High brightness, high power LEDs (HB-LEDs), although quite efficient light sources, still generate a lot of heat. When not controlled, high LED-die temperatures can significantly reduce the luminous flux output and lifetime of the LEDs. To ensure a highly reliable LED lighting system, the LED-die temperature needs to be controlled.
One of the most efficient ways to control the temperature is to use an NTC thermistor within near proximity of the LEDs. This can be achieved by placing an NTC SMD close to the LED on the same substrate.
Different topologies can be used based on specific IC LED drivers. As shown in Figure 1, the NTC can operate in a resistor network where the sense voltage can indirectly control the LED current by influencing the PWM ratio. Another option is shown in Figure 2 where the NTC thermistor is used in the LED current sense branch to influence the feedback signal at higher temperatures.
Figure 3 outlines the LED current and luminous flux at varying ambient temperatures. Up to a medium temperature, HB-LEDs can be driven with their maximum current and have extra luminous flux output. When temperature compensation is not applied or sufficient cooling cannot be guaranteed, the luminous flux output should be stabilized at 100% corresponding with a nominal current level, for example 40% of max current to avoid overheating of the LED-die. In such cases the full brightness capabilities of the LEDs cannot be exploited.
With the use of an NTC compensation circuit, the LEDs can be driven with a current close to the maximum specified without influencing their lifetime. In summary, NTC thermistors are an excellent choice for thermal management when designing HB-LED lighting systems to ensure LED longevity and optimal performance.
- Surface mount ceramic NTC with glass encapsulation for added stability and nickel barrier terminations for superior solderability
- Wide temperature range from -40°C to +150°C
- Several resistance and beta value (curve) combinations with tolerances down to 1%
- Standard case sizes: 0402, 0603, 0805 and 1206
- UL recognized and AEC-Q200 compliant
- LED lighting systems
- Power supply and conversion
- Heatsink thermal management
Figure 1: NTC used to influence PWM over LED temperature
Figure 2: NTC used to influence current sense signal at higher LED temperature
Figure 3: LED current and luminous flux at different ambient temperatures and mounting conditions