Interpretation of heat dissipation of lamps and adjustable thermal protection technology

Along with the rapid development of the LED lighting market, LED lighting application related products are becoming lighter and thinner, and the power demand is more powerful. Among them, the heat dissipation performance requirements are increasingly severe. In addition, the failure rate of the LED power module is also improved with the limited heat dissipation performance of the lamp material. Therefore, the thermal protection sensing technology related to the LED lighting control IC has recently become a hot spot for the design and application of various LED lamps.

LED lamp cooling problem

In general, conventional LED lamps use aluminum heat-dissipating materials as lamp housings. Although they have good heat dissipation effects, they are complicated in processing, high in cost, and heavy in materials. In principle, the heat dissipation of LED lighting fixtures requires the heat conduction mechanism of the carrier through the heat conduction inside the material.

Among them, the metal heat conduction is mainly because the metal has free electrons and can transfer heat energy quickly; while the plastic has no free electrons, the molecular vibration is difficult, and the heat conduction is mainly the result of the lattice vibration of the material itself, and the phonon is the main thermal energy loader.

In contrast, the plastic lamp housing has other advantages in design, performance and cost, such as light weight (20-40% lighter than aluminum), easy to form, low cost, and the most important feature is that the plastic is not conductive. With good electrical insulation, it is an irresistible trend to use a plastic case for non-isolated LED lighting drivers.

However, the thermal conductivity of plastics is generally poor, and the resin type materials directly affect the thermal conductivity. Generally, the thermal conductivity k of plastics is generally only 0.2 W/(m*K), which is about 1/1000 of that of aluminum. Can not effectively dissipate heat and affect the reliability of LED lamps.

LED lamp reliability

The concept of reliability first began in the Second World War. When Germany developed the V-1 rocket, the failure rate of each component directly affected the performance of the entire system. The concept is the same in the power module of the LED luminaire. The change of the ambient temperature will lead to the reduction of the reliability of the LED luminaire. The electrolytic capacitor used in the LED and the driving circuit has the greatest influence on the reliability evaluation.

Figure 1 shows the relationship between Tj temperature and life of high-brightness white LEDs. It can be seen from the figure that the lower the Tj temperature, the longer the service life. In addition, Figure 2 shows the relationship between temperature and service life of electrolytic capacitors, and electrolytic capacitors in power modules. The service life is also inversely proportional to the ambient temperature.

The destruction of the electrolytic capacitor can even cause the lamp to be unusable, so if the lamp can effectively adjust the self temperature with the sensing ambient temperature, the service life of the LED lamp can be prolonged. Therefore, how to adjust the temperature is an important issue in LED lighting applications.

Temperature sensing

In practical applications where temperature is sensed, thermistors are the most common and readily available temperature sensors, with the addition of thermistors in LED lamp power modules as an effective and low-cost solution for sensing temperature applications.

The thermistor is made by mixing metal oxides and sintering at a high temperature. The resulting component has the property of changing the resistance value with temperature.

One of the most common types is the negative temp coefficient thermistor (NTC Thermistor). Its resistance value will decrease as the temperature rises.

Thermistors are used for three major advantages of temperature sensing:

1. It has extremely high sensitivity. If a high-resistance thermistor is used, the sensitivity can be as high as 10kΩ/ °C.

2. Its relative high resistance value. Its resistance at 25 ° C can range from hundreds to several MΩ. Its high resistance value reduces the error caused by the impedance of the wire.

3. Thermistors offer a versatile package of components, with a small package in the form of a patch, even down to the 0201 package, as shown in Figure 3. It can be easily laid out in the circuit design for components that need to sense the heat source.

However, in practical applications, the use of thermistors for temperature sensing has its limitations:

1. The thermistor is a passive resistor. It needs to be driven by current flow. Therefore, if there is current flowing, there is thermal energy emitted by the power loss. The thermistor needs to be connected in series with a resistor with a large resistance. In order to avoid excessive self-heating of the thermistor, it reacts to the self-power consumption temperature, instead of sensing the temperature of the object to be tested.

2. Although the thermistor has high resistance and sensitivity, its temperature characteristics are nonlinear. Figure 4 shows the relationship between the thermistor resistance and temperature of Xingqin Electronics TSMSeries.

As shown in the figure, in the temperature range to be sensed, the resistance value changes drastically with temperature. It is necessary to connect a plurality of thermistors or resistors in parallel to modify the temperature slope curve. During the development process, the development time is extended and increased. cost.

Adjustable overheat protection

LED driver ICs with thermal protection, with thermistor application, need to have limitations such as self-heating of the thermistor. In addition, the thermal protection function is most important, including the ability to adjust the thermal protection on and off. The function's output current regulation start point (TFB) and output current cut-off point (TCut-off) are used for circuit design. Figure 5 shows the design of the adjustable overheat protection. The overheat protection function can be arbitrarily adjusted during the circuit development phase.

In addition, due to the rise in ambient temperature, the LED current drop slope is adjusted to achieve the performance of adjusting the lamp temperature. The brightness of the LED will decrease as the output current decreases. The slope of the drop should match the perceived sensitivity of the human eye to low brightness changes in the gamma curve, as shown in Figure 6.

The thermal protection function needs to be adjusted to make the slope of the high brightness use a slow drop, so that the user can realize the temperature adjustment of the LED lamp without knowing it, in order to prolong the service life.

Adjustable thermal protection vs. The traditional thermal protection function has an LED driver IC with adjustable thermal protection and traditional thermal protection. The output current vs. temperature graph is shown in Figure 7. The measurement results show obvious appreciable, and the LED driver IC with adjustable thermal protection function can meet the user's needs.

in conclusion

Adjustable thermal protection LED driver IC can effectively extend the life of the lamp and improve reliability. It is an excellent choice for users in the LED lighting market where the power is getting bigger.

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