Field of the invention:
The invention relates in general to electronic control circuits, and in particular to a control circuit for reliable dimmable high power LED lamps.
Background and prior art:
LED lamp technology is THE FUTURE OF LIGHTING and to some extent has already arrived due to its advantages over incandescent and fluorescent lamps, as given below:
1. LED lamp consumes just approx. 40 % power compared to the power consumed by Compact Fluorescent Lamps (CFL) and approx. 17% compared to Incandescent Lamps (IL) for the same lumens.
2. Life of LED lamp is approx. 10 times the life of CFL and 50 times the life of IL.
3. LED lamp's dimming is simple and cost effective; CFLs are dimmable only through complex electronics (hence dimmable CFLs are not generally available).
4. LEDs offer a green lighting technology compared to CFLs. Mercury used in CFLs is hazardous.
Though LEDs are comparatively costly at the moment, cost is bound to come down due to large investments being made in the R & D for inventing new cost effective materials, with a view to increase lumens per watt. Innovations in production technology are also happening to achieve lower cost of production, especially with economies of scale. In the STATE OF ART section ,the various problems encountered in
the lamp design concerning parallel sharing, thermal run away, limitations in utilizing LED's power to the maximum possible extent and problems of thermal equalization for the LEDs working in conjunction with constant current source controllers have been enumerated . This invention presents an innovative solution to these problems as enumerated in the OBJECTIVES section.
State Of Art
A high power LED lamp consists of series connected LED devices making a string; many such strings are connected in parallel to arrive at the desired rating of the lamp. These strings share different currents depending on the voltage band of forward conduction characteristic of LEDs. It is not practical to order fully matched LEDs as it is going to be very costly. Even' the LEDs from same production batch are known to have +/- 20% forward voltage mismatch. Hence, though overall current limit of the lamp is not allowed to be exceeded by current source, the individual string's current may differ as much as 100 % due to mismatch of the forward characteristics of individual LEDS.
As LEDs heat up, the problem of parallel sharing gets further aggravated as forward voltage drop of LED becomes less at higher temperature(due to -ve temperature coefficient). The LEDs sharing higher currents due to mismatch run hotter, and thus shall carry even more current due to reduced voltage drop resulting from higher temperature, causing thermal runaway. To reduce the gravity of the situation, all the LEDs are mounted on a common heat sink for thermal equalization as far as possible. This makes the design very complex and costly. Also each individual LED lamp needs a buck or boost- buck switch mode constant current source matched to the lamp-making the
lamp very costly. In the prior art, several different methods of parallel sharing of LED lamps exist.
Accompanying Fig-1, Fig-2 and Fig-3 present 3 prevalent basic methods for ensuring parallel sharing. It can be noticed in Fig-1 that putting a resistor in series with each lamp is very lossy and does not ensure perfect sharing and if resistor value is selected less, the danger of thermal runaway still persists.
A current mirror method using active transistors Ql and Q2 as per Fig-2 ensures much better parallel sharing but needs to have matched transistors and also thermal equalization of the Transistors. This method cannot be used for more than 2 strings in parallel and hence cannot be applied universally. (Ref-LEDs Magazine-issue Feb-2009, article by Steve Robers titled 'A current mirror and monitor can save a parallel string from failure').
Fig-3 presents yet another prevalent method used for non-dimming type LED lamps, wherein switch mode type constant current regulator is provided in each arm (linear types are ruled out due to heavy loss in the regulator) - This configuration shall also ensure equal current sharing, it is however not very cost effective solution as switch mode regulators are complex in nature having large component count-reducing the reliability. Design of constant current type controllers is very much sensitive to input supply voltage variations range and compensation requirements are also different for continuous mode and discontinuous current modes (hence not recommended for dimming as both the modes shall be present during dimming) , on the contrary constant voltage controlled switch mode sources (which are proposed in this invention) do not face these complications. -Hence this is not a preferred solution.
An Japanese patent publication no. JP 2003142281(A) ,dtd 16th May 2003, presents 'A Driving Circuit for LED Display Device 'which can control the brightness of the lamp using a 3 terminal regulator. It can be used for dimming but it also uses a series resistor with each LED string for parallel sharing. It is well known that 3 terminal regulators are very power consuming specially when input voltage variations are large and thus this method is highly inefficient and does not solve the thermal runaway problem.
Another US patent Application no. US2004/0036418 Al, dtd 26th Feb 2004, presents a closed loop control using constant current switching techniques to ensure parallel sharing between multiple number of strings within +/- 20 % based on the sensing of summated current of all the strings(or arrays) in a common inductor through a common current switch controller working in conduction with individual array switch controller with at least one of them having current limit . It does not offer dimming feature and requires a complex controller to be mounted with each lamp. It is basically designed for automotive applications wherein individual arrays can be switched at different times depending on the requirement.
The invention as described in detail in DESCRIPTION section , presents a universal solution which is cost effective. It offers a dimmable LED lamp with perfect parallel sharing between the strings(or arrays) and also offers a design enabling the application of an individual or a common constant Y0,ta9e source increasing the flexibility and reducing the cost when a common controller is used for multiple lamps. It is not possible to use common current source for multiple lamps.
There is also no need of mounting the LEDs on a common heatsink or for thermal equalization of any othe,r component- in fact it is shown
that this feature shall result in to better utilization of LED's power and make the electrical and mechanical design quite flexible and very cost effective.
Objects of the invention:
1. An object of the invention is to provide a high power LED lamp whose parallel strings shall always carry equal current irrespective of different forward characteristics of LEDs even under different dimming conditions.
2. Another object of the invention is to utilize LED's power to maximum possible extent and to achieve flexibility in mechanical and electrical design by avoiding the necessity of mounting the LEDs on a common heatsink.
3. Yet another object is to have cost effective and compact lamp intensity cum parallel sharing control for each lamp which will work in conjunction with the switch mode constant voltage source as per 4 below.
4. A further object is to provide a cheap switch mode current limited constant voltage source instead of constant current source controller for each lamp. Due to this feature , one can also use a common constant voltage source for multiple lamps (which is not possible with the application of existing constant current source technology),making the overall system much cheaper.
5. A still further object is to outline a tuning procedure for the controllers as to achieve minimum power loss in the low drop out regulator
Brief description of the accompanying drawings:
Fig.l shows a circuit for parallel sharing of LED strings in the prior art using only resistors as control elements.
Fig.2 shows the current mirror circuit for parallel sharing in the prior art.
Fig.3 shows constant current regulator for each string in the prior art.
Fig.4 shows a high power LED lamp with the control circuit according to the invention.
Fig.5 shows individual constant voltage source (CVS), interfaced to each high power LED lamp according to the invention.
Fig.6 shows a common constant voltage source (CVS) for a multiple number of high power LED lamps according to the invention.
Fig.7 shows a linear low drop-out voltage regulator used in the control circuit according to the invention.
The invention will now be described in an exemplary embodiment as depicted in the accompanying drawings. There can however be other embodiments of this invention all of which one deemed covered by this description.
Description
The invention with all the details with respect to its realization are as described here below. The objectives of the circuit disclosed herein can be realized in different ways by the practitioners of the art, using different hardware elements or even through mix of hardware and software, to achieve the same objectives.
This description therefore is exemplary and non-limiting.
The high power LED lamp with the invented control circuit (4.0) is presented in the Fig-4. To make the controller work ,it is to be connected to a constant voltage source(CVS) as shown in Fig-5.For the
sake of explanation, we have assumed that the lamp has 4 parallel strings ,each string having 3 LED devices in series. However these series and parallel numbers can be different depending on the power rating of LED lamp, power rating of individual LED device and voltage rating of the direct current (DC) power source. Description as given below is for one string- the same holds good for other 3 strings
1. Each LED lamp uses the invented control circuit(4.0) as per Fig-4. The 4 nos. parallel strings of LED devices (each string using 3 LEDs in series connected to the nodes 3-31/31-13/13-1), has been shown in Fig-4 as an illustration to show the connectivity of control circuit with LED devices, used in the lamp.
2. LED lamp using the invented control circuit as per Fig-4 is to be connected to a matched constant voltage output source (CVS) as per Fig-5 to become operational. We design a CVS from the variable voltage input available with the customer .It can be a buck and /or boost regulator from a DC source or It can be designed suitable for input from the AC mains (in that case the lamp can work from AC Mains). The CVS , according to this invention is to be specially designed to give the matched output voltage to the invented controller, irrespective of input supply
i
and load variations, as described in points 4 and 5 below.
3. Referring to Fig-4,each string of LEDs is provided with a linear low drop out voltage regulator Ul (which has been functionally depicted in Fig-7).Low drop out voltage regulator are available as integrated circuits in different packages. One can find a 0.2V to 0.4V drop voltage(=VI-VO) regulator at LED rated current of say typical 350ma or 750 ma. Pin nos shown in Fig-7 are only illustrative, and only bare minimum essential pins have been shown as some regulators may have more features which are
not important or relevant to this disclosure. Low drop out regulator sets the output voltage based on the ratio of resistors connected at pinl(ADJ)- to Vout i.e. Rl and ADJ to GND i.e. R5 respectively, Vref is generally generated internally by IC. Vref and pinl voltage are fed to -ve and +ve pins of OPAMP ,so that Vout gets adjusted to equalize pinl input to Vref.
4. Value of output voltage of constant voltage dc source is calculated as sum total of forward voltage drop of LED device (maximum value for the batch) at rated current multiplied by no. of LEDs in series, plus VI-VO rating of the regulator plus small line drop voltage from DC source to the string . Output voltage of low drop out regulator is programmed as equal to maximum forward voltage of LED device at the rated current for the given batch multiplied by no. of LED devices in series per string.
5. The output voltage of CVS is designed to remain constant irrespective of load and supply variations.
6. Current of each string is sensed (R17) and fed as input to the + pin of operational amplifier U3A (OPAMP) connected Integral (I) regulator using CI as shown in the Fig-4. Variable current reference is fed to negative pin of OPAMP.
7. Output of I regulator acts on adjustable pin of low drop out regulator through RIO in such a way that output of low drop out regulator gets automatically adjusted to drive the set current in the respective string.
8. The circuit (4.0) presented shall have a few other routine components including input /output filter and bypass capacitors for compensation, which have not been shown for brevity. These are anyway required for successful operation.
9. This circuit is repeated for remaining 3 strings of the LED lamp.
10. Maximum and minimum current limits are set by a common potentiometer (R18).This feature shall ensure that all the strings can be dimmed simultaneously and also shall always carry equal currents.
11. It is possible to shutdown the lamp by applying the appropriate voltage to shut down pin 2,through R9. Hence the lamp can be continuously dimmed as well as fully shutdown.
12. It can be noted that power loss in low dropout voltage regulator shall be negligible if proper precautions in the design are taken as elaborated below. Its maximum value shall be equal to VI-VO times rated current1 of LED device , where VO is the expected out put voltage at rated current, the worst case value of which shall be corresponding minimum forward voltage drop of LED for a particular batch. Minimum theoretical voltage drop in low dropout regulator for say 350 ma device shall be 0.2 to 0.4V(as per specs of low dropout voltage regulator) .This multiplied by current 350ma »0.30watts shall be the minimum loss and maxm loss shall be approx 0.6watts(as calculated below, for a string having 3 devices in series with Fmin=3.1V and VFmax=3.5 volts using the procedure given in SN 3 and herein). Value of constant voltage source out put voltage=3X3.5+0.4+0.3V(say- line drop) * 11.2 volts ,and VO minm =3x3.1 +line drop 0.3V=9.6V,hence loss in the regulator =(11.2-9.6 )x0.350«0.6wattsJ However, to reduce the the loss in low dropout voltage regulator, output voltage of constant voltage source can be fine tuned using a POT in the constant voltage source circuit, based on realistic values of VF maxm for a given batch instead of taking general VF maximum and1 VF minimum limits given by
the supplier(As the suppliers will normally play safe and give broad general purpose limits).
13. A common voltage source for multiple number of lamps can be provided as per Fig.-6 making the design very cost effective and flexible. Intensity of all the lamps connected to a common voltage source can be continuously varied simultaneously through potentiometer (R23) provided on the common source.
14. One can make hybrid circuit using chip dies/or an Application Specific IC (ASIC )based on the basic minimum circuit (as per Fig-4),hence such variations can only be treated as derived from this invention only.
15. Similarly ,one can integrate CVS with the basic minimum circuit as per Fig-5 also in a common hybrid or ASIC and the same should be treated as derived from this invention.
WE CLAIM
A control circuit (4.0) for reliable dimmable high power LED lamps, each LED lamp being is formed with a plurality of LED devices connected in a plurality of strings and each LED lamp being connected (5.0) to a matched constant voltage output source (CVS) to be operational, the control circuit (4.0) comprising
- a linear low drop-out voltage regulator (Ul),one per string
- a plurality of OPAMPS (U3A, U4A,U5A,U6A) configured as Integral Regulator for comparing current feedback from each said string,
- a common potentiometer (R18) for generation of a current reference for all the strings and a plurality of OPAMPS (U3A, U4A, U5A,U6A) configured as Integral Regulator for comparing current feedback from each said string,
characterized in that the control circuit (4.0) ensures equal current sharing amongst the strings even under dimming condition and also ensures that thermal equalization of individual LED devices is not required and LED devices can be mounted on individual heat sink.
2. The control circuit as claimed in claim 1, wherein the said string comprises of a plurality of LED devices connected in series, and a plurality of such strings are connected in parallel to form the said high power LED LAMP.
3. The control circuit as claimed in claim 1, wherein the intensity of the said high power LED lamp can be controlled by varying the common potentiometer (R18).
4. The control circuit as claimed in claim 1, wherein the constant voltage source (CVS) converts the variable input voltage from either a DC or an AC source to a constant output voltage which is predetermined and which lies in the range of 11.2 V and 9.6 V for the illustrated lamp, and which can be calculated for other configurations of the lamp using the procedure outlined in this invention.
5. The control circuit as claimed in claim 1, wherein the constant voltage source (CVS) is of buck/boost switch mode type.
6. The control circuit as claimed in claim 1, wherein the CVS can be connected as a common source (6.0) for a plurality of high
power LED lamps, (LAMPi,LAMP2.......LAMPn) when the intensity
of all the connected lamps can be varied simultaneously through potentiometer (R23) provided on the common source.
7. The control circuit as claimed in claim 1, wherein the high power LED lamp can continuously be dimmed and even shut down by applying an appropriate voltage to shut down pin 2.
8. The control circuit as claimed in claim 1, wherein the maximum and minimum current limits of all the said strings are set by common potentiometer (R18) so that all the strings can be dimmed simultaneously and carry equal currents.

A control circuit (4.0) for reliable dimmable high power LED lamps, each LED lamp being is formed with a plurality of LED devices connected in a plurality of strings and each LED lamp being connected (5.0) to a matched constant voltage output source (CVS) to be operational, the control circuit (4.0) comprising
- a linear low drop-out voltage regulator (Ul),one per string
- a plurality of OPAMPS (U3A, U4A,U5A,U6A) configured as Integral Regulator for comparing current feedback from each said string,
- a common potentiometer (R18) for generation of a current reference for all the strings and a plurality of OPAMPS (U3A, U4A, U5A,U6A) configured as Integral Regulator for comparing current feedback from each said string,
characterized in that the control circuit (4.0) ensures equal current sharing amongst the strings even under dimming condition and also ensures that thermal equalization of individual LED devices is not required and LED devices can be mounted on individual heat sink.