【Expert Thoughts】Design Thinking of Blue Light "LED Incandescent Lamp"

　　Incandescent lamps have been used around the world for more than 100 years since their invention. Incandescent lamps add AC power directly to a tungsten filament resistor, causing it to heat up and emit light, thus illuminating the darkness. They have brought brightness and happiness to hundreds of millions of people around the world at night, as well as convenience in their work.

　　Incandescent lamps are electric light sources that heat the filament to an incandescent state by passing electricity through it, and then use heat radiation to emit visible light. Since 1879, when Thomas Edison of the United States made a carbon fiber (i.e., carbon filament) incandescent lamp, the luminous efficiency of incandescent lamps has been improved accordingly through continuous improvements in filament materials, filament structures, and filling gases. However, the luminous efficiency and other light point parameters of incandescent lamps are inferior to those of new light sources such as energy-saving lamps and halogen tungsten lamps, and are gradually being replaced by new light sources. Starting from October 1, 2012, China will ban the import and sale of ordinary lighting incandescent lamps above 100 watts.

　　Figure 1: 100-year-old incandescent lamp

　　How to design and manufacture LED lamps to be as easy to use as incandescent lamps? This is a major issue facing thousands of LED lighting designers.

　　Blue light "LED incandescent lamp" may be okay.

　　The concept of blue light "LED incandescent lamp"

　　In the spring and summer of 2011, when designing the "blue LED light engine", we had already thought of designing a blue "LED incandescent lamp". In view of the fact that the traditional white LED is directly coated with yellow phosphorus phosphor on the blue LED, the yellow phosphorus phosphor is easy to denature under long-term high temperature baking due to the high heat after the white LED is lit, resulting in a poor color rendering index after long-term use. In LED lamps, if the yellow phosphorus phosphor is not directly coated on the white LED, but dyed on the PVC lampshade, and a certain distance is kept from the blue LED in the lamp, the two can be used in combination, and the white light or warm white light effect of the LED lamp can also be obtained. Since the yellow phosphorus color filter lampshade is far away from the LED heat source, it will not age and denature due to the long-term high temperature baking of the LED light source, and the CRI (color rendering index) will remain basically unchanged; the formula of the yellow phosphorus color filter lampshade is adjustable, and the color and color temperature of the light can be adjusted according to the needs of customers, which can meet customized requirements and produce differentiated products.

　　The blue light "LED incandescent lamp" uses the structural design of the incandescent lamp to facilitate the use of incandescent lamps for hundreds of years. The innovation of its connotation is to give LED new technology that is energy-saving, pollution-free and long-life. It uses blue light HV LEDs as the light source and adds a yellow phosphorus color filter lampshade. The mains power is converted through AC/DC rectification and constant current drive power supply, and the output constant current DC is used to light up the blue light LED light source. The blue light is filtered by the yellow phosphorus color filter lampshade and emits secondary light, making the "LED incandescent lamp" emit cold white light or warm white light. The working principle is shown in Figure 2.

　　Figure 2 Working principle of "New LED Incandescent Lamp"

　　The blue light "LED incandescent lamp" uses a PVC lampshade dyed with yellow phosphorus or sprayed with yellow phosphorus, so the bulb is yellowish yellow when not lit, and emits cold white or warm white light when it is powered on. The unlit blue light "LED incandescent lamp" is shown in Figure 3. The PVC lampshade dyed with yellow phosphorus or sprayed with yellow phosphorus is the key technology.

　　Figure 3: Unlit blue light "LED incandescent lamp"

　　Selection of LED light source

　　The blue light source of the blue "LED incandescent lamp" is preferably blue light HV LEDs, which can be directly lit with high voltage direct current, simplify the power supply, reduce the power supply cost, and reduce the space occupied by the power supply. HV  LEDs are systems on a chip (SOC) or modules made directly on silicon wafers during chip production. This is a revolutionary advancement in LED chip manufacturing technology. On a 2-inch silicon wafer, thousands of LED tube cores can be produced at a time, but using general technology and processes to produce, the discreteness of each LED parameter is very large, so after production, the manufacturer needs to measure the parameters one by one and classify them. Customers can only use three adjacent classes in one unit (lamp) to ensure the consistency of the displayed light color. HV LEDs use a new generation of processes and superb processing technology during production, so that the parameters of thousands of LED tube cores on the same silicon wafer are completely consistent, so that they can be easily formed into SOC HV LEDs nearby during chip production. For example, there are 16 LEDs on the VF50V module, and all of their electrical parameters are exactly the same (Figure 4). Such blue light HV LEDs are easy to use and have controllable costs. Another type of COB blue light HV LED is a blue light LV LED tube core with a larger parameter dispersion produced using the aforementioned general technology. After testing the data, the configurations with the same parameters are put together and recombined into a light source. Since the LED tube cores need to be tested, paired, screened, and combined one by one, and there are many scattered parameters that cannot be paired and combined, it should be said that the production cost will be high. The operating current of HV LEDs is 20-60mA, which is relatively less hot when working compared to the 300-700mA current required by low-voltage LEDs.

　　Figure 4 Blue HVLEDs module

　　Energy Star requires that the light output angle of LED bulbs must be greater than 270 degrees, so the three-dimensional light-emitting design of LED light sources is the only solution to expand the light-emitting angle of the bulb. A copper through-core vertical rod is designed on the bulb head, and LED lamp beads or modules are placed around and on the top of it. In this way, the light output can be designed to be as large as possible, achieving light emission similar to that of an incandescent lamp. The copper through-core vertical rod can help the heat dissipation of LED lamp beads. Due to the limited area of ​​the copper through-core vertical rod, if the blue light "LED incandescent lamp" requires a larger light-emitting angle, it can only be applied to a 3W low-power bulb lamp, which is equivalent to a 25W incandescent lamp. If the light-emitting area of ​​the bulb lamp close to the lamp head is sacrificed to install an aluminum heat sink, a universal low-voltage blue light LED string can be used to design a 5W or 7W blue light "LED incandescent lamp". There are many options for the distribution design of the copper through-core LED light source, and they vary from designer to designer, and each has a patent application. The key technology will not be described in detail.

　　Blue HV LEDs driver power design

　　There are currently three ways to drive blue HV LEDs:

　　1) Constant current diode (CRD)

　　A constant current diode (CRD) is a device that can only provide a stable current to an LED light source within a certain voltage variation range. It is equivalent to a small constant current source or a maximum peak current limiter. It can ensure a certain current stability when the power grid fluctuates slightly. The application circuit is simple, low-cost, and takes up very little space. The performance is only slightly better than the RC step-down circuit. It can meet the requirements of LED constant current when the load is small and the power grid voltage fluctuates slightly. When the power grid voltage fluctuates greatly, it is difficult to stabilize the output current. The output current of the constant current diode is limited, generally 20-40mA, but it can meet the high voltage and low current requirements of HV LEDs. The current required by HV LEDs is generally 20-60mA. The application circuit of CRD is very simple, as shown in Figure 5. When the LED requires a small current such as 20mA and a light load, one CRD can be selected; when a larger current is required, such as 30-60mA, multiple CRDs can be connected in parallel as shown in the figure to expand the current.

　　Figure 5 Simple constant current circuit of CRD 2) High voltage linear constant current source driver chip

　　Linear driver chips may be one of the better solutions for constant current driver power supply of HV  LEDs . EXC100 is one of the more ideal chips, with a very simple application circuit and few peripheral parts (Figure 6). EXC100 uses segmented conduction technology for LED loads as shown in Figure 7. The EXC100 chip has a built-in high-voltage MOS tube, and the application circuit has no transformer or electrolytic capacitor, and few peripheral circuit parts. It meets the needs of designing a small space for blue light "LED incandescent lamps". However, the price of EXC100 purchased in China is too expensive, and the cost of using it to produce popular LED lighting fixtures is high, which is not conducive to the mass sales of products. Only products with good cost performance can be popular all over the world.

　　Figure 6 EXC100 linear constant current drive

　　In recent years, the design level of my country's domestic integrated circuits has been continuously improving, and the products have good cost-effectiveness and market competitiveness, and the technical solutions and technical support that can be used for production are relatively in place for downstream manufacturers.

　　BP5118 is a new linear driver chip designed and produced by a local chip design company specifically for HV LEDs. BP5118 is a high-precision linear constant current LED control chip, mainly used to drive high-voltage, low-current LED light strings or modules powered by the mains. Since the application circuit does not require electrolytic capacitors and magnetic components, the LED driver power supply can achieve small size, long life, and comply with EMI regulations. BP5118 uses a patented input voltage sampling and LED drive mechanism to change the number of connected LED lights when the input voltage changes, so that the LED can be lit during the entire AC cycle. The time, thereby improving the utilization rate of the LED and the total output lumens. In order to adapt to different LED light string forward voltage drops or different mains voltages (110V, 220V), the light string switching voltage can be flexibly set through external voltage divider resistors.

　　Figure 7 EXC100 conducts LED load in segments BP5118 can accurately set LED  current through external resistors. In the mains input application case in Figure 8, input voltage: 200Vac ~ 260Vac; 47Hz ~ 63Hz; output load: LED1 134V, LED2 76V, LED3 76V; output average current: 22mA (set maximum current 30mA). If a higher output current is required, the LED needs to use a heat sink or parallel external power MOS tube. BP5118 has a thermal regulation function (Thermal Regulation). When the input voltage is too high or the LED current is too large, this function will reduce the output current, thereby limiting the temperature rise and protecting the LED driver power supply.

　　The power output high-voltage MOS tube of BP5118 is external, not packaged in the chip, so the load application of LED lamp beads is more flexible. This is the biggest difference from EXC100.

　　Figure 8 Schematic diagram of BP5108 high voltage linear constant current driver

　　3) High voltage switch constant current source driver chip

　　There are not many varieties of high-voltage switch constant current source driver chips, and BP2812 is one of them. BP2812 is suitable for non-isolated step-down LED power supply systems, suitable for full-range AC input or 12V-600V DC power input. Built-in 600V power MOS tube. Critical mode operation, no inductor compensation required. The application circuit requires very few peripheral devices. The chip has a high-precision current sampling circuit and patented constant current control technology, which can achieve high-precision constant current output and excellent load adjustment rate. The system efficiency is 95%. Using source drive technology, the chip operating current is as low as 200uA, and no auxiliary winding power supply is required. It has multiple protection functions such as LED short-circuit protection, CS sampling resistor short-circuit protection, and chip over-temperature protection. The application circuit is simple and the system cost is low.

　　FIG. 9 is a high voltage constant current drive circuit designed for the HV LED of the wafer at the lower right of the figure.

　　Figure 9 Schematic diagram of BP2812 high voltage switch constant current source driver

　　It can be seen from this that it is a better choice to use a high-voltage linear constant current source chip to design the driving power supply for the blue light "LED incandescent lamp".

　　Blue light "LED incandescent lamp" will become a unique product among LED lighting products, competing for the best. Blue light HV LEDs light source and high voltage linear constant current source driver chip required by blue light "LED incandescent lamp" have already been mature products; yellow phosphorus color filter lampshade and LED three-dimensional light-emitting lamp column still have room for innovation in production process and design technology, so many differentiated new products can be derived. The LED lighting market is unlimited.