Abstract:
In fluorescent lamp lighting, it is required that when the lamp reaches its end of life there has to be some mechanism to shut down the power supply to the lamp for safety. In the present invention, an apparatus is proposed to fulfill this requirement. This invention comprises of a lamp, means to sense currents at the lamp filaments and the imbalance of the currents is detected and turn off the power source to the lamp. Several embodiments are included to illustrate the execution of this invention.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     In fluorescent lighting, there is a requirement for end-of-life detection which is listed in ANSI C82.11 (Consolidated-2002). The requirement states that for lamps with tube diameter which is equivalent to T5 size or smaller must have a protection method when the lamp&#39;s end-of-life symptoms occurred to avoid hazardous conditions.  
         [0002]      FIG. 1  shows a prior art ballast configuration with an inductor-capacitor (LC) resonator. A lamp  101  is connected in parallel with the resonance capacitor  102  with an alternating power source  103  and the resonating inductor  104  connected in series. To start the lamp the resonator is activated and a high voltage is established across the capacitor which in turn strikes on the lamp. After the lamp is stroke on the lamp voltage falls to a lower value while it operates in the steady state.  
         [0003]     It is required that the ballast shall not impair safety when abnormal and fault conditions happen. Abnormal conditions are classified (European standard) as:  
         [0004]     a) lamp not inserted;  
         [0005]     b) the lamp does not start because one of the two cathodes are broken;  
         [0006]     c) the lamp does not start although the cathodes are intact;  
         [0007]     d) the lamp operates, but a single cathode is de-activated or broken (rectifying effect).  
         [0008]     It is desired that the ballast shall have appropriate protections against the four scenarios listed above. In scenarios a), b) and c), if the lamp is unable to be stroked on during the startup phase, the ballast may develop a dangerously high voltage arc across the two ends of the lamp holder, which causes an electrical shock hazard to the technician who may try to replace the lamp, therefore protection must then be enabled. Typically, there are two common detection methods:  
         [0009]     1) Current Sensing detection  
         [0010]     2) Voltage Sensing Detection  
         [0011]     A common current sensing detection method utilizes the inverter choke current as a sensing parameter. By placing a sensing resistor connected between the source of the low side driver and the ground, the choke current is monitored throughout the startup phase. If the lamp is not present or it cannot be ignited, high current will continue to flow through the sensing resistor since the LC resonator will operate at its resonant frequency after the preheat phase with infinite lamp resistance.  
         [0012]     Voltage sensing detection is similar to current sensing detection in which the lamp voltage is utilized as the sensing parameter. Over-voltage condition will occur if the lamp does not strike on as the ballast runs in its resonant mode.  
         [0013]     In scenario d), the lamp may suffer an imbalance of current flow in alternating direction which is commonly known as the rectifying effect, where one end of filament act as the cathode and the other end act as an anode. In particular, a broken or disconnected filament in a running lamp is a typical end-of-life failure where current sensing detection and voltage sensing detection may be unable to protect the lamp as the lamp is still in an operation mode. This may result in overheating the filament at one end which may melt the glass tube. Consequently this may cause an electrical shock hazard and overheating hazard to the user.  
         [0014]     In all cases, end-of life detection usually involves a combination of detection of high voltage across the lamp and the choke current. If the lamp is unable to be stroked on, then voltage and current sensing techniques are already enough to confirm the lamp is at its end-of-life status. Yet, if the lamp is able to be stroked on, only the lamp voltage, filaments status or the rectification of the lamp current may show the End-of-life. However, detection of lamp voltage is unreliable in a sense that the lamp voltage is highly changeable with different operating ambient temperature. Moreover, electronic ballast with dimming function nowadays is very common and voltage detection technique may not be adequate as the lamp voltage could change drastically at different dimming levels with a difference of typically 30-40%. The consequence would be the lamp safety is seriously impaired unless every condition is checked.  
         [0015]     In U.S. Pat. No. 6,819,063, Arthur Nemirow has provided a method to sense the filament status in a fluorescent lamp. His invention includes a DC flyback converter to drive the filaments and a separate alternating power source to drive the lamp with multiple outputs act as voltage sources across each of the filament. Due to the open circuit flyback effect and cross-regulation feature of the converter, output voltage will experience a sharp increase in voltage if the load tends to an open load, i.e. the filament is broken. A threshold voltage is then sensed and triggers a protection mechanism to inhibit the output of alternating power to the lamp. There is an advantage in sensing the filament resistance with an isolated circuitry because the lamp usually has high AC voltage across it, i.e. at least one end of the filaments will be at high voltage. Sensing parts at high voltage would not be ideal since the control logic is at low-voltages. However, such method has a few drawbacks where an isolated component, such as opto-coupler is required and it is commonly known that its current transform ratio (CTR) deteriorates against time, produces a change in the sensed voltage level unintentionally. High component counts with an integrated switch at primary side of the DC flyback converter contribute extra cost. Moreover, it provides no information on the potential differences of the two filaments where the amount of difference could indicate one type of rectifying effect.  
         [0016]     Thus, there is a need for improving End-of-life detection mechanism, particularly in sensing the difference between filaments in a fluorescent lamp and also the rectifying effect, which must be immune from factors such as operating temperature and lamp condition. The new method should be relatively economical, while providing a more complete and reliable protections to End-of-life. The present invention addresses these needs as described herein.  
       SUMMARY OF THE INVENTION  
       [0017]     It is therefore an object of the present invention to provide end-of-life protection to fluorescent lamps. When the lamp has reached its end-of-life, the present invention makes an appropriate detection and sends off a signal to shut down the power source for the lamp.  
         [0018]     Briefly, the present invention comprises an alternating power source, a fluorescent lamp, power supply arrangements to provide power to heat up the two filaments in the fluorescent lamp. Furthermore, the present invention comprises a transformer with multiple windings. Two of the transformer windings are connected in such a way to detect filament currents in the two different filaments. The windings are constructed in such a way that the signals of these two windings cancel each other in normal operation. The transformer has a further third winding which is coupled to a signal conditioning circuit which in turn produces an output fault signal which shuts down the power source when the lamp has reached its end-of-life.  
         [0019]     In operation, the alternating power source provides power to the fluorescent lamp. Separate power supply arrangements provide power to the two filaments. Two windings of the transformer detect the filament currents. When the lamp is in a healthy state, the two detected filament currents should cancel out each other and produce no signal at the transformer third winding. However, when the lamp has reached its end-of-life, the filament currents become asymmetrical and are no longer able to cancel out each other. The residue signal is picked up by the third winding of the transformer. This signal goes through a signal conditioning circuit where it is compared with a preset reference. If the residue signal is higher than the preset reference a fault signal is produced which in turn turns off the alternating power source.  
         [0020]     In an alternative embodiment, power supply for the two filaments in the lamp come from the main alternating power source. A capacitor is arranged in such a way to carry the filament current in parallel with the lamp. Similar to the first embodiment, this second embodiment has a transformer with three windings. Two windings detect the filament currents. When the lamp has reached its end-of-life the currents become asymmetrical and a signal is produced at the third winding of the transformer. This signal then goes through a signal conditioning circuits and turns of the alternating power source.  
         [0021]     In a further alternative embodiment, two transformers are used instead of one while the arrangement of an alternating power source and a lamp is basically the same as the first embodiment. The two transformers are arranged to detect current from the alternating power source. One of the transformers has rectifier on its secondary winding arranged to pick up the positive portion of the filament current, while another one of the transformers has rectifier on its secondary winding arranged to pick up the negative portion of the filament current. The positive portion and a negative portion of the current signals are added together which should produce a zero signal when the lamp is healthy while the current waveform is symmetrical. When the lamp has reached its end-of-life the waveform becomes asymmetrical and a non-zero signal is produced. It is picked up by a low pass filter and furthermore passed through a signal conditioning circuit where the signal is compared with an internal reference. A fault signal will be produced if the current imbalance exceeds the internal reference. This fault signal is coupled to the alternating power source and turns it off. 
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0022]      FIG. 1  is a prior art configuration of a fluorescent lamp with filaments driven by an alternating power source.  
         [0023]      FIG. 2  is a schematic diagram which explains the principle of the present invention.  
         [0024]      FIG. 3  is a current waveform drawn by a healthy lamp.  
         [0025]      FIG. 4  is a current waveform drawn by a lamp near its end-of-life.  
         [0026]      FIG. 5  is a first embodiment of the present invention.  
         [0027]      FIG. 6  is an embodiment of a signal conditioning circuit.  
         [0028]      FIG. 7  is a second embodiment of the present invention.  
         [0029]      FIG. 8  is a third embodiment of the present invention.  
         [0030]      FIG. 9  is a band pass signal conditioning circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     The principle of the present invention is explained by apparatus shown in  FIG. 2 . It comprises of a main alternating power source  206  coupled to a fluorescent lamp  101 . The lamp has two filaments  121  and  122  at its ends. Two isolated voltage sources  204  and  205  are coupled respectively the said filaments which provide power to heat up these filaments. This method is known as voltage-mode driven filament. This apparatus also comprises of detector  201  which is a means to detect current supply to the lamp by said alternating power source  206 . This apparatus further comprises detectors  202  and  203  which are means to detect filament current supplies from said isolated voltage sources. When the lamp comes to its end-of-life Rectifying Effect could be detected by sensing the lamp current detected by  201 ,  202  and  203 .  
         [0032]      FIG. 3  and  FIG. 4  illustrate the lamp current waveforms of a healthy lamp and a lamp reaching its end-of-life with rectifying effect respectively. These current signals can be sensed by detector  201 , which can be used to detect current imbalance in the alternating power cycle. A healthy lamp should have symmetrical current waveform in an alternating power cycle. In  FIG. 3  the voltage magnitude V1 should be equal to V2 while the signal average level (DC level) is be detected as zero. When a lamp reaches its end-of-life a rectifying effect shows up as illustrated by  FIG. 4 , where voltage magnitude V3 will not be equal to V4 and the DC level will be either positive or negative with a non-zero value. The principle of the present invention is to detect such non-zero DC level. If this level exceeds a certain threshold the main alternating power source will be shut down for safe lamp operation.  
         [0033]     Now, if the lamp rectifying effect is caused by a deteriorated or broken filament, filament current detectors  202  or  203  will indicate changes in filament current. Threshold detection technique can be easily applied for safe and accurate lamp protection.  
         [0034]     In  FIG. 5 , a first embodiment of the present invention for a fluorescent light system is shown. It comprises a primary alternating power source  206  which provides power to a fluorescent lamp through its two terminals. The fluorescent lamp has two filaments at its ends and they are powered up by two isolated voltage sources  204  &amp;  205 . These two voltage sources produce voltages with the same magnitude but opposite phases. In general these two voltage sources for filament currents are coupled from the primary alternating power source  206  to ensure that they are synchronized. The embodiment further comprises of a transformer  504  which has at least three windings, namely  501 ,  502  and  503 . The filament current signals are picked up by windings  501  and  502  of transformer  504 . Winding  503  produces a signal in fault conditions which is coupled to a signal conditioning circuit  505 , which has an output signal  506  coupled to the primary alternating power source.  
         [0035]     Here operation of the first embodiment is described. In normal operation when the lamp is at a healthy state, filaments at the two ends of the fluorescent lamp have the same characteristics so filament currents through windings  501  and  502  are equal in magnitude but in anti-phase. Windings  501  and  502  therefore generate equal but opposite flux and cancel each other where the third winding  503  sees the overall flux level around the core which is equal to zero. When there is a change in one of the filament resistance, say due to deterioration of the filament, the flux cancellation mechanism is upset causing an AC voltage induced in winding  503 . A signal conditioning circuit picks up this AC voltage signal and decides if the lamp has reached the end of life. If so it sends off a signal and shut down the primary alternating power source to the lamp. This serves the objective of inhibiting the power source to a fluorescent lamp when it has reached its end of life.  
         [0036]     An embodiment of the signal conditioning circuit  505  is shown in  FIG. 6 . The signal conditioning circuit captures current generated in winding  503  of transformer  504 , when this current level exceeds a reference value this indicates a filament has deteriorated and a signal  506  is issued to shut down the primary power source to the lamp. Here a typical configuration is show. A resistor turns current signal in winding  503  into voltage signal. A circuit with a diode and a capacitor pick up the amplitude of the AC voltage sensed at  503 . This voltage is compared with an internal reference voltage  601  by a comparator and a signal  506  will be issued to disable the primary alternating power source if this voltage exceeds the reference voltage. An internal reference voltage  601  can either be preset or made adjustable to respond to different operating conditions. There are many other possible embodiments but the operating principle should prevail under the scope of this invention.  
         [0037]     It is obvious to those having ordinary skill in the art that there are many ways to detect an amplitude voltage. The embodiment so described is illustrative rather than restrictive while the principle of operation prevails.  
         [0038]     A second embodiment of the present invention is shown in  FIG. 7 . It comprises of an Alternating Power Source  206  having two terminals coupled to the filaments of a fluorescent lamp  101 . Like all typical fluorescent lamps this lamp  101  has two filaments at its ends each having a filament terminal pair. Each filament terminal pair has one of its terminals coupled to Alternating Power Source  206  and the other coupled to a capacitor  102 . This embodiment further comprises a transformer  504  at least three windings  501 ,  502  &amp;  503 . A resistor  701  coupled in series with a winding  501  of transformer  504  are coupled in parallel to a filament terminal pair. The other filament terminal pair has a similar arrangement which connects in parallel to a resistor  702  in series with a winding  502  of transformer  504 . In addition to windings  501  and  502  transformer  504  has a third winding  503  which is coupled to a signal conditioning circuit  505 . This signal conditioning circuit  505  has an output signal  506  which is coupled back to Alternating Power Source  206  for control purpose.  
         [0039]     Operation of the second embodiment is described herein. During normal operation, Alternating Power Source  206  drives current to the lamp filaments. At the lamp filaments some current flows through the lamp as lamp current while some flow through capacitor  102  as filament current. Only a small amount of current flows through the two parallel circuits having resistor  701  and winding  501 , or resistor  702  and winding  502  at the other end of the lamp because resistors  701  and  702  are designed to have a higher value than the filament resistance. Symmetrical arrangement of the configuration produces the same current in these two circuits in normal operation when the lamp is healthy. Windings  501  and  502  of transformer  504  are constructed in such a way that current flow in these two windings cancels out the magnetic flux produced by each other. A third winding  503  of transformer picks up the difference in magnetic flux produced by windings  501  and  502 . In normal healthy lamp operation no flux is picked up by winding  503  as flux produced by windings  501  and  502  cancel out each other. When the lamp deteriorates one of the lamp filaments may break or produces a significant change in resistance. This forces current into its parallel circuit and produces different currents in windings  501  and  502 . Winding  503  then picks up the imbalance signal and presents to a signal conditioning circuit which in turn produces a signal  506  to inhibit the Alternating Power Source  206 . This serves the objective of inhibiting the power source to a fluorescent lamp when it has reached its end of life.  
         [0040]     A third embodiment of this invention is shown in  FIG. 8 . It comprises a primary alternating power source  206  which provides power to a fluorescent lamp  101  through its two terminals. The fluorescent lamp has two filaments at its ends and they are powered up by two isolated voltage sources  801  and  802 . The lamp current delivered by the alternating power source  206  is monitored by two current transformers  803  and  806 . The lamp current passes through the respective windings  804  and  807  of these transformers which are connected in series. The current transformers  803  and  806  have secondary windings  805  and  808  respectively. These secondary windings produce the same alternating signal as the transformers monitor the same current. However rectifiers  811  and  812  are connected to secondary windings  805  and  808  in such a way that rectifier  811  picks up the positive portion of the alternating signal produced by secondary winding  805  and rectifier  812  picks up the negative portion of the alternating signal produced by secondary winding  808 . The cathode of rectifier  811  is coupled to the anode of rectifier  812  and further couples to a capacitor  813 . Capacitor  813  is coupled to a low pass filter  814  followed by a Band-Pass Signal Conditioning circuit  815 . This Band-Pass Signal Conditioning circuit has an output signal when the absolute value of the detected signal is greater than an internal preset threshold  901 .  
         [0041]     Operation of the third embodiment is described herein. During normal operation, Alternating Power Source  206  drives current through the lamp. When the lamp is healthy the alternating lamp current should be symmetrical and its waveform has equal positive portion and negative portion. The lamp current is monitored simultaneously by two current transformers  803  and  806  respectively. They produce the same output signal. However this signal is rectified by different rectifiers  811  and  812  such that rectifier  811  captures the positive portion and rectifier  812  captures the negative portion of the signal. If the lamp is healthy the lamp current is symmetrical the combined signal will have a zero level and no fault signal is produced. If the lamp has reached its end-of-life rectifying effect will come out. A signal with a magnitude will appear at the node where rectifiers  811  &amp;  812  are connected. This signal is then passed through a low pass filter  814  which is further coupled to a band-pass signal conditioning circuit  815 .  FIG. 9  illustrates such a band-pass signal conditioning circuit. It comprises of two comparators each having its own voltage reference  901  trigger levels. When the incoming signal exceeds the internal reference threshold level whether this signal is positive or negative, one of the comparators will be triggered and produces a fault signal at the output. The fault signal  816  in  FIG. 8  is then coupled to said Alternating Power Source  206  and turns it off.  
         [0042]     The present invention is not to be limited in scope by the specific embodiments described herein, which are intended as single illustrations of individual aspects of the invention, and functionally equivalent methods and components are within the scope of the invention. Indeed, various modifications of the invention, in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.