Discussion on Thermal Stress Protection of High Brightness LED
Author: Yang Rong spin
In fact, less than 20% of the input LED power is converted to light energy, and the remaining 80% becomes thermal. This is really a problem that lighting system designers need to overcome. If the lighting system is not implemented well, even the most complete thermal design will not be effective. Maintaining a safe LED operating environment and reducing the impact of heat on LED life falls on the LED driver IC.
Validation specification
Just look at the component specifications provided by the high-brightness LED manufacturer, it is not difficult to determine the main design parameters to be aware of, and the negative effects of operating these components at high temperatures. The actual lifetime of the LED is inversely proportional to the power consumption and the temperature of the LED. The manufacturer can display a mean time between failure (MTBF) of approximately 100 million hours of operation at a temperature of 80 °C. In a practical system, the failure of the LED does not necessarily cause a big problem, but in a system where the heat dissipation is insufficient and the Tj rises to 120 ° C or above, the life of the LED will be greatly shortened. In extreme cases, LEDs can fail in real time. The thermal design introduces overcompensation to counter the toughest implementation environments. But in some cases, this is impossible. Taking the downlight as an example, it is generally installed in an insulated ceiling mezzanine space. This space not only hinders heat dissipation, but also does not have enough space to install additional cooling.
The relative brightness is also inversely proportional to the junction temperature. As data data mutates, manufacturers estimate that light output at the maximum junction temperature is reduced by 30%. Similarly, the lumen maintenance effect is inversely proportional to the junction temperature. At a junction temperature of 70 ° C, after an LED has been operated for more than 50,000 hours, it typically loses 30% of the photometric output; at higher temperatures, the loss is greater.
In fact, no matter what the reason, the reduction of light output over time does not necessarily pose a big problem. Users may not even notice a reduction, because the performance of LEDs can be comparable to other lighting devices.
Node temperature control
Based on the above factors, the most important goal of a well-considered designer is the heat dissipation of the LED, keeping the junction temperature below the maximum rating to avoid premature failure. The electronic components used to generate the required LED current can be introduced to detect excessive temperature, effectively reducing the LED drive current and maintaining a stable operating temperature. Although the light output will be slightly weakened, the "life" of LEDs is very strong and can operate for a long time.
Taking the circuit of (Fig. 1) as an example, the buck converter is equipped with a temperature control function. The circuit is designed to drive LEDs with drive currents up to 1 amp and supply voltages ranging from 4 to 6 volts.
(Figure 1) Introducing temperature control to reduce the configuration of the converter
Buck converter operation
After the Q1 switch is activated, current flows through the LEDs and L1 and rises to a specific level, allowing the voltage across Rsense to reach the threshold of U1. The ZXSC300 controller will then remove the drive to Q1 and then open the circuit. The energy stored in L1 produces a discharge that flows through D1 and the LED. The ZXSC300 has a fixed open circuit period of 1.7?s, then Q1 will restart and the entire cycle will be repeated. The exchange frequency for this application is approximately 150 kHz.
Add heat control
The circuit uses a 150k? NTC thermal control tube for temperature sensing. The component is placed in contact with the LED to maintain high voltage thermal energy. The current flowing through the superheat control tube is multiplied and then added to the peak exchange current to adjust the LED current.
As the temperature increases, the resistance of the thermal tube decreases, allowing more current to flow to increase the Isense voltage, causing the controller to turn off at lower LED currents. The Rgain and Rsense values ​​of the thermal control tube are set to maintain the operating temperature of the LED at a safe operating limit. As shown in the control image, the variation in supply voltage will only have a small effect on temperature control.
The components used in the circuit of Figure 1 are calculated using the following simple equations.
(Formula 1)
This circuit uses a Yuden 150k? thermal control tube as the temperature sensor. The target control temperature is 75 ° C and the output current is 833 mA. Rgain is 10?, Rsense is 20m?, and Vsense is 20mV. (Table 1) shows the temperature characteristics of the thermal control tube and the effect of the 6V power supply on the peak current. The results are shown in (Figure 2), mainly for different voltage values ​​in the range of 4 to 6 volts.
This example illustrates the components required to drive a 833mA LED current. The circuit can be easily adapted to drive lower currents by changing the Rsense value. Simply change the Rgain value to select different temperature breakpoints.
Table 1 Temperature Characteristics of Thermal Control Tubes and Their Influence on Peak Current
(Figure 2) Effect of thermal control characteristics on peak current in the voltage range of 4 to 6 volts
to sum up
Protect your valuable high-brightness LEDs with the addition of simple electronic components. This technique can be applied to many different control systems and is suitable for buck and boost modes of operation using any of the ZXSC family of LED driver ICs. This thermal protection design helps designers of lighting systems achieve smaller, lower cost solutions. In some cases, it is even possible to use an area that cannot be used in an environment without thermal protection.
All In One Business Computer is the one of the important All In One PC series, can meet all enter and normal business application scenarios. Therefore, more and more clients choose and recommend this Business All In One Computer for business or education projects. The Best All In One Computer For Business is the similar Apple design, competitive and attractive, the hottest parameters is 19 or 21.5 inch intel i3 or i5 2th or 3th or 4th 4 gb ram 128GB ssd . There are many other all In One Computers For Business with lighter weight. Of course, you can see Colorful All In One Gaming PC, All In One Desktop Touch Screen and All In One Business Pc. Believe you can always find the right one at this store. To save time, You can also contact us by email or calling directly to get matched and valuable information fast.
Any other special configuration interest, also feel free to let us know, will try our best to back you up.
Except all in one Custom All In One PC, 14 inch Education Laptop and 15.6 inch business laptop also available here.
All In One Business Computer,Business All In One Computer,Best All In One Computer For Business,all In One Computers For Business,All In One Business Pc
Henan Shuyi Electronics Co., Ltd. , https://www.shuyitablet.com