Patent Publication Number: US-8536933-B2

Title: Method and circuit for an operating area limiter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/045,588, filed Mar. 10, 2008 entitled “Method and Circuit for an Operating Area Limiter”, the contents of which are incorporated herein by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to constant current sources, and more particularly, to controlling the operating area of a transistor used in constant current sources such as those used in light emitting diode (“LED”) strings for backlighting electronic displays. 
     BACKGROUND OF THE INVENTION 
     Backlights are used to illuminate liquid crystal displays (“LCDs”). LCDs with backlights are used in small displays for cell phones and personal digital assistants (“PDAs”) as well as in large displays for computer monitors and televisions. Often, the light source for the backlight includes one or more cold cathode fluorescent lamps (“CCFLs”). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (“ELP”), or one or more hot cathode fluorescent lamps (“HCFLs”). 
     The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, they require adequate idle time to ignite, and they require delicate handling. Moreover, LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. Because of this, displays with LED backlights can consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display. 
     Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings.  FIG. 1  shows an exemplary flat panel display  10  with a backlighting system having three independent strings of LEDs  1 ,  2  and  3 . The first string of LEDs  1  includes seven LEDs  4 ,  5 ,  6 ,  7 ,  8 ,  9  and  11  discretely scattered across the display  10  and connected in series. The first string  1  is controlled by the drive circuit  12 . The second string  2  is controlled by the drive circuit  13  and the third string  3  is controlled by the drive circuit  14 . The LEDs of the LED strings  1 ,  2  and  3  can be connected in series by wires, traces or other connecting elements. 
       FIG. 2  shows another exemplary flat panel display  20  with a backlighting system having three independent strings of LEDs  21 ,  22  and  23 . In this embodiment, the strings  21 ,  22  and  23  are arranged in a vertical fashion. The three strings  21 ,  22  and  23  are parallel to each other. The first string  21  includes seven LEDs  24 ,  25 ,  26 ,  27 ,  28 ,  29  and  31  connected in series, and is controlled by the drive circuit, or driver,  32 . The second string  22  is controlled by the drive circuit  33  and the third string  23  is controlled by the drive circuit  34 . One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration. 
     An important feature for displays is the ability to control the brightness. In LCDs, the brightness is controlled by changing the intensity of the backlight. The intensity of an LED, or luminosity, is a function of the current flowing through the LED.  FIG. 3  shows a representative plot of luminous intensity as a function of forward current for an LED. As the current in the LED increases, the intensity of the light produced by the LED increases. 
     To generate a stable current, circuits for driving LEDs use constant current sources.  FIG. 4  is a representation of a circuit used to generate a constant current. A constant current source is a source that maintains current at a constant level irrespective of changes in the drive voltage V SET . Constant current sources are used in a wide variety of applications; the description of applications of constant current sources as used in LED arrays is only illustrative. The operational amplifier  40  of  FIG. 4  has a non-inverting input  41 , an inverting input  42 , and an output  43 . To create a constant current source, the output of the amplifier  40  may be connected to the gate of a transistor  44 . The transistor  44  is shown in  FIG. 4  as a field effect transistors (“FET”), but other types of transistors may be used as well. Examples of transistors include IGBTs, nMOS devices, JFETs and bipolar devices. The drain of the transistor is connected to the load  45 , which in  FIG. 4  is an array of LEDs. The inverting input of the amplifier  40  is connected to the source of the transistor  44 . The source of the transistor  44  is also connected to ground through a sensing resistor R S    46 . When a reference voltage, is applied to the non-inverting input of the amplifier  40 , the amplifier increases the output voltage until the voltage at the inverting input matches the voltage at the non-inverting input. As the voltage at the output of the amplifier  40  increases, the voltage at the gate of the transistor  44  increases. As the voltage at the gate of the transistor  44  increases, the current from the drain to the source of the transistor  44  increases. 
     For an LED backlit display to operate at a given brightness, the current in the drain current of the transistor  44  must be maintained at a set level: the design current. The design current may be a fixed value or it may change depending upon the brightness settings of the display. 
       FIG. 5  illustrates a typical relationship between the drain current and the gate voltage for an exemplary transistor. Since little to no current flows into the inverting input of the amplifier  40 , the increased current passes through the sensing resistor R S . As the current across the sensing resistor R S  increases, the voltage drop across the sensing resistor also increases according to Ohm&#39;s law: voltage drop (V)=current (i)*resistance (R). This process continues until the voltage at the inverting input of the amplifier  40  equals the voltage at the non-inverting input. If, however, the voltage at the inverting input is higher than that at the non-inverting input, the voltage at the output of the amplifier  40  decreases. That in turn decreases the source voltage of the transistor  44  and hence decreases the current that passes from the drain to the source of the transistor  44 . Therefore, the circuit of  FIG. 4  keeps the voltage at the inverting input and the source side of the transistor  44  equal to the voltage applied to the non-inverting input of the amplifier  40  irrespective of changes in the drive voltage V SET . 
     Large displays with LED backlights use multiple constant current sources like that of  FIG. 4 . Therefore, large LED-backlit displays use many transistors  44 . Transistors are limited in the maximum drain-to-source voltage and drain current that the transistor can safely handle. Curves that show a transistor&#39;s limitations of simultaneous high voltage and high current, up to the rating of the device, are often provided to circuit designers by transistor manufacturers. These curves are generally known as safe operating area curves. The safe operating area (“SOA”) of the transistor is the area below the curve. An example of an SOA curve is shown in  FIG. 6 . 
       FIG. 6  illustrates a SOA curves for two different operating conditions: continuous current mode  60  and discontinuous pulse current mode  61 . Multiple SOA curves for discontinuous pulse current modes  61  based upon the relative pulse duration are generally provided by the transistor manufacturer. For a given forward drain current, the SOA curve instructs circuit designers on the maximum drain-to-source voltage that the transistor can safely handle. For example, at the continuous drain current  62  in  FIG. 6 , the maximum safe drain-to source voltage  63  for the transistor is determined from the SOA curve. If the maximum safe drain-to-source voltage  63  is exceeded at the drain current  62  shown, the transistor is at risk of failure or degradation. Therefore, circuit designers must ensure the operation of the transistor is within its SOA. 
     To expand the area under the SOA curve for higher maximum drain current ratings, the size of the transistor must be increased. Larger transistors are more expensive and require a larger die size if integrated into a single die or integrated circuit. To extend the area under the SOA curve for higher maximum drain-to-source voltages, an enhanced or more complex fabrication process must be used. Transistors fabricated for larger drain-to-source voltages might not be readily available or cost effective for many designs. To reduce device size and costs, circuit designers often choose the basic minimum-geometry transistor that can safely operate at the design drain-to-source voltage and design drain current. However, this often limits the available overhead room for increased drain-to-source voltage at the design drain current. 
     Occasionally, the drain-to-source voltage of the transistor  44  may unexpectedly increase above the design level. This may happen because of inadvertent over-voltage of the drive voltage V SET  or due to shorting of the load  45 . Shorting of the load  45  can happen for many reasons including foreign material shorting the load path, improper soldering during assembly of the circuit, and damage in the load. When the drain-to-source voltage increases from the design voltage due to a short, it may increase all the way to the drive voltage V SET . When the drain-to-source voltage inadvertently increases at a given drain current, the operating point of the transistor may go beyond the safe operating area. An example of this for a transistor operated in continuous current mode is shown at point  64  in  FIG. 6 . At point  64 , the drain-to-source voltage has increased to the drive voltage V SET . The drain current is at the design current  62 . Since the operating condition  64  of the transistor is outside of the safe operating area, the transistor has a high probability of immediate failure or degradation. If a transistor fails or degrades, the current source will no longer function properly. Transistor failure or degradation causes safety and reliability problems and therefore increases recall and warranty costs for device manufacturers. 
     For a circuit that could safely operate at the design current  62  and drain-to-source voltage V SET , circuit designers would have to use a much larger transistor with a SOA that encompassed the point defined by the design current  62  and drain-to-source voltage V SET . A larger transistor would be more expensive and more difficult to integrate into a device designed to be integrated into a single chip. 
     SUMMARY OF THE INVENTION 
     The present invention relates to circuits and methods for limiting the operating area of a transistor in a constant current source circuit. The circuits and methods use a detector and a driver to limit the operating area of a transistor. The detector and driver have parameters selected so that, when the voltage at the drain of the transistor satisfies a reference condition, the driver causes drain current of the transistor to decrease. The reference condition is determined relative to the maximum safe drain-to-source voltage at the design drain current of the constant current source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  illustrates an exemplary display implementing LED strings; 
         FIG. 2  illustrates another exemplary display implementing LED strings; 
         FIG. 3  illustrates a graph showing the relationship between current and luminous intensity in an LED; 
         FIG. 4  illustrates a prior art technique for providing constant current source; 
         FIG. 5  illustrates a graph showing the relationship between gate voltage and source current in a transistor; and 
         FIG. 6  illustrates a safe operating curve for a transistor in continuous and discontinuous pulse current modes. 
         FIG. 7  illustrates an exemplary embodiment of the operating area limiter of the present invention. 
         FIG. 8  illustrates an exemplary embodiment of the operating area limiter of the present invention. 
         FIG. 9  illustrates an exemplary embodiment of the operating area limiter of the present invention. 
         FIG. 10  illustrates an exemplary embodiment of the operating area limiter of the present invention. 
         FIG. 11  illustrates an exemplary embodiment of the operating area limiter of the present invention. 
         FIG. 12  illustrates the effect of an exemplary embodiment of the operating area limiter of the present invention on drain current of a transistor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The methods and circuits of the present invention relate to the regulation of the operating area of a transistor. The constant current sources described may be used in LED strings of the backlights of electronic displays or they may be used to drive any electronics load. The methods and circuits of the present invention prevent the degradation and failure of transistors by preventing the drain-to-source voltage and drain current of the transistor from exceeding the safe operating area of the transistor. 
       FIG. 7  shows an exemplary example of the operating area limiter  700  of the present invention. The exemplary circuit of the present invention  700  limits the operating area of a transistor  730  like the one used in the constant current source  710 . The transistor  730  of the constant current source has a drain, a source, and a gate terminal. The operating area limiter circuit  700  uses a detector  740  to detect changes in the voltage at the drain of transistor  730  and a driver  760  to control the drain current of the transistor  730 . The drain-to-source voltage of the transistor  730  is a function of the drain voltage because the drain voltage of the transistor  730  equals the drain-to-source voltage minus the drain current times the resistance of the sensing resistor R S . 
     The connection of the detector  740  to the drain of the transistor  730 , as well as other connections described herein may be direct or indirect. Connections may be electronic, electromagnetic, electrooptical, mechanical, or any mixture of the above. 
     The detector  740  and the driver  760  are designed and configured so that the driver reduces the drain current of the transistor  730  when the drain voltage of the transistor  730  satisfies a reference condition as determined by the detector  740 . The reference condition is determined by the maximum safe drain-to-source voltage at the design drain current of the constant current source. The reference condition may be a maximum drain voltage set relative to the maximum safe drain-to-source voltage at the design drain current of the transistor  730 . The reference condition may also include durational limits so that the reference condition is not satisfied unless the drain voltage achieves a certain value for a certain amount of time. Moreover, the reference condition may include any combination of magnitude and duration limits. 
     When the voltage at the drain of the transistor  730  satisfies the reference condition, the driver  760  causes the drain current in the transistor  730  to decrease. The decrease in the drain current maintains the operating conditions of the transistor within the safe operating area thereby avoiding failure or degradation of the transistor  730 . 
     As shown in  FIG. 7 , the operating area limiter  700  may include a signal processor  770 . The signal processor  770  may be part of the detector  740  as shown in  FIG. 7  or the signal processor  770  may be a separate component of the operating area limiter  700 . The signal processor  770  may be any combination of digital or analog devices. The signal processor  770  may include latch and hold, de-bounce or de-glitch functions, noise reduction, and/or misfire detection. The purposes of the signal processor  770  include making sure the signal is proper, to tell subsequent devices how and when to react, and to determine reset conditions. For example, if the drain voltage of the transistor  730  fluctuates, intermittently satisfying the reference condition, the output of the detector  740  could also fluctuate. In this situation, the signal processing may include means to hold the output of the detector  740  at a set value. 
     The signal processor  770  may also keep the drain current at a set level until a reset condition is met, even if the drain voltage of the transistor returns to its design level or no longer satisfies the reference condition. The reset signal may result from central or local control in the system of which the operating area limiter is a part. 
     Additional advantages of the operating area limiter set/reset ability are that it allows detection and correction of the fault that caused the high drain voltage and it allows reinitiation of the system without damage to the transistor. For example, in the LED load  780  in  FIG. 7 , when the reference condition is met, the drain current in the transistor  730 , and hence the LED current, is decreased thereby decreasing the light output of the LEDs  780 . The system or a user could detect the reduced light output from the LEDs  780 , correct the problem and then reset the operating area limiter  700 . The drain current in the transistor  730  and the LED  780  current return to the design setting after reset. 
     As shown in  FIG. 8 , the detector  840  of the operating area limiter  800  may include a comparator  841 . In  FIG. 8 , the voltage of the constant voltage source  842  is determined relative to the maximum safe drain-to-source voltage at the design drain current of the constant current source. The comparator  841  compares the voltage at the drain of the transistor  830  to the voltage of the constant voltage source  842 . When the voltage at the drain of the transistor  830  exceeds a set value relative to the voltage of the constant voltage source, the output of the comparator  841  causes the driver  860  to decrease the drain current in the transistor  830 . The decrease in the drain current maintains the operating conditions of the transistor within the safe operating area thereby avoiding degradation of the transistor  830 . 
     The driver  760  of the operating area limiter  700  may cause the drain current of the transistor  730  to decrease by any of a number of possible means. As shown in  FIG. 8 , the driver  860  may decrease the drain current of the transistor  830  by decreasing the reference voltage  820  of the constant current source  810 . The driver may include a variable voltage source  861  to reduce the reference voltage  820  of the constant current source  810 . The reference voltage  820  of the constant current source  810  may be the non-inverting input of an operational amplifier  850  used in the constant current source  810 . 
     Alternatively, as shown in  FIG. 9 , the driver  960  of the operating area limiter circuit  900  may include a switch  961  and a constant current source  962 . When engaged, the switch  961  reduces the resistance of the current path form the constant current source  962  thereby reducing the reference voltage  980  of the constant current source  910 . Another alternative method for reducing the reference voltage  980  is to use a potentiometer or variable resistor to control the resistance of the current path form the constant current source  962 . In that case, the output of the detector  940  controls the resistance of the potentiometer thereby controlling the reference voltage  980 . Alternatively, as shown in  FIG. 10 , the driver  1060  in the operating area limiter  1000  may include a current source  1062  that, when engaged, bleeds off current supplied by the current supply  1061  thereby reducing the reference voltage  1080  of the constant current source  1010 . The detector  1040  controls the changes to the current source  1062  of the driver  1060 . 
     Referring again to  FIG. 7 , the driver  760  may alternatively cause the drain current of the transistor  730  to decrease by increasing the resistance of the sensing resistor R S . The sensing resistor R S  may be a variable resistor or potentiometer with a resistance that changes in response to the output of the detector  740 . The sensing resistor R S  may also be implemented by multiple resistors some of which are only engaged based on the output of the detector  740 . In  FIG. 7 , the sensing resistor R S  is shown as part of the constant current source circuit  710 . In implementations where the drain current of the transistor  730  is controlled by modifying the resistance of the sensing resistor R S , the sensing resistor R S  may also be a part of the operating area limiter circuit  700 . 
     The operating area limiter  700  of the present invention may be implemented using analog devices and circuits. Alternatively, the operating area limiter  1100  may be implemented using digital devices and circuits or a combination of analog and digital devices and circuits as shown in  FIG. 11 . In  FIG. 11 , the output of the detector  1140  controls a multiplexer  1170 . The multiplexer  1170  has an input data bit for normal conditions  1180  and an input data bit for fault conditions  1190 . At normal operating conditions, the multiplexer  1170  passes the input data bit for normal conditions  1180  to the digital-to-analog converter  1120 . A fault condition occurs when the drain-to-source voltage of the transistor  1130  satisfies the reference condition of the detector  1140 . In a fault condition, the multiplexer  1170  passes the input data bit for fault  5  conditions  1190  to a digital-to-analog converter  1120 . When the fault bit  1190  is passed to the digital-to-analog converter  1120 , the output of the converter  1120  is a reduced voltage, which reduces the reference voltage  1150  of the constant current source  1110 . 
     The effect of the exemplary operating area limiter  700  circuit of  FIG. 7  is shown in  FIG. 12 .  FIG. 12  shows the drain-to-source voltage  1210  and drain current  1220  of the transistor  730  as a function of time. Before time T 1  the transistor  730  is operating at its design drain-to-source voltage  1230  and design drain current  1240 . After time T 1 , the drain-to-source voltage  1210  increases. The increase may be due to an inadvertent short or other over-voltage condition as described previously. When the drain-to-source voltage  1210  satisfies the reference condition  1250  at time T 2 , the operating area limiter  700  causes the drain current of the constant current source  710  to be reduced to a level  1260  that will maintain the operating conditions of the transistor  730  within the safe operating area. The drain current may remain at the reduced level  1260  until there is a system or sub-system reset. 
     One of ordinary skill in the art will appreciate that the techniques, structures and methods of the present invention above are exemplary. The present inventions can be implemented in various embodiments without deviating from the scope of the invention.