operation' the lamp can be operated in the steady state in the normal operation mode using a high frequency current.

Fig. 1 shows an embodiment of a lamp driving circuit with an FBCF topology, as will be explained in further detail below.

Some lamp types and old lamps have a tendency to extinguish after ignition in the run-up mode, e.g. due to electrode behavior. This can even happen at up to one minute after the lamp was ignited.

When the lamp extinguishes soon after the ignition process (e.g. less than two seconds after an ignition has been detected), the lamp has not yet warmed up and the lamp control returns to the ignition mode. In the prior art products there is a delay period of several seconds (e.g. 6 seconds) before the next ignition attempt is made. A reason for this delay period can be recognized in a risk that the cause of the extinction of the lamp is a lamp defect, which could seriously harm the lamp driving circuit when no delay period is used. In this delay period, the lamp is off, and the user cannot notice any activity of the lamp control. As a result, the user might therefore switch off the lamp control, assuming the lamp or the lamp control has a defect. After ignition, the lamp can also extinguish at a later stage, in the run-up mode at e.g. 30 seconds after ignition. In that case, the lamp is already warm and will not ignite at the next ignition attempts with a 6 seconds delay period. Accordingly, the uncertainty of the user with regard to the condition of the lamp increases.

The lamp can further extinguish at a still later stage, in the normal operation mode, e.g. as a result of a power supply voltage dip. Also in that case, the lamp is warm, and after a delay period the lamp will not ignite at the next ignition attempts, again leading to uncertainty of the user. OBJECT OF THE INVENTION

It is desirable to improve the lamp performance, increasing user comfort, in case a lamp extinguishes in a run-up mode or in a normal operation mode.

In an embodiment, the present invention provides a method of operating a gas discharge lamp, wherein, after ignition of the lamp, an extinguishing of the lamp is monitored; and wherein, after detecting an extinguishing of the lamp, a high frequency voltage is generated across the lamp. The high frequency voltage may be applied as a burst (i.e. having a predetermined time duration). If no re-ignition of the lamp occurs after applying a burst of high frequency voltage, one or more further bursts may be applied, up to a predetermined maximum number of bursts (for example, 30 bursts) if it would take longer to re-ignite the lamp.

By generating a high frequency voltage across the lamp within a time period of less than 50 ms, it has appeared possible to re-ignite the lamp without a user noticing any extinguishing of the lamp. When the extinguishing of the lamp occurs in the run-up mode of the lamp control, after re-ignition the lamp may be run up further for the control to reach the normal operation mode. When the extinguishing of the lamp occurs in the normal operation mode of the lamp control, after re-ignition the lamp control may resume the normal operation mode. Thus, essentially no visible interruption of the light emitted by the lamp occurs for the user, and the user comfort is greatly enhanced by the present invention. In another embodiment, the present invention provides a ballast for operating a gas discharge lamp, the ballast comprising a detection circuit for monitoring an extinguishing of the lamp, and a high frequency voltage generating circuit coupled to the detection circuit, the high frequency voltage generating circuit being configured to apply a high frequency voltage across the lamp after detecting an extinguishing of the lamp by the detection circuit.

SUMMARY OF THE INVENTION

The above features and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a lamp driver and lamp in an FBCF topology. Figure 2 is a chart showing graphs of lamp voltage and lamp current in case of an extinguishing lamp during run-up mode, with a horizontal time scale of 200 ms/div.

Figures 3 a and 3b are charts showing graphs of a lamp voltage and a corresponding lamp current with re-ignition in accordance with an embodiment of the present invention, with horizontal time scales of 200 ms/div and 500 ms/div, respectively.

Figure 3 c is a chart showing graphs which are details of a part IIIc of the graphs of Figure 3b on an enlarged horizontal time scale of 5 ms/div.

Figure 4a is a chart showing graphs of a lamp current and a corresponding lamp voltage with re-ignition in accordance with an embodiment of the present invention, with a horizontal time scale of 50 ms/div.

Figure 4b is a chart showing graphs which are details of a part IVb of the graphs of Figure 4a on an enlarged horizontal time scale of 2 ms/div, and additionally a signal indicative of the lamp voltage.

Figure 5a is a chart showing graphs of a lamp current and a corresponding lamp voltage with re-ignition in accordance with an embodiment of the present invention, with a horizontal time scale of 50 ms/div.

Figures 5b, 5c and 5d are charts showing graphs which are details of parts El and E2 the graphs of Fig. 5a on an enlarged horizontal time scale of 2 ms/div, 10 ms/div, and 1 ms/div, respectively. Figures 6 and 7 are charts showing graphs of a lamp current and a corresponding lamp voltage with re-ignition in accordance with an embodiment of the present invention, each with horizontal time scales of 20 ms/div.

DETAILED DESCRIPTION OF EXAMPLES Fig. 1 shows a lamp driver, comprising a lamp driving circuit 2 and a control circuit 4, and a lamp 6 connected to the lamp driver.

The lamp driving circuit 2 comprises power supply terminals 8 and lamp terminals 10. The lamp 6 is connected between the lamp terminals 10. A voltage stabilizing capacitor 12 is connected between the power supply terminals 8. Further, a series connection of switches Ql and Q2, and a series connection of switches Q3 and Q4 are connected between the power supply terminals 8. An inductor 14 is connected between a common node of the switches Ql and Q2 on the one hand, and one of the lamp terminals 10 on the other hand. An inductor 16 is connected between a common node of the switches Q3 and Q4 on the one hand, and the other one of the lamp terminals 10 on the other hand. A series connection of capacitors 18 and 20 is connected between the power supply terminals 8, with a common node of the capacitors 18 and 20 being connected to one of the lamp terminals 10. A capacitor 22 is connected between the other one of the lamp terminals 10 and one of the power supply terminals 8. Each of the switches may be embodied as a Field Effect Transistor (FET), in particular a MOSFET (Metal Oxide Semiconductor FET).

The inductors 14, 16 and the capacitors 18, 20, 22 may each be embodied as concentrated component or distributed component (i.e. comprising several concentrated components), and may in part or in whole be inherent to the circuit. The inductor 16 and capacitor 22 are essentially used to generate an ignition voltage across the lamp. The inductor 14 and capacitors 18 and 20 in combination with Ql and Q2 form a down-converter.

The control circuit 4 comprises (as only schematically indicated) a shunt feedback circuit 24 configured to obtain a signal indicating lamp current. The current indicating signal is fed to a power control unit 26, as indicated by arrow 28. The power control unit 26 is configured to control alternate switching of switches Ql and Q2, as indicated by double arrow 30, on the basis of the current indicating signal and possibly other parameters. A signal indicating lamp voltage is obtained by an (only schematically indicated) lamp voltage feedback circuit 32. The voltage indicating signal is fed to a controller 34, as indicated by arrow 36. From the voltage indicating signal, the controller 34 generates (e.g. via a look-up table and/or after calibration) a reference value. The controller 34 is configured to control alternate switching of switches Q3 and Q4 on the basis of said reference value, as indicated by double arrow 38, on the basis of the voltage indicating signal and possibly other parameters. A communication between the power control unit 26 and the controller 34 is indicated by arrows 40. The power control unit 26 controls the lamp power such that the voltage generated by the shunt feedback circuit 24 is equal to said reference value generated by the controller 34.

In operation, for driving the lamp 6 with the lamp driver, in an ignition mode of the control circuit 4 an HF (High Frequency) ignition voltage is generated with the switches Q3 and Q4, and Ql and Q2 are off. After ignition of the lamp 6, as detected with the shunt feedback circuit 24 and/or the lamp voltage feedback circuit 32 and possibly a short period of HF run-up, the switches Ql and Q2 take over driving the lamp in a run-up mode of the control circuit 4. The switches Ql and Q2 are further used at HF in a normal operation mode following the run-up mode, while the switches Q3 and Q4 in the normal operation mode are operated at LF (Low Frequency). Figure 2 shows a lamp voltage F2 and a lamp current F3 as a function of time. A time E is indicated. Before time E (at the left-hand side of E, a gas discharge lamp is in a run-up mode, and the lamp is burning. At time E, the lamp extinguishes. After time E, no measures are taken to re-ignite the lamp during a time period, so the lamp voltage F2 and the lamp current F3 fall to zero.

Figure 3a shows a lamp voltage F2 and a lamp current F3 as a function of time for a particular lamp. Figure 3b shows a lamp voltage F2 and a lamp current F3 as a function of time for another lamp. Both lamps are operated with a lamp driver as shown in Figure 1. The arrows in Figures 3a and 3b indicate re-ignition attempts of the control circuit of the lamp driver after a current zero has been detected.

As shown in Fig. 3c, in a third period of average positive voltage and current, the current becomes zero for an extended period of time, as indicated at Z in Figure 3c. This is detected by the shunt feedback circuit 24, and immediately a re-ignition attempt is started by controlling the switches Q3 and Q4 to generate a high frequency re-ignition voltage. As can be seen in the Figures 3a-3c, the lamp is re-ignited, and a user of the lamp in practice does not detect the extinction of the lamp at Z.

It is to be noted here, that also other ways of detecting an extinguishing of the lamp, like monitoring lamp voltage, ballast input current, etc.

In Figs. 4a and 4b, CH4 indicates a lamp current, and CHl indicates a lamp voltage at lamp terminal 10 connected to inductor 16. As can be seen, after several seconds after ignition still one of the lamp electrodes does not emit very well, as evidenced by peaks in the lamp voltage CHl, and small dips occurring in the lamp current CH4. Lamps might extinguish when these dips are too large. The lamp is operated on an FBCF driver as shown in Fig. 1. In case of the lamp extinguishing, a high frequency (e.g. more than 50 kHz) voltage is generated across the lamp with the MOSFETs Q3 and Q4 of the FBCF driver. The high frequency voltage duration is about 50 ms followed by 10 low frequency (e.g. less than 1 kHz) periods.

As can be seen in Fig. 4b, the lamp is almost extinguished, but due to the high frequency voltage the lamp is re-ignited. In this case, the high frequency voltage is only generated if the current dip (current approximately equal to zero) has too long a duration (approximately 900 microseconds or more). Other current dips, as can be seen at the left- hand side of the lamp current CH4 signal, have durations that do not trigger the generation of a high frequency voltage. Furthermore, Fig. 4b shows a voltage at capacitor 20 as detected by lamp voltage feedback circuit 32 (Fig. 1). In Figs. 5a-5d, a signal CH4 represents a lamp current, and a signal CHl represents a voltage at lamp terminal 10 connected to inductor 16. A signal CH2 represents a voltage at capacitor 20 as detected by lamp voltage feedback circuit 32 (Fig. 1).

According to Fig. 5a, two periods of high frequency voltage signal are necessary to prevent re-extinguishing of the lamp.

Fig. 5b illustrates the first extinguishing of the lamp, as indicated at time El in Fig. 5a, in more detail. A dip in the lamp current CH4 exceeding a predetermined time period, and/or an associated rise in the lamp voltage CHl, trigger a generation of a high frequency voltage, similar to the situation illustrated in Fig. 4b. Fig. 5c illustrates a second extinguishing of the lamp, as indicated at time E2 in Fig. 5a, in more detail. After terminating a burst of high frequency voltage, the lamp extinguishes in the first low frequent period. Consequently, a second burst of high frequency voltage is applied to the lamp.

In Fig. 5d, F4 is the lamp current. Fl is the voltage at lamp terminal 10 connected to inductor 16. F2 represents a voltage at capacitor 20 as detected by lamp voltage feedback circuit 32 (Fig. 1). As can be seen in Fig. 5d, the lamp current F4 becomes zero for a first time, but not long enough to trigger a generation of high frequency voltage for re- ignition. Then, for a second time, the lamp current F4 becomes zero, for a time period which is longer than a predetermined time period, e.g. 900 microseconds. The voltage F2 decreases, and the lamp voltage increases. After the predetermined time period, the high frequency voltage generation is started again.

Also during normal operation mode, when the lamp is operating in steady state, the present invention can be applied.

The results of measurement done on hot lamps that extinguished during steady state operation are given in Figs. 6 and 7. In Figs. 6 and 7, graph Fl shows lamp voltage against time, and graph F2 shows lamp current against time. Both the lamp current and the lamp voltage are substantially square wave shaped before the extinguishing of the lamp. The extinguishing of the lamp can e.g. be sensed by sensing a decrease of the lamp current below a reference level or an increase in lamp voltage to a level higher than a reference level for longer that a predetermined time period.

Figs. 6 and 7 show measurements obtained for re-ignition attempts of High Intensity Discharge (HID) lamps that extinguished during normal operation mode, to illustrate an effect of using a delay time (10 ms and 20 ms, respectively) in applying the high frequency voltage to the lamp. From the graphs of Fl and F2 in Figs. 6 and 7, it can be seen that a delay time of 10 ms and 20 ms, respectively, still allowed a re-ignition of the lamp, whereas after a delay time of 30 ms no re-ignition could be effected.

Of course, a maximum allowable delay time (e.g. 50 ms) is dependent on parameters such as lamp type, the amplitude of the high frequency voltage applied to the lamp after the delay time, the lamp temperature, and others.

When it is sensed that the lamp is extinguishing or has extinguished, the control circuit is immediately, or after a brief delay time period, changed to generate a high frequency voltage across the lamp. Then, upon ignition of the lamp, the current through the lamp is increased (e.g. stepwise increased), and the voltage across the lamp is decreased (e.g. stepwise decreased). During a time period immediately following re-ignition of the lamp, the lamp driving circuit is operated such that the lamp current is a high frequency current and the lamp voltage is a high frequency voltage. After this time period, the control circuit once more controls the lamp driving circuit in a 'commutating forward operation', and both the lamp current and the lamp voltage are substantially square wave shaped, also referred to as a Low Frequency Square Wave (LFSW) operation. When an extinguishing lamp is detected in a lamp driving circuit having an FBCF topology, the left side of the FBCF circuit is switched off and the right side is driven with a high frequency for a predetermined period of time to generate a high frequency open circuit voltage. Key to a successful re-ignition is to react fast on a lamp extinguishing, so real time monitoring of lamp behavior is necessary.

As already pointed out above, the present invention can also be used in other lamp driving circuit topologies than an FBCF topology, such as a Half Bridge Commutating Forward (HBCF) topology. In fact, the present invention can be implemented in any lamp driver capable of applying a high frequency voltage across the lamp, e.g. lamp driving circuits using HF ignition mode and/or HF run-up mode like a down-converter with a full- bridge commutator and LC igniter. The fast re-ignition according to the present invention can also be done with HF circuitry that supplies a high frequency current to the lamp during normal operation mode. Generally, the frequency of the normal operation mode current differs from the ignition frequency. In such case, when an extinguishing lamp is detected, the normal operation mode frequency must be shifted to the ignition mode frequency.

A control circuit comprises control circuitry having a program stored therein comprising instructions adapted to provide functions in accordance with the present invention. As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention.

The terms "a", "an", "first", "second" etc. as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. In addition, singular references do not exclude a plurality. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. The term program as used herein, are defined as a sequence of instructions designed for execution on a controller or computer system. A program may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a controller or computer system.

Claims

CLAIMS:

1. A method of operating a gas discharge lamp, comprising: after ignition of the lamp, monitoring an extinguishing of the lamp; and after detecting an extinguishing of the lamp, generating a high frequency voltage across the lamp.

2. The method of claim 1, wherein the high frequency voltage across the lamp is generated immediately after detecting an extinguishing of the lamp.

3. The method of claim 1, wherein the high frequency voltage across the lamp is generated after a delay time period following detecting an extinguishing of the lamp.

4. The method of claim 3, wherein the delay time period is between 0 and 50 ms.

5. The method of claim 1, wherein the high frequency voltage generation is maintained for a predetermined period of time.

6. The method of claim 1 , wherein the lamp is operated in an ignition mode in which the lamp is ignited, a run-up mode following the ignition mode in which the lamp runs up to a normal operation, and a normal operation mode following the run-up mode in which the lamp operates in a steady state, and wherein a monitoring of an extinguishing of the lamp is performed in the run-up mode and in the normal operation mode.

7. The method of claim 1 , wherein the monitoring of an extinguishing of a lamp comprises sensing a lamp current, and wherein detecting an extinguishing of a lamp comprises sensing a decrease of the lamp current below a predetermined reference level longer than a predetermined period of time.

8. The method of claim 1 , wherein the monitoring of an extinguishing of a lamp comprises sensing a lamp voltage, and wherein detecting an extinguishing of a lamp comprises sensing an increase of the lamp voltage above a predetermined reference level longer than a predetermined period of time.

9. A ballast for operating a gas discharge lamp, the ballast comprising a detection circuit for monitoring an extinguishing of the lamp, and a high frequency voltage generating circuit coupled to the detection circuit, the high frequency voltage generating circuit being configured to apply a high frequency voltage across the lamp after detecting an extinguishing of the lamp by the detection circuit.

10. The ballast of claim 9, wherein the detection circuit is adapted to monitor a lamp current.

11. The ballast of claim 9, wherein the detection circuit is adapted to monitor a lamp voltage.

12. The ballast of claim 9, wherein the high frequency voltage generating circuit is a circuit used for igniting the lamp.

13. The ballast of claim 9, further comprising: power supply terminals for connecting a power supply to the ballast; lamp terminals for connecting a lamp to the ballast; the high frequency voltage generating circuit being connected between the power supply terminals and the lamp terminals; a control circuit connected to the detection circuit and the high frequency voltage generating circuit, the control circuit being configured to: control the detection circuit to monitor an extinguishing of the lamp; and control the high frequency voltage generating circuit to apply a high frequency voltage across the lamp after detecting an extinguishing of the lamp by the detection circuit.

14. The ballast of claim 13, further comprising a bridge circuit, the bridge circuit comprising a pair of switches connected in series between the power supply terminals, one of the lamp terminals being connected to a node interconnecting the switches of the pair of switches, wherein the pair of switches is operable to generate the high frequency voltage across the lamp terminals.