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