Thoughts on the design of affordable LED bulbs

0 Preface

LED as a new generation of energy-saving, environmentally friendly, long-life lighting source has become the consensus of all mankind. LED lighting fixtures can not be widely promoted and applied, can not enter thousands of households, the main reason is that LED lighting prices remain high, a 5W LED bulb, its brightness is equivalent to 25-30W incandescent lamps, the current market The retail price is RMB 50-100, which is much more expensive than fluorescent energy-saving lamps. Now the people buy fluorescent energy-saving lamps, the government still has subsidies, and there are no LED lights to buy! Therefore, it is recommended to develop cheap LED bulbs and LED fluorescent lamps that the people can afford. The LED industry is in urgent need of doing practical things.

Today, LED light sources are continually cutting prices, and the price of new generation chips for LED drive power is also falling, and the price of matching hardware and plastic parts is also falling. Therefore, the time to design and produce affordable LED lighting fixtures has matured.

The market follows the pyramid rule, and high-priced products can only occupy a small part of the spire, and the price drops a little every time, and the market expansion is multiplied by squares (Figure 1). The low-priced LED lighting fixtures will be a popular product that civilians can enjoy, and a new generation of lighting fixtures that ordinary people can afford.

Figure 1 Pyramid rule of the market

With the large price reduction of the LED light source and the high integration of the LED chip power supply chip function, the power cost of the power supply is also gradually reduced. If it can be 5W-10W (equivalent to the brightness of 25W-60W incandescent lamp) The production cost of low-priced LED lighting fixtures is controlled within RMB 8-25 yuan, and the retail price of the market is controlled within 15-35 yuan, so that the retail price of the cheap LED bulbs is close to the retail price of fluorescent energy-saving lamps of the same power, then LED as The replacement of incandescent lamps and fluorescent lamps by the fourth generation of light sources will be greatly accelerated, and the price of cheap LED lighting fixtures into the trillions of ordinary people will become a reality. If the government can supplement the financial subsidies for affordable LED lighting, it will enable the people to benefit earlier and enjoy it earlier, so that the society can save more energy.

1 design ideas for low-cost LED lighting

The design idea of ​​affordable LED lighting fixtures can be used with the principle of quality, quality first, and low cost. The production cost of 5W-10W parity LED lighting fixtures equivalent to the brightness of 25W-60W incandescent lamps is controlled within RMB 8-25. The market retail price is controlled within 15-35 yuan. Taking the cheap LED bulb as an example, it consists of an LED light source module, a constant current driving power supply, a lamp holder, a casing, a radiator, and a lamp cover (Fig. 2). The overall composition of LED fluorescent lamps is also basically the same. How to choose each of these components is the key to designing the success of this affordable LED lighting fixture. The LED light source is selected to have high luminous efficiency, low heat generation, good heat dissipation and good cost performance. The constant current driving power source is selected to use primary side control (PSR) isolation circuit, few peripheral parts, high power factor, high efficiency, and good cost performance. For the guidelines.

Figure 2 The main parts of the cheap LED bulb

2 LED light source module selection

Nowadays, the technology for producing LED light source modules is quite mature, and the price has been in the civilian population. The price per watt (W) of RMB 1.00 is the design cost that can be accepted by currently designing affordable LED Lamps. How to choose the suitable and cost-effective LED light source module among many COB, MCOB, COF and LEDs is one of the keys to the success of this low-priced LED bulb design. LEDs are high voltage LED modules (HV LEDs) that are not discussed in this case.

LED light source module or LED light source module, common is COB package, recently developed COF, LEDs, these are designed for downstream LED application factory LED lighting final product is convenient and fast, and LED upstream company uses its large-scale Designed and produced by the advantages of producing LED chips. The key technology is to integrate a single LED chip with the same electrical parameter performance in one unit when the LED wafer is produced or after production.

The COB (Chip On Board) package is a common LED module package. The LED chip particles with the same electrical parameters are paired, and the matrix arrangement of the combined light sources is designed according to the power. The N-particle LEDs are bound to the aluminum through the COB packaging process. On the substrate or ceramic substrate, it becomes a new LED light source module. The aluminum substrate is made of a material with good heat conductivity, such as Bergs, Laird, etc. in the United States, and the insulating layer on the aluminum substrate to which the LED chip surface is attached is usually about 75-100 um. The copper foil coated on the aluminum substrate is usually 35 um. The thickness of the LED chip is generally 80-100 um. The copper-on-aluminum substrate is prefabricated on the aluminum substrate. Usually, a COB light source has only two connection points to the driving power source (Fig. 3). COB has N LED beads on one plane, one LED light beam has 4 light-emitting surfaces, N LED beads have 4 times N light-emitting surfaces, and the light-emitting surface is larger than the single LED light-emitting surface, and the light-emitting density High, COB package can increase the light extraction efficiency by 15% compared to a single lamp bead of the same power. If the optical cone concentrating pit is preset on the LED chip mounting point of the aluminum substrate or the bracket, the silver plating on the tapered ring can increase the reflection of light and improve the light extraction rate. This kind of placing the LED chip directly in the optical concentrating cup pit is also called MCOB package. The COB package can also reduce the cost of the bracket of the original single LED package, save one of the N secondary optical lenses originally required, and share the same substrate with multiple chips, so that the cost of the multi-wattage lamp bead can be saved, and the labor cost of assembly is also saved. . The LED chip directly dissipates heat through the aluminum substrate, which is more conducive to heat conduction and diffusion.

Figure 3 LED COB package

COF (Chip On Frame) is an innovative development of COB packaging technology. The COF is directly bonded to the copper bracket like the integrated circuit die. The thickness of the copper bracket is about 0.4mm, according to the pre-designed light source. Matrix arrangement, punching the spotlight pit of the LED lamp bead, the depth of the concentrating cup pit is about 0.4mm, the copper foil prefabricated the connection between each LED lamp bead, and only one contact and drive of one COF module Power connection. The copper bracket is stamped and formed, the processing is simple and fast, and the manufacturing cost is not high. Figure 4 is a 5W COF module. The left side is the geometrical drawing of the COF copper bracket, and the right is the physical photo of the COF module.

Figure 4 LED COF package

The COF module is an innovative development of COB module packaging technology. As shown in Fig. 5, a schematic cross-sectional view of the skeleton structure of the COB and COF package substrates assumes that the LED chips (Die) are not placed in the concentrating cup pit of the substrate skeleton, wherein the skeleton of the COF is a copper bracket, and the skeleton of the COB is aluminum. The substrate and copper foil are used to connect the circuit traces. The LED chip on the COB is bound on the aluminum substrate. The LED chip on the COF is bound to the copper bracket. The thermal conductivity of copper is much higher than that of aluminum. Therefore, the thermal resistance of COF can be reduced by 50% compared with COB; COF is N LED lamp bead. Fixed in the matrix concentrating cup pit, the concentrating cup pit adopts silver plating to increase the light reflection, and the multi-faceted light of the LED lamp bead can increase the light efficiency of the COF by 15%; the COF has no aluminum substrate and reduces several channels. The production process saves some labor and materials; therefore, the COF of the same power can reduce the overall cost of COF by 30% compared with COB, so the COF module has a good cost performance.

The module light source of COB and COF has various series suitable for bulb, ceiling lamp, downlight, fluorescent lamp, flat lamp, etc., and the application is very convenient. COF is the first of its kind.

Figure 5 is a schematic cross-sectional view of the two substrate skeleton structures of COB and COF

3 constant current drive power chip selection

The main electrical parameters of the LED light-emitting diode chip operating as an illumination source are forward voltage (VF) and forward current (IF), and other related colors/wavelength/brightness/luminous angle/efficiency/power consumption. The VF forward voltage establishes a normal operating state for LED illumination. The IF forward current is responsible for causing the LED to emit light, and the luminance of the light is proportional to the current flowing through it. White LED VF nominal voltage: 3.2V ± 0.2V (WLED). The LED IF working current is selected according to the application. If it is too large, it will cause the early light decay of the LED. If it is too small, it will not light up. The characteristics of an LED light-emitting diode determine that it must operate in a constant current state. Therefore, the selection of a suitable function, stable and reliable performance, and moderate cost of constant current drive power is the key to the success of this affordable LED bulb design.

The selection function can be used, the peripheral parts are few, and the primary side control isolation circuit of the built-in MOS tube drives the power chip, which is a more rational choice. The technical level of local LED lighting driver chip designers is not much different from that of international manufacturers, and it has obvious advantages in cost performance. Such as Jingfeng Mingyuan, Meixin 晟 and other local LED dedicated constant current source chip design company products BP3102, BP3112, MT7953, etc., they are professional, useful, and have a good cost performance.

The BP3102 is a high-precision primary-side feedback LED constant current control switch. The chip operates in the inductor current interrupt mode and is suitable for flyback isolated LED constant current power supplies with a full input voltage range of 5W or less. The BP3102 chip integrates a 600V power MOS switch, adopts the primary side feedback mode, eliminates the need for a secondary feedback circuit, and does not require a compensation circuit. It requires only a few external components to achieve constant current, saving the cost and volume of the system. The BP3102 has a high-precision current sampling circuit that allows the LED output current accuracy to be within ±3%. The chip uses a patented constant current control method to achieve excellent linearity adjustment and load regulation. The BP3102 has multiple protection functions including LED open circuit protection, LED short circuit protection, chip over temperature protection, over voltage protection, under voltage protection and FB short circuit protection. The internal structure block diagram of BP3102 is shown in Figure 6. The BP3102 is available in an SOP-8 package and the pin functions are described in Table 1.

Table 1 BP3102 pin description

Figure 6 BP3102 internal structure block diagram

The principle diagram of the constant current power supply for designing a 5W affordable LED bulb with BP3102 is shown in Figure 7. This is an isolated constant current power supply scheme. Because the BP3102 has integrated the constant current, LED open circuit protection, LED short circuit protection, chip over temperature protection, over voltage protection, under voltage protection and FB short circuit protection required by the LED constant current drive circuit, the whole application circuit is very simple. . In addition to the necessary AC/DC rectifier bridges, resistors, and capacitors, the driver chip and transformer are the two main components.

Fig. 7 Schematic diagram of constant current power supply of 5W parity LED bulb