7 diagrams to understand the problem of lighting and adjustable thermal protection technology

With the rapid development of LED lighting market, LED lighting applications tend to become more and more thin and short, and the power demand is more powerful, in which the increasingly stringent requirements on the effectiveness of heat dissipation. In addition, the failure rate of LED power supply module with lighting materials Co. and improve heat radiating efficiency, therefore, in the LED lighting control IC thermal protection sensing technology, has recently become the focus in the design of LED lamps.

LED lamp cooling problem

The traditional LED lamp with radiating aluminum material as the shell, has good cooling effect, but the complex processing, high cost and heavy material etc.. The principle of heat conduction of the heat conduction in the material is needed to heat the LED lighting fixture.

Among them, mainly because of good heat conductivity of metal metal with free electrons which can transfer heat; and the plastic because there is no free electrons, the molecular vibration is difficult, the heat conduction is the main material itself and the results of lattice vibration, heat load are mainly phonon.

In contrast, plastic shell other advantages in the aspects of design, performance and cost, such as light weight (20~40%, lighter than aluminum) with easy forming, low cost, and the most important characteristic is because the plastic is not conductive, good electrical insulating ability, so the non isolated type LED lighting driver using plastic shell is represent the general trend.

However, the general plastic its thermal conductivity is quite poor, resin type material also has a direct impact on thermal conductivity, the general thermal conductivity of the K value of the general 0.2W/ (m*K), about 1/1000 of aluminum metal. Can not effectively heat and affect the reliability of LED lamps.

LED lamp reliability

The concept of reliability, first started in the two World War, Germany in the development of V-1 rockets, the failure rate for each component directly affects the performance of the entire system. In the LED lamp power module in the same concept, in which the temperature of the environment due to the change will lead to reduce the reliability of LED lamps, LED and drive circuit used in electrolytic capacitors in the reliability evaluation of the greatest impact.

In Figure 1 the high brightness white LED Tj temperature and the service life of the diagram, from the figure we can see the Tj temperature is lower then the service life is long, in addition, figure 2 electrolytic capacitor temperature and service life of the diagram, the service life of the electrolytic capacitor power supply module, also is inversely proportional to the ambient temperature.

Electrolytic capacitor damage and even lead to the lamp can not be used, so if the lamp can effectively adjust the temperature of the environment with the perceived temperature can also extend the life of LED lamps. Therefore, how to adjust the temperature is an important issue in LED lighting applications.

Temperature sensing

The actual application of sensing the temperature of thermistor temperature sensor is the most common and easy to obtain, which joined the LED lamp power module thermistor as sensing temperature has effective and low cost solution.

Thermistors are made of high temperature sintered metal oxides. The formed component has the characteristic of changing the resistance value with the temperature.

One of the most common is a negative temperature coefficient thermistor (negative temp coefficient), referred to as NTC Thermistor (thermistor). The resistance will decrease with the increase of temperature.

Three advantages of using thermistors for temperature sensing:

1 it has a very high sensitivity. If the use of high resistance thermistor, the sensitivity can be as high as 10K / C.

2 its relative high resistance. Its resistance at 25 C, can span hundreds to several M selectivity. It has a high resistance, reducing the error caused by the wire impedance.

3 thermistors provide a variety of packaging components, in which the SMD type of minimal outline packaging, and even size can be reduced to 0201 packaging, as shown in figure 3. In the circuit design can easily be Layout to need to sense the heat source components around.

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

1 thermistors for passive resistors, using should be driven by current flows, so that current flows through the heat emanating from the power loss, the use of thermistor series resistor with high resistance to current limiting, to avoid excessive thermistor self heating, reaction temperature and power consumption of autologous, non sensing analyte temperature.

The resistance and sensitivity of the 2 thermistor has high temperature, but its characteristic is nonlinear, which figure 4 is the relationship between xingqin electronic TSMSeries thermistor for temperature.

As shown in the temperature range for sensing the resistance value of the temperature changes, parallel multiple thermistors or resistors to modify the temperature variation of the slope of the curve, the development process, the development of elongated aging, increase the cost of.

Adjustable overheat protection function

Provide protection for LED driver IC, required can solve the problem of self heating thermistor thermistor application collocation and other restrictions, in addition, thermal protection function can be adjusted is most important, which includes adjustable opening and closing the output current protection function adjustment starting point (TFB) and TCut-off (output current cut-off point) for circuit design, figure 5 is the schematic design of overheat protection adjustment. The circuit development stage can be adjusted automatically overheating protection function.

In addition, due to the increase in ambient temperature, the slope of the LED current is adjusted to adjust the lamp