Abstract:
A driver for driving a plurality of light emitting diodes (LEDs) is formed of a plurality of LED controllers connected in series between a power supply and a reference voltage. Each controller drives one or more LEDs directly connected to it. Each controller has a voltage input terminal coupled to an output terminal of an adjacent upstream controller, and an output terminal coupled to the voltage input terminal of an adjacent downstream controller. Each controller has a normally-on bypass switch coupled between its voltage input terminal and the voltage input terminal of the adjacent upstream controller. The bypass switch completely bypasses the adjacent upstream controller when the adjacent downstream controller detects that its input voltage is below a threshold insufficient to drive the LED in the adjacent upstream controller. The bypass switch is turned off if the voltage is above the threshold.

Description:
FIELD OF THE INVENTION 
   This invention relates to light emitting diode (LED) drivers and, in particular, to stacked LED controllers that are automatically and successively enabled based on the magnitude of the supply voltage. 
   BACKGROUND 
     FIG. 1  illustrates a conventional string of LEDs (LED 1 -LEDN) driven by a supply voltage source  12  and a current source. In the example of  FIG. 1 , the current source is a MOSFET  14  whose conductivity is controlled using a current detector  16  (e.g., a low value resistor), a controller  18 , and an Iset signal. The voltage drop across the detector  16  is compared to a reference, provided by the Iset signal. The controller  18  controls the MOSFET  14  to cause the voltage drop to correspond to the Iset signal. Many other types of current controllers can be used. 
   The brightness of the LEDs is controlled by controlling the current through the LEDs. The voltage supplied by the voltage source  12  must be at least as great as the total voltage drop across all the LEDs plus the voltage needed for operation of the current source. The voltage drop of conventional LEDs is between 2-4 volts. Depending on the type of LED, the currents can range from 20 mA-100 mA, for low power LEDs, to 300 mA-1 A for high power LEDs. 
   LEDs are frequently connected in series and parallel, depending on the available power supply voltage, the required brightness, the colors to be controlled, and other factors. One increasingly popular use of LEDs is in a light fixture, driven by household current, where many LEDs are connected in series due to the high voltage. Connecting multiple LEDs in series is also common for large backlights of LCDs where high brightness is required, and where LEDs of the same color (e.g., red, green, or blue) are connected in series so they can be controlled using a single current source for each individual color. LEDs of different colors have different electrical characteristics, such as voltage drops, since they are formed of different materials. 
   Since LEDs of different colors and from different manufactures have different electrical characteristics, it is difficult to design an efficient LED drive system that can be used with any type of LED. Inefficiency increases when excess power supply voltage is used since the excess voltage is dropped across the current source MOSFET. The prior art systems require excess voltage when driving a serial string of LEDs since, if the supply voltage is even barely insufficient to drive the entire string of LEDs, all the LEDs are off. 
   In cases where the supply voltage is not regulated, such as a battery or a rectified AC signal, all the LEDs in the string will be turned off once the instantaneous supply voltage level drops below a threshold level. 
   It would be desirable to have an efficient LED driver for driving many LEDs, of any type, where only those LEDs that can be driven by the power supply are energized. It is also desirable to have an LED driver that can use a rectified AC voltage where all the LEDs do not turn off together once the instantaneous AC voltage drops below a threshold. 
   SUMMARY 
   In one embodiment of the invention, an LED driver system comprises a serially connected string of LED controllers. Each controller drives one or more LEDs directly connected to it. In the following descriptions, it is assumed that each controller drives one LED; however, each controller can drive any number of LEDs. 
   Each controller comprises a current source for its LED, a voltage detector that detects whether its input voltage exceeds a threshold needed for driving the LED, and a bypass switch controlled by the voltage detector for bypassing the adjacent upstream controller depending on the detected input voltage level. In one embodiment, the voltage detector also shunts excess current through the controller if the upstream and downstream current is greater than the current set for the LED. This allows for different LEDs connected to the stacked controllers to be driven by different currents. In contrast, the prior art series LEDs all had to conduct the same current. 
   If the power supply voltage is sufficiently above the combined voltage drops of all the LEDs, all of the normally-on bypass switches are turned off, so all the controllers and LEDs are energized. If the supply voltage is less than that needed to drive all the LEDs, only those controllers/LEDs that can be adequately driven by the power supply are energized, starting from the most downstream controller, and the remainder are bypassed by the switches. 
   Accordingly, the maximum number of LEDs connected to the stacked controllers will be energized by the available power supply voltage. This prevents total failure of the LED string for under-voltage situations and provides greater flexibility in the design of LED circuits. Further, the lighting designer does not have to provide a power supply voltage for worst case scenarios to ensure the LEDs are energized, since any power supply voltage less than required for the worst case scenario is still guaranteed to energize some LEDs. Any excess voltage above that required to drive all LEDs increases inefficiency. 
   In an example of the controllers being used for an LED light fixture driven by rectified but unfiltered household current, the LEDs will successively turn on, starting from the most downstream LED, and then successively turn off starting from the most upstream turned-on LED, as a result of the varying instantaneous voltage. This is a vast improvement compared to driving one or more serial strings of LEDs using a rectified AC signal, since in such a prior art configuration all the LEDs in a string would only turn on when the instantaneous voltage exceeded the combined voltage drops of all the LEDs. 
   Also, as compared to the prior art, the LEDs used in the present invention can be driven at a lower peak current when an AC supply is used, while achieving the same brightness level as the prior art systems with the same number of LEDs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional serial string of LEDs driven by a power supply and a current source. 
       FIG. 2  illustrates a serial connection of controllers for LEDs in accordance with one embodiment of the invention. 
       FIG. 3  illustrates the “bottom” three controllers of a serial connection of any number of controllers and the circuitry in each controller in accordance with one embodiment of the invention. 
       FIG. 4  illustrates a top controller connected to a power supply via a high voltage depletion mode MOSFET. 
       FIG. 5  illustrates one type of current source (using a simple linear regulator) that may be used in a controller of  FIG. 3 . 
       FIG. 6  illustrates a type of generic current source that may be used in a controller of  FIG. 3 . 
       FIG. 7  illustrates an LED light fixture that is connected to standard household current. 
       FIG. 8  is a flow chart illustrating basic steps performed by the circuit of  FIG. 2 ,  3 , or  6  for dynamically enabling only those LEDs that can be driven by the power supply voltage. 
   

   DETAILED DESCRIPTION 
     FIG. 2  illustrates identical controllers  20 A- 20 N, each connected to a respective LED (LEDs  1 -N). There may be any number of controllers  20  and LEDs. Instead of a single LED connected to a controller  20 , multiple LEDs may be connected in series and/or parallel to a single controller, and the controller circuitry would be suitable modified, such as modified to provide an increased current for driving multiple LEDs in parallel. In another embodiment, the current supplied by a controller to its respective LED may be different from the current supplied by another controller to a different type of LED. 
   Additionally, RGB LEDs connected to each controller  20  may be driven individually by the controller  20  to achieve virtually any color, including white, by controlling the relative brightness of each RGB color component. 
   The controllers  20 A- 20 N are connected in series between a supply voltage source  24  and ground. The supply voltage may be a constant DC voltage, a rippling voltage, a rectified AC voltage, a non-regulated voltage, or any other type of voltage. Instead of ground, any reference level may be used. 
   An optional current controller  26  may be used if it is desired to dynamically adjust the LED currents for varying brightness rather than have fixed currents. The current control signal may be a reference signal, a resistance, a current, a voltage, a PWM signal, an analog signal, a digital signal, or any other control signal related to the currents supplied by the controllers  20  to their respective LEDs. The power supply current path is shown by vertical path  28 , while the current control path is shown by vertical path  30 . 
   A switchable bypass connection  32  is shown for selectively bypassing each controller  20 , except the bottom controller  20 A. Each controller includes a bypass switch for bypassing the adjacent upstream controller  20 . Any number of controllers  20  except the bottom controller  20 A can be bypassed if there is insufficient voltage to power all the LEDs. Depending on the available voltage, the controllers  20 , starting from the bottom controller  20 A, are successively energized until there is no longer sufficient voltage to drive any additional LEDs, and any upstream controllers  20  are bypassed by their bypass connection  32 . For example, if the supply voltage source  24  only supplied enough voltage to drive two LEDs, then all the controllers  20  above controllers  20 A and  20 B would be bypassed by their bypass switch connections  32 . 
   Each controller  20  can be formed of discrete components or any combination of integrated circuitry and discrete components, with any suitable pins for the LED connection and optional current setting signals/components. In one embodiment, all controllers  20  and all components except for the LEDs are formed in a single integrated circuit. Further, a single package may house an integrated controller and its controlled LEDs. Using advanced fabrication techniques, a controller and its LEDs may be integrated on a single chip. 
   An LED does not have to be coupled to every controller  20  for the circuit to operate properly, and one or more LEDs may fail without disabling the entire system. 
     FIG. 3  illustrates the circuitry inside each controller  20 , in accordance with one embodiment. There are many ways to implement the basic functions of the controller  20 , and all those ways are envisioned by the present invention. The current controller  26  and current control path  30 , shown in  FIG. 2 , is not employed in the circuit of  FIG. 3  for simplicity, but providing an external circuit to control the LED current supplied by each controller in  FIG. 3  is a simple task. 
   Only the bottom three controllers  20 A,  20 B, and  20 C in a serial string of controllers are shown in  FIG. 3 . There may be any number of additional controllers, and they may be identical or supply different currents to their respective LEDs. A power supply voltage source  38  is connected to the top controller in the string, and the bottom controller is connected to ground or another reference voltage. The voltage  28  coupled to controller  20 C is that voltage that has been dropped across any upstream controllers or any conducting bypass switches. 
   The bypass switches Q 1  are normally-on types, such as n-channel depletion mode MOSFETs. An n-channel depletion mode MOSFET has a conducting n-channel when its gate is either at or above its source potential. The MOSFET turns off when the gate is more negative than the source by a threshold amount. 
   When a voltage is initially applied to the topmost controller in the stack (e.g., controller  20 N in  FIG. 2 ), all the bypass switches Q 1  in the stack of controllers are on, so the full voltage is applied to the bottom controller  20 A via the normally-on bypass switches. 
   A zener diode  34  in controller  20 A has an on-threshold slightly higher than the voltage needed to turn on the LED in controller  20 A, so the zener diode  34  does not affect the current through the LED in controller  20 A. 
   The current through the LED in controller  20 A is controlled by a low dropout regulator  36  (LDO  36 ) and a low value sense resistor R 1 . A simple LDO is shown in  FIG. 5 , to be discussed later. Any other current source may also be suitable. The input voltage to the LDO  36  is applied to a terminal of a pass transistor internal to the LDO  36 , and the output of the LDO  36  is a second terminal of the pass transistor. The anode of the LED is connected to the output of the LDO  36 . The current through the LED flows through the sense resistor R 1 . The voltage drop across the resistor R 1  is applied to a voltage sense input of the LDO  36 . The LDO  36  controls the conductivity of the pass transistor so that the sense voltage equals a fixed reference voltage, typically generated internal to the LDO  36 . In this way, current through the LED is precisely set by the value of the resistor R 1 . If the controllers  20  are formed as integrated circuits, the resistor R 1  may optionally be external to the IC package to enable the user to set the current. 
   Capacitors C 1  and C 2  are used for smoothing any voltage spikes, typically caused by the switching of the bypass switches Q 1 , and to prevent oscillations in the LDO  36 . 
   The voltage applied to the controller  20 A is assumed to be at least slightly higher than that needed to drive a single LED. The excess voltage applied to the controller  20 A turns on the zener diode  34 , which conducts a current through a resistor R 2 . When the voltage drop across the resistor R 2  equals the Vbe of the bipolar transistor Q 2 , the bipolar transistor Q 2  turns on. This pulls the gate of the MOSFET Q 1  to a low level (lower than its source) to turn the MOSFET Q 1  off, thus enabling the controller  20 B. If the bipolar transistor Q 2  were later turned off, a resistor R 3 , connected between the gate and source of the MOSFET Q 1 , would cause the gate and source of the MOSFET Q 1  to be at equal voltages so as to turn the MOSFET Q 1  back on. 
   The combination of the zener diode  34 , resistor R 2 , and bipolar transistor Q 2  serves as both an “excess voltage” detector to control the bypass switch MOSFET Q 1  and as a shunt element to shunt any excess current around the LED to the output of the controller  20 , to be further explained later. The threshold of the zener diode  34  must be such that (V ZD +V BE )&gt;(V SENSE +V LED +V LDO     —     DROP ), to ensure that there is sufficient voltage to turn on the LED. The zener diode  34  in a controller  20  must turn on at a voltage somewhere between the voltage needed to turn on the LED driven by the controller and the voltage needed to also turn on the LED in the adjacent upstream controller. In one embodiment, the voltage needed to turn on the zener diode  34  is about 1 volt or less above the voltage needed to turn on the LED. 
   Only when the MOSFET Q 1  in controller  20 A is turned off is current allowed to energize the upstream controller  20 B. If the voltage across controller  20 B is above that needed to turn on its LED, the controller  20 B will energize its LED, and current will flow through the LED and through the downstream controller  20 A. If the voltage across the controller  20 B is sufficient to turn on its zener diode and bipolar transistor Q 2 , the bypass MOSFET Q 1  in controller  20 B will be turned off to cause the next upstream controller  20 C to receive current. The same scenario applies to each controller  20  in succession towards to the power supply until there is equilibrium, where the maximum number of LEDs are driven. 
   In the event that the bipolar transistor Q 2  in the controller  20 A attempts to shut off its bypass MOSFET Q 1  but there is insufficient voltage remaining to turn on the LED or zener diode  34  in the upstream controller  20 B, then shutting off of the MOSFET Q 1  in the controller  20 A would result in no current being be passed by controller  20 B to controller  20 A. Therefore, in such an event, the controller  20 A is inherently prevented from turning off its bypass MOSFET Q 1  if the upstream controller  20 B will not have enough voltage to drive its LED. This applies to any of the controllers. 
   As seen, the turning on of the zener diode  34  and bipolar transistor Q 2  in each successive controller  20 , based upon the voltage available for the upstream controllers, results in only those controllers  20  that can adequately drive their LEDs to not be bypassed by a turned off MOSFET Q 1 . 
   In the event that the current setting resistor R 1  in controller  20 B is selected to cause the LED in controller  20 B to be driven by a current that is higher than the current set for the LED in controller  20 A, this excess current is shunted by the conducting zener diode  34  and base-emitter diode of transistor Q 2  in the controller  20 A. This shunting feature is applicable to all the controllers. Therefore, the controllers  20  allow each LED to be driven by a different current. In prior art strings of LEDs, such as shown in  FIG. 1 , this would be not be an available option since the same current must flow through all the LEDs connected in series. Additionally, the shunting feature allows an LED to fail as an open circuit without disabling the downstream controllers. 
   As an additional feature of the circuit of  FIGS. 2 and 3 , since the bottommost controller  20 A is never bypassed and can operate at very low supply voltages, the bottommost controller  20 A can be used for additional functions requiring power. For example, the controller  20  A may also dynamically control the LED current of the whole light fixture (e.g., perform the function of the current control  26  in  FIG. 2 ). The controller  20 A can control any suitable circuitry or components in addition to those shown within the controller  20 A in  FIG. 3 . 
   The MOSFET Q 1  of the topmost controller (shown as Qtop in  FIG. 3 ) connected to the voltage supply  38  dissipates the difference between the total supply voltage and the sum of the controller drops, which would be slightly higher than the LED drops. 
   In one embodiment, shown in  FIG. 4 , all controllers  20  are identical, using standard low voltage technology, but the drain of the low voltage MOSFET Q 1  of the top controller  20 N is not connected. Instead, the MOSFET Q 1  gate control terminal of the top controller  20 N is connected to an external high voltage depletion mode MOSFET, labeled Qtop (HV) in  FIG. 4 . The MOSFET Qtop (HV) is connected between the voltage supply  38  and the upper supply input terminal of the top controller  20 N. The high voltage MOSFET Qtop extends the voltage range and power dissipation capability, since it drops the voltage difference between the controllers  20  and the voltage supply  38 . This also adds flexibility to the design since the MOSFET Qtop (HV) may be chosen separately from the controllers when implementing the system for a particular application. 
   To optimize efficiency, the voltage drops across all components should be made as low as possible while still achieving the proper function. Any of the controller components may be other than those used in the example to accomplish the basic functions of the controllers. 
   Using the present invention, the power supply voltage V PS  is distributed between the active controllers  20  and the “on” bypass switches. Even an on bypass switch drops a small voltage. If M of N controllers  20  are activated, then V PS &gt;V 1 +V 2 + . . . +VM+(N−M)*V S , where V 1  through VM is the voltage drop across each activated controller  20  and V S  is the voltage drop across each on bypass switch. 
   Because of the controllers  20  being activated seriatim, based on their ability to be driven by the available voltage, virtually any number of controllers may be connected serially without the user worrying whether the power supply can drive all of the LEDs. 
     FIG. 5  illustrates a simple current source that can be used in each controller  20  to set the current through its LED. An LDO comprises a pass transistor  50  and an error amplifier  52 . The input voltage Vin into the controller is applied to one terminal of the transistor  50 , and the LED  54  is connected to the other terminal of the transistor  50 . The current through the LED  54  flows through the sense resistor  56 . The voltage dropped across the resistor  56  is compared with a reference voltage V REF , and the error amplifier  52  controls the conductivity of the transistor  50  to keep the sensed voltage equal to the reference voltage. The resistor  56  “ground terminal” is just the “common voltage” of the LDO (to which V REF  is referenced) and may not be zero volts. 
     FIG. 6  is similar to  FIG. 5  but envisions that any suitable circuitry may be used in amplifier  60  to generate a controlled current through LED  54 . Current mirrors or other circuitry may be used in amplifier  60  to generate the output current. The current source may even be a small switching regulator. 
   The present invention is particularly advantageous when used in an LED light fixture driven by 120 VAC at 60 Hz (or 115 VAC/230 VAC at 50 Hz in Europe). As shown in  FIG. 7 , the LED light fixture  66  may use a simple full bridge rectifier  68  without filtering to create a rippling DC at 120 Hz. Not using a filter allows the fixture to be small and inexpensive since large filter capacitors are not used. The maximum number of controllers  20 A- 20 N in series between the rectified AC terminals is that needed to drop the peak voltage of about 168 volts when all the controllers are enabled. If each controller requires 4 volts to drive its LED(s), there may be up to 42 controllers and at least 42 LEDs. There may of course be fewer or more controllers and LEDs. Each controller may drive multiple LEDs connected in series or parallel. All controller components may be mounted on a single small printed circuit board. As the voltage cyclically changes between 0 and 168 volts, the controllers will successively become enabled and disabled by the switching of the bypass switches. Thus the LED light will smoothly pulsate at 120 Hz, and only the average brightness will be perceived by the human eye. If the rectified 120 Hz voltage were used to drive a prior art type series connection of LEDs, fewer LED must be connected in series since they would have to turn on well prior to the peak voltage, and all would turn on and off at the same time. By using the present invention, more LEDs can be used in the light fixture, and the overall light output will be brighter. There will also be greater efficiency since there will be no large voltage drops using the present invention. 
   When using the invention with a rectified 120 Hz voltage (or 100 Hz in Europe), the LEDs closer to the neutral potential will have a higher duty cycle than the upstream LEDs, causing those downstream LEDs to appear brighter than the upstream LEDs. If this is not a desirable appearance, the LEDs may be arranged helically with the brighter LEDs toward the center to create symmetry. Alternatively, to equalize the perceived brightness of each LED, the upstream LEDs can be driven with progressively more current during each pulse of power. The product of the duty cycle times the instantaneous LED current would be the same for each LED. So, the decreased duty cycle will be offset by the increased brightness emitted during each cycle. The overall brightness of each LED will appear to be the same to the human eye. 
   The resistors R 1  for setting currents may be individually adjustable to separately set a desired current through each LED. This may be used to create a certain overall color if the LEDs were different colors, such as RGB. In another embodiment, each LED is a white light LED, typically using a phosphor. The overall brightness level can be dynamically controlled, such as with a dimmer control, by varying a current control signal to each controller  20 , as previously discussed. The circuit allows the light fixture to be dimmed using a regular AC light dimmer. 
   The color of LEDs changes slightly with the current through the LED. This is particularly problematic for prior art LED strings driven by an AC source, since the current through the LEDs changes as the instantaneous voltage changes once the LEDs are on. The present invention allows the current through each LED to be set to a well defined level, independent of the instantaneous supply voltage, so that the color emitted by the LED system does not change with the supply voltage. 
   Another application of the circuit is a voltage level detector, since the number of LEDs illuminated generally indicates the power supply voltage level. 
   A temperature sensor that either senses ambient temperature or the temperature of one or more of the LEDs may be incorporated into each controller to control the current to the LEDs to ensure that a threshold temperature of the LEDs is not exceeded. 
     FIG. 8  is a self-explanatory flow chart identifying the basic steps performed by the circuits of  FIGS. 2 ,  3 , and  7 . 
   Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. For example, a negative power supply may be used with the polarities of the components reversed. The various switches, transistors, and current sources may be any suitable types. Any component may be electrically coupled to another component using a direct wire connection, a resistance, or a non-linear element, as appropriate for an actual implementation. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.