Abstract:
A constant power drive for light emitting diodes, such that there is automatic compensation for variation in forward voltage of the LED, both in a single unit with temperature, and also due to unit-to-unit variations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to providing constant power to light emitting diodes (LEDs), and more particularly, to eliminating temperature and manufacturing variation effects in the light output of LEDs. 
       BACKGROUND OF THE INVENTION 
       [0002]    An LED consists of a semiconductor junction, which emits light due to a current flowing through the junction. Since the purpose of an LED is to emit light, it is often desirable for this light to be as constant as possible, both during operation of a device and also from unit to unit. Many designers of LED circuits use a constant current circuit for this purpose, because this gives a better regulated amount of light output than driving it with a voltage limited by a resistor. 
         [0003]    However, the constant current drive still has a number of drawbacks. Among the chief of these is that, although the current through the LED is constant, the forward voltage of the junction is not. The light output of the LED is dependent on its input power, and this power depends on both the junction current and the forward voltage. Any variation of forward voltage thus directly results in variation in output light. 
         [0004]    The variation in forward voltage in the LED has two main sources. One is the temperature of the junction. As the LED warms up, its forward voltage decreases, typically 2 to 4 mV/° C., or 0.06 to 0.11%/° C. While this seems small, LED temperatures in normal operation will typically range from 25° C. to at least 85° C., and over this temperature range, the variation in forward voltage can be as much as 6.7%. A variation of this size in light output, when combined with other factors, can be quite undesirable. 
         [0005]    The other main source of variation in forward voltage in LEDs is manufacturing tolerance. A typical white LED may have a forward voltage specified to be between 2.8V and 4.0V. This variation translates directly to a variation in light output when using a constant current drive. As a consequence, LED manufacturers typically bin their parts, typically in 100 mV bins. This can reduce the variation to some 2.8%, but taken together, the two effects may still account for almost a 10% variation of light from unit to unit and from cold to hot. 
         [0006]    One solution to this problem is to measure the forward voltage of the LED and provide a drive such that the product of this forward voltage and the drive current is constant. In practice, however, because the LEDs may not be ground-referenced, it becomes necessary to use expensive components to level shift the forward voltage signal to where it can be used by the control circuit. 
         [0007]    Another partial solution is to measure the temperature of the LED, for example with a thermistor, and use the measurement as a feedback to the control circuit to adjust the drive current. While this concept works in some situations, it can be difficult to implement if the LEDs are not conveniently located. To measure the temperature requires two additional connections from the location of the LEDs for the thermistor, in addition to the two connections required to power the LEDs. Additionally, the control circuit must be configured to accept the input from the thermistor. If the signal is not acceptable, it must be conditioned with additional circuitry, or with a microcontroller. However, this method does not compensate for factory variations in forward voltage. 
       SUMMARY OF THE INVENTION 
       [0008]    This invention has the object of developing a constant power drive for light emitting diodes (LEDs), such that the above-described primary problem is effectively solved. It provides an inexpensive circuit that automatically compensates for variation in forward voltage of the LED, both in a single unit with temperature, and also due to unit-to-unit variations. The invention includes a current sensor, such as a resistor, and an integrator, such as a resistor-capacitor low-pass filter. While the current sensor produces a signal proportional to the LED drive current, the integrator produces a signal proportional to the duty cycle, which in turn is proportional to the forward voltage of the LED. When the current sensor input is fed to the integrator, the output is a signal proportional to the product of the LED drive current and the LED forward voltage, which is the LED power. 
         [0009]    The time constant of the integrator must be set appropriately. In particular, it must be substantially longer than the sort of noise filter typically used in such applications, which are typically timed to be roughly the speed of the rising and falling edges of the switching element. In a preferred embodiment, the time constant is 3-10 times as long as the switching period of the switching element. 
         [0010]    In a circuit in which the power source to run the LED is the AC line, and the drive circuit is power factor corrected (PFC), an additional constraint is that the time constant of the integrator must be short compared with the AC line frequency. In the preferred embodiment, this condition is naturally fulfilled. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawing is included to provide a further understanding of the invention, and is incorporated in and constitutes a part of this specification. The drawing illustrates an embodiment of the invention and, together with the description, serves to explain the principles of the invention. 
           [0012]      FIG. 1  is a circuit schematic of a constant power circuit for driving a string of LEDs, such that neither variations in temperature of the LEDs, nor lot-to-lot variations of the forward voltage of the LEDs, substantially affects the power with which the LEDs are driven. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    Reference will now be made in detail to the present preferred embodiments of the invention, an example of which is illustrated in the accompanying drawing. Wherever possible, the same reference numbers are used in the drawing and the description to refer to the same or like parts. 
         [0014]    According to the design characteristics, a detailed description of the preferred embodiment is given below. 
         [0015]      FIG. 1  is a schematic of a constant power LED circuit  10 . In accordance with a preferred embodiment, at least one LED  30  is powered from an input power source  20 . When a transistor switch  60  is turned on by a control circuit  70 , current  22  through the at least one LED  30  is ramped up because of inductor  40 . When the transistor switch  60  is turned off by the control circuit  70 , current  22  through the at least one LED  30  is ramped down because of inductor  40 . In the turned-on configuration (“on configuration”), current  22  from the at least one LED  30  and inductor  40  passes through the transistor switch  60 . In the turned-off configuration (or “off configuration”), current  22  from the at least one LED  30  and inductor  40  passes through diode  50 . The average current  22  through the at least one LED  30  is set by the relative amounts of time the transistor switch  60  spends in the on configuration and the off configuration, the two together being known as a complete switching period. It can be appreciated that in accordance with an exemplary embodiment, the input power source  20 , the inductor  40 , the diode  50 , and the transistor switch  60  combined forms a switch-mode power supply  12 . 
         [0016]    In accordance with one embodiment, during the period when the transistor switch  60  is in the on configuration, the current  22  passing through the at least one LED  30 , the inductor  40 , and the transistor switch  60  also passes through a sense resistor (or current sensor)  80  to ground. In accordance with an exemplary embodiment, the sense resistor  80  converts the current  22  from the at least one LED  30  into a voltage signal  24 . The voltage signal  24  is then filtered by an integrator  90 . In accordance with an exemplary embodiment, the integrator  90  receives (i.e., takes) a signal from the current sensor  80  and combines it with a signal proportional to the duty cycle and forms an output. The output of the integrator  90  is then used as feedback  100 , to determine the relative amount of time the transistor switch  60  spends in the on configuration and the off configuration. 
         [0017]    In accordance with a preferred embodiment, the integrator  90  consists of a series resistor  92  and a parallel capacitor  91 . In accordance with an exemplary embodiment, the time constant of the integrator  90  (or resistor-capacitor circuit) is a multiple of the inverse of the switching frequency of the switch-mode power supply. For example, the time constant of the integrator  90  is preferably set to be approximately 3-10 times longer than the complete switching period of the transistor switch  60 . 
         [0018]    The current  22  sensed by the current sense resistor  80  is conditioned by the integrator  90 . Since the current  22  is present only during the time that the transistor switch  60  is in the on configuration, the integrator  90  produces a voltage  24  that is proportional to the time the transistor switch  60  is in the on configuration. In accordance with an exemplary embodiment, the time the transistor  60  is in the on configuration is dependent on the ratio of the forward voltage  26  of the at least one LED  30  and the voltage of the input power source  20 . Thus, the output  100  is proportional to the product of the current through the at least one LED  30  and the forward voltage  26  of the at least one LED  30 . Thus, the control circuit  70  regulates the power into the at least one LED  30 . 
         [0019]    In accordance with an exemplary embodiment, the constant power LED circuit  10  is designed to be a buck converter with a transistor switch (i.e., a buck-derived converter). However, it can be appreciated that any switching circuit providing a signal proportional to the LED current can also be used in a similar circuit. In accordance with another embodiment, the circuit  10  can use LEDs which are ground-referenced, or can use an amplifier or use a current-sense transformer to determine the LED current. The circuit  10  can also use AC-line power, and can be power-factor corrected, so long as the integrator time constant is short compared with the AC-line frequency. 
         [0020]    It will be apparent to those skilled in the art that various modifications and variation can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.