Light-emitting diodes (LEDs) have the advantages of lightness, shortness, power saving, good color saturation, long service life and environmental protection. Recently, with the vigorous development of high-brightness LED-related technologies and the gradual decline in the overall production cost of LEDs, LED lighting has been widely used in various fields, including indoor and outdoor lighting, architectural lighting, automotive lighting, street lights and portable lighting. And more and more manufacturers are competing to join the design and development of LED lighting modules.
At present, most of the LED lighting modules on the market are mostly independent modules with their driving circuits and light sources. Among them, traditional LED drive circuits are composed of a large number of discrete components. Although driven by the vigorous development of the LED lighting market in recent years, major IC designers around the world have published highly integrated LED drive control ICs to reduce the discreteness in the drive circuit. The number of components reduces cost and assembly complexity; however, LED lighting modules that are integrated with individual drive circuits and LED light sources still require more parts and higher assembly costs.
In order to further improve the cost structure of LED lighting products and accelerate the popularization of applications, related companies have actively developed photoelectric integrated LED lighting modules (Figure 1). The so-called photoelectric integrated LED lighting module uses System on Board (SoB) technology to place the LED light source and the LED drive circuit on the same substrate, thereby reducing assembly parts and assembly costs, and reducing modules Assembly costs.
Figure 1 Basic architecture of optoelectronic integrated LED lighting module
Overcoming the photoelectric integrated heat dissipation / dimension problem
Since the LED light source and the LED driving circuit are integrated on the same substrate, in the development process of the entire optoelectronic integrated LED lighting module, what type of LED light source should be selected and which driving circuit architecture must be used for integration , Will be the key to the design of the entire LED lighting module.
At present, in LED lighting applications, commonly used LED light sources can be roughly divided into two types. The first type is a low-voltage high-power LED that uses low forward voltage and high forward current, such as each forward current is 350, 700 or up to 1. 2,000 milliampere (mA) scheme; the second is a high forward voltage, low forward current high voltage medium and low power LED formed by connecting several or even dozens of low power LEDs in series.
Since the FR4 printed circuit board (PCB) and the aluminum substrate may be used as substrates in the photoelectric integrated LED lighting module, if low-voltage high-power LEDs with low forward voltage and high forward current are used as the light source, the heat dissipation problem will be It is quite a tricky bottleneck; in contrast, using high-voltage, medium- and low-power LEDs with high forward voltage and low forward current as the light source to allow LED heat energy to be evenly distributed will greatly reduce the difficulty of handling heat dissipation problems. In addition, under the same output power, the use of medium and low power LED light sources with high forward voltage and low forward current can also make the light source distribution of the entire LED lighting module more even, so most manufacturers are developing photoelectric integrated LED lighting For the module, the medium and low power LED light source with high forward voltage and low forward current is preferred.
In addition to choosing the appropriate LED light source, which LED drive circuit architecture should be integrated into the optoelectronic integrated LED lighting module is also the key to the design. Since the drive circuit must be integrated with the LED light source on the same printed circuit board, if the size of the drive circuit is too large, the drive circuit will block or absorb the LED light, affecting the light type of the lighting module and reducing the light. Output, so the number of discrete components used in the drive circuit should be reduced as much as possible, and avoid the use of bulky passive components, so that the drive circuit and LED light source can achieve a high degree of integration, reducing the drive circuit to the LED lighting module light type and light output influences.
Linear constant current scheme simplifies LED circuit
Since the driving circuit and the LED light source in the photoelectric integrated LED lighting module are combined with each other, the service life of the two modules will affect the service life of the entire lighting module. Designers want to extend the effective use time of the entire LED lighting module. In addition to selecting LED light sources with long-life life, they must also try to extend the life of the drive circuit.
Common high-voltage LED drive circuit architectures on the market can be roughly divided into three categories. The first is a circuit architecture using current-limiting resistors, the second is a circuit architecture using switching converters, and the third is a linear constant current. LED drive circuit architecture. Among them, the circuit structure of the current limiting resistor mainly adjusts the LED forward current through the series resistance. Therefore, the LED current will vary with the input voltage, resulting in the disadvantage that it cannot be maintained at a fixed value. At present, this solution has been gradually linearized Replaced by LED drive circuit.
As for the circuit architecture of the switching converter compared with the other two, it can be found that it has the advantages of high circuit conversion efficiency, and can meet any situation where the input voltage is greater than, less than or equal to the output voltage according to different circuit types. However, the circuit architecture of the switching converter must use magnetic components to achieve energy transfer, so the circuit volume is relatively large and complicated, and it also needs to be imported into more external discrete components, so it is not suitable for an optoelectronic integrated LED lighting model Drive circuit in the group.
The last linear constant current LED drive circuit architecture, because no additional energy storage components are needed, can simplify the circuit design, so it combines the advantages of small size, low cost and few external discrete components. It has become a designer's development of photoelectric integrated LEDs Ideal for lighting modules.
Removal of electrolytic capacitor / magnetic element LED driver circuit for longer life
The linear constant current LED drive circuit is a circuit structure using a constant current diode (CRD). The structure is the simplest, but often the LED can be driven smoothly only when the input voltage is greater than the LED forward voltage. Therefore, when AC is input, When there are many LEDs in series, the LED input voltage must be close to the peak value before it will be driven. This will shorten the effective lighting time of the LED lamp and the total output lumen.
To improve this problem, the offline high-voltage linear constant current LED driver circuit shown in Figure 2 can be used. It consists of a bridge rectifier, a high-voltage linear constant current LED driver and an LED lamp. Since only a few SMD resistors and The ceramic capacitor (MLCC) does not introduce electrolytic capacitors and magnetic components, so it will not be limited by the service life of the electrolytic capacitor, resulting in shortened driving circuit time, and can greatly reduce the size of the driving circuit and reduce the cost of the driving circuit. In addition, integrating this driving circuit with the LED light source on the same substrate to form a photoelectric integrated design has little effect on the light type and light output of the LED lighting module.
Figure 2 Offline high-voltage linear constant current LED drive circuit architecture
The new generation of LED drivers can also automatically control the number of LED strings to be lit according to different input voltages. When the input voltage is low, only some LED strings are lit, and the remaining strings are turned off; on the contrary, When the input voltage is high enough, all LED strings can be lit. In this way, in an LED lamp composed of multiple light strings, the number of light strings that are lit will vary according to the voltage of the input power supply, as shown in Figure 3 for each half of the cycle of the entire AC input power supply The current fluctuation of the light string.
Figure 3 Current waveform of LED lamp
In summary, the LED driver circuit drives the LED lamps connected in series in a segmented manner, which allows the driver circuit to start driving part of the lamp string when the input voltage is low, thereby increasing the LED lamp during the entire AC power cycle The time of being lighted within; in this way, the total output lumens of the LED lamp can be improved without increasing the current. Figure 4 is an example of an LED photoelectric integrated LED lighting module of the LED bulb.
Fig. 4 LED photoelectric integrated LED lighting module
The cost of introducing LED photoelectric integrated design drops dramatically
The photoelectric integrated LED lighting module can reduce the production cost of the LED lighting module by using the SoB technology to place the LED light source and the LED driving circuit on the same substrate. In addition, the design of the high-voltage linear constant current LED driver circuit without magnetic components and electrolytic capacitors is also very important. Because this circuit uses only a few SMD resistors and ceramic capacitors, it has small size, low cost and few external discrete components. Advantages, very suitable for use in LED lighting modules integrated photoelectric. By combining this high-voltage linear constant current LED drive circuit in an optoelectronic integrated LED lighting module, it is believed that the price of indoor lighting fixtures can be more in line with general public expectations.
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