Overheating is a common issue that can negatively impact both the light output and lifetime of LEDs. Due to the instant on-off capability of LEDs, one can quickly turn on and off an LED in a periodic manner. Applications in fluorescence, which typically require higher light outputs, commonly use pulsed mode (duty cycle) operation to more safely increase LED current. Duty cycle is the percentage in one period in which an LED is turned on; where a period is the total time it takes to complete an on-and-off cycle. An LED operating at a 50% duty cycle, for example, would be turned on exactly half of the time and off half of the time. Figure 2 shows the normalized light output at various drive currents and duty cycles.
Figure 2 Here we see the impact of a varied duty cycle on normalized light output, while on time is held constant at 500 μs. Normalized power is relative light output power compared to the light output at maximum rated operating current of 100 mA with the appropriate heat sink.
Operating the LED at high currents impacts the LED junction temperature, which consequently impacts lifetime and light output. Optimizing the duty cycle can minimize the impact of the increased drive current on the junction temperature, thereby preserving the LED performance. Figure 3 illustrates the impact duty cycle can have on maintaining the junction temperature of the LED. By operating at a 5% duty cycle, one could achieve over three times the light output (as seen in Figure 2) with minimal impact on junction temperature.
Figure 3 This graph shows the impact of varied duty cycle on junction temperature, while on time is held constant at 500 μs.
Excess heat negatively impacts the light output and lifetime of an LED.