Abstract:
Methods and apparatus for driving discharge lamps are disclosed herein. In one embodiment, a method for driving a discharge lamp includes generating a switching signal to drive a discharge lamp, monitoring a working status of the discharge lamp, and determining whether the discharge lamp is operating abnormally based at least in part on the monitored working status. The method also includes decreasing a duty cycle of the switching signal when the discharge lamp is determined to operate abnormally.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to Chinese patent application No. 200810046109.3, filed Sep. 19, 2008, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure generally relates to switching circuits for driving discharge lamps and associated methods of operation. 
       BACKGROUND 
       [0003]    Cold cathode fluorescent lamp (CCFL), external electrode fluorescent lamp (EEFL), and other types of discharge lamps are widely used to backlight liquid crystal displays (LCD). Such discharge lamps all require a driving mechanism for supplying an alternating current (AC) driving voltage and a stable high-frequency lamp current. 
         [0004]    Typically, discharge lamps require a striking voltage (e.g., a few hundred volts) to initiate or strike an electrical arc in the discharge lamps. The striking voltage can be even higher (e.g., 1000-2000 volts) under low temperature and/or aging conditions. Once an electrical arc is struck inside the discharge lamps, the terminal voltage may fall to an operation voltage (e.g., a few hundred volts), and the brightness produced depends on the current flowing through the discharge lamps. When a driving circuit detects that a discharge lamp is in an open circuit (e.g., the electric arc has not been struck yet; the lamp is not properly coupled to the terminals; or the lamp malfunctions), the driving circuit would provide the striking voltage to the terminals and attempts to re-strike the electric arc in the discharge lamp. If the driving circuit still detects an open circuit after a preset amount of time, the driving circuit would determine that the lamp is not properly coupled to the terminal or the lamp has malfunctioned, and cease attempting to re-strike the electric arc in the discharge lamp for self-protection. 
         [0005]    Typically, conventional driving circuits adjust the brightness of discharge lamps based on a lamp current feedback signal in normal operation, and adjust the terminal voltage based on a lamp voltage feedback signal in open circuit conditions. The driving circuits can also include protection circuitry that monitors the terminal voltage and terminates the driving circuits when the terminal voltage exceeds a threshold for longer than a preset amount of time (e.g., 1 second). To provide a sufficient striking voltage, the driving circuits often utilize frequency hopping techniques in which the working frequency is increased to a preset value after an open circuit is detected. 
         [0006]      FIG. 1  is a block diagram of a driving circuit for driving a single discharge lamp in accordance with the prior art. As shown in  FIG. 1 , the driving circuit includes a switching circuit  101 , a control circuit  102 , a transformer  103 , a resonant circuit  104 , and a load  105  that includes a single discharge lamp L. The switching circuit  101  comprises at least one switch that receives a direct current (DC) input voltage V in  and generates a switching signal SW. The control circuit  102  is electrically coupled to the switching circuit  101  and controls the on/off of the at least one switch. The transformer  103  is electrically coupled between the switching circuit  101  and the resonant circuit  104 . The primary winding of the transformer  103  receives the switching signal SW, and the secondary winding of the transformer  103  accordingly generates an AC signal. The resonant circuit  104  is electrically coupled between the transformer  103  and the load  105 . The resonant circuit  104  receives the AC signal and generates an output voltage V out  to drive the load  105 . 
         [0007]    When the input voltage V in  and circuit parameters are constant, the output voltage V out  of the driving circuit is determined by the duty cycle of the switching signal SW and the voltage gain of the resonant circuit  104  and the load  105 . The voltage gain is related to the operating conditions of the load  105  (whether the lamp L is open) and the switching frequency of the switching signal SW. Typically, the lamp current or the lamp voltage is monitored and compared with a threshold to detect whether the lamp is under open circuit condition. However, in a transient open circuit state, the duty cycle of the switching signal does not have time to adjust, and there is a delay between the lamp reaching open circuit and the driving circuit detecting the open circuit condition. 
         [0008]      FIG. 2  is a curve showing a relationship between the switching frequency and the voltage gain of the resonant circuit  104  and the load  105  as a function of the switching frequency. In normal operation, the gain curve is curve a, the switching frequency is the operation frequency f s , and the voltage gain is G 1 . The corresponding output voltage V out  is the normal working voltage V o,normal . The operation frequency f s  is generally set to be slightly higher than the resonant frequency of the resonant circuit  104  and the load  105 . Under open circuit conditions, the gain curve is the curve b. If the switching frequency is maintained at the operation frequency f s , the voltage gain will be G 2 &gt;G 1 . The difference between G 2  and G 1  is determined by the resonant parameters of the resonant circuit  104  and also the characteristic of the lamp L. 
         [0009]    Generally, G 2  is not large enough to allow the output voltage V out  to reach the striking voltage, so a frequency hopping technique is usually used. Once the open circuit condition is detected, the switching frequency is set to a higher frequency f s,open  to obtain a voltage gain G 3 , and G 3 &gt;G 1 ,G 2 . The frequency f s,open  may be set by external resistors or voltages, or it may be set internally. If the frequency f s.open  is set internally, under some conditions (related to the resonant parameters of the resonant circuit  104 ), the instant output voltage V out  during frequency hopping may be too high to cause the failure of the lamp L and/or the other electrical elements. 
         [0010]      FIG. 3  is a diagram illustrating a waveform of the peak output voltage V out  with respect to time during lamp initiation. At t=0, the driving circuit is powered on, the lamp L is not ignited, and the open circuit condition is not detected. The switching frequency is the operation frequency f s  and the voltage gain is G 2 . During 0&lt;t&lt;t 1 , the duty cycle of the switching signal SW is increased by the control circuit  102 , and the output voltage V out  is increased accordingly. At t=t 1 , the open circuit condition is detected, the frequency is set to the frequency f s,open , and the voltage gain is G 3 . If G 3  is large enough, there will be an overshoot V os1  across the lamp L. Then, the duty cycle of the switching signal SW is decreased by the control circuit  102  until the output voltage V out  is regulated to the striking voltage V o,strike . At t=t 2 , the lamp L is ignited, the switching frequency is set to be the operation frequency f s  again, the voltage gain is G 1  and the output voltage V out  is the operation voltage V o,normal . 
         [0011]      FIG. 4  is a diagram illustrating a waveform of the peak output voltage V out  with respect to time before and after a lamp opening. Before t=t 3 , the driving circuit is in normal operation, the switching frequency is the operation frequency f s , the voltage gain is G 1  and the output voltage is the operation voltage V o,normal . At t=t 3 , the lamp L is open, but the open circuit condition is not detected, the switching frequency is maintained at the operation frequency f s , the voltage gain is G 2  and the output voltage is V os2 . During t 3 &lt;t&lt;t 4 , the duty cycle of the switching signal SW is increased by the control circuit  102 , and the output voltage V out  is increased accordingly. At t=t 4 , the open circuit condition is detected, the frequency is set to the frequency f s,open , and the voltage gain is G 3 . If the difference between G 3  and G 2  is large enough, there will be an overshoot V os3  across the lamp L. Then, the duty cycle of the switching signal SW is decreased by the control circuit  102 , until the output voltage V out  is regulated to the striking voltage V o,strike . At t=t 5 , the lamp L is ignited again, the switching frequency is set to be the operation frequency f s , the voltage gain is G 1  and the output voltage V out  is the operation voltage V o,normal . 
         [0012]      FIG. 5  is a block diagram of a driving circuit for driving two serially connected discharge lamps. The driving circuit is similar to the one shown in  FIG. 1 , except that the load  105  comprises two serially connected discharge lamps L 1  and L 2 . The lamps L 1  and L 2  may not be ignited at the same time because of their characteristic differences. If L 1  is ignited first, its instant impedance will be decreased during ignition, which will cause an overshoot across L 2 . L 1  may be ignited before or after the open circuit condition is detected. If the frequency hopping technique is used, there will be two overshoots across L 2 , one caused by the frequency hopping, and the other caused by the ignition of L 1 . If one of the two discharge lamps opens during normal operation (e.g., L 2  is open), its instant impedance will increase during circuit opening to cause a voltage overshoot across L 2 . After a delay, if the driving circuit detects the opening circuit condition and uses the frequency hopping technique, it will cause another overshoot across L 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a driving circuit for driving a single discharge lamp in accordance with the prior art 
           [0014]      FIG. 2  is a curve showing a relationship between the switching frequency and the voltage gain of the driving circuit in  FIG. 1 . 
           [0015]      FIG. 3  is a diagram illustrating a waveform of the peak output voltage with respect to time during ignition in the driving circuit in  FIG. 1 . 
           [0016]      FIG. 4  is a diagram illustrating a waveform of the peak output voltage with respect to time before and after a lamp opening in the driving circuit in  FIG. 1 . 
           [0017]      FIG. 5  is a block diagram of a driving circuit for driving two serially connected discharge lamps in accordance with the prior art. 
           [0018]      FIG. 6  is a block diagram of a driving circuit for driving discharge lamps in accordance with embodiments of the disclosure. 
           [0019]      FIG. 7  is a block diagram of a driving circuit for driving a single discharge lamp in accordance with embodiments of the disclosure. 
           [0020]      FIG. 8  is a diagram illustrating a waveform of the peak output voltage with respect to time during ignition in the driving circuit in  FIG. 7  in accordance with embodiments of the disclosure. 
           [0021]      FIG. 9  is a block diagram of a driving circuit for driving two serially connected discharge lamps in accordance with embodiments of the disclosure. 
           [0022]      FIG. 10  illustrates a portion of the control circuit in  FIG. 6  in accordance with embodiments of the disclosure. 
           [0023]      FIG. 11  is a block diagram of a driving circuit for driving four discharge lamps in accordance with embodiments of the disclosure. 
           [0024]      FIG. 12  is a flowchart showing a method for driving discharge lamps in accordance with embodiments of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Specific details of several embodiments of the disclosure are described below with reference to driving circuits for driving discharge lamps and associated methods of operation. Moreover, several other embodiments of the converters may have different configurations, components, or procedures than those described in this section. A person of ordinary skill in the art, therefore, will accordingly understand that the converters and the associated methods of operation may have other embodiments with additional elements, or the invention may have other embodiments without several of the elements shown and described below with reference to  FIGS. 6-12 . 
         [0026]      FIG. 6  is a block diagram of a driving circuit for driving discharge lamps in accordance with embodiments of the disclosure. As shown in  FIG. 6 , the driving circuit comprises a switching circuit  101 , a control circuit  102 , a transformer  103 , a resonant circuit  104 , a load  105 , and a status monitoring circuit  606 . The switching circuit  101  comprises at least one switch that receives a DC input voltage V in  and generates a switching signal SW. The switching circuit  101  may be configured in half-bridge, full-bridge, push-pull, and/or other suitable DC/AC topology. 
         [0027]    The control circuit  102  is electrically coupled to the switching circuit  101  and the status monitoring circuit  606 . The control circuit  102  receives an adjustment signal ADJUST from the status monitoring circuit  606  and generates a control signal to control the switching circuit  101 . The control signal can be adjusted to reduce the duty cycle of the switching signal SW when the adjustment signal ADJUST is valid, so as to reduce the output voltage V out  and avoid overshoot. In one embodiment, the duty cycle of the switching signal SW is reduced to one half of its original value when the adjustment signal ADJUST is valid. In other embodiments, the duty cycle of the switching signal SW is reduced to other suitable values when the adjustment signal ADJUST is valid. 
         [0028]    The control circuit  102  can be electrically coupled to the load  105  to sense the electrical parameters of the lamps (such as current, voltage, and/or power) and to generate the control signal accordingly. In one embodiment, the control circuit  102  regulates the lamp current to control the lamp brightness if no open circuit condition is detected, and regulates the lamp voltage to the striking voltage if the open circuit condition is detected. 
         [0029]    The transformer  103  is electrically coupled between the switching circuit  101  and the resonant circuit  104 . The primary winding of the transformer  103  receives the switching signal SW, and the secondary winding generates an AC signal accordingly. The transformer  103  may comprise multiple primary and secondary windings. 
         [0030]    The resonant circuit  104  is electrically coupled between the transformer  103  and the load  105 . The resonant circuit  104  receives the AC signal and generates an output voltage V out  to drive the load  105 . The resonant circuit  104  generally comprises a resonant inductance and a resonant capacitance. The resonant inductance may be a free inductance, or composed of the leakage inductance and/or the excitation inductance of the transformer. The resonant capacitance may be a free capacitance, or composed of the distributed and parasitic capacitance of the discharge lamp. The load  105  may comprise a single discharge lamp or multiple discharge lamps. In one embodiment, the resonant circuit  104  is electrically coupled between the switching circuit  101  and the transformer  103 , while the load  105  is electrically coupled to the transformer  103 . In other embodiments, the load  105  may be suitably connected to other components of the driving circuit. 
         [0031]    The status monitoring circuit  606  is electrically coupled to the load  105  and the control circuit  102 . The status monitoring circuit  606  monitors the working status of the load  105  and generates the adjustment signal ADJUST. The adjustment signal ADJUST is valid when the abnormal working status of the load  105  is detected. In one embodiment, the status monitoring circuit  606  detects whether the open circuit condition exists and validates the adjustment signal ADJUST when the open circuit condition is detected. In another embodiment, the status monitoring circuit  606  detects whether the voltage across the lamp is over-voltage and validates the adjustment signal when the over-voltage condition is detected. In still another embodiment, the status monitoring circuit  606  detects whether the open circuit condition exists and whether the voltage across the lamp is over-voltage. The adjustment signal ADJUST is validated when the open circuit condition or over-voltage condition is detected. In certain embodiments, the control circuit  102  may respond to a valid adjustment signal ADJUST only once, until the normal working status of the discharge lamps resumes. 
         [0032]      FIG. 7  is a block diagram of a driving circuit for driving a single discharge lamp in accordance with embodiments of the disclosure. A frequency hopping method is used in the following description though other suitable methods may also be used. The load  105  comprises a discharge lamp L. The status monitoring circuit  606  comprises a current sensing circuit and a current comparison circuit. The current sensing circuit is electrically coupled to the lamp L to sense the current flowing through the lamp L, and to generate a current sensing signal. The current comparison circuit is electrically coupled to the current sensing circuit and the control circuit  102 . The current comparison circuit compares the current sensing signal with a threshold signal V th  to detect whether an open circuit condition exists. When an open circuit condition is detected, the current comparison circuit validates the adjustment signal ADJUST to let the control circuit  102  reduce the duty cycle of the switching signal SW, so as to at least reduce the overshoot caused by frequency hopping. 
         [0033]    In one embodiment, the current sensing circuit comprises a resistor R s , and the current comparison circuit comprises a comparator COM. The resistor R s  is electrically connected between the lamp L and the ground. The inverting input terminal of the comparator COM is electrically connected to the resistor R s  and the lamp L, while the non-inverting input terminal receives the threshold V th . The output signal of the comparator COM is the adjustment signal ADJUST. When the voltage across the resistor R s  becomes smaller than the threshold V th , indicating that an open circuit condition is detected, the adjustment signal ADJUST is valid (e.g., the rising edge), and the control circuit  102  adjusts the control signal to reduce the duty cycle of the switching signal SW, so as to at least reduce or eliminate the overshoot caused by frequency hopping. 
         [0034]      FIG. 8  is a diagram illustrating a waveform of the peak output voltage with respect to time during ignition in the driving circuit in  FIG. 7  in accordance with embodiments of the disclosure. At t=0, the driving circuit is powered on, the lamp L is not ignited, the open circuit condition is not detected, and the adjustment signal ADJUST is invalid. The switching frequency is the operation frequency f s  and the voltage gain is G 2 . 
         [0035]    During 0&lt;t&lt;t 1 , the duty cycle of the switching signal SW is increased by the control circuit  102 , and the output voltage V out  is increased accordingly. At t=t 1 , the open circuit condition is detected, the adjustment signal ADJUST is valid, the frequency is set to the frequency f s,open , and the voltage gain is G 3 . The control circuit  102  adjusts the control signal to reduce the duty cycle of the switching signal SW. As a result, the overshoot can be at least reduced or even eliminated. Then, the duty cycle of the switching signal SW is increased by the control circuit  102  until the output voltage V out  is regulated to the striking voltage V o,strike . At t=t 2 , the lamp L is ignited, the switching frequency is set to be the operation frequency f s  again, the voltage gain is G 1 , and the output voltage V out  is the operation voltage V o,normal . 
         [0036]      FIG. 9  is a block diagram of a driving circuit for driving two serially connected discharge lamps in accordance with embodiments of the disclosure. Even though a frequency hopping method is used in the following description, in certain embodiments, other suitable methods may also be used. 
         [0037]    The load  105  comprises two serially connected discharge lamps L 1  and L 2 . The status monitoring circuit  606  comprises a current sensing circuit, a current comparison circuit, a voltage sensing circuit, a voltage comparison circuit and a signal processing circuit. The current sensing circuit is electrically coupled to the lamps L 1  and L 2 . The current sensing circuit senses the current flowing through the lamps and generates current sensing signals representative of them. The current comparison circuit is electrically coupled to the current sensing circuit and the signal processing circuit. The current comparison circuit compares the current sensing signals with a threshold voltage V th1  to determine whether an open circuit condition exists. 
         [0038]    The voltage sensing circuit is electrically coupled to the lamps L 1  and L 2 . The voltage sensing circuit senses the voltage across the lamps and generates voltage sensing signals representative of them. The voltage comparison circuit is electrically coupled to the voltage sensing circuit and the signal processing circuit. The voltage comparison circuit compares the voltage sensing signals with a threshold voltage V th2  to determine whether an over-voltage condition exists. The signal processing circuit is electrically coupled to the current comparison circuit and the voltage comparison circuit. The signal processing circuit receives their comparison results and validates the adjustment signal ADJUST when the open circuit or over-voltage condition is detected, and thereby allowing the control circuit  102  to reduce the duty cycle of the switching signal SW, so as to at least reduce or even eliminate any overshoot. 
         [0039]    In one embodiment, the current sensing circuit comprises resistors R s1  and R s2 . The current comparison circuit comprises comparators COM 1  and COM 2 . The voltage sensing circuit comprises capacitors C s11 , C s12 , C s21  and C s22 . The voltage sensing circuit comprises comparators COM 3  and COM 4 , and the signal processing circuit comprises an OR gate U 1 , electrically connected as shown in  FIG. 9 . When the voltage across the resistor R s1  or R s2  becomes smaller than the threshold V th1 , which indicates the open circuit condition exits, or the voltage generated by the voltage divider comprising C s11 , and C s12 , or C s21  and C s22  becomes larger than the threshold V th2 , which indicates an over-voltage condition exists, the adjustment signal ADJUST is set to be valid (such as rising edge). The control circuit  102  accordingly adjusts the control signal to reduce the duty cycle of the switching signal SW. 
         [0040]      FIG. 10  illustrates a portion of the control circuit  102  in  FIG. 6  in accordance with embodiments of the disclosure. The control circuit  102  comprises an amplifier circuit AMP, a selective switch S 1 , and a comparator COM 5 . The amplifier circuit AMP may be any circuit that can realize the signal amplification. The gain of the amplifier circuit AMP is m, wherein m is a positive constant which is smaller than one. In one embodiment, m is 0.5. In other embodiments, m can be 0.75, 0.8, 0.85, or other suitable values. 
         [0041]    In operation, the amplifier circuit AMP receives a CMP signal and generates an amplified CMP signal to one input terminal of the selective switch S 1 . The other input terminal of the selective switch S 1  receives the CMP signal, the output terminal of the selective switch S 1  is electrically connected to the non-inverting input terminal of the comparator COM 5 , the control terminal of the selective switch S 1  is electrically coupled to the status monitoring circuit  606  to receive the adjustment signal ADJUST. 
         [0042]    The inverting input terminal of the comparator COM 5  receives a triangular signal V tri , and the output terminal of the comparator COM 5  outputs a control signal CTRL to control the on and off of the at least one switch in the switching circuit  101 . The amplified CMP signal is transmitted to the non-inverting input terminal of the comparator COM 5  by the selective switch S 1  if the abnormal working status of the discharge lamps is detected (such as the high level period of the adjustment signal ADJUST). The CMP signal is transmitted to the non-inverting input terminal of the comparator COM 5  by the selective switch S 1  if no abnormal working status of the discharge lamps is detected (such as the low level period of the adjustment signal ADJUST). 
         [0043]    The CMP signal may be a predetermined voltage signal, or a signal generated by the control circuit  102  through sensing, comparing, and/or compensating of the electrical parameters of the lamp. In one embodiment, if the open circuit condition is detected, the control circuit  102  senses the voltage across the lamp, compares the voltage sensing signal with a threshold representative of the striking voltage, compensates the comparison signal and uses the compensated signal as the CMP signal. If no open circuit condition is detected, the control circuit  102  senses the current flowing through the lamp, compares the current sensing signal with a threshold representative of the expected lamp current, compensates the comparison signal and uses the compensated signal as the CMP signal. 
         [0044]      FIG. 11  is a block diagram of a driving circuit for driving four discharge lamps in accordance with embodiments of the disclosure. The load  105  comprises four discharge lamps L 3 -L 6 . The transformer  103  comprises two secondary windings, each of which is electrically connected to two serially connected discharge lamps. The status monitoring circuit  606  comprises a current sensing circuit, a current comparison circuit, a voltage sensing circuit, a voltage comparison circuit and a signal processing circuit. 
         [0045]    The current sensing circuit comprises resistors R s3 , R s4 , R s5  and R s6 . The current sensing circuit senses the current flowing through the discharge lamps L 3 -L 6 . The voltage sensing circuit comprises capacitors C s31 , C s32 , C s41 , C s42 , C s51 , C s52 , C s61 , C s62 , every two of which may form a voltage divider to sense the voltage across a discharge lamp. The current comparison circuit comprises diodes D 31 , D 41 , D 51 , D 61 , and comparators COM 1 , COM 2 . The current comparison circuit detects whether the open circuit condition exists. The voltage comparison circuit comprises diodes D 32 , D 42 , D 52 , D 62 , and comparators COM 3 , COM 4 . The voltage comparison circuit detects whether the over-voltage condition exists. The signal processing circuit comprises an OR gate U 1 . The signal processing circuit validates the adjustment signal ADJUST when the open circuit or over-voltage condition is detected. 
         [0046]    The control circuit  102  comprises an amplifier circuit AMP, selective switches S 1  and S 2 , a comparator COM 5 , a voltage loop, a current loop and an open circuit monitoring circuit. The open circuit monitoring circuit is electrically coupled to the current comparison circuit to detect whether the open circuit condition exists. In one embodiment, it comprises an OR gate U 2 . The selective switch S 2  is switched to the voltage loop if the open circuit condition is detected, and switched to the current loop if no open circuit condition is detected. If the working status of the discharge lamps L 3 -L 6  is normal, the CMP signal is transmitted by the selective switch S 1 , else, the amplified CMP signal is transmitted by the selective switch S 1 . 
         [0047]      FIG. 12  is a flowchart showing a method for driving discharge lamps in accordance with embodiments of the disclosure. As shown in  FIG. 12 , the method includes using a switching circuit to generate a switching signal to drive discharge lamps. The method also includes monitoring the working status of the discharge lamps. The method further includes decreasing a duty cycle of the switching signal when the abnormal working status of the discharge lamps is detected. In one embodiment, the abnormal working status of the discharge lamps comprises an open circuit condition. In another embodiment, the abnormal working status of the discharge lamps comprises an over-voltage condition. In still another embodiment, the abnormal working status of the discharge lamps comprises an open circuit or over-voltage condition. In certain embodiments, the duty cycle of the switching signal is decreased to one half of its original value when the abnormal working status of the discharge lamps is detected. In other embodiments, the duty cycle of the switching signal can be decreased to ⅓, ¼, ¾ of its original value or other suitable values when the abnormal working status of the discharge lamps is detected 
         [0048]    From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the invention. For example, many of the elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments. Accordingly, the invention is not limited except as by the appended claims.