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Posted on 29 June 2019

Pulsed Over-Current Driving of LEDs

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Exploring the needs of three types of over-current conditions

Certain product requirements, like emergency vehicle lighting, specialized stroboscopic illumination or pulsed modulation for general-illumination dimming, call for an LED operation above the maximum data-sheet rating. Multiple variables affect both initial and long-term performance and reliability on an LED. These include thermal resistance, pulse duration, as well as current amplitude, frequency and duty cycle. In this article we will explore three types of over-current conditions: Single-pulsed events like electrical overstress (EOS), repetitive pulsing and ripple effects.

By: Mitch Sayers, Field Applications Engineer, CREE Europe

 

Single Pulse over current events

Single-pulse over-current events are often the result of an unintentional application of excessive electrical energy to LEDs and typically lead to a catastrophic failure of the device. Beyond a certain threshold, a singlepulse event will lead to an immediate catastrophic failure of the LED.

The main factor limiting an LED’s ability to withstand an EOS event is the current-carrying capability of the LED chip and internal interconnections. Conventional electrical conductors have a finite resistance. Current must be kept sufficiently low to prevent the conductors from melting or fusing, or the insulating material from breaking down. For example, at high current densities material forming the interconnections can move. This phenomenon is known as electromigration.

Current crowding, a localized increase of current density especially at the vicinity of the contacts and over the p-n junctions, can lead to localized overheating and formation of thermal hot spots. In catastrophic cases this can lead to thermal runaway, and can also aggravate electromigration effects and formation of voids. The increased resistance around a void is a self-feeding cycle, causing further localized temperature rise, which in turn accelerates the formation of the void and eventually can lead to an open-circuit failure.

Conversely, localized lowering of current density, with an implied current-density gradient, may lead to deposition of migrated atoms from current-“crowded” regions. This can lead to further lowering of current density and further deposition of migrated ions, even the formation of small protuberances, which in turn can cause short circuits.

Two examples of metal migration: (A) device subjected to repeated transient currents below maximum threshold; (B) device subjected 20 times normal forward voltage

These effects can always be mitigated with proper power-supply design, which prevents electrical transients from reaching the LED component.

Repetitive Pulsing

Apart from the risk of an early catastrophic failure of the LED, repetitive high-current pulsing may result in a shortened life expectancy for the LED compared to the usual expected lifetimes on the order of tens or hundreds of thousands of hours. A particular device could be subjected to repeated transients at an amplitude some percentage above the data-sheet limits but below the threshold required for single-pulse failure, yet it will still eventually fail. The failure mechanism will be most likely due to electromigration as enough metal ions are eventually shifted away from their original lattice positions. Excessive heating of the p-n junction, which causes the LED’s output to degrade below 70 percent of its original luminous flux, also reduces its lifetime.

Cree tested different types of XLamp LEDs at pulsed currents over a broad range of currents, including levels well above the maximum- rated continuous current. The data shows that above certain levels, there is little gain in light output and efficiency decreases. Thus it is not advisable to operate LEDs at such extreme levels.

The relationship between light output and forward current is non-linear, as is the relationship between forward voltage and current. Increasing the drive current to the LED by a factor of two will result in an increase in power greater than a factor of two. The same holds true for light output. Doubling the current will not double the output, and in fact, above a certain point, light output may even begin to decrease as the internal temperature of the LED rises.

Relationship between current amplitude and luminous efficiency of white XLamp XP-G at three different duty cycles at a frequency of 1000 Hz

Driving LEDs at high currents can also cause chromaticity shifts. As the LED forward current increases, the x and y color coordinates begin to shift to the left and downwards on the CIE 1931 color space, resulting in an increase in correlated color temperature (CCT).

Typical XLamp XP-G, 20% duty cycle, 1Khz pulse (maximum rated continuous current at 1500 mA)

An even greater long-term reliability concern is the effect of operating LEDs at elevated currents. This is due to heating of the p-n junction, especially for duty cycles greater than 25%. In order to determine if the maximum junction tem-perature (Tj) will be exceeded, one must measure the current (If), voltage (Vf) and case temperature (Tc) of the LED. For a pulsed LED, the power will be proportional to the duty cycle (D); therefore, to calculate Tj the following formula can be used:

To calculate Tj the following formula can be used

However, this is only part of the equation. The ambient temperature and thermal resistance from the case to the ambient air must also be factored in. Proper thermal-management techniques must still be followed.

Chromatic shift of coolwhite and warm white XLamp XP-E under various pulsed-current conditions

Ripple current

Ripple current is a periodic peak-to-peak variation in the current waveform. Unfiltered, rectified 120-Hz signals are routinely used to drive LED illumination applications and within specification boundaries pose no problems whatsoever to the LEDs. However, excessive ripple current can also be a concern for electrolytic capacitors on the output filter stage of an LED driver. High ripple currents can lead to overheating of the capacitors and exacerbate early failure of the driver circuitry. There is also a chance that if the filter capacitors fail, transient currents that would otherwise have been attenuated will become present and may in turn cause damage to the LED.

High ripple current, 525 mA, peak to peak

Large swings in peak-to-peak current may also affect the luminous efficiency of the LED. As an example, take a sample XP-E Lamp with the luminous in Table 3.

XP-E luminous flux in steady state

If the lamp is driven at a steady DC current of 500 mA, the output will be 142 lumens. On the other hand, if it is driven with a current that oscillates between a minimum of 200 mA and a maximum of 800 mA, then the light output will swing between 66 to 203 lumens. The mean lumen output is only 134.5, which is about five percent less than when the LED is driven with a steady DC current.

The maximum switching frequency for pulsing LEDs will be limited by the turn-on time of the device as well as the rise- and falltime limits of the switching circuitry. The typical turn-on time for an XLamp device is on the order of 100 nanoseconds or less, which would limit the maximum switching frequency to approximately 10 MHz.

Summary

While Cree’s XLamp products are capable of withstanding current transients well above the maximum rated continuous current, there are physical limits that must not be exceeded in order to avoid EOS. It is possible to operate LEDs in a continuous pulsed mode at higher levels, but there will be trade-offs that may adversely affect efficiency, chromaticity and long-term reliability.

For certain specialized applications, there may be a good reason to exceed maximumcurrent ratings to achieve a desired level of performance. However, for these cases, Cree recommends that customers perform their own life testing when proving out a design that will deliver any of the three overcurrent conditions described here. It is the customer’s responsibility to determine if the tradeoffs will be acceptable.

 

References:

1) Wire bonding in microelectronics: materials, processes, reliability and yield, George C. Harman, pp. 58-61.
2) Advanced wire bond interconnection technology, Shankara K. Prasad pp. 25-26.
3)“Physical Analysis of Data on Fused-Open Bond Wires”, Eugene LohIEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-6, No. 2 June 2019.
4) “Electrical Overstress in Integrated Circuits,” J.T. May Handbook of Nitride Semiconductors and Devices, Physics and Technology of GaNBased Optical and Electronic De-vices, Vol. 3, H. Morkoc.
5) “Current spreading and thermal effects in blue LED dice,” K. A. Bulashevich, I. Yu. Evstratov, V. F. Mymrin, S. Yu. Karpov.

 

 

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2 Responses

  1. avatar Jonathan says:

    This post is blatanly copied from a cree technical document without any reference to it.

    http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/XLamp%20Application%20Notes/XLampPulsedCurrent.pdf

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    • avatar PowerDahl says:

      Hello Jonathan,

      This post was submitted by CREE as an article for the April 2011 issue of Bodo's Power Systems magazine. It appears that the content was taken from the CREE document that you referenced, but there are no copyright issues present here. PowerGuru archives, with permission, all technical articles from Bodo's Power Systems magazine.

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