Practical first! Introducing a simple LED desk lamp design

As the fourth generation of new energy-saving light source, LED light source was used as the light source of various lamps since its birth. The incandescent lamp as a light source has a luminous efficiency of only 5%, and the luminous efficiency of the LED light source is almost 90%. LED lighting has become the focus of everyone's attention because of its high energy saving, long life and environmental protection. The desk lamp is an ordinary lamp used by every household. In recent years, the high-brightness LED light source has been leaps and bounds due to its manufacturing technology, and its production cost has been reduced. Now, the table lamp can use the LED light source as high brightness, high efficiency and power saving. A carbon-free lighting source.

LED table lamp working principle

Following the design concept of safety first civil electrical appliances, LED light source is a kind of low-voltage DC constant current source light-emitting device, which can not be directly lit with 100-220V AC high-voltage power. Therefore, the design idea of ​​LED desk lamp scheme is First, the high-voltage AC power is converted into a low-voltage DC constant current source to illuminate the LED light source. The most cost-effective way to step down and perform AC-DC conversion is the primary consideration in design. Today's portable electronic products use AC power AC-DC buck converters - Adapters are both affordable, ready-made, and A good choice. The output voltage of the adapter needs to be stable at DC12V. The output current should be selected according to the power of the LED light source. Generally, a margin of 30% should be given. Take the white LED light source of 3&TImes; 1W as an example. The standard working current of the 1W white LED should be It is 350mA, so the current required for the circuit connection of 3 LED light sources is also 350mA. Considering the extension of LED life and reducing the light decay, it can be designed to be 300mA~330mA, which will not obviously affect the brightness of LED illumination, so the output current of the adapter should be Choose 750mA ~ 1A.

A simple and practical LED table lamp scheme is shown in Figure 1. The AC220V provides a stable 12V DC power supply to the LED desk lamp via a safe step-down converter outside the luminaire. A constant current source power supply board is placed in the base of the lamp base to convert the DC voltage into a stable DC constant current source to meet the LED light source. Technical requirements for luminescence. A power switch can be added before the DC constant current source to turn off the DC power when the desk lamp is not used, but the 220V AC power supply cannot be turned off. Therefore, the power plug of the adapter should be removed from the wall when not in use. This is also the practical solution. The only "disadvantage". If you don't want to use mechanical switches and want a more creative selling point, you can use electronic touch switch, such as finger tap to realize the opening of the desk lamp - "full bright, semi-dark, off"; due to the rapid advancement of electronic technology, electronic touch Switches are now a low-cost device.

LED light source working principle

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 LED VF 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 lead to the difference in brightness of LED lamps produced in the same batch; LED IF 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.

PT4115 is only required to use four parts for the white LED light source driving scheme of 1~6W (Fig. 2), Cin is the input filter capacitor, Rs sets the current IF flowing through the LED, Rs=0.1/ILED; L is the freewheeling inductor D5 is a freewheeling diode. Since the adapter has supplied a DC voltage of 12V, the bridge rectifiers D1 to D4 used for AC voltage input rectification can be omitted. Although the parts are less, the requirements for the parts are higher. The design needs to 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, so that the same LED light source The brightness of the load varies, which increases the time for on-line debugging of high-current source power boards during mass production, affecting 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%.

Inductor selection

In the case of 3&TImes; 1W high-brightness white LED light source, 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, Q value is greater than 50, saturated. A magnetic circuit with a current greater than 800 mA closes the inductor.

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 limitation of the internal current detection circuit of the 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.

Adapter selection

The adapter provides stable AC power for the LED desk lamp of this program--AC-DC step-down conversion, its real-time load output capability will affect the performance of the LED table lamp of this solution, and it is used for the adapter of the 3X1W white LED table lamp of this scheme. When the output current should be greater than 1A, 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, which can also reduce the selection requirements of the adapter to reduce the production supporting cost.

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 to support the production. The LED table lamp 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 generates radiation during operation. Therefore, it can be effectively reduced in the metal case of the lamp base. The mechanical structure should be considered in the mechanical structure of the metal base. shield.