Abstract:
A supply assembly for an LED lighting module includes a control switch for supplying a constant current to the LED lighting module. A dual switching signal composed of low frequency bursts of high frequency pulses is applied to the control switch. By varying the low frequency component of the dual switching signal, the average current through the LED lighting module may be varied in order to vary the light intensity outputted by the LED lighting module.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a continuation-in-part of U.S. patent application Ser. No. 09/773,159, filed Jan. 31, 2001 now U.S. Pat. No. 6,580,309 which is herein incorporated by reference Pub. No. US 2001/0024112 A1. 
   CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation-in-part of U.S. patent application Ser. No. 09/773,159, filed Jan. 31, 2001, now Pub. No. US 2001/0024112 A1, published Sep. 27, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject invention relates to a supply assembly for supplying power to a light emitting diode (LED) lighting module. 
   2. Description of the Related Art 
   LED lighting modules are becoming more common in many applications for replacing less efficient incandescent lamps, for example, in traffic signal lights and automobile lighting. Depending on the amount of light required in the application, the LED lighting modules may consist of a plurality LED&#39;s arranged in parallel or in series, or a combination of both. In either case, the LED lighting module receives operating power from a supply assembly that switches a direct current voltage on and off at a high frequency. Such supply assemblies are known as switched-mode power supplies and are available in a plurality of forms, for example, a flyback converter, a buck converter, a half-bridge converter, etc. Each of these converters is capable of supplying a constant current to the LED lighting module in the form of a pulse width modulated signal. 
   In the use of LED lighting modules, it is desirable to be able to control the intensity of the light being output by the LED lighting module. This may be achieved in a number of ways. For example, the amount of current delivered to the LED lighting module may be adjusted by controlling the pulse width modulation. However, once the current intensity drops below 20% of the nominal current intensity, the relation between the current intensity and the light output becomes largely non-linear, and the efficiency of the LED lighting module becomes far from optimal. 
   U.S. Pat. No. 5,661,645 describes a power supply for a light emitting diode array which includes a circuit for interrupting the supply of power from the power supply to the LED array. As shown in  FIG. 1  herein, the power supply  1  includes a supply of direct current voltage  10 , which may be a battery or rectified line alternating current (AC) voltage connected to a switched-mode converter  12  typically having a control switch  14 , a diode  16 , an inductor  18 , an optional capacitor  20  and an optional transformer  22 . A control input of the control switch  14  receives a high frequency pulse-width modulated (PWM) switching signal. Outputs from the power supply  1  are connected to an LED lighting module  2  having an LED array  24  (shown herein as a single LED) and a controllable switch  26  for interrupting the supply of power to the LED array  24 . The controllable switch  26  receives a low frequency PWM switching signal for controlling the mean current to the LED array  24 .  FIG. 2  shows a plot of the current through the LED array  24  in which the low frequency PWM switching signal causes current pulses D occurring in the period F D , and the high frequency PWM switching signal causes the current variation ΔI D . While this arrangement ensures that the LED array always operates in an efficient manner, it should be understood that the power supply  1  is continually on even when the PWM switching signal has the controllable switch  26  turned off.  FIG. 3  shows an equivalent circuit of the arrangement of FIG.  1 . As should be apparent, while the power from the DC source is stopped when the control switch  14  is open, such is not the case when the controllable switch  26  is open. As such, this arrangement suffers from an unnecessary loss of energy. 
   Published U.S. Patent Application No. 2001/0023112A1 discloses an alternate arrangement to that shown in U.S. Pat. No. 5,661,645. In this alternate arrangement, the power supply itself is turned on and off using the low frequency PWM switching signal.  FIG. 4  shows an example of this alternate arrangement. Similarly as in  FIG. 1 , the power supply  1 ′ includes a supply of direct current voltage  10 , which may be a battery or rectified line alternating current (AC) voltage connected to a switched-mode converter  12 ′ typically having a control switch  14 , a diode  16 , an inductor  18 , an optional capacitor  20  and an optional transformer  22 . A control input of the control switch  14  receives a high frequency pulse-width modulated (PWM) switching signal. Outputs from the power supply  1 ′ are connected to an LED lighting module  2 ′ having an LED array  24  (shown herein as a single LED). The LED lighting module  2 ′ does not include the controllable switch  26  shown in FIG.  1 . Rather, the switched-mode converter  12 ′ includes an input for receiving the low frequency PWM switching signal which effectively controls means for turning on and off the switched-mode converter  12 ′. 
   SUMMARY OF THE INVENTION 
   It is an object of the subject invention to eliminate the means for switching on and off the power supply to an LED array while still effecting the low frequency pulse width modulation of the current to the LED array. 
   This object is achieved in a supply assembly for a LED lighting module comprising a direct current (DC) voltage source having a first and a second supply terminal; a series arrangement of a diode and a controllable switch connected across the first and second supply terminals of the DC voltage source; an inductor connecting the first supply terminal of the DC voltage source to an first output terminal, a node between the diode and the controllable switch forming a second output terminal, said LED lighting module being connectable between the first and second output terminals; and a controller for controlling the switching of the controllable switch, said controller having means for supplying a dual pulse-width modulated switching signal to said controllable switch at two frequencies including a high frequency pulse-width modulated switching signal component for controlling a magnitude of the LED current, and a low frequency pulse-width modulated switching signal component for controlling a duration of the LED current. 
   Applicants have found that the control switch in the switched-mode power supply may be used for both the high frequency PWM switching as well as the low frequency PWM switching thereby eliminating the need for separate means for switching the power supply on and off. To that end, the supply signal to the control switch includes both the high frequency PWM switching signal as well as the low frequency PWM switching signal, i.e., the high frequency switching signal is applied in pulse bursts at the low frequency to the control switch. 
   Applicants have further found that when the power supply is switched on and off by separate means, there is a gradual increase and decrease in the duty cycle, while when a dual PWM switching signal is applied to the control switch, the change in the duty cycle is instantaneous. 
   In a further embodiment of the subject invention, the controller further comprises an input for receiving a current signal indicative of the LED current, and means for modifying said low frequency pulse-width modulated switching signal component in dependence on said current signal. 
   Applicants have found that by detecting the LED current, the duty cycle of the high frequency PWM switching signal component may quickly respond to the LED current leading to the fastest rise/fall time of the LED current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the above and additional object and advantages in mind as will hereinafter appear, the subject invention will be described with reference to the accompanying drawings, in which: 
       FIG. 1  shows a generic block circuit diagram of a prior art power supply for an LED array; 
       FIG. 2  shows a graph of the current through the LED array of  FIG. 1 ; 
       FIG. 3  shows an equivalent circuit of the power supply of  FIG. 1 ; 
       FIG. 4  shows a generic block circuit diagram of another prior art power supply for an LED array; 
       FIG. 5  shows a generic block circuit diagram of a power supply for an LED array incorporating the subject invention; 
       FIG. 6  shows a graph of the dual PWM control signal for the power supply of  FIG. 5 ; 
       FIG. 7  shows a block circuit diagram of a buck converter for an LED array incorporating the subject invention; 
       FIG. 8  shows an equivalent circuit of the power supply of  FIG. 7 ; 
       FIG. 9  shows a block circuit diagram of the power supply of  FIG. 7 , showing a first embodiment of the controller; 
       FIG. 10  shows a block circuit diagram of the power supply of  FIG. 7 , showing a second embodiment of the controller; 
       FIG. 11  shows a block circuit diagram of the power supply of  FIG. 7 , showing a third embodiment of the controller; and 
       FIG. 12A  shows a graph of the LED current,  FIG. 12B  shows the details of the LED current at turn off, and  FIG. 12C  shows the details of the LED current at turn on. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5  shows a generic block circuit diagram of the power supply and LED lighting module of the subject invention. In particular, similarly as in  FIGS. 1 and 4 , the power supply  1 ″ includes a supply of direct current voltage  10 , which may be a battery or rectified line alternating current (AC) voltage connected to a switched-mode converter  12 ″ typically having a control switch  14 , a diode  16 , an inductor  18 , an optional capacitor  20  and an optional transformer  22 . Outputs from the power supply  1 ″ are connected to an LED lighting module  2 ′ having an LED array  24 . A control input of the control switch  14  now receives a dual PWM switching signal. As is more clearly shown in  FIG. 6 , this dual PWM switching signal is, in essence, a combination of a high frequency PWM switching signal component which is applied in pulse bursts at a low frequency, i.e., the low frequency PWM switching component. 
     FIG. 7  shows a block circuit diagram of a buck converter for an LED array incorporating the subject invention. In particular, a DC supply  10  is connected across the series arrangement of a diode D 1  and a control switch  30 , shown as a MOSFET, while a series arrangement of an inductor  32  and the LED lighting module  2 ′ is connected across the diode D 1 . A controller  34  generates the dual PWM switching signal which is applied, via an amplifier  36  to a control input of the control switch  30 . The controller  34  has an input for receiving a signal indicative of the current sensed in the drain terminal of the control switch  30 , which is related to the LED current. Alternatively, as shown in dotted line, this input may receive a signal indicative of the sensed LED current. 
     FIG. 8  shows an equivalent circuit diagram of the power supply/LED lighting module of FIG.  7 . It should be apparent that in this configuration, the inductor current always ramps down to zero when the control switch is turned off, thereby avoiding the current circulation problems of the circuit diagram of  FIG. 3  when the controllable switch is turned off. 
     FIG. 9  shows the block circuit diagram of  FIG. 7  with a first embodiment of the controller  34 . In particular, the controller  34  includes a current mode pulse width modulator  38  which receives an LED current reference signal from a current source  40 , the sensed current, and a high frequency sawtooth signal. The current mode pulse width modulator  38  then supplies the high frequency pulse width modulated switching signal component which is applied to one input of an AND-gate  42 , the other input of which receives the low frequency PWM switching signal component. The output from the AND-gate  42  is then applied through the amplifier  36  to the gate of the control switch  30 . 
     FIG. 10  shows the block circuit diagram of  FIG. 7 , with a second embodiment of the controller  34 . In particular, the controller  34  includes an adder  44  having a positive input for receiving a reference voltage VREF and a negative input for receiving a high frequency ramp signal. An output from the adder  44  is applied to an inverting input of a comparator  46  which receives the sensed current at its non-inverting input. An output of the comparator  46  is applied to the reset input of an RS flip-flop  48  which receives a high frequency clock signal at its set input. The Q output from the RS flip-flop  48  is applied to one input of an AND-gate  50  which receives the low frequency PWM switching signal component at its other input. The output from the AND-gate  50  is then applied through the amplifier  36  to the gate of the control switch  30 . 
   In the embodiment of  FIG. 9 , either peak or average current detection may be used, while in the embodiment of  FIG. 10 , peak current detection is used. 
     FIG. 11  shows the block circuit diagram of  FIG. 7 , showing a third embodiment of the controller  34  in which both peak current detection and average current detection are used. In particular, the sensed current is applied to an integrator  52  which forms an average of the sensed current. An output of the integrator  52  is applied to a low frequency pulse width modulator  54  which receives a reference current from current source  56  and a low frequency sawtooth signal from low frequency sawtooth generator  58  which has a user control  60  coupled thereto. An output from the low frequency pulse width modulator  54  is applied to a first input of an AND-gate  62 . The sensed current is also applied to a sample-and-hold circuit  64 . An output from the sample-and-hold circuit  64 , which represents the peak sensed current, is applied to a high frequency pulse width modulator  66  which also receives a reference current from current source  68  and a high frequency sawtooth signal from high frequency sawtooth generator  70 . The output from the high frequency pulse width modulator  66  is applied to the second input of the AND-gate  62 , and the output from the AND-gate  62  is then applied through the amplifier  36  to the gate of the control switch  30 . 
   In operation, the user sets a desired intensity level for the LED lighting module using the user control  60 . The resulting sawtooth signal (varying in, for example, the duration of each sawtooth) generated by the low frequency sawtooth generator  58  is applied to the low frequency pulse width modulator  54 . In dependence on this sawtooth signal, the reference current, and the average LED current, the low frequency pulse width modulator generates the low frequency PWM switching signal component with the appropriate pulse width. At the same time, the sensed current is applied and stored in the sample-and-hold circuit  64 . The output from the sample-and-hold circuit  64 , along with the reference current and the high frequency sawtooth signal are processed by the high frequency pulse width modulator  66  to adjust the pulse width of the high frequency PWM switching signal component. The AND-gate  62  then combines the high frequency and low frequency PWM switching components to form the dual PWM switching signal which is applied, via the amplifier  36  to the gate of the control switch  30 . 
     FIG. 12A  shows the overall LED current.  FIG. 12B  shows the LED current at the end of, for example, the first pulse in  FIG. 12A , as compared with the dual switching signal of FIG.  6 . For comparison,  FIG. 12B  also shows the LED current (dotted line) if, instead, the power supply were merely turned off, which then exhibits ringing. Finally,  FIG. 12C  shows the LED current at the beginning of, for example, the second pulse in  FIG. 12A , as compared with the dual switching signal of FIG.  6 . For comparison,  FIG. 12C  also shows the LED current (dotted line) if, instead, the power supply were merely turned on. 
   Numerous alterations and modifications of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the above described embodiments are for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.