"Background
People pay more and more attention to the impact of traditional lighting methods on the environment, and the price of LED is declining. Therefore, for many off-line applications, high-power LED is rapidly becoming a popular lighting solution. High brightness LED can save energy, have long life and benefit the environment. These characteristics continue to promote the development of a wide variety of solid-state lighting (SSL) applications. Therefore, it should not be surprising that the growth rate of LED continues to accelerate. By the end of 2010, the market scale of high brightness LED had reached US $8.2 billion, which is expected to grow to more than US $20 billion by 2015, with an annual compound growth rate of 30.6% (data source: strategies unlimited). In the past few years, LED used as backlight for high-definition television (HDTV) display has been the main driving force of LED market growth. However, with the increasing interest in LED general lighting applications in commercial and residential environments, the growth of LED will accelerate significantly.
The main driving force behind the high growth rate of LED lighting is that the power consumption of LED lighting is greatly reduced compared with traditional lighting methods. To provide the same light output (in lumens) as incandescent lighting, LEDs require less than 25% of the electrical power of incandescent lamps. LED lighting has many other advantages, but there are also some challenges related to LED lighting.
The advantages of LED lighting include that the working life is several orders of magnitude longer than that of incandescent lamps, which greatly reduces the replacement cost. It is also a major cost advantage to use the previously installed triac dimmer to dim the LED, especially in the field of residential lighting. The LED can be switched on immediately, unlike CFL, which requires preheating time, and the LED is not sensitive to power cycle, which is also different from CFL. In addition, LEDs do not contain any toxic materials that need to be managed or disposed of, and CFL needs toxic mercury vapor to work. Finally, LEDs can achieve new, very flat dimensions, which is impossible for other technologies.
Offline power supply can be used
The ability to drive LEDs with off-line power supply has led to a rapid growth in LED applications, because this form of power supply is easy to obtain in commercial and residential buildings. Although LED lamp replacement is relatively simple for end users, the new requirements for LED Driver IC have greatly increased. Because LEDs need a well regulated constant current source to provide a constant amount of light output, using AC input power supply to supply power to LEDs requires some special design methods, and there are some very special design requirements.
Depending on where you are in the world, the range of off-line power supply is about 90vac to 265vac, and the frequency range is 50Hz to 65Hz. Therefore, to produce LED lamps for the global market, it is ideal to provide a single circuit design that can be applied anywhere in the world without modification. This requires that a single LED Driver IC can handle a variety of input voltages and power supply frequencies.
In addition, many off-line LED applications require that the LED be electrically isolated from the drive circuit. This is mainly for safety reasons and is also a requirement of several regulators. Electrical isolation is generally provided by the isolated flyback LED driver topology, which uses a transformer to isolate the main and secondary end parts of the drive circuit.
The driving force behind the use of LED lighting is that the power required to provide a certain amount of light output is greatly reduced. Therefore, it is imperative that the LED Driver IC should provide the highest efficiency. Because the LED driver circuit must convert the high-voltage AC power supply to a well regulated LED current at a lower voltage, the LED Driver IC must be designed to provide an efficiency of more than 80% in order not to waste power.
In addition, in order for LED lamps to use the widely installed triac dimmers common in residential applications, the LED Driver IC must be able to work effectively with these dimmers. Triac dimmers are designed to work well with incandescent and halogen lamps, which are ideal resistive loads. However, LED driver circuits are generally nonlinear and are not purely resistive loads. Its input bridge rectifier usually absorbs high-intensity peak current when the AC input voltage is at its positive and negative peaks. Therefore, the LED Driver IC must be designed to "simulate" a purely resistive load to ensure that the LED starts correctly without any obvious flicker, and uses a triac for proper dimming.
Power factor correction (PFC) is an important performance specification in LED lighting. In short, if the absorbed current is proportional to the input voltage and in phase, a power correction factor equal to 1 can be achieved. Because incandescent lamp is a pure resistive load, all input currents and input voltages are in phase, and PFC is 1. PFC is particularly important when it is related to the amount of electrical power required by the local power supply. That is, in a power supply system, in terms of transmitting the same amount of useful power, the load with low power factor absorbs more current than the load with high power factor. The need for greater current will increase the energy lost in the distribution system, which leads to the need for thicker wires and other larger transmission equipment. Because of the high cost and energy waste of larger equipment, power companies usually charge higher fees to industrial or commercial customers with lower power factor. International standards for LED applications are still under development, but most people believe that PFC 0.90 will be required for most LED lighting applications.
Because the performance of LED driver circuit (including many diodes, transformers and capacitors) will not be the same as that of pure resistive load, its PFC may be as low as 0.5. In order to improve the PFC above 0.9, both active and passive PFC circuits must be designed into the LED driver circuit. It should also be mentioned that high PFC is particularly important in applications using a large number of LED lighting arrays. For example, in a parking garage using more than hundreds of 50W LED lights, the design of high PFC (0.95) led driver will be very advantageous.
In addition to high PFC, it is also important to minimize the harmonic distortion of LED lamps. The International Electrotechnical Commission has formulated IEC 61000-3-2 harmonic specification for class C lighting equipment to ensure that the new LED lighting system meets these low distortion requirements.
In lighting applications, it is very important to accurately adjust the LED current in a wide range of line input voltage, output voltage and temperature changes, because the change of LED brightness must be difficult for human eyes to detect. Similarly, in order to ensure the longest service life of the LED, it is also important not to drive the LED with a current higher than its maximum rating. Adjusting the LED current in isolated flyback applications is not always simple, but often requires an optocoupler to close the desired feedback loop, or an additional conversion level may be added. However, both methods add complexity and reliability problems. Fortunately, some LED IC driver designs adopt new design methods to ensure that the LED current can be accurately adjusted without these additional components and / or increasing design complexity.
One of the biggest obstacles to the rapid transition from incandescent to LED is the cost and size of LED based solutions. Consumers are used to paying less than $0.50 to replace a 60W incandescent lamp and about $3 to replace a CFL lamp with the same wattage. Paying more than $30 to replace an LED lamp is a major obstacle for consumers to overcome. At this price, compared with the replacement cost, it is really economical to replace LED lamps in the service life of LED lamps. However, most consumers are not used to looking at problems in this way. Generally speaking, commercial enterprises such as warehouses and parking garages, especially those paying high energy bills due to lighting, will adopt LED lighting faster because the cost savings are more obvious. With the decline in the purchase cost of LED lamps, more consumers will be willing to turn to LED lighting.
Finally, an equally important factor is the size of LED lighting solutions. Many lighting lamps can be replaced by directly screwing into the lamp holder, so the whole LED solution must be able to be installed in the space with the same volume and shape as the original incandescent lamp. LEDs need a radiator and a much more complex driver circuit, so it may be a challenge to install the whole LED solution including these two parts in the same space as incandescent lamps. Therefore, the required LED Driver IC should be able to provide all these required functions and features in a simple and compact board area solution.
A new solution
In order to meet the requirements of off-line lighting (such as high power factor, high efficiency, isolation and triac dimmer compatibility), previous LED drivers used many external vertical components, resulting in a large and complex solution.
Linglilte's lt3799 integrates all the functions required for offline LED lighting to solve these complexity, space and performance problems. Lt3799 is an isolated flyback LED controller with source power factor correction, which is specially designed to drive LEDs in the general input range of 90vac to 265vac. The device controls an isolated flyback converter in critical on (boundary) mode, which is suitable for LED applications requiring 4W to more than 100W or higher LED power. The novel current detection circuit can provide a well regulated output current to the secondary terminal without using an optocoupler. Its unique energy discharge circuit makes the LED driver compatible with triac dimmer without adding additional components.
Led open circuit and short circuit protection ensure long-term reliability.
Figure 1 shows a complete LED driver solution with an efficiency of 86%. The lt3799 detects the output current from the main terminal switching current waveform. For a flyback converter operating in boundary mode, the output current equation is:
IOUT = 0.5 • IPK • N • （1 – D）
Ipk is the peak switching current, n is the turns ratio from the main terminal to the auxiliary terminal, and D is the duty cycle. The IC adjusts the peak switching current and duty cycle through a novel feedback control circuit to adjust the output current. Unlike other main end detection methods that need to know input power and output voltage information, this new circuit provides much better output current regulation, because the accuracy is almost unaffected by transformer winding resistance, switch RDS (on), forward voltage drop of output diode and led cable voltage drop.
Figure 1: 20W offline LED Driver with lt3799 and triac dimmable
90V to 270V: 90V to 270V
20W LED power: 20W LED power
High power factor, low harmonic
By making the line current follow the applied sine wave voltage, lt3799 achieves high power factor and meets the harmonic requirements of IEC 61000-3-2 class C lighting equipment. If the current drawn is proportional to the input voltage, a power factor equal to 1 can be achieved. The lt3799 modulates the peak switching current with a voltage generated from the input voltage and proportional to the input voltage. As can be seen in Fig. 2, this method provides a power factor of 0.98 or higher. A small bandwidth feedback loop maintains the regulation of the output current without distorting the input current.
Figure 2: VIN and iin waveforms of lt3799 with source power factor correction
Voltage: voltage
Current: current
Time: time
Frequency: frequency
Compatible with triac dimmers
When the triac dimmer is off, it is not completely off. A considerable leakage current flows to the LED driver through its internal filter. This current charges the input capacitor of the LED driver, causing the led to switch and flash randomly. Previous solutions added an energy relief circuit that included a large and expensive high voltage MOSFET. The lt3799 uses the main winding of the transformer and the main switch as a discharge circuit, so no such MOSFET or any other additional components are required. As shown in Fig. 3, when triac is off, the MOSFET gate signal is high and the MOSFET is on, so as to drain the leakage current and maintain the input voltage at 0V. As soon as triac is turned on, MOSFET will seamlessly return to normal power supply device.
Figure 3: MOSFET gate signal and Vin
LED current regulation
In addition, the lt3799 provides LED current regulation over the entire input voltage, output voltage and temperature range. As shown in Figure 4, it can be seen that when the input changes from 90vac to 150vac, the LED current remains within ± 5% of the regulation range, as required by most American lighting applications. The lt3799 uses a unique current detection circuit to replace the optocoupler to provide a well regulated current to the secondary terminal. This not only reduces costs, but also improves reliability.
Figure 4: lt3799 LED current regulation and VIN (AC)
LED current vs input voltage
Led open circuit and short circuit protection
The LED voltage is continuously monitored through the third winding of the transformer. When the main switch is off, the voltage of the third winding is directly proportional to the output voltage, and the output diode conducts current. Once the overvoltage or LED is open circuit, the main switch is disconnected and the capacitor of CT pin begins to discharge. The circuit then enters burping mode. In the case of LED short circuit, the IC operates at the lowest frequency before the VIN pin voltage drops below the UVLO threshold, because the third winding cannot provide sufficient power to the IC. Then the IC enters the start sequencing state.
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