Electrical - Can I use really short pulses and really high currents to drive Luminus LEDs?

LED System Overview

LEDs apart from traditional sources of light are usually designed to operate voltage in the realm of 1.2V to 6.4V in most cases. Since LEDs are effectively diodes, the IV curve becomes a point of interest. Due to the relationship between the current and voltage, the devices are typically run via Constant Current.

Once we get into pulsing, drivers become the key component in the entire system. A change in voltage can cause a change in the supplied current. Due to the relationship between light output proportionally to the drive current, these conditions may cause the output and lifetime to vary and/or degrade faster. The entire system will need to be balanced with respect to the temperatures within the LED.

Switching times and Reliability Testing

Once we start discussing the LEDs in a system case, there are also additional dominant factors for the rapid switching to affect the rise and fall time. E.g., junction and parasitic capacitance, inductance of the traces and the driving topology will all affect this. In characterizing our devices, we do so at a 20ms pulse which is fast enough for the device to switch on and off with minimal thermal effects on the device. This is an industry standard as most integrating spheres are calibrated to this type of pulse.
Once you go lower than this value, we cannot guarantee long-term reliability as we do not test these devices in such conditions. For reference, standardized reliability testing is done so at CW due to the thermal effect being the strongest on the device. Hours of testing is required to reach the 6x multiple to rate our products. Adding a duty cycle to this test will only increase the length of the standardized testing by the amount the duty cycle affects the test. (example, a 50% duty cycle will make the standardized test go 2x as long). 

Rise times are typically in the nanoseconds and fall times in the tens of nanoseconds for direct emission LEDs.

Conditions to Consider

• Temperature

Maximum internal temperatures (a.k.a. junction temperatures) will vary from device to device. In typical Continuous Wave conditions, a simple calculation will determine the junction temperature from the ambient temperature. This can be viewed simply as the formula below.

T_junction = R_thermal * Power + T_ambient

Where

T_junction = Internal Junction Temperature

R_thermal = Thermal Resistance of DUT

Power = Drive Current & Drive Voltage

T_ambient = Ambient System Temperature

From this CW condition, the duty cycle can be incorporated as such

T_junction = R_thermal * Power * Duty Cycle + T_ambient

This provides a ballpark estimation to determine the limit in terms of current in regards to the overall system design. Better or Worse system conditions may will ultimately this.

In example, the final calculation for a limit in thermals with respect to the XST-3535

100°C > 0.5°C/W * Power * Duty Cycle + T_ambient

Where the Power, Duty Cycle and T_ambient will be a representation of the system design.

• High Frequency Phenomenon/Low Duty Cycles

During very high frequencies, portions of the device may exhibit some issues not related to current. For example, the effect of self-inductance of the wire bonds are significant at higher frequencies.
Both the associated inductance and resistance values of the bond wire vary as the frequency values change. Such parameters may be associated with length, diameter and thickness of the wire as well as its height above the substrate.
Metal migration may also occur at very high frequencies which has been observed at sub 1% duty cycle on some preliminary designs.

Coupling this with the tolerances of our devices, it becomes increasingly harder to define a maximum point in relation to lower duty cycles that fits within our guidelines for long term reliability.

This is not to say that there have been companies that have utilized our devices duty cycles down to 1%, but only as a step for dimming, not for continuous operation for typical use.

Key note for UV devices with respect to pulsed operation, including dimming is that at 1% duty cycle, the photon count out of the device becomes lower over time vs a CW condition which will affect the dosage rate of the device.
If the goal for such devices is to cure and/or disinfect, it may not be reasonable to drive the device at such low duty cycles as this will directly affect the amount of photons emitted and reaching the target. Because of this we typically recommend a CW condition for UVC devices.
Some of the Luminus Devices may have yet to find a reasonable limit pulsing our LED due to the reliability conditions that are required to consider and if the use case is for a limited time or for long term use. 
We are attempting to define this limit for the portfolio that Luminus provides, but until we can verify the reliability of our LED at certain pulsed conditions, we urge some system level reliability testing to ensure the integrity. Ultimately you may also contact [e-mail] for in-depth information regarding your system.

Conclusion

Ultimately the final design will be a combination of 1) keeping your thermals in check, and 2) ensuring that the pulses are long enough. We are continuously in the discovery phase for the latter in which we hope to publish our results once defined.

Interesting Read: 

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