Practical desk lamp design. LED

Practical desk lamp design. LED

Discovery
LED light source as the fourth generation of new energy-saving light source, from birth when they were used to do all kinds of lamps and lanterns of glowing light source. As the light source the light only five percent efficiency incandescent lamp, and LED light source close to ninety percent the luminous efficiency almost. LED lighting with its high energy saving, long life, and the characteristics of environmental protection became the focus of attention of the widely 27. Desk lamp is every household in the use of common lamps and lanterns, these a few years high brightness LED light source for its manufacturing technology advance by leaps and bounds, and its production costs and rising, now down to use desk lamp LED light source as high brightness, high efficiency and save electricity and no carbon emissions lighting light source.

The LED lamp working principle
Follow safety first civil electrical design concept, LED light source is a low voltage dc constant current source light-emitting device, can't use 100 ~ 220 V ac high-voltage direct light, therefore, LED the design ideas is the desk lamp, want above all high pressure of alternating current to transform the low voltage dc constant current source to light, LED light source. Use the most cost-effective method to buck and ac/dc transform is the first consideration, the design of the portable electronic product use the ac power dc voltage reducing transformer (Adapter-Adapter) became both economical and practical, and ready, and the first choice of use. The output voltage of the adapter requires steady in DC12V, output current according to the power of the light source LED to choose, general to give 30% of cushion to 3 x the 1 W white LED light source, for example, the 1 W white LED the standard working current should be 350 mA, and three LED light source the circuits of the current series need is 350 mA, consider to prolong life and reduce LED light failure, can design for 300 mA ~ 330 mA, not obvious effect on the LED the brightness, so the adapter output current elected 750 mA ~ 1 A.
A concise and practical LED lamp scheme as shown in figure 1 below. AC220V adapter on outside the lamps and lanterns through security step-down transformation, to provide stability LED lamp 12 V dc power, in the desk lamp base for shell and constant current source power board, will transform the dc voltage stability constant dc current source LED light source, in order to meet the technical requirements. In dc constant current source can add a before the power switch, so that in the desk lamp need not when can shut off the dc power supply, but can't shut off the 220 V ac power, so need not when should take down the wall adapter power plug, this also is the practical scheme of the only "weakness". If you don't want to use of mechanical switch, and want a more creative, can choose the selling point of the electronic touch switch, like the finger can be realized by clicking the desk lamp of open-> all bright, half dark, shut; and Because of the rapid development of electronic technology, electronic touch switch is now a low-cost devices.

LED light source working principle
LED light source of the main parameters work is VF, IF, other relevant is color/color temperature/wavelength/brightness/light Angle/efficiency/power consumption, etc. The LED is a PN junction diode, only on the forward voltage transmission to the enough current, VF the forward voltage is for LED to establish a normal working condition, IF positive current is to promote LED light brightness and flow of current proportional. White LED VF nominal voltage: 3.4 V + 0.2 V. LED light source in the large volume production, each batch of VF has certain LED discrete and is for the customers to use the consistency of the factory, LED must according to different VF fund sells; Customer order with a batch of lamps and lanterns of need LED light source must choose the same class of class or adjacent VF, otherwise, it will lead to the same production LED lamps and lanterns are different brightness; IF work according to the current application LED to choose, different current class cannot mixed with.

LED light source driver circuit
LED light source driving circuit is the 12 V dc voltage transform into stable constant-current source, the design of the circuit in ShanFanJiuJian, saving cost of principle, should complete the circuit from can design requirements of the many LED drive chip the choice in the high level of integration, good performance, application circuit is simple, the price is the price has the advantage of flat chip. So choose drive circuit peripheral equipments of less drive chip is the production cost of the primary consideration. PT4115 used for 1 ~ 6 W white LED light source drive scheme only four parts (figure 2), Cin is input filter capacitance, Rs set the current flows through the LED the IF, Rs = 0.1 / ILED; L is free-wheeling inductance, free-wheeling D5 is the diodes. For adapter has provided a 12 V dc voltage, for the ac voltage rectifying thumper input with the bridge rectifiers D1 ~ can be omitted the D4. Although parts, but less on parts, taller to the requirement of design should be made according to the requirements of table 1 tip choose, can cause the circuit in good working condition. PT4115 switch frequency shake the frequency technology effectively reduce the EMI, bypassing a resistance to EMI circuit.
Drive chip LED light source of constant current accuracy for LED lamps and lanterns for manufacturers is very important, the current LED light source of IC drive constant current precision batch production will have certain discrete and LED lamps and lanterns manufacturers in batch production debugging, the same power and the same load, the same LED light source of constant current source power supply for the same type of board, different drive IC its constant current precision of the individual character difference can lead to constant current source power plate output current produce a certain tolerance, make the same LED light source of light load, the different brightness will increase constant current source power of mass production board when the commissioning of the time, influence online productivity. Therefore, constant current source power board manufacturers should choose constant current high precision driver IC, constant current precision than the plus or minus 3%, at least if plus or minus 1% more ideal, but the price is higher than 3% of the product.-

The inductance of the choice
3 x the 1 W high brightness LED light source white light design case is concerned, three the 1 W LED light source, the working current series can design for 300 ~ 350 mA, L inductance quantity should choose 68 uH ~ 100 uH, Q value than 50, saturated with current more than 800 mA of the magnetic circuit closed inductor.
The design of the PT4115 best work in 1 MHz frequency the large amount of influence the inductance take the working frequency of the project design, inductance in 68, this system work more than uH frequency can control to 1 MHz below. Inductance is small, the job is high, because the frequency PT4115 internal electric current detection circuit response speed limits, the internal current test properly, can't influence appears to better achieve the internal switch conduction/shutoff control. Plus, because of high frequency will bring greater switch loss, make the chips run in high temperature and stress, electricity, which increase stability. Inductance amount is too small to PT4115 SW end of burnt out, and no output.
The smaller the inductance of the DCR the higher, the efficiency. The proposal chooses EPC13 manganese zinc ferrite core 4000. Inductor saturation current choose small D5, schottky diodes current choose small, will cause the whole circuit, free-wheeling shortage LED light source will produce the human eye visible light. Will inductor saturation current and the schottky diodes current suitable enlargement of the whole circuit to increase the free-wheeling current, eliminate the resulting flash.

Adapter choice
For this scheme adapter LED lamp provide stable ac power-> ac/dc step-down transform, it's in real time with this scheme will affect the output capacity of desk lamp, used for performance LED this plan 3 X1W white LED desk lamp, it with load in adapter when output current should be greater than 1 A, voltage should be stable in DC12V. Some take in a less able adapter, connection LED light source load the scheme, when in fact the output voltage will drop down to seven V, even 6.5 V, for working voltage from 8 V began to LED drive IC, and will enter voltage protection status and stop job once driven to stop work, voltage and IC back up to 12 V, LED drive into the work of the state, IC again so round and round, make eye visible LED lamp appear flash. At this time, only replace with load capacity good adapter to make LED the normal work desk lamp. At the same time should be chosen working voltage range from 6 V initial LED drive IC, also can reduce the requirement of adapter choice, to reduce production cost of form a complete set.

The conduction and radiation EMI
This LED lamp overall design scheme should consider when can the conduction and radiation EMI, the conduction and radiation EMI is the key to power converter, and therefore should choose to EMI, even can have CE, UL adapter to form a complete set, so that production LED desk lamp can be exported to Europe and America, market. Constant current source power from the use of the driver board IC is DC/DC switch device, work switch frequency will produce the radiation, so built-in desk lamp base metal shell and can effectively reduce the radiation, the mechanical structure design should be considered when metal base of the magnetic circuit in the shield

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