Patent Publication Number: US-7719206-B2

Title: Method and system for open lamp protection

Description:
CROSS-REFERENCE OF RELATED APPLICATION(S) 
   The present application is a continuation of U.S. Pat. Ser. No. 11/303,329, filed Dec. 15, 2005, entitled METHOD AND SYSTEM FOR OPEN LAMP PROTECTION, which is incorporated herein by reference in its entirety. 

   TECHNICAL FIELD 
   The present invention relates to the driving of fluorescent lamps, and more particularly, to methods and protection schemes for driving cold cathode fluorescent lamps (CCFL), external electrode fluorescent lamps (EEFL), and flat fluorescent lamps (FFL). 
   BACKGROUND OF INVENTION 
   Open lamp voltage schemes are often required in cold cathode fluorescent lamp (CCFL) inverter applications for safety and reliability reasons. In an open lamp condition, there might be a very large undesirable voltage occurring across the outputs if protections are not in place. This undesirable voltage may be several times higher than a nominal output and could be harmful to circuit components. 
   A conventional method to achieve open lamp voltage protection is to monitor the lamp current. The method is shown in  FIG. 1  for in-phase applications and in  FIG. 2  for out-of-phase applications. When lamp current becomes zero, the open lamp protection is triggered. In the open lamp protection circuits shown, an extra diode is needed for every lamp. Also, the open lamp detection circuit and the lamp voltage feedback circuit are independent. This results in undesired complexity of the overall circuit and associated high costs. A simpler open lamp protection method and circuit is needed. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The following figures illustrate embodiments of the invention. These figures and embodiments provide examples of the invention and they are non-limiting and non-exhaustive. 
       FIG. 1  An open lamp detection circuit for in-phase applications. 
       FIG. 2  An open lamp detection circuit for out-of-phase applications. 
       FIG. 3  Gain curves of a CCFL inverter. 
       FIG. 4  The phase relationship between lamp voltage V c  and excitation voltage V in  under normal operation condition. 
       FIG. 5  The phase relationship between lamp voltage V c  and excitation voltage V in  under open lamp condition. 
       FIG. 6  An open lamp protection method using the phase relationship between lamp voltage and excitation voltage. 
       FIG. 7  An open lamp protection circuit in single lamp application. 
       FIG. 8  Waveforms of dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 7  under normal operation condition. 
       FIG. 9  Waveforms of dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 7  under open lamp condition. 
       FIG. 10  An open lamp protection circuit in 4-lamp in-phase application. 
       FIG. 11  Waveforms of V c , dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 10  under normal operation condition. 
       FIG. 12  Waveforms of V c , dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 10  under open lamp condition. 
   

   DETAILED DESCRIPTION 
   Embodiments of a system and method that uses logic and discrete components to achieve open lamp voltage protection are described in detail herein. In the following description, some specific details, such as example circuits and example values for these circuit components, are included to provide a thorough understanding of embodiments of the invention. One skilled in relevant art will recognize, however, that the invention can be practiced without one or more specific details, or with other methods, components, materials, etc. 
   The following embodiments and aspects are illustrated in conjunction with systems, circuits, and methods that are meant to be exemplary and illustrative. In various embodiments, the above problem has been reduced or eliminated, while other embodiments are directed to other improvements. 
   The present invention relates to circuits and methods of open lamp voltage protection in discharge lamp applications. The circuits detect open lamp condition and trigger an open lamp protection process by monitoring the phase relationship between the lamp voltage and the excitation voltage that includes the voltage across the transformer. 
     FIG. 3  shows gain curves of a typical CCFL inverter. Under normal operation, the inverter works with a switching frequency f s , which is close to a resonant frequency f r  in the inductive region of the bottom gain curve. Under an open lamp condition, the inverter works with f s  in the capacitive region of the top gain curve. A CCFL lamp circuit under normal operation is plotted in  FIG. 4(   a ). As indicated in the circuit, the input current i L  and the excitation voltage V in  are almost in phase. Further, the phase of the lamp voltage V c  lags compared to the phase of V in . The relationship between i L , V in , the inductor voltage V L , and V c  under normal operation is illustrated in the vector diagram of  FIG. 4(   b ). 
   The CCFL lamp circuit under an open lamp condition is shown schematically in  FIG. 5(   a ). As indicated in the circuit, i L  and V in  have almost 90 degrees phase difference. And V c  and V in  are almost in phase. The relationship between i L , V in , V L , and V c  under open lamp condition is illustrated in the vector diagram of  FIG. 5(   b ). As seen, there is a significantly different phase relationship between V c  and V in  under normal operation and open lamp condition. In accordance to one embodiment of this invention, the phase difference between V c  and V in  is monitored and used for open lamp protection. The phase difference is used to trigger an open lamp protection process. When the open lamp protection process is triggered, the circuit increases the switching frequency f s  hence the gain of lamp voltage. If the open lamp condition persists after a predetermined waiting time, the circuit shuts down immediately to prevent a potential over-voltage and damages to electronic components. Note that since the gate voltage of the power device has the same phase as that of V in  in some applications, the phase difference between gate voltage and V c  can also be used for open lamp protection. The power device is the one or more power transistors used to invert the DC power source into AC power for transmission into a transformer. Furthermore, the comparison between gate voltage and V c  can be done on the integrated circuit level. 
   One method for monitoring the phase difference between V c  and V in  is illustrated in  FIG. 6 . The slew rate of the lamp voltage dV c /dt is calculated and obtained. There is a detection window t W  located in the middle of the V in  pulse. If dV c /dt changes from positive to negative, or vice versa, within t W , the open lamp protection process is triggered. If dV c /dt changes its sign, outside t W , the open lamp protection process will not be triggered. An embodiment of this invention for a single lamp application is shown in  FIG. 7 . The sensed lamp voltage, V c , is coupled to a differential circuit, which comprises a capacitor and a grounded resistor. The output of the differential circuit, dV c /dt, is coupled to the negative terminal of a comparator whose positive terminal is coupled to ground or a threshold voltage V th . The output of the comparator, V comp , is coupled to an input terminal of an AND gate and a voltage source V cc  through a resistor. The other input terminal of the AND gate is coupled to V center , which is generated by a triangular waveform and a DC level. V center  represents the middle portion of V in . Since the triangular waveform is also used to generate the duty cycle of the discharge lamp inverter, the phase of the pulse is exactly the same as that of V in . The DC level is used to adjust the width of t W . 
     FIG. 8  shows the waveforms of dVc/dt, Vcomp, Vcenter, and Vout in the circuit of  FIG. 7  under normal operation condition. Under normal condition, dV c /dt changes its sign outside t W . The comparator compares dV c /dt and zero voltage to generate the pulse V comp , which is also outside V center . The output of the AND gate, V out , is always low and open lamp protection process is not triggered.  FIG. 9  shows the waveforms of dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 7  under open lamp condition. When an open lamp condition occurs, dV c /dt changes its sign within V center  and V comp  overlaps with V center . A pulse is generated in every cycle to trigger the open lamp protection process. 
   Another embodiment of this invention is shown in  FIG. 10  for multiple lamp applications. For simplicity of discussion, a 4-lamp in-phase application is discussed. Each sensed lamp voltage, V c1  to V c4 , is coupled to the input terminal of a differential circuit through its corresponding diode, D 1  to D 4 . All diodes have an OR gate configuration so that the input signal V c  for the differential circuit follows the largest Vci value, wherein i is between 1 and 4. Like in a single-lamp application, V c  is coupled to a capacitor and a grounded resistor. The output of the differential circuit, dV c /dt, is coupled to the negative terminal of a comparator while the positive terminal of the comparator is coupled to ground or a threshold voltage V th . The output of the comparator, V comp , is coupled to an input terminal of an AND gate and a voltage source V cc  through a resistor. The other input terminal of the AND gate is couple to V center , which is generated by a triangular waveform and a DC level. V center  represents the middle portion of V in . Since the triangular waveform is also used to generate the duty cycle of the discharge lamp inverter, the phase of the pulse is exactly the same as that of V in . The DC level is used to adjust the width of t W .  FIG. 11  shows the waveforms of dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 10  under normal operation condition. Under normal operation condition, dV c /dt changes its sign outside t W . The comparator compares dV c /dt and zero voltage to generate the pulse V comp , which is also outside V center . The output of the AND gate, V out , is always low and open lamp protection process is not triggered.  FIG. 12  shows the waveforms of dV c /dt, V comp , V center , and V out  in the circuit of  FIG. 10  under open lamp condition. When one or more lamps are open, there are two peaks in each waveform cycle of V c . The higher peak is from the sensed voltage from opened lamps while the lower peak is from lamps under normal condition. The slew rate dV c /dt changes its sign within V center  and V comp  overlaps with V center . A pulse is generated in every cycle to trigger the open lamp protection process. 
   In one embodiment of the present invention, a detection circuit is used to monitor the phase relationship between the lamp voltage V c  and the excitation voltage V in  in a single-lamp or multiple-lamp system, and trigger the open lamp protection process when one or more lamps are open. Under normal operation condition, the phase difference between V c  and V in  is large, typical more than 30 degrees; while under open lamp condition, the phase difference is close to zero degrees. In another embodiment of the present invention, the detection circuit calculates the slew rate of the sensed lamp voltage dV c /dt and compares it with a detection window t W  which is located in the middle of V in  pulse. If dV c /dt changes from positive to negative, or vice versa, within t W , the open lamp protection process is triggered. If dV c /dt changes its sign, outside t W , the open lamp protection process will not be triggered. One advantage of the present invention is that the lamp current detection circuit is not needed. The detection circuit can be incorporated into a lamp voltage feedback circuit to monitor and trigger the open lamp protection. Also, the detection circuit can be implemented on the integrated circuit level with less cost and circuitry complexity. 
   The description of the invention and its applications as set forth herein is illustrative open lamp voltage protection and is not intended to limit the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible, and practical alternatives to and equivalents of the various elements of the embodiments are known to those of ordinary skill in the art. Other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.