END-OF-LIFE PROTECTION CIRCUIT FOR HIGH-INTENSITY DISCHARGE LAMP

FIELD OF THE INVENTION

The present invention relates to a technical field of illumination, and more particularly, to a lamp end- of-life protection circuit for being used in a ballast control circuit of a high intensity discharge lamp, as well as a ballast control circuit and a high-intensity discharge lamp thereof.

BACKGROUND OF THE INVENTION

High Intensity Discharge (HID) lamps have been widely used because they have long life span and can effectively transform electrical energy into light. HID lamps are typically classified as mercury (vapor) lamp, sodium lamp and metal halide lamp. An HID lamp generally includes a lamp base, an outer bulb, inner electrodes, arc tube, outer electrodes, etc. Different type Lamps differ from each other in coating of the outer bulb, the inner and outer electrodes, size of the arc tube and type of inert gas filled in the arc tube.

End-of-life of HID lamps is generally caused by three factors: 1. deterioration of the inner electrodes; 2. cracking of the arc tube; 3. evaporation of salt (e.g. sodium salt) in the arc tube. Factors 1 and 3 usually will not result in big problems, because in those cases, although lamp voltage and/or re-ignition voltage peak value may rise and finally cause the lamp to be unusable at some time in the future, the old HID lamp can be replaced so as to make a lighting device containing the lamp work normally.

However, the above factor 2 may possibly cause some serious problems. For example, when a HID lamp is a sodium lamp, if the arc tube cracks, the sodium salt therein will enter the outer bulb which encloses the arc tube; since the outer bulb of an HID lamp is usually filled with low-pressure inert gas, a low-pressure mercury lamp of a certain type will be formed in the outer bulb of the HID lamp. At this moment, the outer electrodes, which are originally used to connect the inner electrodes and external circuit and support the arc tube, will function as inner electrodes. During this stage, there is a great deal of "glow-to-arc" transition. Meanwhile, during this stage, there is only a slow temperature rise in the lamp base, so various parameters are still within the critical conditions of the lamp and the ballast (but the ballast has to be subjected to a large current and high current variation rate di/dt). However, since the metal currently used as the electrodes is not suitable for being used as the electrodes of a low-pressure mercury lamp, the electrodes will evaporate metal particles, which will deposit at a position near the lamp base in the outer bulb to form a metal thin film. With the continuous transition of the "glow-to-arc", the deposition of the metal film continues and finally may

contact the metal supporting frame, and then the metal film will function as a conductor. At this moment, the HID lamp enters "incandescent lamp mode" where the metal thin film functions as a resistor and acquires electrical energy from the ballast. In most cases, impedance of the metal thin film is low,- so the ballast can transfer most of the energy to the metal thin film. Since a great amount of current flows through the metal thin film and the lamp base will be heated to such a high temperature possibly as 60O ⁰ C, which greatly exceeds a temperature (usually 3 ₅ 0°C) that the lamp base can generally withstand. Therefore, the above factor 2 will cause the lamp base to be damaged owing to the over-temperature. Furthermore, due to the lamp base in direct connection with a lamp holder, which provides support to the lamp, the lamp holder may be overheated and damaged. In order to restore a lighting device including the HID lamp back to work, not only Hie HID lamp should be replaced, but the lamp holder should also be replaced, thus increasing the cost.

Generally, if the ballast can be turned off in 4-6 minutes after the HID lamp enters the operation mode of "glow-to-arc" transition, the HID lamp has not entered the "incandescent lamp mode" yet, so the lamp base and lamp holder will not be damaged by the over-temperature and the lamp can be considered as being in a safe mode at the time.

SUMMARY OF THE INVENTION

One problem addressed by the invention is how to quickly turn off the ballast upon the HID lamp entering the "glow-to-arc" transition mode owing to the crack of the arc tube so as to prevent the lamp base from being damaged by the over-temperature. Another problem addressed by the invention is how to use a single timer circuit to realize timing for hot lamp ignition and for quickly turning off the ballast upon the HID lamp entering the "glow-to-arc" transition mode.

There already exists a timer employed in the ballast circuits for igniting a hot lamp, and its timing usually may last 15-30 minutes. If this timer can be used to the above-mentioned situation so as to turn off the ballast in a timing of 4-6 minutes, scale and cost of the circuit will be greatly reduced.

In order to solve the above-mentioned problems, inventor of the present invention analyzed operation states of the ballast control circuit under the hot lamp ignition mode and the "glow-to-arc" transition mode, respectively. The inventor discovered that in the operation state of hot lamp ignition, since it is necessary to ignite the hot lamp, an ignition pulse is needed; while in the "glow-to-arc" transition mode, no ignition pulse is needed. Furthermore, under a normal operation mode and the "glow-to-arc" transition mode of the HID lamp, there is a distinct difference in the power of the HID lamp. Generally speaking, the operating power under the normal operation mode is distinctly greater than that under the "glow-to-arc" transition mode. Based on the

above features, the inventor improved the existing timer for hot lamp ignition, such that it has different timing under different operation modes and the above-mentioned problems are solved accordingly.

Specifically, according to one aspect of the invention, a lamp end-of-life protection circuit for being used in a ballast control circuit of a high intensity discharge lamp is provided, which comprises a feedback circuit for detecting an ignition state of the high intensity discharge lamp and generating a first control signal corresponding to the ignition state; and a timing circuit, which can set various timing values according to the first control signal provided by the feedback circuit, and output a cut-off signal to cut off operation of the ballast in the high intensity discharge lamp when any of the timing values is reached.

According to another aspect of the invention, a ballast control circuit including the lamp end-of-life protection circuit and a high intensity discharge lamp comprising the ballast control circuit are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example, with reference to drawings, wherein Fig. 1 shows a diagram of circuit principle of the ballast control circuit according to a preferred embodiment of the invention; and

Fig. 2 shows a schematic circuit diagram of a timing unit 105 used in the ballast control circuit as shown in Fig. 1 according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to Fig. 1, circuit principle of a ballast control circuit 100 according to a preferred embodiment of the invention is illustrated. The ballast control circuit 100 comprises a control unit 101, a power supplying unit 102, an igniting/maintaining unit 103, a feedback unit 104 and a timing unit 105. The control unit 101 receives an external input signal Sin and controls operations of the other units. The power supplying unit 102 supplies power to the igniting/maintaining unit 103 based on a feedback signal Sr provided by the feedback unit 104 and under the control of the control unit 101. According to characteristics of the HID lamp, before and after ignition of the HID lamp, voltage Vp that the power supplying unit 102 supplies to the igniting/maintaining unit 103 is different. Preferably, before ignition of the HID lamp, the supply voltage Vp is 500V, while after ignition of the HID lamp, the supply voltage Vp falls to 400V. Under the control of the control unit 101, the igniting/maintaining unit 103 ignites the HID lamp and further controls the HID lamp at the normal operation mode after the HID lamp has been ignited.

The feedback unit 104 monitors the ignition state of the HID lamp and obtains a signal SI indicative of the ignition state of the HID lamp from the igniting/maintaining unit 103, and further generates a feedback signal Sr accordingly. Preferably, both SI and Sr are level signals, and before ignition of the lamp,- SI is a low level signal and Sr is a high level signal, while after ignition of the lamp, SI is a high level signal and Sr is a low level signal.

Furthermore, the feedback unit 104 also monitors the operating power of the HID lamp and obtains a signal Sp indicative of the operating power of the HID lamp from the igniting/maintaining unit 103, and further according to the signal Sp generates a control signal Sc for controlling the timing unit 1OS. Preferably, when the HID lamp is in the normal operation state, the operating power is high, so the generated control signal Sc is of a high level, while when the HID lamp is in the "glow-to-arc" transition mode, the operating power is low, so the control signal Sc is of a low level.

The timing unit 1O ₅ starts the timing operation under the control of the control unit 101, and the timing value of the timing unit 1O ₅ depends on the feedback signal Sr provided by the feedback unit 104. Preferably, if the feedback signal Sr is of a low level, the timing value of the timing unit 105 is somewhat small, and such a timing is preferably suitable for the timing needed by the HID lamp in the "glow-to-arc" transition mode, which is, for example 4-6 minutes, preferably 5 minutes. If the feedback signal Sr is of a high level, the timing value of the timing unit 105 is relatively large, compared to the feedback signal Sr being of a low level, and such a timing is preferably suitable for the timing needed by the HID lamp in the heated ignition operation mode, which is, for example, 15-30 minutes, preferably 20 minutes. When the timing unit 105 reaches any of the timing values thereof, it outputs a cut-off control signal Scut to the control unit 101 so as to cut off the power supply to the control unit and to turn off the ballast at the same time.

Meanwhile, the timing unit 105 is also controlled by the control signal Sc provided by the feedback unit 104. Preferably, when the control signal Sc is of a high level, operation of the timing unit 1O ₅ is terminated, and timing value therein is preferably re-set. When the control signal Sc is of a low level, operation of the timing unit 1O ₅ is started. That is to say, the timing unit 1O ₅ is also controlled by the operation modes of the HID lamp, when the HID lamp is operating normally, the timing unit 1O ₅ does not perform the timing operation, so it will not output any cut-off control signal Scut; when the HID lamp is operating abnormally, the timing unit 1O ₅ starts the timing operation.

Operations of the respective circuit components in preferred embodiments of the invention are described in detail below.

When the ballast control circuit 100 is in a hot ignition operation mode, at the beginning of ignition, since the HID lamp has not been ignited yet, the signal SI is low and signal Sr is high, the power supplying unit

102 supplies a high supply voltage (e.g. SOOV) when the signal Sr is high, and the high supply voltage causes the igniting/maintaining unit 103 to perform normal ignition. During the ignition of the HID lamp, the operating power thereof is lower than the normal operating power of the HID lamp after it is ignited, so the control signal Sc provided by the feedback unit 104 maintains to be a low level, and the timing unit 105 is in an operating state. Since the signal Sr is of a high level, the timing value of the timing unit 105 is large so as to meet the need of the HID lamp in the hot ignition operation mode and to preferably cut off the ballast in 20 minutes if the lamp cannot be ignited. After the HID lamp is ignited, signal SI becomes high and signal Sr becomes low, and the power supplying unit 102 supplies a low supply voltage (e.g. 400V), so the igniting/maintaining unit 103 performs the normal maintaining operation instead of the igniting operation. At this moment, the HID lamp is in the normal operation state, so the control signal Sc provided by the feedback unit 104 is of a high level, and the operation of the timing unit 105 is terminated to make the ballast continue its normal operation.

When the HID lamp enters the "glow-to-arc" transition mode due to the crack of the arc tube, the operating power of the HID lamp is lower than the normal operating power, so the control signal Sc provided by the feedback unit 104 becomes a low level, and the timing unit 105 starts to work. Meanwhile, the lamp maintains to be in the ignited state, so signal SI is high and signal Sr is low to cause the timing unit 1O ₅ to have a small timing value to meet the need of the HID lamp in the "glow-to-arc" transition mode and to preferably cut off the ballast in 5 minutes.

It can be seen that by detecting the ignition state and operating power of the HID lamp, and by switching on or off the timing unit 105 according to the operating power of the HID lamp and modifying the timing value of the timing unit 105 according to the ignition state, the timing unit 105 that is originally used only for the timing of hot lamp ignition can now be used to quickly cut off the ballast in the "glow-to-arc" transition mode.

It shall be noted that in the figures, only the parts that are closely related to the subject of the present invention are shown in a form of circuit module. Except for the timing unit 105, specific internal structures of these circuit modules are known in the prior art, so in order to describe this invention concisely, the specific structures of these modules are not shown herein.

Fig. 2 shows a schematic circuit structure of the timing unit 105 according to a preferred embodiment of the invention. As shown in Fig. 2, main components comprised in the timing unit 105 are a timer 201 and an oscillating capacitor 202. The timer 201 comprises an input pin CEOL and an output pin EOL. The input pin CEOL is connected to the oscillating capacitor 202 for receiving oscillating pulses therefrom and connected to the feedback unit 104 for receiving the control signal Sc therefrom. Alternatively, the timer 201 comprises a frequency divider 211 and a counter 212. The timer 201 performs frequency division and counting on the

oscillating pulses from the oscillating capacitor 202, by its frequency divider 211 and counter 212, respectively. When the counting reaches a certain number, the output pin EOL outputs a high level signal Scut to cut off the power supply of the control unit 101 and to turn off the ballast at the same time. Preferably, the frequency divider 211 used in the timer 201 is a 17-level frequency divider, and after the counter 212 counts a certain number like 120,000 oscillating pulses, the high level signal Scut is output at the output pin EOL.

The oscillating capacitor 202 comprises a first capacitor C120 and a second capacitor C121 connected in parallel. The first capacitor C 120 is coupled between the input pin CEOL of the timer 201 and a ground, alternatively, via a resistor R.136. The resistor R136 can prevent a large current from flowing through the input pin CEOL. Furthermore, the oscillating capacitor 202 comprises a switching, such as a transistor Q122, which is connected to the second capacitor C 121 in series and then coupled between the input pin CEOL of the timer 201 and the ground. Therefore, the second capacitor C121 is under the control of the transistor Q122 to connect in parallel to the first capacitor C 121 or not. Alternatively, the signal Sr is first divided by resistors R123 and R124 and then sent to the base of the transistor Q122. Thus, the transistor Q122 will be controlled by the signal Sr to operate accordingly, that is, in a state of being switched on or off. Alternatively, a diode D125 is coupled in parallel between the collector and emitter of the transistor Q 122 and has a function of freewheeling. Alternatively, a capacitor 126 is coupled in parallel between the collector and emitter of the transistor Q 122 for filtering interference pulses from the signal Sr.

With respect to the timer 201, capacitance of the oscillating capacitor 202 connected to the input pin CEOL can be adjusted to adjust the oscillating frequency, thereby the timing value for outputting the high level signal Scut at the output pin EOL can be changed. As shown in Fig. 2, C120 is connected between CEOL and the ground, moreover, C 121 and the transistor Q 122 are also connected in series between CEOL and the ground.

The transistor Q122 is controlled by signal Sr, when Sr is of a high level, transistor Q122 is switched on, in this case, C120 and C121 are connected in parallel, the capacitance connected to CEOL equals to the capacitance of C120 plus the capacitance of C121. If the capacitance connected to CEOL is large, the oscillating frequency is low, so the timing value of the timer is large, e.g. 20 minutes. When Sr is of a low level, transistor Q 122 is switched off, the capacitance connected to CEOL is only C 120, at this time, the capacitance connected to

CEOL is small, and the oscillating frequency is high, so the timing value of the timer is small, e.g. 5 minutes.

Of course, the transistor Q122 can be replaced by any switching element that can achieve the same function. Preferably, in order to obtain the timing value of 20 minutes suitable for the hot lamp ignition mode and the timing value of 5 minutes suitable for the "glow-to-arc" transition mode, the capacitance of C120 is 33nF and the capacitance of C121 is 68nF.

Therefore, by changing the external capacitance connected to the input pin CEOL of the timer 201 according to the level of signal Sr, the timer 201 can be made to be used to the above two different operation modes at the same time. It can be seen that by making some minor changes to the existing timing circuit, the ballast can be cut off quickly when the HID lamp enters the "glow-to-arc" transition mode due to the crack of the arc tube, thus the lamp base and holder is prevented from being damaged by the over-temperature.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The words like "comprising" and "including" do not exclude the presence of elements or steps other than those listed in the claims or the whole specification. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements, and vice versa. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The fact that certain measures are recited in different dependent claims does not indicate that a combination of these measures cannot be used to advantage.