FIELD OF INVENTION
[001] The present invention relates to the LED Drivers. The present invention in particular relates to an auto adjusting digitally controlled LED Driver.
DESCRIPTION OF THE RELATED ART
[002] LED components come into bins according to the minimum and maximum forward voltage which changes the power consumption in case of constant current driver due to which LED driver current needs to be adjusted very frequently in the production. The Auto adjusting feature will allow designers to select LED’s over a range of binning group. Below table is for a reference where the minimum forward voltage is 1.75V and maximum forward voltage is 3.75V of an LED, hence product made with combination of this LED for 280W may vary from 175W to 375W.
Forward voltage group
Minimum Forward Voltage @ 350 mA.
Maximum Forward Voltage @ 350 mA.
B
1.75
2.0
C
2.0
2.25
D
2.25
2.5
E
2.5
2.75
3
F
2.75
3.0
G
3.0
3.25
H
3.25
3.5
J
3.5
3.75
[003] Different LED products also uses different voltage current combinations for the same power level due to which various driver need to be designed and certified for each power and current level.
[004] Maintaining constant power delivery to LED with different LED forward voltage which is a very common problem faced by LED 5 Lighting product design. LED is a semiconductor junction and its VI characteristics is nonlinear. Due to which, once the voltage reaches to forward bias voltage, its current changes a lot faster than the change in applied voltage. Therefore, LEDs are commonly driven by current source rather than a voltage source. LEDs are placed in series and 10 parallel combination in product which leads to different voltage and current combination for same power of LED bank, this leads to variety of LED drivers to be designed for each voltage and current combination. Moreover, LED has high variation in forward voltage. It is sensitive to batch of production as well as operating temperature 15 which leads to piece to piece variation in consumed power at luminary level in the production. Adjusting every product in the production for
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input power is very time consuming and hence this invention targets to overcome this problem by a LED driver with auto adjusting feature.
[005] LED voltage tends to drop with the rising temperature which makes LED product to consume more power at start and reduce as the temperature of LED increases, which also leads to seasonal variation in 5 power consumption of LED lighting product. To overcome this the proposed auto adjusting LED driver continuously adjusts the power delivered to the LED by adjusting the power by sensing LED voltage and current.
[006] Both LED and LED driver’s life are proportional to the operating 10 temperature hence the life of LED product reduces drastically if LED product is operated higher than the rated temperature of the product. The proposed invention has a provision for thermal input to power control loop which adjusts the output target power as the thermal threshold is reached and extend the life of LED product against rising 15 ambient temperature, this makes LED luminary to operate to much higher temperature with auto reduced power making the product more rugged to be used in harsh conditions.
[007] Till now, LED designer uses current adjustable driver or programmable driver which provides option to adjust the output current 20 of LED at the time of manufacturing of LED product. These adjustments are available in form of rotary knobs, external resistor connection, switchable jumper/switches, digital programming port which changes the set output current. We used microcontroller to sense
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both output voltage and output current to dynamically adjust the output current to meet the LED bank requirement to feed the required power.
[008] Constant power LED Drive concept was published in US patent application US20080068298A1 where analog control system was used with voltage and current clamps where made with power transistor 5 which dissipates extra power by clamping the extra voltage and limiting the current. This type of method was lower in efficiency. Our invention is about proving right feedback to an off-line converter to adjust power conversion to just provide required power.
[009] US patent US8305004B2 uses analog control on offline power 10 factor controller which can maintain relatively constant power with change in input voltage only. Our method uses Digital control to sense the LED voltage and current and calculate right feedback required to the offline converter to maintain power constant over input as well as output voltage changes. 15
[010] US20110121760A1 uses thermal management logic for AC dimmable LED drive. This Patent requires LED thermal profile stored in the micro controller memory as Look-up table and based on that it adjusts LED current. In various embodiments, obtaining the thermal operating range of the LED includes referencing a look-up table. The 20 look-up table may include LED thermal-power data. Detecting the temperature of the LED may include receiving input from a thermal sensor. Adjusting power delivered to the LED may include setting the LED to its maximum brightness level within the thermal operating range. This patent requires thermal profile of specific LED used to be 25
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stored in the microcontroller. Our innovation is to cover wide range of LEDs which will detect forward voltage of LED itself and adjust the power delivered to maintain the specified power level of the LED. Thermal sensor senses the temperature and adjusts the LED power based on difference between set thermal limit and current temperature. 5
[011] US20100277077A1 discloses constant current source as an LED driver with fixed current. To maintain the life of the LED in the Luminary it uses another spare LED apart from the main LED of the Luminary and diverts the current to the spare LED to maintain the life of the Main LED in case of higher thermal operation as mentioned in 10 the below section of the application. The present invention can significantly enhance the life of LED chips and LED lamps that operates at high temperature. The invention comprises of a set of controllable spare LEDs to share the current of the main LED matrix drive by a constant current Source and reduce the main LEDs junction 15 temperature. Converter circuit, controller and sensor is included to enable the spare LED to work in the desired operating region.
[012] However, the present invention discloses an auto adjusting LED driver where current of the driver itself changes to maintain required internal temperature whereas above method uses fixed current drive and 20 diverts the current from the main LED load to spare LED bank.
[013] According to the present invention, LED Driver continuously senses the LED voltage and keeps adjusting LED current to maintain the power of luminary constant within a required tolerance which takes care of batch to batch and thermal variation of LED voltage as well as 25
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different combination of series parallel LEDs and allows user to use same driver for various products configuration without adjusting each time.
[014] Along with this there is thermal input to power control loop which adjusts the output current as the thermal threshold is reached and 5 extend the life of LED product against rising ambient temperature, this makes same LED luminary to operate to much higher temperature with reduced power making the product more rugged to be used in harsh conditions.
OBJECTS OF THE INVENTION 10
[015] The principal object of the present invention is to provide an auto adjusting LED Driver for constant power delivery to LED.
[016] Another object of the present invention is to provide an auto adjusting LED Driver which is connected with a DC load whose voltage and current is measured by a microcontroller. 15
[017] Still another object of the present invention is to provide an auto adjusting LED Driver wherein auto adjusting is realized by regulating the offset between target power and actual output power.
[018] Another object of the present invention is to provide an LED Driver wherein thermal protection is achieved through a temperature 20 sensor which controls the target power once the thermal threshold is achieved.
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SUMMARY OF THE INVENTION
[019] In accordance with the said objectives, the present invention provides a LED driver which provides constant power delivery to LED. Auto adjusting digitally controlled LED Driver here is referred to a driver design which consists of driver having a microcontroller. 5
[020] LED Driver is connected to a DC load whose voltage and current is measured by a microcontroller and it auto adjusts by regulating the offset between target power and actual output power. The actual output power is obtained by the product of sensed output voltage and output current. Also, the thermal protection is achieved through a temperature 10 sensor which controls the target power once the thermal threshold is achieved. Once the driver temperature crosses the threshold temperature thermal fold back takes place which reduces the target output power of the LED driver.
BRIEF DESCRIPTION OF THE DRAWINGS 15
[021] It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments.
[022] Figure: 1 Block diagram of the LED driver according to the 20 present invention.
[023] Figure: 2 Flow chart of the LED Driver according to the present invention
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[024] Figure: 3 graph illustrating constant output power with the variation of input line voltage from 100V to 440V
[025] Figure: 4 graph illustrating constant output power with the different output voltages with the variation of output voltage from 55V to 90V 5
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[026] It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative systems embodying the principles of the present subject matter. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition 10 diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and executed by a computer or processor, whether or not such computer or processor is explicitly shown.
[027] In the present disclosure, the word "exemplary" is used herein to 15 mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
[028] While the present disclosure is susceptible to various 20 modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the present disclosure to the particular forms disclosed, but on
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the contrary, the present disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the present disclosure.
[029] The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a 5 setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by “comprises… a” does not, without more 10 constraints, preclude the existence of other elements or additional elements in the system or apparatus.
[030] In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific 15 embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following 20 description is, therefore, not to be taken in a limiting sense.
[031] The present invention will be described in detail below with reference to an embodiment as shown in the drawing.
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[032] Accordingly, the present invention provides a method to maintain constant power delivery to LED with different LED forward voltage which is a very common problem faced by LED Lighting product design while using a LED driver which maintains the constant current delivery to the LED. 5
[033] LED is a semiconductor junction and its VI characteristics are nonlinear. Due to which, once the voltage reaches to forward bias voltage, its current changes a lot faster than the change in applied voltage. Therefore LEDs are commonly driven by current source rather than a voltage source. LEDs are placed in series and parallel 10 combination in product which leads to different voltage and current combination for same power of LED bank, this leads to variety of LED drivers to be designed for each voltage and current combination. Moreover, LED has high variation in forward voltage. It is sensitive to batch of production as well as operating temperature which leads to 15 piece to piece variation in consumed power at luminary level in the production. Adjusting every product in the production for input power is very time consuming and hence this invention targets to overcome this problem by a LED driver with auto adjusting feature.
[034] LED voltage tends to drop with the rising temperature which 20 makes LED product to consume more power at start and reduce as the temperature of LED increases, which also leads to seasonal variation in power consumption of LED lighting product.
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[035] To overcome this the proposed LED driver continuously adjusts the power delivered to the LED by adjusting the power by sensing LED voltage and current.
[036] Both LED and LED driver’s life are proportional to the operating temperature hence the life of LED product reduces drastically if LED 5 product is operated higher than the rated temperature of the product. The proposed invention has a provision for thermal input to power control loop which adjusts the output target power as the thermal threshold is reached and extend the life of LED product against rising ambient temperature, this makes LED luminary to operate to much 10 higher temperature with auto reduced power making the product more rugged to be used in harsh conditions.
[037] Referring to Fig.1, the block diagram of the LED driver according to the present invention, and Fig. 2, the flow chart of the LED Driver according to the present invention is disclosed. 15
[038] The LED Driver for constant power delivery to LED, comprises of LED Voltage (VLED) and LED current (ILED) which are fed to a multiplier (M) through which a product of VLED and ILED is received as current LED power (PLED). The PLED is compared with Target LED power (PTARGET) to determine an offset (OFF) between PLED 20 and PTARGET.
[039] The value of PTARGET is the power which is constantly provided to the LED according to the present invention.
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[040] Once the value offset (OFF), it is sent to a Pulse Width Modulation (PWM) block in a digital control block (10), wherein the digital control box (10) is made from a digital microcontroller. Pulse-width modulation (PWM), is a modulation technique used to encode a message into a pulsing signal. Although this modulation technique can 5 be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. In addition, PWM is one of the two principal algorithms used in photovoltaic solar battery chargers, the other being maximum power point tracking. In this particular 10 application PWM is used to generate and feedback signal proportional to the difference between current power and the target power.
[041] The average value signal is controlled by the ratio of on and off period at a fast rate. The longer the switch is on compared to the off periods, the higher the is the signal. 15
[042] The PWM switching frequency has to be much higher than what would affect the control system response time and control loop stability which is to say that the resultant waveform perceived by the load must be as smooth as possible. The rate (or frequency) at which the signal switches ON and OFF can vary greatly depending on application. 20
[043] The PWM signal is further converted to a proportional analog signal (PAS) through a RC Low Pass Filter (15) in Analog Feedback control block (20) wherein the PAS compares to the ILED and provides the control signal to the power stage to change the ILED to maintain the power constant to the PTARGET. 25
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[044] In one of the embodiments, the temperature data from one or more thermal sensors (TS) is feed to a thermal control block (30) to compare with predetermined thermal limit (TL).
[045] If current temperature (CT) is less than the set thermal limit (TL) then thermal control block (30) sets PTARGET at PSET (the rated 5 power) of the LED driver and if the thermal sensor (TS) input is higher than the Thermal limit (TL) then thermal control block (30) decreases the PTARGT below the PSET level to enable thermal protection and extended temperature operating region with reduced power level.
[046] Further, once the current temperature (CT) crosses the threshold 10 temperature (TL), a thermal fold back takes place which reduces the PTARGET of the LED driver.
[047] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific 15 embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various assembly that, although not explicitly described or shown herein, embody the principles of the present subject 20 matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without 25
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limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the aforementioned description when considered in connection 5 with the accompanying figures.
[048] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled 10 in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[049] It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single 15 component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other 20 dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the 25
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fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that 5 “consist essentially of” or “consist of” the recited feature.
[050] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and 10 methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present 15 subject matter.

WE CLAIM:
1. A LED Driver for constant power delivery to LED, comprising:
LED Voltage (VLED) and LED current (ILED) which are fed to a multiplier (M);
receiving product of VLED and ILED to calculate current LED 5 power (PLED);
comparing PLED with target LED power (PTARGET) and finding the offset (OFF);
sending offset (OFF) to a Pulse Width Modulation (PWM) block in a digital control block (10), wherein the digital control box 10 (10) is made from a digital microcontroller;
characterized in that the PWM signal is converted to a proportional analog signal (PAS) through a RC Low Pass Filter (15) in Analog Feedback control block (20) wherein the PAS compares to the ILED and provides the control signal to the 15 power stage to change the ILED to maintain the power constant to the PTARGET.
2. The LED Driver as claimed in claim 1, wherein data from a thermal sensor (TS) is feed to a thermal control block (30) to compare with a predetermined thermal limit (TL). 20
3. The LED Driver as claimed in claim 2, wherein if current temperature (CT) is less than the set thermal limit (TL) then
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thermal control block (30) sets PTARGET at PSET (the rated power) of the LED driver.
4. The LED Driver as claimed in claim 2, wherein if the thermal sensor (TS) input is higher than the Thermal limit (TL) then thermal control block (30) decreases the PTARGT below the 5 PSET level to enable thermal protection and extended temperature operating region with reduced power level.
5. The LED Driver as claimed in claim 2, wherein once the current temperature (CT) crosses the threshold temperature (TL), a thermal fold back takes place which reduces the PTARGET of 10 the LED driver.
6. The LED driver as claimed in claim 2, wherein LED Driver is connected to an LED matrix load.