Abstract:
A fluorescent lamp driver circuit is provided. The fluorescent lamp driver circuit uses reversed current detecting signal to achieve feedback control and circuit protection so as to simplify the driver circuit and reduces the number of the required electronic components. The driver circuit needs a single control unit to control the whole circuit, which not only reduces cost, but also simplifies circuit design.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fluorescent lamp driver circuit, and more particularly to a multi-lamp cold cathode fluorescent lamp (CCFL) driver circuit. 
     2. Description of Related Art 
     In a backlight device of a liquid crystal display (LCD), a high-frequency sine wave AC power supply is usually adopted for supplying electric power to drive a cold cathode fluorescent lamp (CCFL) to emit light. Therefore, a DC/AC inverter circuit is demanded for converting energy. The typical CCFL driver circuit usually has a resonance module to convert a DC voltage into an AC voltage for driving the CCFL to emit light. Voltage and current detect circuits are usually used for detecting a driving voltage and a driving current of the CCFL, respectively. A pulse width modulation (PWM) controller receives a voltage detection signal and a current detection signal for stabilizing the illumination of the CCFL and for circuit protection. 
     Attending with the development of large-scale LCD panels, the number of CCFLs in the backlight device needed to be driven is increased accordingly. The traditional circuit design with single PWM controller and single resonance module to drive single lamp may incur complicated circuits and high costs of such backlight device. To reduce the cost, U.S. Pat. No. 7,291,991 has disclosed a multi-lamp driver circuit to reduce the number of components in the circuit and simplify the circuit design. 
     With reference to  FIG. 1  for a circuit diagram of a multi-lamp driver circuit in accordance with a U.S. patent, the multi-lamp driver circuit includes a PWM controller  10 , a resonance module  20 , a multi-lamp module including a plurality of lamps L 1 ˜L 4 , and a switch module  40 . The switch module  40  is connected to an input voltage source Vin and is used to control the energy transmitted to the resonance module  20  according to control signals of the PWM controller  10 . The resonance module  20  includes two transformers T 1 , T 2  and a plurality of transistor switches. The lamps L 1 , L 2  are connected in series with a secondary side of the transformer T 1 , and the lamps L 3 , L 4  are connected in series with a secondary side of the transformer T 2 . Current detectors  32 ,  34  are serially connected to the lamps L 1 , L 2  and the lamps L 3 , L 4  respectively for detecting a lamp current passing through the lamps L 1 , L 2  and a lamp current passing through the lamps L 3 , L 4  to generate current detection signals IFB 1 , IFB 2 . Voltage detectors  36 ,  38  are connected in parallel with the lamps L 1 , L 2  and the lamps L 3 , L 4  respectively for detecting lamp voltages of the lamps L 1 , L 2  and the lamps L 3 , L 4  to generate voltage detection signals VFB 1 , VFB 2 . The PWM controller  10  receives the current detection signals IFB 1 , IFB 2  and the voltage detection signals VFB 1 , VFB 2  for performing feedback control to control the electric power transmitted by the switch module  40  so as to stabilize the light emission of the lamps and to protect the circuit under the abnormal conditions. 
     In the aforementioned circuit, one resonance module, one current detector, and one voltage detector are used for driving two lamps simultaneously, and one PWM controller is used for controlling the operation of four lamps. Compared with the conventional circuit, the multi-lamp driver circuit has reduced the number of pins of the PWM controller and the number of electronic components, and also simplified the circuit design. However, it is still an important subject for the CCFL driver circuit research to further reduce the number of pins of the PWM controller and the number of electronic components, and to simplify the circuit design. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to further reduce the number of pins and the number of required electronic components of a multi-lamp driver circuit, so as to lower the cost of the circuit and simplify the circuit layout. The present invention provides a fluorescent lamp driver circuit comprising a switch module, a resonance module, a first fluorescent lamp module, a second fluorescent lamp module, a detection unit, a selection unit, a protection unit, and a control unit. The switch module is coupled to a DC input voltage and controls the magnitude of an output electric power according to a plurality of control signals. The resonance module is coupled to the switch module for converting the output electric power into a first AC signal and a second AC signal, wherein the first AC signal and the second AC signal are almost in opposite phases. In other words, the phase difference between the first AC signal and the second AC signal falls within a predetermined range from 180 degrees. The first fluorescent lamp module is coupled to the resonance module for receiving the first AC signal, and the second fluorescent lamp module is coupled to the resonance module for receiving the second AC signal. The detection unit includes a first detecting portion and a second detecting portion. One end of the first detecting portion and one end of the second detecting portion are coupled with a common ground. The other end of the first detecting portion is serially connected to the first fluorescent lamp module for generating a first detection signal, and the other end of the second detecting portion is serially connected to the second fluorescent lamp module for generating a second detection signal. The selection unit receives the first detection signal and the second detection signal and outputs a select signal. The protection unit receives the first detection signal and the second detection signal and outputs a protection feedback signal. The control unit is coupled to the selection unit and the protection unit, and generates the plurality of control signals according to the select signal for controlling the switching of the switch module. The control unit stops the switching of the switch module if the level of the protection feedback signal is higher than a predetermined value. 
     The present invention further provides a fluorescent lamp driver circuit comprising a switch module, a resonance module, a first fluorescent lamp module, a second fluorescent lamp module, a detection unit, a selection unit, a protection unit, and a control unit. The switch module is coupled to a DC input voltage, and controls the magnitude of an output electric power according to a plurality of control signals. The resonance module is coupled to the switch module for converting the output electric power into a first AC signal and a second AC signal, wherein the phase difference between the first AC signal and the second AC signal falls within a predetermined range from 180 degrees. The first fluorescent lamp module is coupled to the resonance module for receiving the first AC signal, and the second fluorescent lamp module is coupled to the resonance module for receiving the second AC signal. The detection unit includes a first detecting portion and a second detecting portion. One end of the first detecting portion and one end of the second detecting portion are coupled to a common ground. The other end of the first detecting portion is serially connected to the first fluorescent lamp module for generating a first detection signal. The other end of the second detecting portion is serially connected to the second fluorescent lamp module for generating a second detection signal. The selection unit is coupled to the detection unit for receiving the first detection signal and the second detection signal, and outputting a select signal. The protection unit is coupled to the selection unit and the detection unit, for determining whether to transfer the select signal into a protection state or not according to the first detection signal and the second detection signal. The control unit is coupled to the selection unit and generates the plurality of control signals for controlling the switching of the switch module according to the select signal, and stops the switching of the switch module after the select signal transferred into the protection state is detected. 
     The present invention provides another fluorescent lamp driver circuit, comprising a switch module, a resonance module, a first fluorescent lamp module, a second fluorescent lamp module, a detection unit, a protection unit, and a control unit. The switch module is coupled to a DC input voltage, and controls the magnitude of an output electric power according to a plurality of control signals. The resonance module includes a primary side and a secondary side, and the primary side is coupled to the switch module for converting the output electric power into an AC signal and outputting the AC signal from the secondary side. The first fluorescent lamp module is coupled to the secondary side of the resonance module, and the second fluorescent lamp module is coupled to the secondary side of the resonance module. The detection unit includes a first detecting portion and a second detecting portion. One end of the first detecting portion and one end of the second detecting portion are coupled to a common ground. The other end of the first detecting portion is serially connected to the first fluorescent lamp module for generating a first detection signal. The other end of the second detecting portion is serially connected to the second fluorescent lamp module for generating a second detection signal. The phase difference between the first detection signal and the second detection signal falls within a predetermined range from 180 degrees. The protection unit receives the first detection signal and the second detection signal, and outputs a protection feedback signal. The control unit is coupled to the protection unit and outputs the plurality of control signals when the protection feedback signal is in a first state. The control unit stops the switching of the switch module when the protection feedback signal is in a second state. 
     In summation of the description above, the fluorescent lamp driver circuit provided in the present invention can achieve the object of feedback control of multi-lamp and circuit protection by using the detection signal selected by the selection unit, and even adjust and control the level of the output detection signal according to the protection feedback signal to achieve the object of using a single feedback signal to provide the functions of the feedback control and circuit protection. The present invention can also simplify the circuit design and reduce the number of electronic components significantly. 
     The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a conventional multi-lamp driver circuit; 
         FIG. 2  is a circuit diagram of a multi-lamp driver circuit in accordance with a first preferred embodiment of the present invention; 
         FIG. 3  is a circuit diagram of a multi-lamp driver circuit in accordance with a second preferred embodiment of the present invention; 
         FIG. 4A  is a circuit diagram of a multi-lamp driver circuit in accordance with a third preferred embodiment of the present invention; 
         FIG. 4B  is a schematic diagram showing the waveform of signals in the multi-lamp driver circuit of  FIG. 4A ; 
         FIG. 5A  is a circuit diagram of a multi-lamp driver circuit in accordance with a fourth preferred embodiment of the present invention; and 
         FIG. 5B  is a schematic diagram showing the waveform of signals in the multi-lamp driver circuit of  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 2  for a circuit diagram of a multi-lamp driver circuit in accordance with a first preferred embodiment of the present invention, the multi-lamp driver circuit comprises a switch module SW, a resonance module, a first fluorescent lamp module L 1 , a second fluorescent lamp module L 2 , a detection unit, a protection unit, and a control unit  100 . The switch module SW is coupled to a DC input voltage Vin and is switched according to a control signal from the control unit  100  so as to control the magnitude of the output electric power. The switch module SW of this embodiment has a full-bridge architecture, but in practical, a half-bridge architecture or a push-pull architecture can be adopted in the switch module SW as well. The resonance module comprises resonant capacitors C 1 , C 2  and a transformer T with a primary side and a secondary side. The primary side of the transformer T is coupled to the switch module SW, and the secondary side of the transformer T is coupled to the resonant capacitors C 1 , C 2  for receiving the output electric power transmitted from the switch module SW and converting the output electric power into an AC signal outputted from the secondary side. The first fluorescent lamp module L 1  is coupled to one end of a secondary side of the resonance module, and the second fluorescent lamp module L 2  is coupled to the other end of the secondary side of the resonance module. Both the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  receives the AC signal outputted from the secondary side of the resonance module to emit light. The detection unit includes a first detecting portion and a second detecting portion, wherein the first detecting portion includes a first detecting resistor R 1  and the second detecting portion includes a second detecting resistor R 2 . The first detecting portion and the first fluorescent lamp module L 1  are serially connected to the secondary side of the resonance module, and the second detecting portion and the second fluorescent lamp module L 2  are serially connected to the secondary side of the resonance module as well. One end of the first detecting portion and one end of the second detecting portion are coupled to a common ground, and the other ends of the first detecting portion and the second detecting portion are serially connected to the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  for generating a first detection signal FB 1  and a second detection signal FB 2  respectively. It is noted that the currents passing through the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  are in opposite directions. Thus, the first detection signal FB 1  and the second detection signal FB 2  are almost in opposite phase. In other words, the phase difference of the first detection signal FB 1  and the second detection signal FB 2  falls within a range from 180 degrees. Since the impedance of the fluorescent lamp modules is not perfectly matched in practice, the phase difference between the first detection signal FB 1  and the second detection signal FB 2  would not be exactly equal to 180 degrees. The deviation of the phase difference from 180 degrees is dependent to the impedance difference. However, the phase difference will remain in the certain range from 180 degrees. 
     The selection unit SE receives the first detection signal FB 1  and the second detection signal FB 2 , and selectively output one of the two detection signals FB 1  and FB 2  to form a select signal FB according to the timing of the two detection signals FB 1  and FB 2 . In this embodiment, the selection unit SE includes two diodes with positive terminals thereof coupled to a first detecting resistor R 1  and a second detecting resistor R 2  respectively, and negative terminals thereof coupled with each other, such that the selection unit SE would selectively output the first detection signal FB 1  and the second detection signal FB 2  to form a full-wave select signal FB. The protection unit is coupled to the detection unit for receiving the first detection signal FB 1  and the second detection signal FB 2 , and outputting a protection feedback signal PR. The protection unit includes a compensating portion and a filter portion FC, wherein the compensating portion includes impedance compensation components Z 1 , Z 2 , such as resistors, capacitors, or any other components having impedance. The impedance compensation components Z 1  and Z 2  are coupled to the first detecting resistor R 1  and the second detecting resistor R 2  of the detection unit respectively so as to have the first detection signal FB 1  and the second detection signal FB 2  compensated with each other to generate a compensation signal CP. Under a normal operation condition, the first detection signal FB 1  and the second detection signal FB 2  are substantially opposite in phase and have similar magnitude, and the compensation signal CP outputted from the compensating portion would substantially approach zero potential. Meanwhile, the protection feedback signal PR is in a first state representing the normal operation. If there is any open circuit, short circuit, or other abnormality happened in the first fluorescent lamp module L 1  or the second fluorescent lamp module L 2 , impedance mismatch between the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  will become more serious than that under the normal operation condition. Thus, the magnitude difference of the first detection signal FB 1  and the second detection signal FB 2  would be increased and the phase difference there between would be deviated away from the 180 degrees more seriously, and the compensation signal CP with larger amplitude would be resulted. The compensation signal CP is then transmitted to the filter portion FC through the rectifier diode D 1 . After filtering out the high frequency portion, the protection feedback signal PR is resulted. It is noted that the level of the protection feedback signal PR would be increased in contrast with that under the normal operation condition, and thus the protection feedback signal PR is in a second state representing the abnormality. 
     The control unit  100  receives the select signal FB and the protection feedback signal PR and performs feedback control according to the select signal FB to stabilize the current passing through the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  to generate steady illumination. If the level of the protection feedback signal PR is higher than a predetermined value, the protection feedback signal PR is determined to be in the second state indicating abnormal circuit, and the control unit  100  will stop the switching of the switch module SW. Meanwhile, the switch module SW stops outputting energy to the resonance module, and the fluorescent lamp driver circuit enters a protection mode. To prevent the temporary voltage rise of the protection feedback signal PR caused by a sudden disturbance happened in the first fluorescent lamp module L 1 , the second fluorescent lamp module L 2 , and the system circuit or other factors (such as system booting) from resulting in misjudgments because the circuit is not damaged or showing any abnormality under such condition, a predetermined time can be set, such that unless the level of the protection feedback signal PR is higher than the predetermined value and remains the predetermine time, the control unit  100  would not stop the switching of the switch module. 
     With reference to  FIG. 3  for a circuit diagram of a multi-lamp driver circuit in accordance with a second preferred embodiment of the present invention, the difference of this embodiment from the first preferred embodiment is that there are two windings disposed at the secondary side of the transformer T of the resonance module in this embodiment coupled to the resonant capacitors C 1 , C 2  respectively for converting electric power into a first AC signal and a second AC signal. The polarities of the two windings are opposite. Thus, the phases of the first and the second AC signals are opposite. The first fluorescent lamp module L 1  is coupled to one of the two secondary side windings of the transformer T for receiving the first AC signal, and the second fluorescent lamp module L 2  is coupled to the other secondary side winding of the transformer T for receiving the second AC signal. The detection unit includes a first detecting resistor R 1  and a second detecting resistor R 2 , and one end of the first detecting resistor R 1  and one end of the second detecting resistor R 2  are coupled to a common ground. The other end of the first detecting resistor R 1  is serially connected to the first fluorescent lamp module L 1  for generating the first detection signal FB 1 , and the other end of the second detecting resistor R 1  is serially connected to the second fluorescent lamp module L 2  for generating the second detection signal FB 2 . Since the phases of the first AC signal and the second AC signal are opposite, the level of the protection feedback signal PR outputted by protection unit according to the first detection signal FB 1  and the second detection signal FB 2  approaches zero potential under normal operation condition. If any abnormality occurs, the impedance mismatch of the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  becomes more serious to have the magnitude difference of the first detection signal FB 1  and the second detection signal FB 2  would be increased and/or the phase difference there between would be deviated away from the 180 degrees more seriously, and thus causing a level rising of the protection feedback signal PR. Thereby, if the level of the protection feedback signal PR is higher than a predetermined value, the control unit  100  will stop the switching of the switch module SW. As a preferred embodiment, in order to prevent misjudgments, if the level of the protection feedback signal PR is higher than the predetermined value and also remained at such condition after a predetermined time, the control unit  100  will stop the switching of the switch module SW. 
     With reference to  FIG. 4A  for a circuit diagram of a multi-lamp driver circuit in accordance with a third preferred embodiment of the present invention, the first fluorescent lamp module L 1  includes a plurality of fluorescent lamps L 11 , L 12 , and the second fluorescent lamp module L 2  includes a plurality of fluorescent lamps L 21 , L 22 . There are two windings at a secondary side of the transformer T coupled to the resonant capacitors C 1 , C 2  respectively for converting electric power into a first AC signal and a second AC signal. One end of the first detecting resistor R 1  and one end of the second detecting resistor R 2  of the detection unit are coupled to a common ground. The other end of the first detecting resistor R 1  is serially connected to the first fluorescent lamp module L 1  for generating a first detection signal FB 1 , and the other end of the second detecting portion R 2  is serially connected to the second fluorescent lamp module L 2  for generating a second detection signal FB 2 . Because of the coupling among the first detecting resistor R 1 , the second detecting resistor R 2 , and the two secondary side windings, currents passing through the first detecting resistor R 1  and the second detecting resistor R 2  have opposite values. Therefore, the level of the protection feedback signal PR outputted by the protection unit approaches zero under normal operation condition. However, if any abnormality occurs in the circuit, the magnitude difference between the first detection signal FB 1  and the second detection signal FB 2  would be increased and/or the phase difference would be deviated away from the 180 degrees more seriously, and thus causing a level rising of the protection feedback signal PR. Similarly, if the level of the protection feedback signal PR is higher than a predetermined value, the control unit  100  will stop the switching of the switch module SW. In order to prevent misjudgments, the control unit  100  will stop the switching of the switch module SW if the level of the protection feedback signal PR is higher than the predetermined value and remained at such condition after a predetermined time. 
       FIG. 4B  is a schematic diagram showing waveforms of the first detection signal FB 1 , the second detection signal FB 2 , the compensation signal CP, the select signal FB, and the protection feedback signal PR in the multi-lamp driver circuit of  FIG. 4A . Under normal operation condition, there exists a slight impedance mismatch between the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2 . Thus, the amplitude of the first detection signal FB 1  and the second detection signal FB 2  are slightly different, the phase difference is approximately equal to 180 degrees, and the compensation signal CP would be oscillated around zero potential. At time point t 1 , an abnormality (such as a short circuit) of the second fluorescent lamp module L 2  occurs and the current rises suddenly. Meanwhile, the amplitude difference between the first detection signal FB 1  and the second detection signal FB 2  increases, the phase difference is deviated from the 180 degrees, the amplitude of the compensation signal CP increases accordingly, and the voltage of the protection feedback signal PR rises gradually. At time point t 3 , the protection feedback signal PR is higher than a threshold voltage Vth, the control unit  100  begins its countdown to enter into a protection state to stop supplying electric power to the resonance module after a predetermined time. At time point t 2 , abnormality (such as an open circuit) occurs in the first fluorescent lamp module L 1  and the current passing through the first fluorescent lamp module L 1  drops suddenly. At this time, the amplitude difference between the first detection signal FB 1  and the second detection signal FB 2  is quite large, the amplitude of the compensation signal CP increases significantly, and the protection feedback signal PR rises rapidly. The protection feedback signal PR remains at a level higher than the threshold voltage Vth, and the control unit  100  keeps its countdown to enter into the protection state (not shown in the figure) after a predetermined time. 
     As shown in  FIG. 4B , any abnormal circuit, regardless of open circuit or short circuit, will cause an increasing of amplitude difference between the first detection signal FB 1  and the second detection signal FB 2  and/or a signification deviation of the phase difference from the 180 degrees. Thereby, the protection feedback signal PR exceeds the predetermined threshold voltage Vth to enable the protection function of the control unit  100  and achieve the object of circuit protection. 
     With reference to  FIG. 5A  for a circuit diagram of a multi-lamp driver circuit in accordance with a fourth preferred embodiment of the present invention, the difference of this preferred embodiment from the embodiment as shown in  FIG. 4A  is that the compensating portion of the present embodiment adopts two compensation capacitors C 3 , C 4  with one end thereof coupled to the first detecting resistor R 1  and the second detecting resistor R 2  of the detection unit respectively, and other ends thereof coupled with each other. The usage of capacitors as the compensating portion is capable to compensate impedance mismatch between the first fluorescent lamp module L 1  and the second fluorescent lamp module L 2  to equalize the current passing through the two fluorescent lamp modules L 1  and L 2 . The protection unit further includes a control portion Q 1  coupled to the selection unit SE. The control portion Q 1  will pull the level of the select signal FB back to the level substantially equal to zero potential to have the select signal FB transferred into a protection state if the protection feedback signal PR is higher than a protection level. The control unit  100  simply requires a single pin to receive the select signal FB for determining whether to perform feedback control or circuit protection control according to the level of the select signal FB.  FIG. 5B  shows the waveforms of the first detection signal FB 1 , the second detection signal FB 2 , the compensation signal CP, the select signal FB, and the protection feedback signal PR of the multi-lamp driver circuit of  FIG. 5A . As shown, a short circuit occurs in the second fluorescent lamp module L 2  at time point t 1 . Because of the capacitors C 3 , C 4 , the magnitude of current passing through the first detecting portion R 1  and the second detecting portion R 2  is not changed significantly and the amplitudes of the first detection signal FB 1  and the second detection signal FB 2  remains close to each other. However, because the phase difference is deviated significantly from the 180 degrees, the amplitude of the compensation signal CP would be increased and the voltage of the protection feedback signal PR rises gradually. At time point t 3 , the level of the protection feedback signal PR is higher than the threshold voltage Vth, the select signal FB is compulsorily pulled back, and the control unit  100  will enter into a protection state to stop supplying electric power to the resonance module after a predetermined time. At time point t 2 , an abnormal open circuit occurs suddenly in the first fluorescent lamp module L 1 . Similarly, there is no significant change to the amplitude difference between the first detection signal FB 1  and the second detection signal FB 2 , but the phase difference is deviated away from the 180 degrees more seriously, so that the amplitude of the compensation signal CP increases significantly and the protection feedback signal PR rises rapidly. The protection feedback signal PR remains at a level higher than the threshold voltage Vth, and the control unit  100  keeps its countdown to enter into a protection state (not shown in the figure) after a predetermined time. 
     In the fluorescent lamp driver circuit in accordance with the foregoing preferred embodiments of the present invention, the control unit can achieve the feedback control of multi-lamp and circuit protection by the detection signal selected by the selection unit and the protection feedback signal. The control unit can even adjust and control the level of the selected detection signal according to the state of the protection feedback signal. The control unit of the present invention does not have to increase the number of feedback and circuit protection pins as the number of fluorescent lamps increases, but simply have to use two pins or even one pin to achieve the feedback control and circuit protection functions of multi-lamp. Thus, the corresponding circuit design can be simplified, and the number of required electronic components can be reduced significantly. 
     Although the present invention has been described with reference to the preferred embodiments thereof, it shall be understood that the present invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the present invention as defined in the appended claims.