New generation of offline LED fixtures demand more from LED Driver ICs
As environmental concerns over the use of traditional lighting are increasing, the price of LEDs is decreasing, thus high power LEDs are fast becoming a popular lighting solution for many offline applications. The power saving capability, long life and environmental benefits of high brightness LEDs continue to drive the development of a wide range of solid state lighting (SSL) applications. It should come as no surprise that the growth rate of LEDs continues to accelerate. By the end of 2010 the market size for high brightness LEDs reached $8.2 billion and it projected to grow to over $20 billion by 2015 at a 30.6% CAGR ramp (source: Strategies Unlimited). Over the past few years LEDs used to backlight HDTV displays have been the major driver of growth; however, as general lighting LED applications continue to gain traction in both commercial and residential applications, they will dramatically accelerate their growth.
The primary driver behind the high growth rate of LED lighting is the dramatic reduction in power consumption that LED lighting offers over traditional lighting. Compared to incandescent lighting, LEDs require less than 25% of the electrical power to provide the same level of light output (measured in lumens). There are additional advantages which LED lighting offers but there are also some challenges associated with them. Their advantages include an operational lifetime orders of magnitude higher than incandescents, which dramatically reduces replacement costs. The ability to dim LEDs using previously installed TRIAC dimmers is also a major cost benefit, especially in residential lighting. A LED’s instant turn-on eliminates the warm-up period associated with CFLs and they are not sensitive to power cycling like their CFL counterparts. Additionally, LEDs do not contain any toxic materials to manage or dispose of, whereas CFL requires toxic mercury gas to operate. Lastly, LEDs enable new very low profile form factors, something which other technologies could not do.
Offline Power Can be Used
The ability to drive LEDs from an offline power source enables applications to grow exponentially as this form of electrical power is readily available in both commercial and residential structures throughout the world. Although LED replacement fixtures are relatively simple for the end user to implement, the new requirements placed on the LED driver IC go up dramatically. Since LEDs require a well regulated constant current source to deliver a constant level of light output, powering them from an AC input source requires some special design techniques with some very specific design needs.
Depending on where in the world you are, the offline power ranges somewhere between 90VAC and 265VAC with a frequency between 50Hz and 65Hz. Therefore, manufacturing a LED fixture for the worldwide market would ideally have a single circuit design which could enable it to be used anywhere in the world without modifications. This demands that a single LED driver IC must handle a wide range of input voltages and line frequencies.
Furthermore, many offline LED applications require electrical isolation of the LEDs from the drive circuit circuit. This is primarily driven by safety factors and is required by several regulatory agencies. Electrical isolation is generally provided by an isolated flyback LED driver topology which utilizes a transformer separating the primary and secondary sections of the drive circuit.
As the driving force behind LED lighting’s adoption is the dramatic reduction of power required to deliver a specific quantity of light, it is imperative that the LED driver IC delivers the highest level of efficiency. Because the LED driver circuit must convert high voltage AC power to well regulated LED current at a lower voltage, the LED driver IC must be designed to deliver efficiencies above 80% so as not to waste power.
Moreover, in order for LED retrofit lamps to be feasible with the large base of installed TRIAC dimmers commonly found in residential applications, the LED driver IC must operate effectively with these dimmers. TRIAC dimmers are designed to work well with incandescent and halogen lamps, which are perfect resistive loads. However, the LED driver circuit is generally nonlinear and is not a purely resistive load. Its input bridge rectifier typically draws high intensity peaks of current when the AC input voltage is at its positive and negative peaks. Therefore, the LED driver IC must be designed to “mimic” a purely resistive load to ensure that the LED starts properly without exhibiting any visible flicker and dims properly with a TRIAC.
Power Factor Correction (PFC) is an important specification in LED lighting. Simply put, a power correction factor of one is achieved if the current drawn is proportional and in-phase with the input voltage. Because an incandescent lamp is a perfectly resistive load, the input current and the voltage are in-phase and the PFC is 1. PFC is particularly important as it relates to the amount of electrical power required by the local power supplier. Namely, in an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. These higher required currents increase the energy lost in the distribution system, which in turn requires require larger wires and other transmission equipment. Because of the costs of larger equipment and wasted energy, electrical utility companies will usually charge a higher cost to industrial or commercial customers where there is a low power factor. International standards are still developing for LED applications, but most believe that a PFC>0.90 will be required for most LED lighting applications.
Because a LED driver circuit (which includes an array of diodes, transformers and capacitors) does not act like a purely resistive load, it can have a PFC as low as 0.5. In order to raise the PFC above 0.9, either an active or passive PFC circuit must be designed into the LED driver circuit. It should also be noted that a high PFC is particularly important in those applications which utilize large numbers of LED lighting arrays. For example: in a parking garage which uses over several hundred 50W LED fixtures, a high PFC (>0.95) LED driver design will be beneficial.
In addition to the importance of high PFC, it is also important to minimize the harmonic distortion levels in LED fixtures. The International Electrotechnical Commission has developed IEC 61000-3-2, Class C Lighting Equipment Harmonics specification to ensure that new LED lighting fixtures meet these low distortion requirements.
Accurate LED current regulation over a wide variation of line voltage, output voltage and temperature is critical in lighting applications as variations in LED brightness must be imperceptible by the human eye. Similarly, to ensure the longest operation life of the LEDs, it is important not to drive them with current above their maximum ratings. LED current regulation in isolated flyback applications is not always straight forward and often requires an opto-coupler to complete the required feedback loop, or an additional conversion stage can be added. However, both of these methods add complexity and reliability concerns. Fortunately, some LED IC driver designs incorporate new design techniques to ensure accurate LED current regulation without these additional components and/or design complexity.
One of the largest obstacles to the instant transition from incandescent bulbs to LED replacements is the cost and size of the LED based solutions. Consumers are used to paying less than $0.50 for a replacement 60W incandescent bulb and about $3.00 for a CFL equivalent. Paying over $30 for a LED replacement is a large hurdle for consumers to overcome. At these prices, the reduced electricity and replacement costs over the lifetime of the LED does make economic sense, however, most consumers are not used to making this connection. In general, commercial businesses which have a significant energy bill specifically for lighting, like warehouses and parking garages, are much quicker to adopt LED lighting, as the cost savings are more obvious. As the purchase cost of LED replacement solutions comes down, more consumers will be willing to make the transition to LED lighting.
Finally, an equally important factor is the size of the LED lighting solution. Many are direct screw-in replacements, thus the entire solution must fit in the same volumetric form factor of the original incandescent bulb. As LEDs require a heat sink and a much more complicated driver circuit, fitting both of these in the same volume footprint can be a challenge. Therefore, LED driver ICs that can offer all of these required features in a simple and compact solution footprint will be needed.
A New Solution
In order to meet the requirements of offline lighting—such as high power factor, high efficiency, isolation and TRIAC dimmer compatibility—prior LED drivers used many external discrete components, resulting in large, cumbersome solutions. Linear Technology’s LT3799 solves these complexity, space and performance problems by integrating all the required functions for offline LED lighting. It is an isolated flyback LED controller with active PFC which is specifically designed for driving LEDs from a universal input range of 90VAC to 265VAC. The LT3799 controls an isolated flyback converter in critical conduction (boundary) mode, suitable for LED applications requiring 4W to over 100W or more of LED power. Its novel current sensing scheme delivers a well-regulated output current to the secondary side without using an opto-coupler. Its unique bleeder circuit makes the LED driver compatible with TRIAC dimmers without additional components. Open- and shorted-LED protection ensures long term reliability.
Figure 1 shows a complete LED driver solution with up to 86% efficiency. The LT3799 senses the output current from the primary side switch current waveform. For a flyback converter operating in boundary mode, the equation for the output current is:
IOUT = 0.5 • IPK • N • (1 – D)
IPK is the peak switch current, N is the primary to secondary turns ratio and D is the duty cycle. The IC regulates the output current by adjusting the peak switch current and the duty cycle through a novel feedback control. Unlike other primary side sensing methods that need to know input power and output voltage information, this new scheme provides much better output current regulation since the accuracy is barely affected by transformer winding resistance, switch RDS(ON) , output diode forward voltage drop and LED cable voltage drop.
Figure 1. TRIAC Dimmable 20W Offline LED Driver Using the LT3799. For full resolution, click here.
High Power Factor, Low Harmonics
By forcing the line current to follow the applied sine-wave voltage, the LT3799 achieves high power factor and complies with IEC61000-3-2, Class C lighting equipment Harmonics Requirement. A power factor of one is achieved if the current drawn is proportional to the input voltage. The LT3799 modulates the peak switch current with a scaled version of the input voltage. As can be seen in figure 2, this technique provides power factors of 0.98 or greater. A low bandwidth feedback loop keeps the output current regulated without distorting the input current.
Figure 2. LT3799 VIN & IIN Waveforms with Active PFC
TRIAC Dimmer Compatible
When the TRIAC dimmer is in the off state, it’s not completely off. There is considerable leakage current flowing through its internal filter to the LED driver. This current charges up the input capacitor of the LED driver, causing random switching and LED flicker. Prior solutions added a bleeder circuit, including a large, expensive high voltage MOSFET. The LT3799 eliminates the need for this MOSFET or any other extra components by utilizing the transformer primary winding and the main switch as the bleeder circuit. As shown in Figure 3, the MOSFET gate signal is high and the MOSFET is on when the TRIAC is off, bleeding off the leakage current and keeping the input voltage at 0V. As soon as the TRIAC turns on, the MOSFET seamlessly changes back into a normal power delivery device.
Figure 3. MOSFET Gate Signal & VIN
LED Current Regulation
Additionally, the LT3799 offers LED current regulation over input voltage, output voltage and temperature. Looking at figure 4, one can see that the LED current remains within +/-5% of regulation when the input varies from 90VAC to 150VAC as would be required in most US lighting applications. In lieu of an opto-coupler, the LT3799’s unique current sensing scheme to deliver a well regulated current to the secondary side. This not only reduces cost but also improves reliability.
Figure 4. LT3799 LED Current Regulation vs. VIN (AC)
Open- and Shorted-LED Protection
The LED voltage is constantly monitored through the transformer third winding. The third winding voltage is proportional to the output voltage when the main switch is off and the output diode is conducting current. In the event of overvoltage or open-LED, the main switch turns off and the capacitor at the CT pin discharges. The circuit then enters hiccup mode. In a shorted LED event, the IC runs at minimum frequency before the VIN pin voltage drops below the UVLO threshold as the third winding can’t provide enough power to the IC. The IC then enters its start-up sequence.
CTRL Pins & Analog Dimming
The LT3799’s output can be adjusted through multiple CTRL pins. For example, the output current would follow a DC control voltage applied to any CTRL pin for analog dimming. Overtemperature protection and line brownout protection can also be easily implemented using these CTRL pins.
Compact and Cost Effective Solution
The LT3799 utilizes a single stage design with the entire LED drive circuit (including the EMI filter), requiring only 40 external components, keeping the solution footprint simple, compact and cost effective. The overall dimensions of the 20W circuit in figure 1 is only 30mm x 75mm with a profile of only 30mm, making it ideal for a wide variety of LED applications. With a few changes in externals, this circuit can be optimized further for 120VAC , 240VAC or even 377VAC applications or virtually any commonly found AC input.
Conclusion
Offline powered LEDs for general purpose lighting applications are constantly driving demand for high performance and cost effective LED driver IC solutions. These LED drivers must provide electrical isolation, high efficiency, PFC > 0.90, and TRIAC dimming capability. Additionally, they must deliver well regulated LED current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations while simultaneously offering a variety of protection features to enhance system reliability. The economics of this transition to LED lighting also demand that LED driver circuits be very cost effective. Fortunately, these LEDs drivers are here now.
About the author: Jeff Gruetter is Senior Product Marketing Engineer, Power Products for Linear Technology Corporation
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