Abstract:
A device and method for providing a temperature compensated LED constant current source. A positive temperature coefficient (PTC) resistor is utilized to reduce the current being supplied through a driver circuit to an LED lighting element under varying environmental conditions. The PTC resistor prevents the LED elements from overheating due to excessive ambient temperature, improper thermal design or excessive voltage. A voltage regulator may be included to linearize the temperature vs. resistance characteristics of the PTC resistor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to lighting control systems for LED lighting applications. 
       BACKGROUND 
       [0002]    The use of light emitting diode (LED) based lighting devices has become prevalent in recent years. LED lighting offers a number of advantages, including increased life and high efficiency. However, the LED elements can also be sensitive to extreme voltage or ambient temperature fluctuations. 
         [0003]    The LED elements are often powered using a driver circuit, such as a linear constant current source. Such current sources typically use a transistor to dissipate any voltage (and power) beyond that needed to safely drive the LED elements. This allows for voltage fluctuations beyond a reference value while still maintaining a constant light output. 
         [0004]    The amount of temperature rise that a constant current biased light source experiences is a function of the ambient temperature around the lamp, the amount of applied voltage, and ability of the package to dissipate excess heat and power. The lighting device supplier can specify the parameters under which an LED lamp should be operated, however the end user may utilize the device under conditions which are outside those specified by the manufacturer. Without proper protection, the temperature of the lighting elements may increase beyond their rated temperature causing uneven light output or even component failure. 
         [0005]    One disadvantage to prior art constant current integrated circuit designs is the lack of user controlled thermal pullback with discrete components. Thermal pullback refers to the gradual reduction of current through a circuit over time, thereby preventing excessive heat buildup in the lighting elements. Voltage regulators can be used to control the voltage and/or current applied to the lighting elements. However, most commercially available regulators do not initiate thermal pullback until the ambient temperature has reached about 160 degrees Celsius. At this point, the temperature of the LEDs has typically exceeded the recommended threshold. Other types of regulators will strobe the LEDs on and off or even shut the LEDs off completely in an over temperature situation. Still other designs incorporate discrete temperature sensors which activate a switch when a threshold temperature is reached, thereby causing a drastic decrease in light output. This can be unacceptable from both a safety and user experience standpoint, especially in automotive or other industrial applications. 
         [0006]    A solution is needed that will gradually reduce the power to the lighting elements as the temperature increases, preferably slow enough that a user will not notice an abrupt change in light output. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, according to one embodiment, an electrical circuit for driving at least one light emitting diode (LED) is disclosed, comprising: a transistor connected in the current path of the light emitting diode, said transistor comprising a control terminal and at least one load terminal; and a PTC resistor connected between a supply voltage source and the control terminal of the transistor; wherein the at least one load terminal of the transistor is connected in the current path of the light emitting diode; and wherein the PTC resistor restricts the current supplied to the control terminal of the transistor as the temperature of the PTC resistor increases; and wherein the transistor restricts the current in the current path of the light emitting diode as the current supplied to the control terminal of the transistor decreases. 
         [0008]    According to another embodiment, an electrical circuit for driving at least one light emitting diode is disclosed, comprising: a transistor connected in the current path of the light emitting diode, said transistor comprising a control terminal and at least one load terminal; a PTC resistor connected between a supply voltage source and the control terminal of the transistor; and a regulator connected to the control terminal of the transistor; and a load resistor connected in the current path of the light emitting diode and the at least one transistor load terminal; wherein the at least one load terminal of the transistor is connected in the current path of the light emitting diode; wherein the PTC resistor restricts the current supplied to the control terminal of the transistor as the temperature of the PTC resistor increases; wherein the transistor restricts the current in the current path of the light emitting diode as the current supplied to the control terminal of the transistor decreases; wherein a reference terminal of the regulator is further connected across the load resistor; and wherein the regulator is configured to compensate for the non-linear characteristics of the PTC resistor by increasing or decreasing the current supplied to the control terminal of the transistor below a specified temperature. 
         [0009]    According to another embodiment, a method for driving at least one light emitting diode is disclosed, comprising the steps of: varying the current being supplied to a control terminal of a transistor in the current path of the light emitting diode; wherein the current is varied due to the varying resistance of a PTC resistor connected between a voltage source and the control terminal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram depicting a temperature compensated LED constant current source circuit according to one embodiment of the present disclosure. 
           [0011]      FIG. 2  is a graph showing the non-linear temperature vs. resistance characteristics of a typical PTC resistor. 
           [0012]      FIG. 3  is a schematic diagram depicting a temperature compensated LED constant current source circuit according to another embodiment of the present disclosure 
           [0013]      FIG. 4  is a schematic diagram depicting a temperature compensated LED constant current source circuit according to yet another embodiment of the present disclosure 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    For the purposes of promoting and understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. The present invention can be implemented with various mixtures of analog and digital circuitry. 
         [0015]      FIG. 1  illustrates a schematic diagram of a circuit  100  for driving an LED  105  from external voltage supply  115 . As shown, circuit  100  comprises an LED  105 , transistor  135 , PTC resistor  140 , and resistor  160 . Regulator  150  may be optionally included as described hereinbelow. 
         [0016]    LED  105  may comprise any type of LED known in the art for use in lighting applications. It shall be understood that while a single LED element is depicted, LED  105  may comprise additional LED elements connected in series or in parallel as required by the particular application. 
         [0017]    Referring to  FIG. 1 , positive terminal  110  of voltage supply  115  is connected to the anode  120  of LED  105 . The cathode  125  of LED  105  is connected to the collector  130  of the transistor  135 , shown here as an NPN bipolar junction transistor. A positive temperature coefficient (PTC) resistor  140  is connected between the positive terminal  110  of the voltage supply  115  and the cathode  145  of regulator  150  as shown. The base  155  of the transistor  135  is connected to the PTC resistor  140  and the cathode  145  of the regulator  150 . Resistor  160  is connected between the emitter  165  of transistor  135  and ground  170 . The reference terminal  175  of regulator  150  is connected to the emitter  165  of the transistor  135  and resistor  160  as shown. The anode  180  of regulator  150  is connected to ground  170 . 
         [0018]    Transistor  135  is illustrated here as an NPN bipolar junction transistor, although it shall be understood that other types of linear control components may be used including, but not limited to, PNP bipolar transistors, insulated gate bipolar transistors (IGBT), junction gate field-effect transistors (JFET), and metal-oxide-semiconductor field-effect transistors (MOSFET), or any other device or circuit (discrete or integrated) that allows for linear control of the current through the LED. 
         [0019]    In a preferred embodiment, PTC resistor  140  comprises any resistor having a positive temperature coefficient, thereby increasing its resistance as a function of increasing temperature. Resistor  160  is a standard resistor, with a value selected according to the needs of the particular application. In certain embodiments, resistor  160  may be replaced by an LED whose resistance is selected to match the resistance required to produce the desired current through the transistor  135  and LED  105 . In other embodiments, the LED  105  may be omitted when the resistor  160  is implemented as an LED. In still further embodiments, the current through the LED  105  may be controlled from a separate circuit. 
         [0020]    Regulator  150  may comprise a voltage regulation device, such as a shunt regulator based on a zener diode, although other types of active or passive voltage regulation devices may be used and are considered to be within the scope of the present disclosure. One example of an acceptable regulator is the LM431 Shunt Zener Regulator supplied by National Semiconductor, 2900 Semiconductor Drive, Santa Clara, Calif. 95051. 
         [0021]    In operation, current is initially drawn from the voltage source  115 , through PTC resistor  140 , and into the control terminal of the transistor  135  (illustrated here as base  155 ). The amount of current entering the base  155  is initially determined by the resistance value of the PTC resistor  140 . Transistor  135  then allows current to flow from the voltage source  115 , through the LED  105 , through the load terminals (collector  130  and emitter  165 ) of the transistor  135 , and through the resistor  160  to ground  170 . 
         [0022]    When PTC resistor  140  heats up, it resistance will increase, thereby decreasing the current flowing into the base  155 . It is a known characteristic of an NPN transistor, if manufactured as a bipolar transistor, that current flowing out of the emitter (I e ) is approximately equal to the current flowing into the collector (I c ) plus the current flowing into the base (I b ), as defined by equation (1) below. 
         [0000]        I   e   =I   c   +I   b   (1)
 
         [0023]    Furthermore, the collector current (I c ) is proportional to the base current (I b ) based on a gain value β as defined by equation (2) below. 
         [0000]        I   c   =βI   b   (2)
 
         [0024]    The value of β may vary based on the characteristics of the particular transistor being used, but is typically in the range of 10 to 100. Therefore, equations (1) and (2) may be combined to arrive at equation (3) below. 
         [0000]        I   e   =I   c +( I   c /β)  (3)
 
         [0025]    Because the value of β is high, the value of I e  will roughly approximate the value of I c . Therefore, as the base current is restricted due to the increasing resistance of PTC resistor  140 , the collector and emitter current values will also decrease. 
         [0026]    One drawback to using the PTC resistor  140  as the sole thermal control and current limiting device is the non-linear relationship between the resistance and temperature of a PTC resistor, particularly in the lower temperature range.  FIG. 2  shows a resistance vs. temperature curve of a typical PTC resistor. Note also that the graph of  FIG. 2  is displayed with a logarithmic vertical scale due to the large resistance changes which occur in the PTC resistor as a result of corresponding temperature changes. As can be seen from the graph, the PTC resistor only exhibits the rated positive temperature coefficient above a specified temperature (shown here as T Rmin ). Below T Rmin , the PTC resistor will actually exhibit a negative temperature coefficient, which would cause the unintended effect of decreasing the light output of LED  105  with decreasing temperature. This is particularly problematic in applications where a constant light output must be maintained despite large fluctuations in operating temperatures. 
         [0027]    To overcome the non-linear characteristics of the PTC resistor  140  in the lower temperature range, regulator  150  is used in combination with PTC resistor  140 . The voltage drop across the resistor  160  (and thereby the current flowing through the resistor  160 ) is sensed by regulator  150  via reference terminal  175 . Regulator  150  uses this voltage as the feedback for controlling the current being allowed to reach base  155 . 
         [0028]    In a preferred embodiment, the regulator  150  has a factory-set reference voltage that the regulator  150  will attempt to maintain across the resistor  160 , with resistor  160  being sized to result in the desired current through the LED  105 . The regulator  150  achieves this regulation by increasing or decreasing the current allowed into the cathode  145  and out the anode  180  as needed in order to maintain the specified reference voltage across the resistor  160 . As long as the resistance of PTC resistor  140  does not increase to the point that the transistor  135  cannot drive the proper collector current, the regulator  150  will keep the current through the resistor  160  (and similarly, the current through the LED  105 ) constant in the temperature range below T ref . This in turn has the effect of maintaining a constant light output until the temperature increases beyond T ref . The design characteristics of the transistor  135  and the PTC resistor  140  must be selected such that the base current to the transistor  135  is sufficiently supplied as long as the resistance of the PTC resistor  140  is below R ref  as shown in  FIG. 2 . 
         [0029]    When the operating temperature surpasses T ref  (thereby causing the resistance of the PTC resistor  140  to surpasses R ref ), the regulator  150  will no longer shunt additional excess current to ground  170 . This will allow the current through the LED  105  to gradually be reduced until a thermal equilibrium is met. The decrease in light output will be gradual due to the characteristics of the PTC resistor  140 . Because the human eye does not respond to gradual changes in light when compared to drastic changes, the user may not notice the decrease as the current settles to the equilibrium. The equilibrium point will be a function of the ambient temperature, the operating voltage and the component package design. 
         [0030]    In order to increase the response of the circuit  100  to changes in the temperature of LED  105 , the PTC resistor  140  may optionally be thermally coupled to LED  105 . In one embodiment, the thermal coupling may be accomplished by placing the PTC resistor  140  and LED  105  sufficiently close to one another and/or within a common casing, causing the temperature of the PTC resistor to approximately equal the operating temperature of LED  105 . In other embodiments, PTC  140  is thermally connected to LED  105  using a metallic conductor or other suitable thermally-conductive medium known in the art. 
         [0031]      FIG. 3  illustrates a schematic diagram of a circuit  300  for driving an LED  305  from external voltage supply  310  according to another embodiment. As shown, circuit  300  comprises an LED  305 , regulator  315 , PTC resistor  320 , resistors  325  and  330 , and capacitors  335  and  340 . 
         [0032]    Regulator  315  is similar to a model LT3080 integrated linear regulator supplied by Linear Technology, 1630 McCarthy Blvd., Milpitas, Calif. 95035-7417. As shown, integrated regulator  315  includes input terminal  355 , output terminal  360 , control terminal  365 , and set terminal  370 . The regulator  315  comprises an integrated transistor and differential amplifier circuit, the details of which are omitted for simplicity. 
         [0033]    Referring to  FIG. 3 , positive terminal  375  of voltage supply  310  is connected to the anode  380  of LED  305 . The cathode  385  of LED  305  is connected the input terminal  355  of regulator  315 . A PTC resistor  320  is connected between the positive terminal  375  of voltage supply  310  and the control terminal  365  of the regulator  315  as shown. 
         [0034]    Resistor  325  is connected between the output terminal  360  of regulator  315  and ground  345 . The value of resistor  325  is selected to provide the desired amount of current through the LED  380  for a given current being received at the control terminal  365 . Resistor  330  is connected between the set terminal  370  of regulator  315  and ground  345 . The value of the resistor  330  is selected in order to provide the proper output voltage across resistor  325  according to the needs of the particular application. 
         [0035]    Capacitor  335  is optionally connected between output terminal  360  of regulator  315  and ground  345  to provide filtering and prevent unwanted oscillations in output. Likewise, capacitor  340  is optionally connected between the input terminal  355  of regulator  315  and ground  345 . 
         [0036]    In operation, the current directed to the control terminal  365  biases an internal transistor (not shown) of the regulator  315 . As the temperature increases, the resistance of the PTC resistor  320  also increases, allowing less current to reach the control terminal  365  of the regulator  315 . The regulator  315  accordingly reduces the current being directed through the LED  305  and resistor  325 , while maintaining the desired linear response as discussed hereinabove. 
         [0037]      FIG. 4  illustrates a schematic diagram of a circuit  400  for driving an LED  405  from external voltage supply  410  using a current source arrangement according to another embodiment. As shown, circuit  400  comprises an LED  405 , PTC resistor  415 , operational amplifier  420 , and transistor  430  (shown here as a metal oxide semiconductor field effect transistor, or MOSFET, although other transistor configurations may be used). PTC resistor  415  is connected between the positive terminal  425  of voltage supply  410  and the positive supply terminal  435  of the operational amplifier  420  as shown. The LED  405  is connected between the positive terminal  425  of the voltage supply  410  and the drain terminal  440  of the transistor  430 . 
         [0038]    Resistor  445  is connected between the positive supply terminal  425  of voltage supply  410  and the non-inverting input terminal  455  of operational amplifier  420 . Resistor  450  is connected between the non-inverting input terminal  455  of operational amplifier  420  and ground  460 . The inverting input  465  of differential amplifier  420  is connected to the source terminal  470  of transistor  430  in a feedback configuration. The output terminal  475  of operational amplifier  420  is connected to the gate terminal  480  of operational amplifier  430 . The negative source terminal  485  of operational amplifier  420  is connected to ground  460 . Resistor  490  is connected between the output terminal  475  of operational amplifier  420  and ground  460 . Resistor  495  is connected between the source terminal  470  of transistor  430  and ground  460 . 
         [0039]    The non-inverting input terminal  455  of operational amplifier  420  senses a reference voltage across the resistor  450 , while the non-inverting input works as a feedback input to sense the voltage across the resistor  495 . The operational amplifier  420  uses the voltage differential between terminal  455  and terminal  465  to determine the gain required to maintain a linear response at the output terminal  475 . As the temperature increases, however, the PTC resistor  415  will increase in resistance, thereby limiting the supply voltage available to the operational amplifier  420 . This further reduces the current being directed to the gate  480  of transistor  430  and reduces the current through the LED  405 . Eventually, as the temperature decreases due to the reduced current through the LED  405 , a thermal equilibrium will be reached. 
         [0040]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all equivalents, changes, and modifications that come within the spirit of the inventions as described herein and/or by the following claims are desired to be protected. 
         [0041]    Hence, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification.