What is the life of a lamp?

Every time you deal with the seller of the lighting company, you will repeatedly mention a vocabulary: warranty. Each manufacturer claims that the warranty time is different. There are two years of warranty, three years of warranty and five years of warranty.

However, in many cases, the seller does not understand, how does this warranty time come from, or is it just that the LED should be warranted for such a long time?

Today, Xiaobian takes you into the world of LEDs to find out how the life of the lamps is defined and judged.

First of all, when it comes to LEDs, we can tell at a glance that it is different from traditional light sources because almost all LEDs have a remarkable feature - the heat sink.

A wide range of heat sinks are not intended for the aesthetics of LED luminaires, but for LEDs to work better.

Then you must be wondering why the previous light source used less radiators, and almost all the lamps in the LED era used radiators?

That is because the previous light sources are all illuminated by thermal radiation. For example, tungsten lamps are based on heat, so they are not afraid of heat. The basic structure of the LED is a semiconductor P-N junction. The temperature is slightly higher and the performance is degraded. Therefore, heat dissipation is very important for the LED.

First, let's take a look at the composition and schematic of the LED.

Tip: The LED chip will generate heat when it works. We refer to the temperature of its internal PN junction as the junction temperature (Tj).

In our daily life, the luminous flux of LEDs will decrease with the increase of junction temperature. As shown in the figure, when the junction temperature is 25 °C, the luminous flux is only 80% when the junction temperature reaches 100 °C. Left and right.

And, most importantly, the lifetime of LED luminaires is closely related to junction temperature.

One concept that needs to be understood is that when we routinely talk about the life of an LED, it is not that it burns out at the end of life. Generally, when the LED light output reaches 70%, we think it is the end of life.

As can be seen from the above figure, if the junction temperature is controlled at 105 °C, the luminous flux will be attenuated to 70% for about 10,000 hours, and if the junction temperature is controlled at about 60 °C, then it will reach 100000+ hours. Will decay to 70%. Life expectancy has increased by a factor of 10.

In daily life, the most common thing we encounter is that the LED has a life of 50,000 hours, which is actually a data when the junction temperature is controlled at 85 °C.

Since junction temperature plays such an important role in the life of LED lamps, how can we reduce the junction temperature? Don't worry, let's first look at how the heat is dissipated. Knowing how to dissipate heat, you naturally know how to lower the junction temperature.

Way of cooling the lamp

The three basic modes of heat transfer are: conduction, convection, and radiation.

The main transmission path of the radiator is conduction and convection heat dissipation, and the natural convection radiation dissipates heat.

Basic principle of transferring heat

Conduction

The way heat travels from a higher part to a lower temperature part.

What are the factors that affect heat transfer?

1. Thermal conductivity of heat dissipating material

2. Thermal resistance caused by heat dissipation structure

3. Shape and size of heat conductive materials

radiation

High-temperature objects directly emit heat outward

What are the factors that affect heat radiation?

1. Thermal resistance of the surrounding environment and medium (mainly considering air)

2. The characteristics of the material of the heat radiation itself (the general darkness of the external radiation is a little greater, in fact, the radiation transmission is not particularly important, because the temperature of the lamp is not too high, the radiation is not very strong)

convection

The way heat is transferred by the flow of a gas or liquid

Factors affecting heat convection

1. Gas flow and its velocity

2. Specific heat capacity and flow velocity and volume of liquid

â–² As shown in the figure, the heat sink with the same shape is better than the one on the left.

In LED luminaires, the heat sink occupies a large part of the cost of the luminaire. In order to save costs, some manufacturers shoddy the structure, materials and design of the radiator, resulting in serious after-sales problems of the lamps.

Having said that, it’s just a stepping stone, and now it’s the focus.

How to judge whether the heat dissipation of the lamp is good or not?

The most authoritative course is to test the junction temperature using professional equipment.

However, such professional equipment may be prohibitive for the average person, then all we have left is to use the most traditional method to touch the light to sense the temperature.

So, is the light fixture hot or not hot?

Hand touch radiator is very hot, certainly not good

The hand touch radiator is very hot, the heat dissipation system must be bad, or the heat dissipation capacity of the heat sink is insufficient, the heat of the chip cannot be dissipated in time; or the effective heat dissipation area is insufficient, and there is a lack of structural design.

The hand touches the lamp body is not hot, not necessarily good

When the LED luminaire is working normally, a good heat sink must have a lower temperature, but a lower temperature heat sink may not be good.

The chip has not much heat, good conduction, sufficient heat dissipation, and low temperature. This is a good cooling system, the only "disadvantage" is a bit of wasted material.

If there is impurities under the substrate, there is no good contact with the heat sink, the heat cannot be transmitted, and it is concentrated on the chip. The outside is not hot, in fact, the chip inside has long been hot.

Here, Xiaobian recommends a very easy to use method - "half-hour illumination method" to determine whether the heat dissipation is good.

Note: "Half-hour illumination method" comes from the article: How to judge the heat dissipation of LED lamps? ← Click on the title to read Author: Dingde Liang

Half hour illumination method

As we said before, the general LED junction temperature rises and the luminous flux decreases. Then, as long as we measure the illuminance change of the luminaire in the same position, we can reverse the change of junction temperature.

â–² "Half-hour illumination method" indirectly measures junction temperature changes

First, choose a place that is not subject to external light and illuminate the fixture.

After lighting up, take a light meter and measure it, for example 1000 lx.

Keep the position of the luminaire and illuminance meter unchanged. After half an hour, take the illuminance meter and measure it. 500 lx, indicating that the luminous flux has dropped by 50%. It is already hot enough. The hand touch is good, indicating that the heat is not coming out. difference.

If the measurement is 900 lx, the illumination is only reduced by 10%, which means that it is a normal data, and the heat dissipation is very good.

The scope of application of the "half-hour illumination method"

We cite the "Luminous Flux vs. Junction Temperature" curve of several commonly used chips. From this curve, we can see how much lumens of luminous flux drop, and we can indirectly know how many degrees Celsius the junction temperature has risen.

Column one:

▲ OSRAM S5 (30 30) chip, the luminous flux is 20% lower than 25 ° C, the junction temperature has exceeded 120 ° C.

Column two:

▲ OSRAM S8 (50 50) chip, the luminous flux is 20% lower than 25 ° C, the junction temperature has exceeded 120 ° C.

Column three:

▲ OSRAM E5 (56 30) chip, the luminous flux is 20% lower than 25 ° C, the junction temperature has exceeded 140 ° C.

Column four:

▲OSLOM SSL 90 white chip, the luminous flux is 15% lower than 25 °C, the junction temperature has exceeded 120 °C.

Column 5:

▲Luminus Sensus Serise chip, the luminous flux is 15% lower than 25 °C, and the junction temperature has exceeded 105 °C.

As can be seen from the above figures, if the thermal illuminance after half an hour is 20% lower than the cold state, basically the junction temperature has exceeded the tolerance range of the chip. Basically, it can be judged that the heat dissipation system is unqualified.

Of course, this is the most common situation. Everything has special cases, as shown in the figure:

Of course, for most LEDs , we can use the half-hour illumination method and 20% drop to judge whether it is good or not.

Have you learned it? When you choose a lamp, you must pay attention to it. You can't just look at the appearance of the lamp. You should use your eye to pick the lamp.