Analyzing the problem of driving power supply for LED lighting

In order to save energy and electricity, LED has been widely promoted, but LED needs a power supply, the quality of which will directly affect the life of the LED, so how to make a good LED driver power supply is the top priority of LED power supply designers. This article introduces some problems with LED driver power supplies, hoping to provide some help to engineers.

1. The driving circuit directly affects the life of LED

The LED drivers we are talking about include digital drivers and analog drivers. Digital drivers refer to digital circuit drivers, including digital dimming control, RGB full-color changes, etc. Analog drivers refer to analog circuit drivers, including AC constant current switching power supplies and DC constant current control circuits. The driving circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life. The failure of any component will cause the failure of the entire circuit or partial function failure. The service life of LED is 50,000-100,000 hours. If it is continuously lit at 50,000 hours, it has a service 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 supplies sold on the market is generally 2-3 years. The power supply with a 6-year warranty is military-grade, and the price is 4-6 times that of ordinary power supplies. It is difficult for ordinary lighting manufacturers to accept it. Therefore, the failure of LED lamps is mostly a failure of the driving circuit.

2. Heat dissipation problem

LED is a cold light source, and the working junction temperature cannot exceed the limit. A certain margin should be left when designing. The design of the entire lamp should consider many aspects such as beautiful appearance, convenient installation, light distribution, heat dissipation, etc., and a balance should be sought among many factors, so that the overall lamp is the best. The development time of LED lamps is not long, and there is not much experience to learn from. Many designs are constantly being improved. Some LED lamp manufacturers use power supplies that are outsourced or purchased. The lamp designers do not know much about the power supply, so they give the LED a larger heat dissipation space and the power supply a smaller heat dissipation space. Generally, after the lamp is designed, a suitable power supply is found to match it, which brings certain difficulties to the matching of the power supply. It is often encountered that the internal space of the lamp is small or the internal temperature is high, and the cost control is low, so it is impossible to match the suitable power supply. Some LED lamp factories have the ability to develop power supplies. They can solve the above problems by evaluating the initial design of the lamp and designing the power supply simultaneously. 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 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 LED has high light efficiency, there is still 80-85% heat loss, which causes a 20-30 degree temperature rise inside the lamp. If the room temperature is 25 degrees, the temperature inside the lamp will be 45-55 degrees. If the power supply works in a high temperature environment for a long time, the power margin must be increased to ensure the life span, generally leaving a margin of 1.5-2 times.

b. Component selection. When the internal temperature of the lamp is 45-55 degrees, the internal temperature of the power supply will rise by about 20 degrees, and the temperature near the component must reach 65-75 degrees. Some components will drift in parameters under high temperature conditions, and some will have a shortened lifespan, so components should be selected that can be used for a long time at higher temperatures, and special attention should be paid to electrolytic capacitors and wires.

C. Electrical performance design. The parameters of the switching power supply are designed for LEDs, 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 will be uneven. Moreover, temperature changes can also cause the output current of the power supply to shift. Generally, the batch error is controlled within +/-5% to ensure consistent brightness of the lamp. The forward voltage drop of the LED has deviations, and the constant current voltage range of the power supply design must include the voltage range of the LED. When multiple LEDs are used in series, the minimum voltage drop multiplied by the number of series connections is the lower limit voltage, and the maximum voltage drop multiplied by the number of series connections is the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range, and generally 1-2V margin is left for the upper and lower limits.

d. PCB layout design. The size of the power supply for LED lamps is relatively small (unless the power supply is external), so the requirements for PCB design are relatively high and there are many factors to consider. The safety distance must be sufficient. The power supply that requires input and output isolation, the primary circuit and the secondary circuit are required to withstand a voltage of 1500-2000VAC, and at least 3MM distance must be left on the PCB. If it is a lamp with a metal shell, the layout of the entire power supply must also consider the safety distance between the high-voltage part and the shell. If there is no space to ensure the safety distance, other measures must be used to ensure insulation, such as punching holes in the PCB, adding insulating paper, and potting insulating glue. In addition, the layout must also consider heat balance. The heating components must be evenly distributed and cannot be placed in a concentrated manner to avoid local temperature increases. Electrolytic capacitors are kept away from heat sources to slow down aging and extend their 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 lamps sold in China refers to the standards of lighting lamps, and those exported are CE or UL certified. Some are made in accordance with foreign LED lamp standards. Therefore, in this case, it is difficult to design a switching power supply to meet all the above standards at the same time. We can only meet different certifications according to different requirements.

4. Problems in power supply application

5. Use parameters.

When choosing an external power supply, you should mainly consider the constant current and the voltage range of the constant current. The constant current value should be lower than the standard current of the LED. The voltage range should be moderate, and try not to choose a larger range to avoid power waste.