Summary of four major problems in LED driver power supply design

- Jan 07, 2019 -

 

The quality of the LED driver power supply will directly affect the life of the LED, so how to do a good LED driver power is the top priority of LED power supply designers.

In the current life, in order to save energy and save electricity, LED has been greatly promoted, but LEDs need to have a power driver, its quality will directly affect the life of LED, so how to do a LED driver is LED power designer The top priority. This article describes some of the issues with LED driver power and hopes to provide some help to engineers.

1, the drive circuit directly affects the LED life

The LED driver we refer to includes digital drive and analog drive. Digital drive refers to digital circuit drive, including digital dimming control, RGB full color change and so on. Analog drive refers to analog circuit drive, including AC constant current switching power supply, DC constant current control circuit. The driving circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life, and failure of any one device may cause failure or partial failure of the entire circuit. The service life of the LED is 50,000-100,000 hours. It is continuously lit for 50,000 hours, and has a life span of nearly 6 years. The life of the switching power supply is difficult to reach 6 years. The warranty period of the switching power supply on the market is generally 2-3 years. The power supply with 6 years warranty is military grade, and the price is 4-6 times that of the ordinary power supply. The lighting factory is very difficult to accept. Therefore, the fault of the LED luminaire is mostly the failure of the drive circuit.

2, heat problem

The LED is a cold light source, the working junction temperature can not exceed the limit, and a certain margin must be left in the design. The design of the whole luminaire should consider the appearance of beautiful, easy to install, light distribution, heat dissipation and many other problems. It is necessary to find a balance among many factors, so that the overall luminaire is the best. The development time of LED luminaires is not long, can learn from There are not many experiences, and many designs are constantly improving. Some LED lighting manufacturers use power supply for outsourcing or outsourcing. The lighting designer does not know much about the power supply, and the cooling space for the LED is large, and the heat dissipation space for the power supply is small. Generally, after designing the lamp, find a suitable power supply to match, which will bring certain difficulties to the power supply. Frequently encountered due to the small internal space of the lamp or the high internal temperature, and low cost control, can not be equipped with a suitable power supply. Some LED luminaires have power supply research and development capabilities. They can be evaluated at the beginning of designing the luminaires, and the power supply design can be synchronized to solve the above problems. In the design, the heat dissipation of the LED and the heat dissipation of the power supply should be comprehensively considered, and the temperature rise of the luminaire should be controlled as a whole, so that a better luminaire can be designed.

3. Problems in power supply design

a, power design. Although the LED has high luminous efficiency, there is still 80-85% of thermal energy loss, resulting in a temperature rise of 20-30 ($10.6626) inside the lamp. If the room temperature is 25 degrees, the lamp has 45-55 degrees inside, and the power supply is long. To work in a high temperature environment, it is necessary to increase the power head to ensure the life, and generally leave a margin of 1.5-2 times.

b, component selection. When the internal temperature of the lamp is 45-55 degrees, the internal temperature rise of the power supply is about 20 degrees, and the temperature near the component should reach 65-75 degrees. Some components will drift in the high temperature condition, and some life will be shortened, so the device should be selected for a long time at a higher temperature, paying special attention to electrolytic capacitors and wires.

c, electrical performance design. The switching power supply is designed for the parameters of the LED, mainly the constant current parameter. The magnitude of the current determines the brightness of the LED. If the batch current error is large, the brightness of the whole batch of lamps is not uniform. Moreover, changes in temperature can also cause the output current of the power supply to shift. Generally, the batch error control is within +/- 5% to ensure the brightness of the lamp is consistent. The forward voltage drop of the LED is deviated, and the constant current voltage range of the power supply design should include the voltage range of the LED. When multiple LEDs are used in series, the minimum voltage drop multiplied by the number of series is the lower limit voltage, and the maximum voltage drop multiplied by the number of series is the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range, generally upper and lower limits. Leave 1-2V margin.

d. PCB layout design. The size of the LED luminaire left to the power supply is small (unless it is external to the power supply), so the PCB design requirements are higher, and there are many factors to consider. The safety distance should be enough to require input and output isolation power supply. The primary side circuit and the secondary side circuit require a withstand voltage of 1500-2000VAC, and at least 3MM distance must be left on the PCB. In the case of a metal-enclosed luminaire, the layout of the entire power supply must also take into account the safe distance between the high-voltage part and the outer casing. If there is no space to ensure the safety distance, other measures should be taken to ensure the insulation, such as punching holes in the PCB, adding insulation paper, potting insulating glue, etc. In addition, the layout of the board should also consider the heat balance, the heating elements should be evenly distributed, and can not be placed in a concentrated manner to avoid local temperature rise. Electrolytic capacitors are kept away from heat sources, slowing down aging and prolonging service life.

e. Certification issues. At present, there is no standard for LED lamps in China. The relevant national departments are researching and formulating. The domestic sales of lamps and lanterns certification is based on the standards of lighting fixtures. The export is done by CE or UL certification, and some are based on foreign LED lamp standards. Therefore, for this situation, it is difficult to design the switching power supply to meet these standards at the same time. We can only meet different certifications for different requirements.

4, the use of parameters

The selection of the purchased power source mainly depends on the voltage range of the constant current and the constant current. The constant current value is chosen to be the standard current of the LED. The selection of the voltage range should be moderate, try not to choose a larger range, and avoid the waste of power.