Practical LED desk lamp design

As the fourth generation of new energy-saving light sources, LED light sources have been used as light sources for various lamps since their birth. The luminous efficiency of incandescent lamps as light sources is only 5%, while the luminous efficiency of LED light sources is almost 90%. LED lighting has become the focus of widespread attention for its high energy saving, long life and environmental protection. Table lamps are common lamps used in every household. In recent years, the manufacturing technology of high-brightness LED light sources has made great progress, and its production cost has been declining. Now table lamps can use LED light sources as high-brightness, high-efficiency, energy-saving and carbon-free lighting sources.

Working principle of LED desk lamp

Following the design concept of safety first for civil electrical appliances, LED light source is a low voltage DC constant current source light emitting device, which cannot be directly lit by 100~220V AC high voltage electricity. Therefore, the design idea of ​​LED desk lamp is to first convert high voltage AC into low voltage DC constant current source before lighting up LED light source. Using the most economical and effective method to reduce voltage and perform AC/DC conversion is the primary consideration of design. Today's portable electronic products use AC power supply AC/DC step-down converter - adapter (Adapter) has become the first choice that is both economical, ready-made and easy to use. The output voltage of the adapter is required to be stable at DC12V, and the output current should be selected according to the power of the LED light source. Generally, a 30% margin should be given. Taking 3×1W white light LED light source as an example, the standard operating current of 1W white light LED should be 350mA, so the current required for the circuit of 3 LED light sources in series is also 350mA. Considering the extension of LED life and the reduction of light decay, it can be designed to be 300mA~330mA, which will not significantly affect the brightness of the LED light, so the output current of the adapter should be 750mA~1A.

A simple and practical LED desk lamp solution is shown in Figure 1. AC220V is safely stepped down by the adapter outside the lamp to provide a stable 12V DC power supply to the LED desk lamp. A constant current source power board is placed in the base shell of the desk lamp to convert the DC voltage into a stable DC constant current source to meet the technical requirements of LED light source lighting. A power switch can be added in front of the DC constant current source so that the DC power supply can be turned off when the desk lamp is not in use, 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 only "disadvantage" of this practical solution. If you do not want to use a mechanical switch and want a more creative selling point, you can choose an electronic touch switch, such as a finger tap to realize the desk lamp on -> full light, half dark, off; due to the rapid advancement of electronic technology, electronic touch switches are now a low-cost device.

Working principle of LED light source

The main parameters of LED light source are VF and IF, and other related parameters are color/color temperature/wavelength/brightness/light-emitting angle/efficiency/power consumption, etc. LED is a PN junction diode, and can only conduct current when sufficient forward voltage is applied. VF forward voltage is to establish a normal working state for LED light emission, and IF forward current is to make LED light emission, and the brightness of light emission is proportional to the current flowing through. White light LED VF nominal voltage: 3.4V±0.2V. When LED light sources are mass-produced, the VF of each batch of LEDs has a certain degree of discreteness. In order to meet the consistency required by customers, LEDs must be sold in different VF grades when leaving the factory; when customers order the same batch of lamps, the LED light sources required must be selected from the same grade VF or adjacent grades, otherwise it will lead to differences in the brightness of LED lamps produced in the same batch; LED IF working current is selected according to application needs, and different current grades cannot be mixed.

LED light source driving circuit

The driving circuit of LED light source converts 12V DC voltage into a stable constant current source. The design of the circuit is based on the principle of simplifying and saving costs. From the many LED driver chips that can meet the requirements of this circuit design, we should choose a chip with high integration, good performance, simple application circuit, and low price. Therefore, choosing a driver chip with fewer peripheral devices in the driving circuit is the primary consideration for production cost.

When PT4115 is used in 1~6W white light LED light source driving solution, only four parts are needed (Figure 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, and D5 is the freewheeling diode. Because the adapter has provided 12V DC voltage, the bridge rectifier D1~D4 used for AC voltage input rectification in the original figure can be omitted. Although there are fewer parts, the requirements for parts are higher. When designing, they must be selected according to the requirements in Table 1 to make the circuit enter a good working state. The switching frequency of PT4115 uses frequency jitter technology to effectively reduce EMI, and the anti-EMI circuit is omitted.

The constant current accuracy of LED light source driver chip is very important for LED lamp manufacturers. At present, the constant current accuracy of LED light source driver IC will have a certain degree of discreteness during mass production. When LED lamp manufacturers are debugging in mass production, the same power supply, the same LED light source load, and the same constant current source power supply board will have a certain tolerance in the output current of the constant current source power supply board due to the individual differences in the constant current accuracy of different driver ICs of the same model, so that the luminous brightness of the same LED light source load is different, which will increase the time of online debugging of the constant current source power supply board during mass production and affect productivity. Therefore, constant current source power supply board manufacturers should choose driver ICs with high constant current accuracy, and the constant current accuracy should be at least less than ±3%. If it is ±1%, it is more ideal, but its price will be higher than ±3% products.

Inductor selection

Taking the design case of 3×1W high-brightness white LED light source as an example, three 1W LED light sources are connected in series, and the operating current can be designed to be 300~350mA. The inductance of L should be 68uH~100uH, with a Q value greater than 50 and a magnetic circuit closed inductor with a saturation current greater than 800mA.

The optimal operating frequency of PT4115 is below 1MHz. The larger the inductance, the smaller the operating frequency. The inductance of this solution is designed to be above 68uH, so that the system operating frequency can be controlled below 1MHz. When the inductance is small, the operating frequency tends to be higher. Due to the response speed limitation of the internal current detection circuit of PT4115, the normal detection of the internal current is affected, and the on/off control of the internal switch cannot be better realized. In addition, due to the high frequency, the switching loss will be larger, and the chip will run at a higher junction temperature, the electrical stress will increase, which is not conducive to stable operation. Too small an inductance will also cause the SW end of PT4115 to burn out, and there will be no output.

The smaller the DCR of the inductor, the higher the efficiency. It is recommended to use EPC13 MnZn 4000 core.

If the saturation current of the inductor is too small, the current of the D5 Schottky diode will be too small, which will lead to insufficient freewheeling of the entire circuit and the LED light source will produce visible flashes. By appropriately increasing the saturation current of the inductor and the current of the Schottky diode, the freewheeling current of the entire circuit can be increased and the resulting flashes can be eliminated.

Adapter selection

The adapter provides a stable AC power supply for the LED desk lamp in this scheme -> AC-DC step-down conversion. Its real-time load output capacity will affect the performance of the LED desk lamp in this scheme. The adapter used for the 3X1W white light LED desk lamp in this scheme should have an output current greater than 1A when loaded, and the voltage should be stable at DC12V. Some adapters with poor load capacity will drop to 7V or even 6.5V in real time when connected to the LED light source load of this scheme. For the LED driver IC with a working voltage starting from 8V, it will enter the undervoltage protection state and stop working. Once the driver IC stops working, the voltage rises back to 12V, and the LED driver IC enters the working state again. This cycle repeats, making the LED desk lamp appear visible to the human eye. At this time, only by replacing the adapter with a good load capacity can the LED desk lamp work normally. At the same time, the LED driver IC with an operating voltage range starting from 6V should be selected, which can also reduce the selection requirements of the adapter to reduce the production supporting costs.

EMI conduction and radiation

The overall design of this LED desk lamp solution should consider the conduction and radiation of EMI. The key to the conduction and radiation of EMI is the power converter, so it is necessary to select an adapter that can pass EMI and even CE and UL, so that the produced LED desk lamp can be exported to the European, American and Japanese markets. The constant current source power board uses a DC/DC switching device as the driver IC, and the switching frequency will generate radiation when working. Therefore, it is built into the metal shell of the desk lamp base to effectively reduce radiation. The magnetic circuit shielding in the metal base should be considered when designing the mechanical structure.