EOT Characteristics of LEDs

This makes measuring LED output an important activity but their small size, directionality and narrow spectral emissions that traditional photometric and colorimetric measurement methods are not always the most appropriate. Measurements of major characteristics of a LED that manufacturers and end-users find most important are like total spectral flux, luminous flux, chromaticity, CCT (Correlated Colour Temperature), CRI (Colour Rendering Index), peak wavelength, dominant wavelength, current (I), voltage (V) and luminous efficacy in the field of lighting.

Techniques for measuring EOT

  The lamp measurement system will depend on three things that is measurement needed, physical size of the lamp, and magnitude of the lamp output. Lamp physical dimensions must also be considered. The sphere diameter must accommodate the maximum length of the lamp under test. The CIE recommends that for tubular fluorescent lamps, the sphere diameter should be at least twice the longest dimension of the lamp, and that for compact lamps the sphere diameter should be at least ten times the largest dimension of the lamp. There are different methods and considerations for lamp measurement radiometry and photometry. In general, the measurement of a lamp's luminous flux and its spectral radiant flux are identical except in how the radiation is detected. There are two methods for measuring the total flux of a lamp. The first employs a goniometer that scans the whole sphere of radiation from the source. Integration over this scan yields a very accurate measurement of the lamp output, but the process is not always easy or quick. The second method employs an integrating sphere to capture all the radiation from the source at the same time associated colour parameters are measured. A detector is used to measure the output of the sphere. By comparing the output with a test lamp in the sphere to the output of a known standard in the sphere, the total flux emitted from the lamp can be calculated. This procedure can be done easily and quickly.

Integrating Sphere

  The integrating sphere integrates the total light output of a tested source to produce a single measurement. It is a hollow sphere coated with Barium Sulphate of fundamental optical technology, commonly used in photometry, radiometry, for applications including the test and measurement of light sources (lamps, LEDs), sensors (spectrometers, camera systems), and material properties (reflectance and transmittance). It has become standard instrument in photometry and radiometry. Luminous flux is the total photometric power emitted in all directions from a light source, measured in lumens. With the proliferation of high brightness, LEDs are used for illumination, and measuring the total flux of LEDs has become widespread. The variation in the spatial and angular response of many detector leads to incorrect results when the flux to be measured is non-uniform or the beam shifts slightly. Non-uniformity can originate from the source or optical path. Beam movement can come from moving components or refractive index variation in the air path. Measurements with fiber optics can be influenced by launch or fiber bend conditions. The recorded output pattern changes as the fiber output pattern changes, and light moves across or off a detector.

  The integrating sphere light measurement systems are in sphere sizes ranging from 0.5 meters. The system consist of a spectrometers, source meter, thermal element, DC power supply, Auxiliary lamp (AUX), and calibration lamp (CAL). The integrating sphere must include a lamp socket fixture, baffles, and a viewing port for the photodetector. In high accuracy measurements, an auxiliary lamp may be used to correct for lamp self absorption.

Fig. 1: Integrating Sphere (Courtesy: www.labsphere.com)

  A baffle is placed between the lamp and the detector port. Because the total luminous flux of the lamp is directly proportional to the illumination of the sphere wall, the detector must be baffled from direct illumination by the lamp. The CIE recommendation for the placement of this baffle is that it be 1/4 to 1/6 the sphere diameter from the photometer head. The baffle is coated with the same material as the integrating sphere wall. The most common mounting arrangement places the lamp at the centre of the integrating sphere and the baffle at approximately one-third of the radius from the viewing port.

Measuring procedure

  At first initialization of integration sphere is very important one because to get accurate measurement. For initialization purpose the CAL lamp can be calibrated for accurate total luminous flux and total spectral flux, proceeding with AUX lamp calibration for self-absorption correction. Each lamp screened, seasoned and calibrated then only integrating sphere ready for measurement.

  The LED is placed in the centre of the sphere and connected to the power port. After fixing the LED updating the auxiliary calibration compensates for errors due to size, shape and colour of device under test. Using temperature controller sets the temperature of the LED mounting block. LED power supply panel control the current or voltage to the LED. Then LED total flux is calculated from signals with a standard (known) flux source. But, anything placed in the sphere affects its throughput.

  Fig. 1 shows Integrating spheres measurement setup that collect the total flux emitted from a lamp. Because an integrating sphere reflects and integrates all the light entering the sphere, the light received by a small area of the sphere is directly proportional to the total flux from a light source mounted within the sphere. The total flux from a test lamp is determined by comparing it to a calibrated working standard. Then the instrument produced the output screen, it consists of total spectral flux, total luminous flux graph and parameters, CRI block and CIE chromaticity diagram and parameters.

  TOC analysis is the thermal transient response of the LEDs resulting from a step function excitation carried a large amount of information about the thermal behaviour of the device. The six types of tests that can be performed are:

• ILV @ constant T: Step and control I, wait for T to stabilize, measure L and V 
• VLI @ constant T: Step and control V, wait for T to stabilize, measure L and I 
• TLV @ constant I: Step and control T, wait for T to stabilize, measure L and V 
• TLI @ constant V: Step and control T, wait for T to stabilize, measure L and I 
• ILV/T: Perform an ILV @ constant T, then step T and repeat at each T 
• VLI/T: Perform a VLI @ constant T, then step T and repeat at each T

  Where, L=Lumens, V=Voltage, I=Current,T =Temperature.

Results and Discussion

  The EOT characteristics of LEDs are measured using laboratory Integrating sphere as shown in Fig. 2. The total spectral flux,total luminous flux, CIE-XYZ chromaticity, color rendering index and TOC analysis (case temperature control vs. electrical and optical parameters) of LEDs are measured.

Fig. 2: Laboratory Integrating Sphere

  Spectroradiometry is the techniques of how the energy distributed over visible range & the total flux is the measurement types of light emitted in all directions. Spectroradiometry + Total Flux = Total Spectral Flux (Watts/nm).

  Photometry involves the physical measurement of visible light energy and attempts to compensate for the psychophysical attributes of the human response and physical units of power. Photometry is just like radiometry except that everything is weighted by the spectral response (luminosity function) of the human eye as defined by the CIE. luminosity function as the transfer function of filters which approximate the sensitivity of the human eye. Photometry + Total Flux = Total Luminous Flux (lumens/ nm).

(a) Total spectral flux output

(b) Total luminous flux output 
Fig. 3: Single Green LED with Heatsink

  Spectroradiometry, photometry measurement result of single green LED with heatsink and single yellow LED with heatsink are shown in Fig. 3 and Fig. 4 respectively. Fig. 3(a) and 4(a), total spectral flux waveform x-axis represents visible wavelength range from 360 to 1000 (nm), y-axis represents spectral power (W), this graph indicates spectral parameters in terms of lumens, Kelvin’s, dominant wave, peak wavelength, purity. CIE-XYZ chromaticity diagram x-axis represents the normalized value of X-coordinates, y-axis represents the normalized value of Y-coordinates, this diagram indicates the value of CIEx, CIEy, DUV prime, u prime, v prime and also measure full width half max (FWHM) and center wavelength, centroid wavelength and temperature. Fig. 3(b) and 4(b), Total luminous flux waveform x-axis represents visible wavelength range from 360 to 1000(nm), y-axis represents total flux (lumen/nm). Luminous Flux is the flow of light from a source per unit of time and is measured in Lumens. Then other waveform & measurement parameters are same as that of total spectral flux waveform.

(a) Total spectral flux output

(b) Total luminous flux output
Fig. 4: Single Yellow LED with Heatsink

  From the Figures 3 & 4, it is proved that LED light sources are much more efficient at converting watts to lumens. Different materials can be used within the LED sources themselves, each of which has its own light extraction efficacy. For these and other reasons, Single green LED with heatsink and Single yellow LED with heatsink can consume the same power but differ widely in lumen output. Because power (Watts) can’t be used as an index of light output, evaluating the “brightness” of LED sources.

  In data analysis of redstrip, whitestrip, single LED without heatsink, single green LED with heatsink, single yellow LED with heatsink are display screen in Fig. 5. This LEDs spectral waveform of the total spectral flux (units Watts/nm) over the visible wavelength range from 360 to 1000 (nm).

Fig. 5: Analysis the characteristics of LEDs

  Fig. 5 reveals that lumens of the red strip is high compared than others, because array of led but the value of CCT is very low so it come under warm colour. White strip has lumens is low than LED arrays because is just a module of LED but the value of CCT is very high compared than others so it come under the cool colour. Single LED with heat sink is used to get the same lumens of module LEDs and it has its own power supply ratings. Single LED without heat sink its lumens level is very low. With different colour LED measurement results the lumen is same, the CCT and CRI value of each LED can be varied, so come under different category of quality.

  Table 1 gives EOT parameters of LED, i.e., CIE chromaticity parameters, CCT in Kelvin’s, lumens, wavelength, purity, integration time, temperature, CRI, voltage, current, power. Measures are characterized according to the measurement value, it is useful for customers to identifying the quality of LED and also this measurement is useful for production level industries to improve quality of LED.

• Chromaticity is an objective specification of the quality of a colour regardless of its luminance. The CIE Chromaticity Diagram showing all visible colours. x and y are the normalized amounts of the X and Y primaries present, and hence z = 1 - x - y gives the amount of the Z primary required. These values used for colour matching purpose. 
• The Correlated Colour Temperature (CCT) is a specification of the colour appearance of a light source, relating the colour to a reference source heated to a particular temperature, measured by the thermal unit Kelvin. The measurement can also be described as the warm, cool and neutral of a light source. Generally, sources below 3500K are considered "warm;" while those above 4100K are considered "cool" sources, in between that values are considered neutral. 
• Lumen is a unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. As lamps and fixtures age and become dirty, their lumen output decreases. 
• Dominant wavelength is the spectral colour which can be mixed with white light in order to reproduce the desired colour. 
• Centre wavelength is the average of two wavelengths determined in the 3 dB width of measurements.
• FWHM describes the spectral width of the half power points of the LED. Half Power points are where power spectral density is one half of the peak amplitude. FWHM in the LEDs are classed as high power level or low power level. 
• Purity is defined as the ratio of the distance from the equal energy point E to the color coordinate. All intensity fairly close to the dominant wavelength (spectral colour is 100%). 
• The integration time is the main parameter for the user to adapt the modulation of a spectrometric system to the signal level of the measurement. In many applications it is necessary to have a linear relation between the integration time and the signal, especially in radiometric measurements.

Conclusion

  The electrical, optical and thermal characteristics of various LEDs like18 Volts red LED strip(redstrip), 12 Volts white LED strip (whitestrip), 3 Volts single LED without heatsink, 3 Volts single green LED with heat sink, 3 Volts single yellow LED with heat sink. From the experimental results it is observed that two different LED sources can consume the same power but differ widely in lumen output. The illumination level of LED with heat sink holds good when compared to other LEDs. From the obtained results best lamp can be chosen for particular lighting purpose and also this measurement is useful for production level industries to improve the quality of LED.

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Dr P S Manoharan is working as Associate professor in Thiagarajar College of Engineering, Madurai. N Uma, is pursuing Post-graduate (Control and Instrumentation) in Thiagarajar College of Engineering, Madurai.

Source: http://lightingindia.in/blog/post/id/8478/eot-characteristics-of-leds

Achievening High Quality Light Dimming with Texas Instruments LED Drivers

The growing popularity of LED light sources is rooted in energy savings, long life, and new fixture options that enable it to be used in almost any application. A 25W LED lamp can replace the light output of a 100W incandescent lamp, deliver a useful lifetime averaging 50,000 hours (compared to 10,000-20,000 hours for fluorescent lamps and 3000 hours for halogen lamps) these advantages ensure a strong future for LEDs but there are challenges associated with using LEDs to meet market expectations. Compatibility between LED lamps, drivers, and legacy controls can be confusing, if they are specified improperly performance will suffer. Issues with compatibility are probably the greatest source of frustration among lighting designers and their customers. Mock-up installations are expensive and time-consuming to smooth the pathway of a project, customers are look for lighting manufacturers who have already done the appropriate testing and research and can ensure successful LED lamp, driver, & control installation.

  Many consumers have turned to dimmers or automated dimming controls over standard light switches because dimmed lighting can reduce energy use and offer ambiance. When using an energy-efficient LED lights, the customer generally expects an experience similar to what incandescent lamps provide but correct combination of controls, drivers and LED sources are necessary. Although some LED lamps are marked as compatible with incandescent dimmers, there are various degrees of what can be defined as “compatible.” Dimmable LED lamps tend to interact quite differently when used with these legacy devices. A number of undesirable results may occur when you use a dimmable LED lamp with an incandescent dimmer, including reduced dimming range, flickering or fluttering of the lamp, inconsistent performance based on the number and assortment of lamps being controlled by one incandescent dimmer.

  Dimming LEDs, similar to the process with incandescent sources, saves energy at a roughly 1:1 ratio. This means that if you dim LEDs down to 50% of their light output, you save nearly 50% of the associated energy use. While it is true that LEDs are already very efficient compared to almost any other light source, you save even more energy by dimming them. Dimming LEDs also makes them run cooler, extending the life of the electronic components in the driver, as well as the phosphor in the LEDs. This will potentially double or triple the useful life of the LED lamp or module. The problem, however, is that nearly all dimmers in the market were designed for standard incandescent lamps. Unfortunately, the market expectation of dimming performance isn't being achieved by LED products over incandescent lamps with existing triac or phase-cut dimmers.

  Here we are analyzing the compatibility issues and the solutions currently industry is offering. The solutions must meet the current technological needs while being mindful of both past and future technological challenges.

Facts and Challenges

  Dimming methodologies can impact flicker. In the output of an LED driver the percentage of ripple at twice the line frequency is the parameter that corresponds to the flicker in the light output. Many LED drivers produce dimming by switching the LED light on and off at a relatively high frequency with the process called pulse-width modulation (PWM) dimming or digital dimming. The human eye is completely oblivious to these high frequencies and simply perceives less light. Dimmable LED drivers exist that simply modulate the light on and off at twice the line frequency at low dim levels, the result can be a lot like the light output of old magnetic ballasts where the flicker may be easily perceived. In addition, if used with a triac dimmer, which doesn't dim positive and negative half-cycles equally, it may introduce a line frequency component to the PWM that will be perceptible to anyone. Other LED drivers produce a uniform DC current level, which is then adjusted downward to produce dimming. This methodology is sometimes referred to as analog dimming. For task and office lighting, this approach is the most trouble-free kind of dimming to use, though it's likely to be more expensive than digital dimming.

  There are applications where LED lamps will operate with an incandescent dimmer, in general, an incandescent dimmer will provide inconsistent performance with SSL. The incandescent lamp by nature represents a simple resistive load with a linear response to the dimmer set point. Standard incandescent dimmers work particularly well with this type of load by switching on at an adjustable phase angle after the start of each alternating current half-cycle, thereby altering the voltage waveform applied to lamps. By switching instead of absorbing part of the voltage supplied, minimal power is wasted, and dimming can occur almost instantaneously. In contrast, LED lamp loads can vary greatly across different manufacturers and designs. But most can be characterized by a diode-capacitor power supply feeding a constant current source. The diodes rectify the applied AC voltage, allowing it to charge the storage capacitor, while the LED elements draw a constant current from the power supply that is related to the desired dimming level and brightness.

  LED light loads are significantly different from incandescent lamps in which the applied voltage and the current flowing into the load are related. In incandescent lamps, the applied voltage across the load and the resulting current flowing through the load are related linearly by Ohms Law (V = IR). In this case, the resistance sets the scale, and the current waveform follows the voltage waveform, differing only by scale. In LED loads, the applied voltage and resulting current flow are not related by a simple linear relationship. In the diode-capacitor power supply model of the LED lamp, current flows from the applied voltage to the load only when the magnitude of the applied voltage exceeds the stored voltage on the power supply capacitor. The stored voltage on the power supply capacitor, in turn, depends on the current drawn by the LED elements themselves, which is a function of the LED brightness. Therefore, the current flowing from the supply to the lamp depends both on the instantaneous value of the input AC voltage waveform and the brightness of the LED lamp. Changing the intensity or dimming level of the LED lamp affects where in the AC line cycle the load begins to draw current. This inflection point also affects the amount of current that surges into the lamp.

Wiring, Dimmers and Regulations – Contributes to Light Flickering

  Wiring:-The inconsistency issues among dimmers and lamps are that most of the existing residential wiring infrastructure was built without a neutral wire at the switch box. The absence of the neutral wire is referred to as two-wire lighting control while the inclusion of a neutral at the switch box is referred to as three-wire lighting control. The need of supporting two different wiring scenarios poses certain challenges that lighting-control designers need to account for in planning to control a broader range of lamp types with a single dimmer.

  Some dimmers are designed to work with one type or the other only, while some are designed to work in both types of installations. But, for all dimmers, even those that are designed for both two-and three-wire installations, there are significant differences in performance between these two installations in terms of how the dimmer circuitry is powered and how the dimmer synchronizes with the line voltage. When used to drive incandescent lamp loads, these differences are mostly negligible. But, when used to drive LED loads, they present significant challenges to stable dimming and lighting control. Regardless of the circuit type, all phase-controlled dimmers need to synchronize with the AC line in order to work correctly. Without the ability to sense the AC line and its zero-crossings, a phase-controlled dimmer would not detect the correct timing for switching the AC voltage, and it would lose its ability to control and dim the lamp load. The end result is flickering and fluttering of the light output.

  In three-wire installations the line, load and neutral wires are connected to the wall control electrical box. The line wire comes from the AC power source and supplies power for both the dimmer and the load. The load wire is connected to the lamp load and provides a return path for the power delivered to the load. The third wire, the neutral connection, provides the essential return path for the dimmer even when the load is disconnected or is in a state that doesn’t draw any current. The neutral is an important feature of three-wire installations. It ensures that the dimmer device has a direct connection to the AC power source regardless of the state of the load. This third wire not only ensures that the dimmer has power to drive its own internal circuitry even when the load is disconnected or off, it also provides a clean signal of the incoming AC power source for detection of zero-crossings and synchronization with the line. Both of these are essential for stable phase-controlled dimming, and are easier to obtain in three-wire designs.

  In two-wire installations only two wires are present in the electrical box, the line wire and the load wire. In this case, the dimmer is simply placed in series between the line and the load. With only two wires, the dimmer must rely on the current passing through the load to both power its own internal circuitry and to detect zero-crossings for synchronization with the AC line. When LED lamps perform poorly with a dimmer, often times the blame is placed on the dimmer circuit. But the source of the problem really lies in how the LED load current differs from the incandescent lamp in two-wire applications. If the load current is regular, as in the case for incandescent lamps, then stable line synchronization and ample power for the dimmer’s internal circuitry are both easy to obtain. With LED lamps, however, the load current is much smaller and much less regular, and line synchronization becomes difficult. Similarly, the load current of LED lamps in their off state can be so small, that even obtaining a few milliamps to supply the internal dimmer circuitry can be challenging. Without adequate supply and stable line synchronization, lamp flickering may result.

Triac Dimmers

  In AC phase control widely used form of brightness control is the familiar triac-based dimmer that is present in many residential applications. Triac dimmers operate by cutting out a portion of the AC waveform.

Fig. 1: Typical TRIAC dimmer circuit

  During the start of AC cycle TRIAC will be off and during the operation cycle (refer Fig 1) C1 charges through R1 and light bulb, when voltage on C1 exceeds DIAC threshold voltage the TRIAC starts conducting. R1 is a variable resistor which controls when TRIAC turns ON, dimming function and defines the conduction angle.

  The most common type cuts out a portion of the leading edge of the AC waveform, as shown in Fig 2. The dimmer senses each zero-crossing of the AC input, and waits for a variable delay period before turning on the triac switch and delivering the AC to the load. The AC input to the light therefore has a bite out of the leading edge of each half sine wave. This forward phase dimming typically operates on two wires & avoids the labor associated with adding a third wire.

Fig. 2: Forward-phase dimming cuts the front edge of each half-cycle of the AC line input

  A second similar type of dimmer operates in the reverse manner, by cutting a portion of the trailing edge of each half sine wave, as shown in Fig 3. This type of dimming is sometimes called reverse phase control, and is designed for use in electronic low voltage (ELV) applications. Reverse phase dimming is considerably more expensive but minimizes electromagnetic interference (EMI) issues.

Fig. 3: Reverse-phase dimming cuts the trailing edge of each half-cycle

  Phase-control dimmers were originally developed for incandescent lighting, where the lamp brightness is directly dependent on the average power in the AC input. By cutting out a portion of the waveform, the power is reduced and the lamp becomes dimmer. However, this is not the case with LED lighting, because LED luminaires contain a power supply and driver whose primary function is to supply constant current to the LEDs regardless of the AC input voltage. If you connect a constant-current or constant-voltage power supply to the output of a phase-control dimmer, the power supply will attempt to compensate for the missing portions of the AC waveform. As the amount of phase-cut increases, the power supply will maintain its output voltage by drawing higher input current, and the LEDs will remain at normal brightness. Eventually, when the dimmer setting is very low, the power-supply feedback circuits will no longer be able to compensate and the power supply output will collapse.

  Performance of phase dimming circuits depends on certain TRIAC parameters, which are critical and should match the spec requirements.

• To turn on the Triac, a gate signal is required and must exceed specified IGT and VGT requirements. 
• Latching current (IL) is required to maintain the Triac in the on state immediately after the switching from off state to on state has occurred and the triggering signal has been removed. 
• Then, Holding current (IH) is the required to maintain (hold) the Thyristor in the on state.

  For an LED luminaire to respond correctly to a phase-control dimmer, it is necessary to add several functional blocks into the driver electronics. A sensor block monitors the AC input waveform and generates an output signal proportional to the amount of phase cut and feeds to PWM controller and then drives the MOSFET. There is also the issue of how dimming information is conveyed to drivers. The driver selected must have the ability to work with the dimmers deployed in an application, especially in retrofit scenarios.

Regulations

  Compatibility issues between lamps and dimming devices are certainly due lack of dimming performance standards within the lighting industry and how each uniquely corresponds with LED drivers. This lack of standardization can be seen not only in varying characteristics between manufacturers, but also by product within some manufacturers’ product lines. Complications arise from the fact that any given lamp can require a set of electrical and electronic characteristics – current, voltage and control signals – that are vastly different from any other lamp. While one lamp may be able to be dimmed by a particular dimming device, others cannot.

  Under current UL standards, notably UL 14725, intended to regulate the safety of dimmers, an LED lamp driver is categorized as an “electronic ballast.” One notable issue addressed by UL 1472 is in-rush current which is generated at the startup of many LED lamp loads. High in-rush current can result in failure of switch contacts, which is a safety hazard in many field applications – such as dimmers – where the switch is serving as the disconnect means. To evaluate the safety of the combination of dimmer and electronic ballasts, UL has taken the systems approach by requiring dimmer manufacturers to provide information on the intended electronic load (i.e., CFL, LED or electronic ballast) for each dimmer. UL listing investigation will involve the use of the specified electronic ballasts or a synthetic load exhibiting the same in-rush and steady-state characteristics in the overload, endurance and temperature tests.

Texas Instruments LED Driver solutions to overcome current dimming challenges

  As energy efficient lamps continue to penetrate the lighting market, the availability of new lighting controls that meet the specific needs of these lamps is increasing. Consumers can take fuller advantage of all of the benefits of the newer, more energy efficient lamps by using TI’s industry-leading TRIAC dimmable offline LED driver solution which is perfect for any application where an LED driver must interface to a standard TRIAC wall dimmer. TI’s new TRIAC dimmable LED driver delivers a wide, uniform dimming range free of flicker, best-in-class dimming performance, & high efficiency all while maintaining ENERGY STAR® power factor requirements in a typical application.

Full Range Dimming Capability

  TRIAC dimmable LED driver offers 100:1 full range dimming capability, going from full light to nearly imperceptible light in a continuous range without being extinguished and maintains a constant current to large strings of LEDs driven in series off of a standard line voltage.

Easy to Use

  TRIAC dimmable LED driver enables a direct replacement of incandescent or halogen lamp systems that are currently interfaced to a TRIAC dimmer without having to change the original infrastructure or sacrifice performance. In addition the new TRIAC dimmable LED driver is available in WEBENCH® LED Designer to allow for easy and quick design in.

Uniform Dimming Without Flicker

  TRIAC dimmable LED driver allows master-slave operation control in multi-chipsolutions which enables a single TRIAC dimmer to control multiple strings of LEDs with-smooth consistent dimming, free of flicker.

  The LM3447 is TI’s newest product for offline, phase-dimmable LED lighting. It is a future genera-ti on LED lighting product as it offers significant features,

• Designed for constant power operation with better line regulation,
• Overall lower isolated solution cost.
• Power Factor Correction.
• Valley switching to improve efficiency and EMI performance
• Improved dimmer hold circuitry
• Addition of thermal fold-back to protect LED arrays.

  The LM3447 is a versatile power factor correction controller IC designed to meet the performance requirements of a residential and commercial (phase-cut) dimmer compatible LED lamps. The device incorporates a phase decoder circuit and adjustable hold current circuit to provide smooth and flicker free dimming operation. A proprietary primary side control technique based on input voltage feed forward is used to regulate the input power drawn by the LED driver and achieve line regulation over a wide range of input voltage. Valley switching operation is implemented to minimize switching loss and to reduce EMI. It contains an internal thermal foldback feature designed to protect the LEDs from damage based on the temperature sensed by a single external NTC resistor. Additional features include load open and short circuit protection, cycle-by-cycle FET over-current protection, burst mode fault operation and internal thermal shutdown. The LM3447 is ideal for implementing dimmable, isolated single-state LED lamp drivers where simplicity, low component count, and small solution size are of primary importance. The device is available in 14-pin TSSOP package.

  The LM3447 is TI's next generation controller for off-line, isolated, phase-dimmable retrofit LED lighting applications. Using primary side power regulation, the LM3447 can implement compact, isolated, constant power flyback designs which give lighting designers up to 10% better utilization of a given string of LEDs. Protection of LED arrays is enhanced using the integrated thermal fold-back capability which protects against high temperature conditions by reducing power, and therefore light output, to the LEDs until the high temperature condition is alleviated. Most competing solutions either lack this capability, or completely turn off under high temperature conditions. The LM3447 is well suited for retrofit bulb designs as A19, E26/27, PAR30/38 as well as TRIAC dimmable down lighting.

  Considering current market challenges and industries technological expectations LM3447 is robust with additional intelligent blocksfor flickerfree Triac dimming.

Angle Detection Block

  The LM3447 uses the input voltage, VREC, to detect the conduction phase angle. Fig 4 shows the LM3447 angle detect circuit, where the input voltage, VREC, is scaled by the current mirror circuits and re-generated across an internal 42kΩ resistor. This replica of the input vol-tage is compared with internal 280mV reference to obtain the conduction information. The resulting PWM signal, with its on-time proportional to the conduction period, is buffered and supplied through the FLT1 pin, as shown in Fig4. To match the external phase dimmer characteristics with the LM3447 decoding circuit and prevent EMI filter capacitors from interfering with dimming operation, it is necessary to select an angle detection threshold, VADET(TH). This threshold can then be programmed using the resistor, RAC, such that- 

Fig. 4: Phase Angle Detection and Hold Current Circuit

  For best results, set VADET(TH) as follows:

• 25V to 40V for 120V systems
• 50V to 80V for 230V systems

  Resistor RAC should also limit the VAC current under worst case operating conditions. The value of RAC should be optimized to meet both angle detect, VADET, & VAC current, IVAC constraints.

Hold Current Block

  The LM3447 incorporates an efficient hold current circuit to enhance compatibility with TRIAC based leading edge dimmers. Holding current from an external dimmer is drawn before the Flyback PFC circuit through the pass transistor, QPASS and limited by resistors RHLD1 and RHLD2, as shown in Fig 4. It should be noted that the additional current drawn has no effect on the rectified input voltage and therefore does not interfere with the input power regulation control scheme. To provide high efficiency, the hold circuit is enabled only when the presence of an external dimmer is detected based on the FLT2 input. The ENHOLD signal is asserted and hold operation is permitted when VFLT2 falls below 1V. The hold operation is halted when VFLT2 rises above 1.2V. During dimming, the hold current is drawn during the interval when rectified input voltage is below the VHOLD(TH), based on the external resistor RAC. The FET turn on is controlled by an internal comparator with a reference of 400mV (higher than angle detect reference), such that hold current is always asserted before angle detect threshold VADET(TH). The hold circuit operation is summarized in Fig 5. The hold trun-on threshold, VHOLD(TH) is given by

  The hold current is based on the BIAS voltage and set by the sum of resistors RHLD1 and RHLD2,

Fig. 5: Angle Detection circuit and Holding Current Circuit Operation

  In selecting the hold current level, it is critical to consider its impact on the average power dissipation and the operating junction temperature of pass transistor, QPASS under worst case operating conditions. The current should be limited to a safe value based on the pass transistor specifications or the ABS MAX rating of LM3447 (70mA). For best performance, it is recommended to set the hold current magnitude between 5mA and 20mA. A capacitor, CHLD of 2.2μF to 10μF, from RHLD2 to GND is connected to limit the rate of change of input current (diin/dt) caused by the step insertion of holding current. This prevents TRIAC based dimmers from misfiring at low dimming level.

Angle Decoding Block

  The LM3447 incorporates a linear decoding circuit that translates the sensed conduction angle into an internal dimming command, VDIM. The conduction angle information, represented by the PWM signal at FLT1 output, is processed by an external low pass filter consisting of resistor, RFLT and capacitor, CFLT, which attenuates the twice line frequency component from the signal. The resulting analog signal at FLT2 is converted into the dimming command by a linear analog processing circuit. The piecewise linear relationship between the FLT2 input and the dimming command is shown graphically in Fig 6.

Fig. 6: Relationship Between VFLT2 and VDIM

  The dimming command, VDIM is-

• Held constant at 1V for VFLT2 ranging from 1.75V to 1.45V (conduction angle 180O to 150O)
• Linearly varied with gain of 0.877 for VFLT2 ranging from 1.45V to 280mV (conduction angle 150O to 30O)
• Saturated at 13mV for VFLT2 lower than 280mV (conduction angle less than 30O).

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Shinu Mathew is Analog Application Engineer, Texas Instruments (India).

Source: http://lightingindia.in/blog/post/id/8539/achievening-high-quality-light-dimming--with-texas-instruments-led-drivers

Equalizing Pressure - Hidden Design Challenge for Robust LED Luminaires

Although the LEDs themselves can last for more than 20 years, this service life only provides added value to the lighting system if the electronic components and power supply drivers are as reliable as the LEDs. Also, LEDs do not usually fail catastrophically; the lumen output gradually decreases until the LEDs are classified as having failed. All of the components in the luminaire must be able to withstand the harsh conditions of the environments in which they are installed, whether in the tropical climates of the Caribbean or the extreme cold of Alaska.

Challenges of Harsh Environments

  According to a study performed by RTI International, the leading causes of solid-state LED luminaire failures are issues with drivers, such as capacitor and interconnect failure, electrical stress, and ingress of moisture or other chemicals. Traditional power supply drivers come with a five-year warranty, and tests have shown that these power supplies have a high failure rate. These issues are often caused by seals, joints, and connection points being compromised by challenging environmental conditions. Most engineers protect the drivers and electronics by enhancing the durability of the luminaire housing in one of three ways — integrating more rugged O-rings or gaskets to improve seals, increasing the thickness of the enclosure’s walls to minimize movement of the housing around the seal, or installing additional bolts around the O-rings or gaskets to maintain a more durable seal. However, even with these added features, the seals can begin to fail and become a leak point, providing a path for water to enter and eventually cause condensation on the lens and reflectors. These failures occur because sealing the device prevents its ability to handle pressure fluctuations, both positive pressure and negative pressure (i.e., vacuum). As these pressure fluctuations occur, they put significant stress on seals and other connection points, which in turn compromises their effectiveness. Over time, the frequent expanding and contracting of the seals causes their weakest point to begin to allow water and contaminants to enter the housing, which can lead to corrosion, shorts, and potential failure of the electronics.

  Temperature changes — whether external, internal, or both — are one of the most common cause of pressure differentials. External temperature changes can be dramatic (e.g., a sudden thunderstorm on a hot summer day) or more gradual over the course of different seasons. Daily temperatures can drop as much as 10 to 20 degrees Centigrade from day to night, and seasonal temperatures can differ 40 degrees or more between summer and winter. As the temperature rises, the air inside the luminaire expands, putting positive pressure on the seals, joints, and connection points. As the temperature drops, the internal air compresses and creates a vacuum, again putting pressure on the seals. A 30 °C change in temperature would create approximately ten percent of volumetric flow of air in or out of the luminaire. A quick drop in the outside temperature can create a vacuum of 0.5 psi or greater inside the enclosure. When a vacuum occurs, water and contaminants can enter the housing. In mild cases, corrosion is accelerated and in severe cases, catastrophic failure could occur. The corrosion can shorten the life of the luminaire by damaging wiring, leads, and other electronics contained within the power supply driver and the LEDs.

Fig. 1: Temperature fluctuations inside a sealed luminaire

  As a luminaire turns on and off, the changes in the temperatures result in sudden pressure differentials (Figure 2). The housing tries to relieve the pressure by drawing air inside, which increases stress on the seals (a process sometimes referred to pressure equalization).

Fig. 2: Pressure differentials caused as luminaire turns on and off. The inability for the luminaire to hold pressure indicates the existence of leak path.

  Altitude changes are another common cause of pressure buildup. Although luminaires are usually stationary once installed, altitude issues occur as the luminaires are shipped from the manufacturing facility to the installation site. Because most shipping containers on cargo plane are not pressurized, luminaires can be exposed to significant pressure differentials during take-off and landing, decreasing from almost 15 psi (1,034 mbar) when on the ground to about 3 psi (137.9 mbar) at an altitude of 40,000 feet (Figure 3).

Fig. 3: Pressure experienced during altitude changes

  A third cause of pressure differentials is the thermal shock experienced when the luminaire is exposed to rain, snow or washing cycles. Figure 4 shows the impact on a luminaire’s internal pressure from a water spray using an IEC 60529 Standard IPX5 nozzle at a distance of approximately three meters for three minutes, with a tap water flow of 21 liters/minute. Rapid recovery of pressure indicates the failure of gasket to hold the vacuum, drawing water along with air in at the leak path(s).

Fig. 4: Pressure differentials caused by tap-water spray using an IPX5 nozzle 

Condensation — Another Consequence of Pressure Differentials

  In addition to the potential damage to electronics if water and contaminants enter the luminaire, moisture vapor from the water can cause condensation on lenses and reflectors, decreasing light efficiency and the aesthetic quality of the luminaire (Figure 5). The relative humidity inside the luminaire determines the potential for condensation occurring on the lenses and reflectors. When the relative humidity inside the luminaire reaches 100 percent, condensation occurs. Once this happens, the condensation will remain unless there is a path for the moisture vapor to escape. Sealed enclosures do not provide this path; therefore, once condensation occurs, it will remain on the lenses and reflectors for extended periods (Figure 6).

Fig. 5: Condensation inside an LED luminaire

Fig. 6: Condensation events inside a luminaire

Preventing Pressure Differentials

  The challenge for maintaining constant pressure inside a luminaire is to allow air to flow freely in and out of the luminaire while blocking water and contaminants. Drilling a hole in the housing or creating a hole with a torturous path definitely addresses the pressure differential issue; however, this open system concept allows contaminants, including dust, dirt, water, and even insects, to enter the enclosure and compromise the electronics. Many engineers consider hermetic seals or potting compound to be the best way to protect electronics. However, hermetically sealing the device requires the use of non-permeable materials — which means no plastic — and the housing is welded shut. This option is unrealistic for most luminaires because the enclosures would be too heavy, not serviceable, and very expensive. Like hermetic seals, potting compound eliminates the ability to service the electronics, is expensive and heavy, and requires specialized equipment in the manufacturing process.

  Installing a felt element, sintered vent, or mechanical valve is another alternative. The felt element and sintered vent address the pressure differentials, but like tortuous paths, they allow water and contaminants to enter and clog the air path. And the mechanical valve is a one-way solution — allowing air to escape only if internal pressure increases to a certain point, but remaining closed if a vacuum occurs.

  Finally, a vent made of expanded polytetrafluoroethylene (ePTFE) — a two-way breathable membrane — continuously equalizes pressure inside the luminaire housing while maintaining an environmental seal. Expanded PTFE is a unique, microporous membrane that is inherently waterproof and chemically inert, and can be coated to provide oleophobicity. Its node-and-fibril microstructure is open enough to allow gas molecules and vapor to pass through it easily, but the openings are so small that liquid and other particulates are repelled.

Venting to Improve Reliability and Durability

  Equalizing pressure using an ePTFE vent reduces the potential for seals to fail and for moisture vapor to condense on the lenses and reflectors. W. L. Gore & Associates, Inc., has performed extended life testing on two commercially available LED luminaires to evaluate the effect of using ePTFE venting products as a solution to improve reliability and durability by relieving pressure. The vent is integrated into the power supply chamber of the luminaire. It is extremely difficult to leak-proof any entry point of an enclosure, because the gasket elastomer and the wire gland relax over time (Figure 7). A sealed housing may pass ingress testing at the factory but fail rapidly in real life when challenged by a wide range of temperatures.

  Depending on the quality of the seal, negative 34 mbar (approximately 0.5 psi) is commonly accepted as the point at which a vacuum can cause a leak in a sealed enclosure. Although the on/off cycle of a luminaire causes temperatures to rise and fall in both a vented and a sealed enclosure, the amount of pressure placed on the seals and joints is significantly different (Figure 8). The testing indicated that pressure in the sealed luminaire spiked as much as 0.09 psi when the light was turned on and dipped approximately -0.1 psi when turned off. However, the vented luminaire showed a change of only ±0.01 psi. Comparing the relative humidity inside the sealed and vented luminaires after an IPX5 water ingress test demonstrates the significance of pressure differentials. Following the water spray test performed by Gore, the relative humidity in a sealed luminaire remained significantly higher than that in an identical luminaire that was vented (Figure 9). Over the course of ten days, the relative humidity of the sealed luminaire almost always remained at 100 percent, which indicated condensation inside the luminaire caused by water entering during the test. Although the relative humidity in the vented luminaire rose after the shock test, it decreased as the moisture vapor escaped from the enclosure and there was no evidence of condensation.

Fig. 7: Leak path in sealed luminaire

Fig. 8: Impact of pressure on vented and sealed housings

Fig. 9: Extended periods of condensation in a sealed luminaire 

Conclusion

  Pressure differentials compromise the housing seals, joints, and other connection points in LED luminaire systems. Leaving these pressure fluctuations unchecked can reduce long-term performance of the power supply drivers and other electronics. They also lead to condensation on lenses and reflectors that can decrease light efficiency and the aesthetic quality of the luminaire. As demonstrated through the real-life IPX5 testing done by Gore, integrating an ePTFE vent into the luminaire’s housing equalizes pressure by allowing continuous airflow in both directions. This two-way airflow prevented water from entering as the pressure changes inside or outside the enclosure. Also, the vent reduced condensation events inside the luminaire because moisture vapor was able to escape before condensing.

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Courtesy: Gary Y. Chan; W. L. Gore & Associates, Inc.

Source: http://lightingindia.in/blog/post/id/8557/equalizing-pressure-hidden-design-challenge-for-robust-led-luminaires

Go Green with LED Lights

 LED lights are the most energy efficient and eco-friendly by nature. The field of lighting has witnessed dramatic technology developments in the past few decades, especially in the sub-fields of solid state lighting and other energy efficient lighting technologies. India’s demand for energy is forecast to grow exponentially in line with India’s urbanization over the coming decades. Industries are rapidly embracing energy efficiency as the key to increasing their energy security and improving their economic productivity and competitiveness. The usage of electricity is increasing day-by-day to meet growing energy demands. These days energy efficiency assumes greater importance because it is most economical and reliable way of meeting the particular global local climate change. Hence utilization of energy for productive lighting is important and happens to be an important portion of the modern society. Today, energy efficiency is at the forefront of our company policy. Concern about global climate change and the environment has brought the issue of energy efficiency front, GlacialTech is more focusing on producing high energy efficient products which have low energy loss and high efficiency for instance we have LED Drivers which have efficiency higher than 90-95% like our LS series and RS series, these models are with higher efficiency and minimal loss of energy, this helps lamps to perform better & the total lighting system efficacy increases.

  LED lights are multilayered semi-conductive materials forming lattice that allow nano phosphors to stick on the surface through a process called solid adsorption. The LEDs are pollution free and provide viable lighting option. LED Lights have super long life span of up to 80,000 hours and have no filaments so it can withstand a greater intensity of vibration and shock than standard lights making them durable with less risk of breaking and need to replace. LED Lights will start at full brightness, instantly; every time; therefore there is no need for backup lighting and they are also an eco-friendly form of lighting as they do not contain mercury or other harmful gasses.

  When we talk about the benefits of LED lights, the first thing that comes to our mind is the longer life-span, more durability, no hazardous materials to clean up if you break one etc. The long operational life time span mentioned above means also that one LED light bulb can save material and production of 25 incandescent light bulbs. These lights are ecological-friendly as they are free of toxic chemicals. Most conventional fluorescent lighting bulbs contain a multitude of materials like e.g. mercury that are dangerous for the environment. LEDs are extremely durable and built with sturdy components that are highly rugged and can withstand even the roughest conditions. As they are resistant to shock, vibrations and external impacts, they make great outdoor lighting systems for rough conditions and exposure to weather, wind, rain or even external vandalism, traffic related public exposure and construction or manufacturing sites.

  LED’s are currently a very well known trend in the lighting business, but the industry is experiencing a high number of players and it is very competitive. Therefore it is not easy to gain new business prospects, especially for the SME’s like Glacial Light. So our approach is to offer more functional lighting solutions by improvising the technical background in our product line, offering high-end and eco-friendly lighting with higher wattage, and strengthening our marketing strategies. GlacialTech recently comes up with innovative products of LED Lights in market:

• Thermal Heat Sinks: GlacialTech is currently developing cold forging technology for LED thermal modules. Cold forging produces heat sinks that have better heat dissipation than die-casting and aluminum extrusion. Using AA1070 aluminum, thermal conductivity can be improved 2.36x compared to die casting from 96.2 W/mK to 227 W/mK. GlacialTech‘s unique copper/aluminum cold forging seamless joining technology greatly enhances thermal conduction and reliability when copper is embedded to make sure the two materials are joined together seamlessly during the manufacturing process. The two materials work together to improve heatsink performance. The copper component conducts heat quickly from the bottom to the entire transverse surface. The high thermal conductivity of the aluminum fins lets heat dissipate quickly. GlacialTech's new copper/aluminum cold forging technology is the most cost effective and lightweight thermal solution when space is at a premium.
• DALI Interface Converter with PWM and DC Output: GlacialLight, a division of the Taiwanese technology manufacturer GlacialTech Inc., is introducing the GL-DA02 DALI interface converter to its product lineup. As an open standard, DALI is internally recognized as the premier new lighting control interface and is cross-compatible across lighting components from different manufacturers. Highly scalable, it simplifies wiring compared to conventional lighting control systems, making installation easier and reducing maintenance costs. DALI can not only control lighting but also monitor it, allowing for intelligent lighting systems that maximize service life and save energy. Compared to legacy solutions, a DALI network is more precise and allows fine grain control over the individual components in a complete lighting system. GlacialLight’s DALI Interface Convertor is fully IEC62386 (102, 206) compliant. Taking a digital DALI signal, it can output either PWM, 0-10V DC, or 1-10V DC signals and is suitable for controlling 3-in-1 (DC/PWM/Resistor) LED drivers. Dimming can be set on a linear or logarithmic curve. With a built-in relay, devices down the line can be turned off completely, giving complete lighting control and reducing energy costs. As an indoor digital lighting control system, the GL-DA02 convertor is well suited for applications including office buildings, conference rooms, factories, and intelligent home lighting. 
• GP-LC7028-Q5D: GlacialPower, a division of technology manufacturer GlacialTech, announces today a new dual-mode LED driver powering LED lighting from 7W to 20W. Featuring either constant current or constant voltage operation, the LED driver mode and power output can be easily adjusted to fit a variety of lighting and signage applications. Power input and output lines are connected via robust and easy-to-use clips.
• 2-in-1: The GP-LC7028 LED driver is two LED Drivers in one, with easy DIP switch configuration to either constant current or constant voltage mode for enhanced functionality. It can be easily customized to specific LED lighting needs with 8 modes of constant current operation from 250mA to 700mA, and constant voltage modes from 12V, 24V and 28V output available. For constant current mode with 1-10V dimming and push dimming can also easily be enabled with a dimmer.

• GP-LC Series: GlacialPower, a division of Taiwanese technology manufacturer GlacialTech, announces two new wall mount LED adapters for desk lamps, floor lamps, strip lighting and other indoor lighting applications under 9 watts. The 8W GP-LC3536-0A and the 9W GP-LC7021-0A are compact wall mount power supplies providing constant current for LED lighting. These fully-isolated LED adapters feature a compact two-prong design with Overload Protection (OLP) and Short Circuit Protection (SCP). 

• Universally Compatible: GlacialPower’s GP-LC LED adapters are worldwide compatible, with universal AC input from 90 to 264V AC power accepted. Input plug adapters can be customized to US or EU electrical outlet standards. The power supply output plug to the device can be selected by the customer to suit their needs.
• Low Bay Lights GL-BL50: GlacialLight, the lighting division of GlacialTech Inc., is introducing the Arcturus series of GL-BL50 Low Bay Lights. These dimmable 50 watt LED low bay lights come in three colors, and an artistic design making them suited for a variety of indoor environments. Shopping malls, restaurants, offices and even homes can all benefit from the even lighting and contemporary styling of the GL-BL50. The GL-BL50 is compatible with international mains voltage from 100-240V and comes in a variety of configurations for almost any indoor environment. A choice of 3 color temperature options are available– Warm White(3000K), Neutral White(4000K), and Cool White(5000K). The GL-BL50 can be hanged with a pendant rod, cable, or chain type installation. IP54 rating means it’s tough enough for restaurant, garage, or kitchen use. And, we hope to see better results for the coming year ahead. 

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Nikhil Malhotra is Regional Sales Manager of GlacialTech Inc., Taiwan.

Source: http://lightingindia.in/blog/post/id/8563/go-green-with-led-lights