Discussion on thermal stress protection of high brightness LED

Less than 20% of the power input to an LED is converted into light energy, while the remaining 80% is converted into heat. This is a real problem that lighting system designers need to overcome. Even the most perfect thermal design will not be effective if the lighting system is not implemented well. Maintaining a safe LED operating environment and reducing the impact of heat on LED life falls on the LED driver IC .

Reviewing Specifications By simply looking at the component product specifications provided by the manufacturer of high-brightness LEDs , it is not difficult to determine the main design parameters that need to be paid attention to and the negative impact of operating these components at high temperatures. The actual life of the LED is inversely proportional to the power consumption and the temperature of the LED. Manufacturers can show a mean time between failures (MTBF) of about 100 million hours at a Tj of 80℃. In practical systems, LED failure will not necessarily cause major problems, but in systems with insufficient cooling and Tj rising to 120℃ or above, the LED life will be greatly shortened. In extreme cases, the LED will fail immediately. Thermal design can introduce overcompensation functions to counter the worst implementation environment. However, in some cases, this is not possible. Downlights, for example, are typically installed in an insulated ceiling mezzanine space. This space not only impedes heat dissipation, but also leaves little room for additional heat dissipation. Relative brightness is also inversely proportional to junction temperature. As data vary, manufacturers estimate that light output will decrease by 30% at maximum junction temperature. Similarly, lumen maintenance is inversely proportional to junction temperature. At a junction temperature of 70°C, an LED will typically lose 30% of its light output after operating for more than 50,000 hours; the loss is greater at higher temperatures. In practice, regardless of the cause, a decrease in light output over time is not necessarily a major problem. Users may not even notice the decrease, as the performance of the LED is comparable to that of other lighting devices. Control of junction temperature Given the above factors, the most important goal for a thoughtful designer is to dissipate heat from the LED to keep the junction temperature below the maximum rating to avoid premature failure. The electronics used to generate the required LED current can incorporate a method to detect overtemperature, effectively reducing the LED drive current to maintain a stable operating temperature. Although the light output is slightly reduced, the LED is still very "liveable" and can operate for a long time. The buck converter is equipped with temperature control. The circuit is designed to drive LEDs with currents up to 1 amp and supply voltages between 4 and 6 volts. Buck Converter Operation When the Q1 switch is turned on, current flows through the LED and L1 and rises to a level that causes the voltage across Rsense to reach the threshold of U1. The ZXSC300 controller then removes the drive to Q1 and turns it off. The energy stored in L1 discharges through D1 and the LED. The ZXSC300 has a fixed off period of 1.7?s, after which Q1 is turned on again and the cycle repeats. The switching frequency for this application is about 150kHz. Adding Thermal Control The circuit uses a 150k? NTC thermistor for temperature sensing, which is positioned to maintain high-voltage thermal contact with the LED. The current through the thermistor is multiplied and added to the peak switching current to regulate the LED current. As temperature increases, the resistance of the thermistor decreases, allowing more current to flow, increasing the Isense voltage and causing the controller to shut down at a lower LED current. The Rgain and Rsense values ​​of the thermistor are set to keep the LED operating temperature within safe operating limits. As shown in the control image, variations in the supply voltage have only a small effect on temperature control. This circuit uses a Yuden 150k? thermistor as the temperature sensor. The target control temperature is 75°C and the output current is 833mA. Rgain is 10?, Rsense is 20m?, and Vsense is 20mV. (Table 1) shows the temperature characteristics of the thermistor and the effect of a 6V supply on the peak current. The results are shown in (Figure 2) for different voltage values ​​in the range of 4 to 6V. This example shows the components required to drive an 833mA LED current. The circuit can be easily adapted to drive lower currents by changing the Rsense value. Different temperature breakpoints can be selected by simply changing the Rgain value. Summary With the addition of simple electronic components, valuable high brightness LEDs can be protected. This technique can be applied to many different control systems and is applicable to both buck and boost operation modes using any ZXSC series LED driver IC. This thermal protection design helps lighting system designers achieve smaller and lower production cost solutions. In some cases, even areas that cannot be used without thermal protection can be used.