With off-line power supply to the LED
Can be used off-line LED driver power supply so that the application can be exponentially rapid growth, because whether it is commercial or residential buildings, this form of power can get very conveniently. While the LED lamp replacement attachment is relatively simple, easy to install the end user, but the requirements for the new LED driver IC massively increased. Since the LED requires a well regulated constant current source to provide a constant light output, the AC input power supply for the LED need to take some special design methods, to meet some very special design requirements.
In the different regions of the world, there is a difference in offline power parameters, typically between 90Vac to 265VAC, and the frequency is 50Hz to 65 Hz. Therefore, it is best to manufacture LED attachment to the global market, it is best to have such a circuit design: no need to modify it anywhere in the world. This requires a single LED driver IC can handle a variety of input voltage and frequency.
In addition, many applications require offline LED electrical isolation between the LED and the driving circuit. This is mainly for security reasons, and is one of several regulatory requirements. Electrical isolation is typically provided by isolated flyback LED driver topology, with the use of a transformer to isolate the primary side and the secondary-side driving circuit portion.
As a result of the driving force is to provide LED lighting required for a specific light output power can be greatly reduced, thus imperative: LED driver IC to offer the highest efficiency. Because the LED driver circuit must be high voltage AC power supply to a lower voltage and a well regulated LED current, the LED driver IC must be designed to: provide efficiencies higher than 80% so as not to waste power.
Further, in order to house common TRIAC dimmer applications have largely the case in place of the LED retrofit lamp becomes possible, LED driver IC must operate efficiently with these dimmers. TRIAC dimmer designed for incandescent and halogen lamps and work well with the design, the two lamps are ideal resistive load. However, LED driver circuit is typically nonlinear and not purely resistive load. Input bridge rectifier AC input voltage is at its positive peak and a high peak currents typically absorb negative peak intensity. Thus, LED driver IC must be "imitation" By designing to a purely resistive load, to ensure proper start LED does not produce any significant flicker under the circumstances, and the use of a proper TRIAC dimming.
It is LED lighting terms, PFC is an important performance specifications. In short, if the current drawn is proportional to the input voltage and in phase, the power correction coefficient is 1. Since the incandescent lamp is an ideal resistive load, the input current and voltage are in phase, and as a PFC. PFC is particularly important because it is related to the electric power required for local electricity supplier to offer. That is, in a power system, compared to a load having a high power factor, under the same circumstances the useful power transmission, a larger current drawn by the load having a low power factor. Because the higher the required current, energy losses in the distribution system is also improved, which in turn require thicker wire and other transmission equipment. Larger equipment and wasted energy cost increases, so power companies tend to lower the power factor of industrial or commercial customers charge higher fees. International Standard for LED applications are still under development, but most people believe: In terms of LED lighting applications, will require PFC "0.90.
Performance LED driver circuit (including a large number of diodes, capacitors and transformers) is not a purely resistive load, so the PFC may be as low as 0.5. In order to increase the PFC to 0.9 above, active or passive PFC circuit must be designed into the LED driver circuit. It should also be noted is that in use the application more than a few hundred 50W LED lamp, high PFC is particularly important. In this case, a high PFC ( "0.95) LED driver design is advantageous.
In lighting applications, can accurately regulate the LED current is critical in a wide input line voltage, output voltage and temperature range, because of changes in the brightness of the LED must be imperceptible to the human eye. Similarly, in order to ensure that the LED has a maximum operating life, without using a current higher than its maximum rating of driving the LED it is very important. In isolated flyback applications, LED current regulation is not always carried out directly, often require an optocoupler to close the feedback loop required, or may add an extra conversion stage. However, both methods have brought complexity and reliability issues. Fortunately, some of the LED driver IC designed with a new design method without these additional components, or without increasing the complexity of the design, can accurately regulate the LED current.
Finally, the rapid shift from incandescent to LED lights, the biggest obstacle is the cost and size of the LED-based solutions. Consumers are accustomed to spend less than $ 1.00 for a 60W incandescent lamp, for about $ 3.00 for a CFL lamp. And spend more than 30 US dollars for a LED light, the consumer is a big obstacle. Taking into account the cost of electricity and replacement costs are reduced in the LED's lifetime, at this price for a LED lamp does have economic significance, but consumers are not accustomed to this link to see the problem. Overall, warehouses, garages and other lighting electricity pay a lot of commercial enterprises are turning to LED lighting speed is much faster, because of the cost savings is obvious. With the decline in the purchase as an alternative lighting solutions, LED lamp costs, there will be more consumers are willing to LED lighting.
A new solution
Fortunately there is a new solution, Linear Technology's LT3799, can solve all these problems offline LED driver is. The device is provided with an active PFC isolated flyback LED controller, designed for driving the LED in a universal input 90VAC ~ 265VAC designed. LT3799 operates in the critical conduction mode, minimizing the size of the external transformer. It is ideally suited for a variety of driving power from 4W LED applications to hundreds of watts. 20W LED drive up the entire circuit shown in Figure 2. The specific circuits employed TRIAC dimming, and can provide up to 1A LED current within a general input range of 90VAC ~ 265VAC, while achieving efficiency over 86% (see FIG. 3). Slight changes of external components, this circuit can be further optimized for use in 120VAC, 377VAC or even 240VAC under almost any circumstances common AC input.
LT3799 using the single-stage design, the LED drive circuit (including an EMI filter) 40 only external components, so that the solution is simple, compact footprint and cost-effective. The total size of the circuit of Figure 2 is only 20W 30mm x 75mm, a height of only 30mm, making it ideal for a wide variety of applications of the LED.
Figure 1: General Purpose Input and having a LT3799 PFC and TRIAC dimming capability, isolated flyback LED controller circuit
FIG 2: LT3799 efficiency over a universal range of input voltage
FIG 3: LT3799 VIN having a source and a PFC waveforms IIN
in conclusion
There has been an unprecedented growth in the general LED lighting applications market off-line power supply, the driving force behind it is a continuing need for higher performance and cost-effective application of lighting. However, the emergence of these new performance requirements must use the new LED driver IC to meet. These LED drivers must provide electrical isolation, high efficiency, and the PFC TRIAC dimming capability. Further, regardless of the input voltage or LED forward voltage variations, these drives must also provide a well regulated LED current, in order to maintain uniform brightness, while providing various protection functions to improve system reliability. The LED driver circuit must be able to constitute a very compact and cost-effective solution. Fortunately, now there is such an offline LED driver
Editor of responsibility; ZL
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