Discussion on the working principle and key factors of LED light source

The main parameters of LED light source operation are VF, IF, and other related colors are color/color temperature/wavelength/brightness/luminous angle/efficiency/power consumption. The LED is a PN junction diode. Only a sufficient forward voltage is applied to conduct the current. The VF forward voltage establishes a normal working state for the LED illumination. The IF forward current is to cause the LED to emit light, and the luminance and current flow are Proportional. White LEDVV nominal voltage: 3.4V ± 0.2V. When the LED light source is produced in large quantities, the VF of each batch of LEDs has a certain dispersion. In order to meet the consistency required by the customer, the LEDs must be sold according to different VF binnings when the products are shipped. LED light source must use the same grade VF or adjacent grade, otherwise it will affect the brightness of the same batch of LED lamps; LEDIF working current is selected according to application requirements, different current grades can not be mixed.
LED light source driving circuit
The driving circuit of the LED light source converts the 12V DC voltage into a stable constant current source. The design of the circuit is based on the principle of simple and cost-saving, and should be selected from a large number of LED driver chips that can complete the design requirements of this circuit. A chip that is better, has a simple application circuit, and has a relatively low price/performance ratio. Therefore, selecting a driver chip with few peripheral components of the driver circuit is the primary consideration for production cost.
Cin is the input filter capacitor, Rs sets the current IF flowing through the LED, Rs=0.1/ILED; L is the freewheeling inductor, and D5 is the freewheeling diode. Since the adapter has supplied 12V DC voltage, the original picture is AC voltage input rectification.
The bridge rectifiers D1-D4 used can be omitted. Although the parts are few, but the requirements for the parts are high, the design should be selected according to the requirements of Table 1, in order to make the circuit enter a good working condition. The switching frequency of the PT4115 uses frequency-hopping technology to effectively reduce EMI, omitting anti-EMI circuits.
The constant current accuracy of the LED light source driving chip is very important for the LED lamp manufacturer. At present, the constant current precision mass production of the LED light source driving IC will have a certain dispersion, and the LED lamp manufacturer is in mass production debugging. The same power supply, the same LED light source load, and the same constant current source power supply board, the difference in the constant current accuracy of the different drive ICs of the same model will cause the constant current source power board output current to have a certain tolerance, which will make the same LED The brightness of the light source load is different, which will increase the time for online debugging of the constant current source power board during mass production, which affects productivity. Therefore, the constant current source power board manufacturer should use a driver IC with high constant current accuracy, and the constant current accuracy should be at least ±3%. If it is ±1%, the price will be higher than ±3%.
Key technologies
1) Choice of inductance
In the case of 3X1W high-brightness white LED light source design, three 1W LED light sources are connected in series, and the working current can be designed to be 300-350mA. The inductance of L should be 68uH-100uH, and the Q value is greater than 50.
And a magnetic circuit that closes the inductor with a current greater than 800 mA. The optimal operating frequency of the PT4115 is below 1MHz. The large inductance will affect its operating frequency. The inductor design of this scheme is above 68uH, so that the operating frequency of the system can be controlled below 1MHz. The inductance is small and the operating frequency is high. Due to the response speed limit of the internal current detection circuit of PT4115, the internal current detection is affected, and the on/off control of the internal switch cannot be better realized. In addition, because the high frequency will bring about a large switching loss, the chip operates at a higher junction temperature, and the electrical stress is increased, which is not conducive to stable operation. Too small an inductance will cause the SW terminal of the PT4115 to burn out without output.
The smaller the DCR of the inductor, the higher the efficiency. It is recommended to use EPC13 manganese zinc 4000 core. The saturation current of the inductor is chosen to be small, and the current of the D5 Schottky diode is selected to be small, which will result in insufficient freewheeling of the entire circuit, and the LED light source will produce a flash visible to the human eye. Increasing the saturation current of the inductor and the current of the Schottky diode increases the freewheeling current of the entire circuit and eliminates the resulting flash.
2) Adapter selection
The adapter (Adapter) provides stable AC power for the LED desk lamp of the scheme->AC-DC step-down conversion. Its real-time load output capability will affect the performance of the LED desk lamp of this scheme. It is used as the adapter of the 3X1W white LED table lamp of this scheme. The output current should be greater than 1A at load and the voltage should be stable at DC12V. Some adapters with poor load capacity, when connected to the LED light source load of this solution, its real-time output voltage will drop to 7V, or even 6.5V. For the LED driver IC whose working voltage starts from 8V, it will enter the undervoltage protection state and stop. Work, once the driver IC stops working, the voltage rises back to 12V, and the LED driver IC enters the working state again, so that the LED desk lamp has a visible flash of light. At this time, only the replacement of the adapter with good load capacity can make the LED desk lamp work normally. At the same time, the LED driver IC with the working voltage range from 6V should be selected, and the adapter selection requirements can be reduced to reduce the production supporting cost.
3) About EMI conduction and radiation
The overall design of this LED desk lamp solution should consider the conduction and radiation that can pass EMI. The key to the conduction and radiation of EMI is the power converter. Therefore, it is necessary to use the adapter that can pass EMI and even CE and UL. The LED table lamps produced can be exported to the European, American and Japanese markets. The constant current source power supply board is a DC/DC switching device. The switching frequency is radiated during operation. Therefore, it can be effectively reduced in the metal case of the lamp base. The mechanical circuit design should consider the magnetic circuit shielding in the metal base. .

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