In order to save energy and power, LED has been greatly promoted, but LEDs need to have a power supply driver, and its quality will directly affect the life of the LED. Therefore, how to make an LED drive power supply is the top priority for LED power designers . This article introduces some LED drive power problems, and hopes to provide a little help to engineers.

 

 

 

1. The drive circuit directly affects the LED life

 

The LED driver we mentioned includes digital driver and analog driver. Digital driver refers to digital circuit driver, including digital dimming control, RGB full-color change, etc. Analog drive refers to analog circuit drive, including AC constant current switching power supply, DC constant current control circuit. The drive circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life, any one device failure will lead to the failure of the entire circuit or part of the function. The service life of the LED is 50,000-100,000 hours, which is calculated according to 50,000 hours, continuously lit, and has a life 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 that reaches the 6-year warranty is military grade, and the price is 4-6 times that of the ordinary power supply. The general Lamp factory is very difficult to accept. Therefore, the failure of LED lamps is mostly the failure of the driving circuit.

 

2. Heat dissipation

 

LED is a cold light source, the operating junction temperature cannot exceed the limit, and a certain margin should be left when designing. The design of the entire luminaire should take into account many aspects such as beautiful appearance, easy installation, light distribution, heat dissipation and so on. We must find a balance among many factors, so that the overall luminaire is the best. The development time of LED luminaires is not long, you can learn Has little experience, and many designs are constantly improved. Some LED lighting manufacturers use external power supply or outsourcing. The lighting designers don't know much about power supply. The heat dissipation space for LED is large and the power supply space is small. Generally, it is necessary to find a suitable power supply to match after designing the lamp, which will bring certain difficulty to the matching of the power supply. It is often encountered that due to the small internal space of the lamp or the high internal temperature, and the low cost control, it is impossible to allocate a suitable power supply. Some LED lamp manufacturers have power supply research and development capabilities, which can be evaluated at the beginning of the design of the lamp and the design of the power supply can be synchronized to solve the above problems. In the design, the heat dissipation of the LED and the power supply must be comprehensively considered, and the temperature rise of the lamp should be controlled as a whole, so that a better lamp can be designed.

 

3. Problems in power supply design

 

A, power design. Although the LED light efficiency is high, there is still 80-85% of heat energy loss, resulting in a temperature rise of 20-30 degrees inside the lamp. If the room temperature is 25 degrees, the inside of the lamp will be 45-55 degrees. Under the work, to ensure the life, you must increase the power margin, 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 still about 20 degrees, and the temperature near the component must reach 65-75 degrees. The parameters of some components will drift at high temperature, and some will shorten the life, so the device should be selected to be used at a higher temperature for a long time, pay special attention to electrolytic capacitors and wires.

 

C, electrical performance design. The switching power supply is designed for LED parameters, mainly constant current parameters. The size of the current determines the brightness of the LED. If the batch current error is large, the brightness of the entire batch of lamps is uneven. And the temperature change can also cause the power supply output current to shift. Generally, the batch error is controlled within +/- 5% to ensure the consistency of the lamp brightness. The forward voltage drop of the LED has deviations, and the constant current voltage range of the power supply design should include the LED voltage range. When multiple LEDs are used in series, the minimum voltage drop times the number of series connections is the lower limit voltage, and the maximum voltage drop is times the number of series connections as the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range. Leave a margin of 1-2V.

 

D, PCB layout design. The size of the LED lamps reserved for the power supply is small (unless the power supply is external), so the PCB design has higher requirements and there are many factors to consider. The safety distance should be sufficient, requiring input and output isolated power supplies, 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. If the lamp is a metal shell, the safety distance between the high-voltage part and the shell should also be considered for the layout of the entire power supply. If there is no space to ensure a safe distance, other measures must be used to ensure insulation, such as drilling holes on the PCB, adding insulating paper, and potting insulating glue. In addition, the layout also needs to consider the heat balance, the heating elements should be evenly distributed, can not be placed centrally, to avoid local temperature rise. The electrolytic capacitor is far away from the heat source, slowing down the aging and prolonging the service life.

 

E, certification issues. At present, there is no standard for LED lamps in China. The relevant national departments are studying and formulating. The certification of domestically sold lamps is based on the standards of lighting lamps, and the certification of CE or UL is exported, and some are based on foreign LED lamps. Therefore, in view of this situation, it is difficult for the design of the switching power supply to meet these standards at the same time. We can only meet different certifications for different requirements.

 

4. Use parameters.

 

The choice of outsourcing power supply mainly depends on the constant current and the voltage range of the constant current. The constant current value is selected as the standard current of the LED is lower. The choice of voltage range should be moderate, try not to choose a larger range, to avoid waste of power.