Abstract:
The invention disclosed herein is a dynamic dummy load to allow a phase control dimmer to be used with LED lighting. The invention includes providing a dynamic dummy load to provide a load to the dimmer when the LED electronics do not provide sufficient load due to start up issues or ringing in the circuit, the dynamic dummy load providing a reduced flow of current when the LED and its converter electronics provide sufficient current draw from the dimmer. The system generally includes a power source electrically connected to a phase control dimmer, the phase control dimmer electrically connected to converter circuitry to convert the AC power output of the dimmer to DC power output for powering the LED lighting, a dynamic dummy load electrically connected in parallel with the converter circuitry, the dummy load varying its current draw in response to operation of the converter circuitry.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application 60/572,557 filed on May 19, 2004, which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is in the field of LED lighting, particularly circuitry to allow low electrical current LED lighting to work with dimming switches. 
     BACKGROUND OF THE INVENTION 
     This invention is related to making the AC to DC converter used on LED lighting compatible with the phase control dimmer which is widely used for incandescent wall dimming applications. A typical circuit for use of a dimmer is shown in  FIG. 1 . 
     As shown in  FIG. 1 , the dimmer  1  is electrically connected in series between the electrical load  2 , and the power source  3 . In this example, the power source is AC household current as one would find in a typical household dimming application. The load  2  in the example is an incandescent light bulb, but one skilled in the art will recognize other loads may be used. Dimming is achieved by adjusting the conduction angle of the dimmer  1  so that the RMS voltage across the load  2  varies with the adjustment of the conduction angle. In the case of the incandescent bulb being the load  2 , the light intensity of the light bulb will change as the RMS voltage is varied across the light bulb. A reduced RMS voltage across the light bulb results in a dimmer light bulb. 
     As shown in  FIG. 2 , the dimmer can create different waveforms across the load, depending upon the conduction angle adjustment of the dimmer. The first example  20  shown in  FIG. 2  shows the waveform of a 115 volt 60 hertz domestic AC power supply without a dimmer. The second  21  and third  22  examples in  FIG. 2  show the output waveforms generated when the 115 volt 60 hertz domestic AC power line is adjusted by a phase control dimmer. In summary, the dimmer clips the waveform for a certain period after a zero crossing, thus resulting in a reduced RMS voltage at the output. One skilled in the art will recognize that the dimmer may clip the waveform at different times and by different amounts than what is shown in  FIG. 2 . 
     Although there is a wide variety of circuit techniques that can achieve the dimming function, the switch or circuit element that controls the power on-off inside a typical phase control dimmer is typically a type of thyristor device commonly known in the art as a TRIAC. TRIACs are generally available from a number of sources, and have well understood characteristics. Example TRIACs are models MAC12V, MAC12M and MAC12N, available from On Semiconductor, which may be found at the home page http://onsemi.com. The TRIACs discussed herein are generally representative of the TRIACs that are available, but are in no way meant to limit the scope of the invention described herein. TRIACs generally have a first main terminal MT 1  a second main terminal MT 2  and a gate terminal G. As known to one skilled in the art, TRIACs generally exhibit the following basic characteristics:
         Bidirectional conduction through the main terminals, allowing AC to pass through.   The TRIAC is turned on and conduction is present between the main terminals when there is a trigger current present between gate G and second main terminal  2  MT 2 .   Once triggered, the TRIAC remains on until a zero crossing of the AC power line at which point the device turns off and awaits the next trigger pulse or zero crossing of the AC power line. This characteristic allows phase angle control to be achieved.       

     The TRIAC has one more important parameter that directly relates to LED lighting, that is the hold current. A TRIAC will not remain in the on state after triggering without a current larger than the hold current passing through the main terminals. Because of the need to hold a current, TRIACs have difficulty remaining on when a low current is drawn through the main terminals, such as in the case of LED lighting. With reference to the data sheet for TRIAC MAC12D, the hold current is typically 20 milliamps. 
     There a number of reasons that dimmers cause problems for LED lighting, especially low wattage LED lighting. Some of these reasons are set forth below. 
     1. LED lighting is more energy efficient that incandescent light, therefore drawing a much smaller current. A typical incandescent light bulb can easily draw more than 200 mA during conduction. This value largely exceeds the holding current of typical dimmers. Therefore, there is usually no problem in dimming an incandescent bulb. LED lighting generally draws less current, typically ranging from 10 to 150 mA depending on the circuit design. 
     At smaller current levels, once the dimmer conducts, the load current does not satisfy the hold current requirement of the device, namely the TRIAC in the dimmer, and the dimmer enters a retriggering state that causes flickering of the LED light. The problem may be solved by placing a dummy load in parallel across the LED lighting so as to provide a sufficient current draw to exceed the hold current of the TRIAC in the dimmer. However, this is not a desired option. Since LED lighting is meant to be energy efficient, putting a dummy load across the LED lighting device will cause some issues such as reduced energy efficiency, due to the power draw of the dummy load, and degeneration of heat inside LED lighting which is undesirable to the thermal management of the power electronics inside. 
     2. Dimmable LED lighting requires an electronic AC to DC converter to operate. The AC to DC converter is basically a step-down switch mode power supply that converts AC input voltage to low voltage high current that drives the LED emitters. A representative circuit is shown in  FIG. 3 . As with the circuit in  FIG. 1 , it includes a power source  3 , a dimmer  1  and a load all connected in series. The load in the representative circuit is LED lighting electronics  6  to convert the AC to DC and the LED  7 . The Figure also shows the small amount of inductance  9  that is present due to the character of the wire. Inside the converter electronics there is small amount of capacitance  8  that will cause the load current to ring when the dimmer starts conduction.  FIG. 4  shows the output current waveform  10  of a dimmer, and the dimmer output waveform  11  when ringing is present at the firing or starting of the dimmer. If the ringing is large enough to cause current flow to fall below the hold current threshold  12  of the TRIAC, dimmer conduction will cease, causing flickering of the LED light. 
     3. The control circuit inside the dimmer requires a small bias current as its power supply to power up the dimmer. This implies the LED lighting load presented to the dimmer has to provide such minimum current. However, the electronic converter inside the LED converter usually has very low current consumption. This prevents the dimmer circuit from firing properly, again causing ringing. 
     4. The LED converter takes time to start, therefore its current consumption requires a finite time to reach a level exceeding the hold current of the dimmer. This delay in providing sufficient current needs to be taken into account in any circuitry. 
     In view of these shortcomings, it is desirable to include a dynamic load for use with a phase control dimmer and LED lighting, the dynamic load providing sufficient load to the dimmer at appropriate times to provide sufficient hold current, and prevent ringing in the circuit. 
     SUMMARY OF THE INVENTION 
     The invention described herein is a dynamic load or snubbing circuit for use with a phase control dimmer and LED lighting. The dynamic load provides sufficient current draw for the dimmer circuit and provides a current draw that will be varied depending on the current draw needs of the dimmer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a basic electrical circuit showing a phase control dimmer electrically connected in series between a power source and a load. 
         FIG. 2  is a plot showing a household AC wave form, and waveforms resulting from applying a phase control dimmer to the household AC power supply. 
         FIG. 3  is a representative circuit showing a dimmer used with LED lighting and converter electronics. 
         FIG. 4  is a plot showing the current output of a dimmer, and a waveform for an output exhibiting ringing. 
         FIG. 5  is a block representation of a dimmable LED lighting circuit, including a dynamic snubbing element. 
         FIG. 6  is a plot showing the relationship between the dimmer voltage output, dummy load current, and control signal with respect to time. 
         FIG. 7  is a schematic showing a preferred implementation of the invention. 
         FIG. 8  is a schematic showing an alternate embodiment of the invention, including a surge limiting circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in  FIG. 5 , the dynamic dummy load circuit  30  of the present invention is connected to an AC power source  32  electrically connected to a dimmer  33  which is electrically connected to a bridge rectifier BR 1 . The LED lighting converter  35  having a LED  36  is connected in parallel across the bridge rectifier BR 1 . The dynamic dummy load  40  is placed in parallel with the LED light converter  35 . A feedback channel  45  is provided between the LED light converter  35  and the dummy load  40 , so that the dummy load may be adjusted to provide an appropriate load when needed and a reduced load when not needed, thereby conserving power. The lighting converter  35  includes electronics to regulate the power received from the bridge rectifier BR 1  and includes electronics to provide a control or feedback signal to the dynamic dummy load  40 . The load presented to the power source or bridge rectifier BR 1  by the dynamic dummy load  40  is varied based on the control signal, thus changing the amount of current that flows or is drawn by the dynamic dummy load  40 . 
       FIG. 6  shows three plots representing the voltage output  50  of the dimmer  40 , the current  51  through the dummy load, and the feedback or control signal  52  from the LED lighting electronics  35  to the dummy load  40 , all with respect to time. One skilled in the art will recognize that the control signal  52  could be a voltage or current signal. For example, with reference to the plots shown in  FIG. 6 , the dummy load provides adequate bias current  55  for the dimmer circuit prior to operation, as discussed in the background section of this application. Thus, the dummy load will not have a high power consumption prior to dimmer conduction, as the power consumption is being limited by the low flow of current through the dimmer circuit when the dimmer is off. However, if the dummy load  40  were not present, there would be no conduction, as the LED would not conduct at the bias current level. 
     When the dimmer fires  57 , the dynamic dummy load  40  provides additional current  59  consumption that when combined with the current consumption of the LED converter  35 , provides sufficient current drawn through the dimmer  33  to exceed the hold current of the dimmer  33 . The current provided through the dynamic dummy load  40  will stay at a sufficiently high value, until the LED converter  35  starts and its current consumption exceeds the hold current of the dimmer. At this point in time  60  a feedback signal  52  is sent via feedback channel  45  to the dynamic dummy load  40 , thereby reducing the current draw  60  of the dynamic dummy load. A typical profile is shown in  FIG. 8 . So long as the feedback signal  52  is maintained  62  at a sufficient level, the dummy load  40  current draw is maintained at a low level  63 . Preferably below the bias current level  55 . When the dimmer  33  output reaches the zero crossing  65 , the control signal  52  is reduced  70 , allowing the dummy load  40  to pass bias level current  55 . When the dimmer  40  fires again  72 , the process is repeated. 
     In an optional embodiment, a surge limiting circuit  50 , shown in  FIG. 5 , may be included in series between the bridge rectifier BR 1  and the LED lighting converter  35 . The surge limiting circuit  50  limits the current peak when the dimmer fires, and leads to reduction of the ringing current magnitude. This reduces the need for a higher dummy load current. An example of a surge limiting circuit can be a constant current source. 
     Also shown in  FIG. 5 , diode D 1  connected between the rectifier DC output of the input rectifier circuit and the bypass capacitor. This diode stops the current from reversing from the bypass capacitor C 1  of the LED lighting converter  35 . Diode D 1  stops the current from reversing from the bypass capacitor C 1  of the LED lighting converter  35  into the input bridge rectifier and thus significantly reduces the ringing magnitude. Preferably, diode D 1  belongs to the type that exhibit low junction capacitants when reversed biased, thus greatly reducing the equivalent capacitants seen into the LED lighting converter  35  when there is a tendency for the current to reverse. 
     A schematic showing an embodiment of a circuit utilizing the dynamic dummy load of the invention is shown in  FIG. 7 . the dimmer is not shown in the figure, but ne skilled in the art would recognize the dimmer would be provided between terminals CN 1  and CN 4 . Similarly, the LED is not shown, but would be provided between terminals CN 2  and CN 3 . 
     With reference to  FIG. 7 , the dynamic dummy load is a current source including MOSFET Q 6  and transistor Q 5 . Dynamic dummy load current modulation is introduced from the auxiliary power supply of the LED converter from the junction of diodes D 12  and D 9 . A brief time delay is generated by the time constant of resistor R 13  and capacitor C 12 . 
     Before the LED converter starts, no current is flowing into resistor R 13 , and the dummy load current source including MOSFET Q 6  and transistor Q 5  is sinking its maximum current. When the dimmer, not shown, fires, the LED converter starts operating, and a voltage equal to a derivative of the LED voltage, as defined by the currents ratio of transformer T 1 , is generated at the cathode junction of diodes D 9  and D 12 . This voltage supplies the control ICU 1  as well as injecting a current determined by the derived auxiliary voltage and resister values of resisters R 12  and R 13 . Since the action of the dummy load current source is to maintain a predetermined voltage drop across a resister R 14 , injecting current from another source other than MOSFET Q 6  will reduce the current flowing MOSFET Q 6 , and thereby current is reduced when a converter starts. 
     A brief delay introduced by resister R 13  and capacitor C 12  insures that current does not fall immediately as the LED converter starts, thus reducing any ringing current magnitude. 
     Diode D 13  stops current from reversing from the LED converter electronics into the bridge rectifier BR 1  thus reducing any ringing current magnitude. 
       FIG. 8  shows an alternate embodiment of the invention described herein. The dimmer is not shown in the figure, but would be connected in series with a power source to terminals CN 1  and CN 4 . The circuit includes a bridge rectifier  80  connected to the output of the dimmer to convert AC current to DC current. Bridge rectifier  80  then provides current to the dummy load  81  and the LED converter electronics  82 . The dummy load  81  is electrically connected to a surge limiting circuit  82  which functions to limit the maximum current through the circuit. The schematic does not show the LED, which if present would be connected between terminals CN 5  and CN 6  of the LED converter electronics  82 . A feedback channel  90  is provided between the LED converter electronics and the dynamic dummy load  81 . 
     Although one skilled in the art would understand the functions of the various devices used to form the circuit elements described above, a brief description of major elements is included to aid in understanding of the circuit elements. The LED lighting converter electronics  82  includes an adjustable voltage regulator U 2  along with its associated diodes D 7  and D 8  and resistors R 22  and R 23  to provide a regulated DC current to converter ICU 1 . In turn, ICU 1  functions as a regulator of current provided to LED terminals CN 5  and C 6 . Converter ICU 1  also includes an output connected to MOSFET Q 1 , which acts as a main switch, controlled by converter ICU 1 . 
     Transformer T 1  is preferably a high frequency transformer provided to step down the voltage input into the transformer to a lower voltage for powering the LED connected between terminals CN 5  and CN 6 . The transformer T 1  also provides a feedback signal to the dynamic dummy load  81  via the feedback channel  90 , which in the present implementation includes diodes D 9  and resistor R 27 . The output of the transformer T 1  is electrically connected to power rectifiers D 6  and D 3  which rectifies the high frequency AC output to direct current to be provided to the LED at terminals CN 5  and CN 6 . Since the rectified DC output provided by rectifiers D 6  and D 3  will be pulsing, capacitor C 3  is electrically connected between the rectifier output and ground to filter and smooth the output from rectifiers D 6  and D 3 . 
     Circuit elements diode D 5  and capacitor C 5  are connected to another output of transformer T 1 , and provide an auxiliary power supply to converter ICU 1 , thereby reducing the power dissipation of linear regulator U 2 . 
     Capacitor C 4 , and resistors R 2  and diode D 2  act to suppress high voltage spikes across main switch MOSFET Q 1 . 
     Dummy load  81  includes a two transistor current source, wherein the current through MOSFET Q 2  is regulated by a quantity determined by resistor R 28  and the base to emitter voltage drop of transistor Q 3 . 
     The surge limiting circuit is electrically connected to the dynamic dummy load  81  and the bridge rectifier  80 . The surge limiter includes a two transistor current source formed by MOSFET Q 4  and transistor Q 5 . The surge limiter limits the maximum current allowed through the LED converter electronics. The surge limiting circuit includes zener diode D 12  which functions to limit the voltage across the gate and source of MOSFET Q 4  is maintained at a safe level. 
     The embodiments disclosed herein are merely examples of implementations of the invention claimed, and are not meant to limit the scope of the invention. One skilled in the art will recognize that other implementation will achieve the claimed invention.