Abstract:
There is provided apparatus for controlling a set of light emitting diodes (LEDs) comprising at least one current source for powering the set of LEDs; a main controller for receiving dimming and/or color mixing information and for translating the information into LED control information and transmitting the LED control information to control the set of LEDs; wherein the LED control information is based on and off times.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Patent Application No. 60/735,220 filed Nov. 10, 2005, and U.S. Provisional Patent Application No. 60/796,550 filed May 2, 2006, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to controlling LEDs. More particularly, the present invention relates to a method and apparatus for dimming and/or colour mixing LEDs. 
     BACKGROUND OF THE INVENTION 
     In the field of lighting technology, there are many different ways for an individual to provide light to a space, such as a room in a house. In the past, lighting levels were limited to only two positions, namely ON and OFF. Therefore, when a person wished to light up the space, they simply turned the light on. In the more recent past, in order to allow individuals to control the amount of lighting in a space, and to assist in lowering the cost to power the lighting, techniques were created which allowed the lighting to be dimmed so that the light was not always operating at a maximum level. These dimming techniques allow for lighting to also be colour-mixed so that different coloured lighting can be combined with each other to provide a plurality of different colours to light up the space. 
     The creation of light emitting diodes (LEDs) has also helped enhance the field lighting technology and has also assisted in lowering overall lighting costs. The combination of dimming techniques and LEDs has provided an improvement over existing lighting. Some existing techniques for dimming and/or colour mixing LEDs include Pulse Width Modulation, Variable Frequency Modulation, Bit Amplitude Modulation and Pulse Amplitude Modulation. 
     Pulse Width Modulation (PWM) is a method that uses a signal at a constant frequency with dimming achieved by varying the pulse width and therefore, the duty cycle of the pulse. However, when performing PWM in the digital domain (DPWM), problems occur when the system is operating at a low intensity. 
     If DPWM is used with 1024 steps, for example, the change in intensity from 1 step out of 1024 to 2 steps out of 1024 is a factor of two. The human eye is capable of detecting a change in intensity as little as 1% so this will be detected as a very large step change in intensity. A clock is used to determine when the pulse is to turn on and off. The clock counts at fixed intervals, and repeats at a multiple of that interval. For example, a 10-bit clock containing 1024 steps will go from 1021 to 1022 to 1023 and back to 0. The pulse on time duration begins when the clock resets to 0 and may end when the clock reaches some modulated value, for example “2” representing an intensity level. If a lower intensity level is required, the only option is to go to “1” which represents a 50% drop in intensity level and is easily seen as a large step. 
     Variable Frequency Modulation is a method which uses a signal having a constant pulse width. Dimming is achieved by varying the off time and therefore, frequency and duty cycle of the signal. However, this technique suffers from the problem that a wide dynamic frequency range is required for the dimming current to achieve an acceptable range of light intensities. For example, assuming a constant pulse width of 50 microseconds (in some topologies it takes a certain amount of time for current to ramp up and down into the LED since the LED cannot reach peak current instantly), a 1% light intensity value will have a dimming current frequency of 200 Hz (50/5000 microseconds). 
     In the case of a 50% LED light intensity, the dimming current frequency will be 10 Khz (50/100 microseconds). Electromagnetic compatibility (EMC) issues limit maximum high frequency for LEDs. Some LED manufacturers recommend maximum frequencies for LEDs in the order of 1 KHz since minimizing frequency variation is a desirable goal. 
     Bit Angle Modulation utilizes a binary pulse train that contains the light intensity value. Each bit of the pulse train is stretched proportionally to the binary significance of each bit and each bit of the binary word is therefore allocated a fixed range of phase angles within the drive cycle. 
     Another method is pulse amplitude modulation (PAM) in which a signal is converted to a digital signal and an analog channel through simple transformations, and vice versa. 
     It is also possible to vary the intensity of an LED by varying the amount of current passing through it but this can lead to a problems such as colour shifting. 
     It is, therefore, desirable to provide a novel method of modulation for the dimming and/or colour mixing of LEDs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate or mitigate at least one disadvantage of previous methods of modulation for controlling loads, such as LEDs. 
     In a first aspect, the present invention provides apparatus for controlling a set of light emitting diodes (LEDs) comprising at least one current source for powering the set of LEDs; and at least one controller for controlling the at least one current source via LED control information; wherein the LED control information comprises both on and off times and period, whereby the on and off times and period are not held constant. 
     In a further embodiment, there is provided a method of controlling a set of LEDs comprising the steps of receiving or generating dimming and/or colour mixing information; translating the dimming and/or colour mixing information into LED control information based on both on and off times and period, whereby the on and off times and period are not constant; and transmitting the LED control information to at least one current source for powering the set of LEDs. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIG. 1  is a schematic diagram of a first embodiment of apparatus for controlling LEDs; 
         FIG. 2  is a diagram showing waveforms for different DMX Levels; 
         FIG. 3  is a table illustrating different ON/OFF times for various light intensities; 
         FIG. 4  is a diagram showing simplified 1-Bit DAC waveforms before filtering 
         FIG. 5  is schematic diagram of a second embodiment of apparatus for controlling LEDs; 
         FIG. 6  is a flowchart outlining a method of controlling LEDs; 
         FIG. 7  is a diagram showing an output current dimming waveform; and 
         FIG. 8  is a schematic diagram of yet a further embodiment of apparatus for controlling LEDs. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, the present invention provides a method and apparatus for controlling the dimming and/or colour mixing of LEDs. 
     Turning to  FIG. 1 , a schematic diagram of a first embodiment of apparatus for dimming and/or colour mixing LEDs is shown. The apparatus  10  comprises a controller  12  which is used to control the operation of a plurality of loads  26 , such as LEDs. The apparatus  10  further comprises an interface  14  which is in communication with the controller  12 . The interface  14  serves to connect the apparatus  10  to an external processor or transmitter  16 , such as a DMX512A transmitter. As will be understood DMX512A is a method of digital data transmission between controllers and control equipment. It is designed to carry repetitive control data from a single controller (transmitter) to one or more receivers. The interface  14  receives signals, in the form of data packets, from the transmitter  16  containing dimming and/or colour mixing information for the apparatus  10 . The data packets are then transmitted from the interface  14  to the controller  12 . 
     The controller  12  is also connected to a plurality of signal generators  18 , individually denoted as  18   a  to  18   n  where n equals any number and not simply the number  14  as might be assumed. Each signal generator  18  is connected to an individual current source  20 , individually denoted as  20   a  to  20   n . The current sources  20  preferably include ancillary circuitry for operation and comprise a buck circuit power stage with hysteretic control. In operation, the signal generator  18  typically transmits a digital signal  22  and an analog signal  24  to the current source  20 . The digital  22  and analog  24  signal combining to deliver load control information. The output of the current source  20  is connected to the external load  26 , seen as loads  26   a  to  26   n , and a current sense  28 , individually denoted as  28   a  to  28   n . Each current sense  28  is connected to the controller  12  and forms a part of a digital control feedback loop. As will be understood, the loads  26 , such as a set of LEDs, is what is being controlled by the apparatus  10 . A power supply  29  is also located within the apparatus  10  to provide the necessary power for operation of the apparatus. 
     In the following description, as schematically shown in  FIG. 6 , operation of the apparatus  10  is described with respect to a single load  26 . It will be understood by one skilled in the art that control of each of the loads is performed in an identical manner. 
     Initially, dimming and colour mixing information is received by the interface  14  from an external source such as the external transmitter  16  (step  100 ). This dimming and/or colour mixing information is then transmitted to the controller  12  which translates this dimming and/or colour mixing information to load control information, based on on and off times and period for the load  26  (step  102 ) whereby the on and off times and the period are not held constant. The load control information is transmitted from the controller  12  to the signal generator  18  (step  104 ) in the form of instructions to generate the digital signal  22  and the analog signal  24 . The digital signal  22  and the analog signal  24  are preferably generated via a digital control algorithm and 1 Bit DAC algorithm, respectively. 
     In one embodiment, the instructions for the digital signal  22  are preferably generated by the controller  12  accessing a digital look up table to translate required light intensity levels (as specified by the external transmitter  16 ) to on-time and off-time information. Alternatively, in other embodiments the on and off time information may be computed via known methods. 
     The digital signal  22 , including the on-time and off-time information, is then transmitted to the current source  20  (step  106 ).  FIG. 3  provides a sample table illustrating on and off times for various intensities. Therefore, if the external transmitter  16  has requested that a load  26  be turned on at an intensity of 24.51%, the on-time value equals 25×32 microseconds and the off-time value equals 77×32 microseconds. Many alternate frequencies and steps are possible. For example, one way to get 25.9% is for the on-time value to equal 265×500 nanoseconds and the off-time value to equal 755×500 nanoseconds resulting in an output frequency near 2 kHz. 
     Digital signal  22  is implemented by a counter which is decremented at regular intervals during a time interrupt such that when the counter reaches 0, the digital signal is toggled. If the digital signal is logic high, the counter is loaded with a new on-time value and if the digital signal is logic low, the counter is loaded with a new off-time value. In other words, on-time and off-time values for the current source  20  are controlled by loading a count-down timer with appropriate new values every time the timer reaches 0. This results in a dimming of the load, such as shown, for example, in  FIG. 2  which provides sample digital signal waveforms for different intensity levels. 
     It will be understood that neither the frequency at which the load is operating nor the time period for which it is operating is a constant and that the method of the current invention allows for the maintenance of the output dimming frequency current within a narrow dynamic range. It will be further understood that specifying any two of on-time, off-time, and period is mathematically equivalent, and that period and frequency are inversely related, and thus, it is equivalent to specify, for example, on-time and frequency, or off-time and period, in place of on-time and off-time. 
     In a preferred embodiment, the controller  12  modifies the on-time and off-time values during transitions from one light intensity to another to reduce or prevent transient flickers due to sudden changes in the output phase of the current being supplied to the load. 
     The analog signal provides an analog reference level which is translated to a peak output current during the on-time of the current from the current source. Alternatively, a maximum and minimum peak current may be determined. In one embodiment, the analog signal  24  is typically calculated by the controller  12  using the following algorithm:
 
Analog signal[ i ]=Analog signal[ i− 1]+Setpoint[ i ]×on-time[ i ]−current_sense_value[ i ]×(on-time[ i ]+off-time[ i ])
 
     where i represents the current time, or state, of the analog signal and i−1 is the previous value of the analog signal and Setpoint equals the required output current with no dimming pulse or off-time. Setpoint is typically dependent on the recommended operating current as set by a manufacturer of the LED being controlled. 
     For example, assume that analog_signal [i−1]=10000, on-time [i]=50, off-time [i]=50, Setpoint [i]=200 and current_sense_value[i]= 95 . In this example, setting the analog signal to 10000 results, over time, in a current_sense_value of 95. However, assuming the desired current is Setpoint[i]×on-time[i]/(on-time[i]+off-time[i])=100, the analog signal[i] is calculated as being 10500. This calculation is repeated at regular intervals at a low enough rate that the effective gain of this control loop does not cause it to oscillate, as will be understood by one skilled in the art of digital control loop design. The calculation of the analog signal based on the current sense value allows the output of the current source to be controlled. 
     After the analog signal is calculated (using the above algorithm), the controller  12  generates an average analog voltage level using a 1-Bit DAC algorithm (as described below). The 1-Bit DAC algorithm is a simple Digital-to-Analog conversion for generating a number of predetermined output digital pulses that are filtered to produce a proportional average analog voltage level. In the preferred embodiment, the DAC algorithm establishes time slots by means of an interrupt every 32 microseconds and updates a pin on the controller  12  twice every interrupt. More specifically, the algorithm divides the time period of 8192 microseconds into 256×31.875 microsecond intervals and 256×125 nanosecond intervals. By selecting which time slots are set high and which are set low, approximately 2 16  analog levels are available, as schematically shown in  FIG. 4 . It will be understood that this DAC algorithm is an implementation detail, and that a hardware DAC may be used to the same effect. 
     After generating the analog signal, this analog signal is transmitted to the current source (step  106 ) to establish an upper and lower peak current range for the hysteretic control and thereby a peak output of the current being provided by the current source  20 . The analog signal is transmitted independent of the digital signal since it varies much more slowly than the digital signal. Concurrently, the digital signal provides the on-time and off-time information to the current source  20  so that the combination of the analog and digital signals provides load control information to the current source for operation and control of the load  26 . 
     After receiving the analog and digital signals, the current source provides a current (in accordance with the analog and digital signals) to the load (step  108 ). While the current is being provided to the load, the current sense  28 , senses the output of the current source  20  (step  110 ) and transmits a signal, the current_sense_value, to the controller  12  (step  112 ) to forward the current level as a part of the feedback control loop so that a constant output peak current from the current source is maintained and compensates for variations in the load  26  and/or current source  20 . 
     Turning to  FIG. 5 , a second embodiment of apparatus for dimming and/or colour mixing LEDs is shown. In this further embodiment, each of the loads are controlled by individual load controllers  30 , individually denoted as  30   a  to  30   n . The functionality of the controller  12  is split between a main controller  32  and individual load controllers  30 . All other parts are identical to the embodiment of  FIG. 1  and are denoted as such. 
     The main controller  32  receives the dimming and/or colour mixing information, in the form of a serial data stream, from the external transmitter  16  via the internal interface  14  and translates this information to controller information, in the form of a more easily decoded synchronous data stream, and transmits this controller information to the individual load controllers  30   a  to  30   n . This simplified controller information, or words of data, is preferably transmitted over a shared “sync” line, a shared “clock” line and a set of parallel “data” lines. The start of each word is preferably delimited by the “sync” line and the start of each bit is delimited by the “clock” line. In yet a further embodiment, the controller information may be transmitted to the individual load controllers  30  via a shared data line or a daisychain arrangement. 
     After the load controller  30  receives the controller information, the load controller  30  performs calculations as outlined above with respect to the controller  12 . Operation of the loads  26  is then controlled by the individual load controllers  30  based on the information transmitted from the main controller  32 . Due to greater processing power available from the individual load controllers  30 , refinements of the formula listed above are possible. For example, the formula for the analog signal may be replaced by:
 
Analog signal[ i ]=Analog signal[ i− 1]+(desired current[ i ]−current_sense_value[ i ])×gain_term
 
     where desired_current is pre-determined as Setpoint×(on-time[i][/(on-time[i]+off-time[i]) and gain_term is a constant such as 2 14 . Further, by removing the need to calculate the analog signal for a short period after the ratio of on-time to off-time has changed, it is also possible to avoid overshoots and undershoots in dimming current. After the digital and analog signals are computed, they are transmitted to the current source which then provides a current to the load, in accordance with the digital and analog signals. As before, the current sense provides a digital feedback loop for each current source  20 . 
     Turning to  FIG. 8 , yet a further embodiment of apparatus for controlling a load is shown. As with the other embodiments, the apparatus  10  comprises an interface  14  for communicating with an external transmitter to receive dimming and/or colour mixing information, a controller  12  for translating the dimming and colour mixing information to load control information, a signal generator  18  which receives the load control information from the controller  12 , a current source  20  for providing the necessary current to power the load  26  and a current sense  28  which forms a part of a feedback loop to assist in controlling he current source. A power supply  29  is also located within the apparatus. 
     In this embodiment, the signal generator  18  preferably includes a complex digital to analog converter which allows only a single signal containing the on-time, the off-time and the minimum and maximum peak output current information to be transmitted to the current source in order to control the current source  20 . 
     In a further embodiment, the current sense may be removed if the current source is sufficiently well regulated. For example, a current source comprising a transistor, a well-regulated voltage source, and a rheostat in series with an LED may be adjusted as to need no further adjustments. In this embodiment, the analog signal computation is omitted and only the digital signal (with on and off times) is used. 
     In another alternative embodiment, each of the current sources may be a removable module or may be a monolithic component of the apparatus. It is understood that the current sources  20  may comprise many alternate topologies, so long as they can be turned “on” and “off” through some digital signal. Furthermore, the control loop (current sense) may be removed if the natural current provided by the current source is the desired peak current for a given application of the LEDs. 
     In yet a further embodiment, the apparatus may be a single controller for controlling a single load by controlling operation of the load via on and off times. 
     In a further embodiment, the controller  12  and the signal generator  18  are located within a microcontroller. 
     In yet another embodiment, the main controller  32  translates the received dimming and/or colour mixing information to LED control information and transmits the LED control information to the individual controllers which then uses this information to control its associated current source. 
     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.