How to Design A LED Power Driver

In today’s life, LED is applied for widely usages in order to save energy. And as we know that it needs a power driver which will directly affect the life of the LED. Therefore, how to make well a LED power driver becomes the most important for LED power supply designers.

1. The power driver will directly affect the life of LED
LED power driver includes digital driving and analog driving two types, Digital driving means digital circuits driving, including digital dimming control, RGB full color changing and so on. Analog driving means analog circuit driving, including AC constant current switching power supply, DC constant current control circuit. Driver circuit includes a semiconductor element, resistors, capacitors, inductors etc. These components have service life, a failure of any of them will cause the failure of the entire or a part of the circuit function. LED’s life is around 50,000-100,000 hours. if say as 50,000 hours and continuously light, it has nearly six years of life. The life of switching power supply is hard to achieve six years. Generally it is 2–3 years in the market. Life of power supply up to 6 years is the highest level, and the price is 4-6 times higher than the normal one, which would be difficult for general lighting factory to accept. So LED lamps mostly fail at power driver circuit.

2. Heat dissipation
LED is cold light source, and its operating junction temperature cannot exceed the limit, so we have to leave a margin for design. To design a lamp, we need to consider its appearance, easy installation, luminous distribution, heat and etc., to strike a balance among all these factors, and to make the best luminaire. The development time of LED lamps is not long enough to learn the experience. Many designs are constantly improved along with the time. Some LED manufacturers would purchase power supply from other company, so lighting designers do not know much of the power supply. They left a larger space for LED heat dissipation, but a smaller for the power supply. Generally, they will find a power source to match the lamp after designed, but it also makes it difficult to find a suitable one.

For example, it happens often because there is small space inside the lamp or the internal temperature is pretty high, but the manufacture controls the cost to be too low for power supply matching. But some LED manufactures have the ability to research and develop the power supply. They will evaluate first at the beginning of lamps design, and to design the power supply simultaneously, so as to solve the above problems. During the design, we should consider heat dissipation of both LED lamp and power supply, to control the temperature rise of the lamp overall, so as to design a better lamp.

3. Power Supply Design
a. Power design. Although LED’s luminous efficiency is high, but there is still 80-85% heat loss, resulting in 20-30 ($ 10.6626) degrees temperature rise inside the lamp. If the room temperature is 25 degrees, then there are 45-55 degrees inside the lamp. If the power supply needs to work for a long time in the high-temperature environment, it has to increase the power margin to 1.5-2 times to ensure the life of LED.

b. Component selection.
When lamps internal temperature is at 45-55 degrees, there are about 20 degrees of the temperature rise inside the power supply, so the temperature near the components should reach at 65-75 degrees. The parameters of some components will be inaccurate in high temperature, and some even its life will be shortened. So the components are selected should be able to work a long time in a high temperature, especially electrolytic capacitors and wires.

c. Performance design.
The LED parameter for design switching power supply mainly is constant current. The current value decides LED brightness. If the current error is large, then the brightness of the entire batch of light is uniform. And the change in temperature can also cause error of the output current. Generally, the batch error should control to be within +/- 5%, in order to ensure uniform brightness of light. The LED forward voltage may have some bias. The constant current voltage range design of power supply should include LED voltage range. When a plurality of LED used in tandem, we can get the lower limit voltage by minimum pressure drop multiplied by the number of tandem, and get the upper limit voltage by maximum voltage drop multiplied by the number of tandem. And the constant current voltage range of power supply can be slightly wider than this range- generally left 1-2V margin for the upper and lower limits.

d. PCB design.
The space that LED lighting left to power supply is small (except external power supply), so the requirement of PCB design is higher, and factors to be considered is more. The safety distance has to be enough. The power supply which requires the input and output isolated, its withstanding voltage requires 1500-2000VAC for primary circuit and secondary circuit, and it should leave at least 3MM distance on the PCB. If the lamp’s shell is metal, it should also consider the safety distance of high-pressure part and the housing for the entire PCB. If there is no enough space to ensure a safe distance, we should use other measures to ensure the insulation, such as punch holes in the PCB, add insulation paper, potting insulation glue etc. Furthermore, we must also consider the heat balance of PCB. The heating components should be placed evenly instead of intensively, to avoid temperature rise partly. Electrolytic capacitors should be away from heat, to slow aging and prolong life.

e. Test standard.
The standard for testing LED power driver is IEC 62384:2006 (DC or AC supplied electronic control gear for LED modules—Performance requirements). Measuring AC & DC input and output characteristics, DC output start characteristics, harmonic test and so on. It has indicated that during the test, the voltage and frequency of power supply should be stable with accuracy ±0.5%, and the total harmonic should not be more than 3%. Harmonic content is defined as the sum of the effective value of the respective components. The fundamental wave is 100%.

4. Parameters
The factors should be considered is constant current and constant voltage range when selecting a power supply. The constant current value shall be lower than the LED standard current. And the voltage range should be moderate, try not to choose a large range, to avoid waste of power. Therefore, Lisun developed WT2080 LED Power Driver Tester, to use together with LSP-500VAR Precision AC variable current and frequency & constant voltage power supply and M9811 DC Electrical Load, to provide a better solution for LED power driver testing.