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Electrical overstress (EOS) is often associated with catastrophic failures such as an electrostatic discharge (ESD), in-rush current, or other types of transient electrical surge.¹ Designers of LED drivers should be sure to take all necessary precautions to protect the LEDs from EOS. The second type of over-current condition is often associated with applications in which the LEDs operate in a flashing mode such as emergency vehicle lights, strobe lights, or signaling beacons. High-frequency pulsing, i.e., greater than 120 hertz, is often used to control the brightness of LEDs in dimming applications. Finally, ripple current is a repetitive, cyclical peak-to-peak variation in a direct-current waveform that may be derived from an insufficiently filtered alternating current source or switched-mode power supply.
Within specification boundaries, none of these conditions is cause for concern in regard to the long-term lumen maintenance or reliability of the LED. But any of these in an “over-current” situation, i.e., beyond data-sheet specification, can decrease LED lifetime.
Single-pulse over-current events
Single-pulse over-current events are often the result of an unintentional application of excessive electrical energy to one or more LEDs and typically lead to a catastrophic failure of the device(s). Cree XLamp LEDs are capable of withstanding current transients that are several times higher than the maximum rated current. But, the exact amplitude that a particular LED device can withstand is also a function of the duration and frequency of the transient. Beyond a certain threshold, a single-pulse event will lead to an immediate catastrophic failure of the LED. In this case, there are two general failure modes: a short circuit or an open circuit.
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. Current density is given as the electric current per cross-sectional area and is usually measured in amperes-per-square-meters (A/m2). Conventional electrical conductors have a finite resistance, causing them to dissipate power in the form of heat. Current (and therefore current density) 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, known as electromigration, occurs when some of the momentum of moving electrons is transferred to nearby ions, causing them to dislocate from their original lattice positions. Electromigration does not occur in semiconductors directly, but in the metal interconnects deposited onto them. Over time, electromigration can transport a significant number of atoms far from their original positions and contributes to a device breakdown.
A localized increase of current density, known as current crowding, is an inhomogeneous distribution of current density through a conductor or semiconductor, especially at the vicinity of the contacts and over the p-n junctions. One of the known factors that limit the efficiency of LEDs, and materials with low charge-carrier mobility, such as indium gallium nitride (InGaN), are especially prone to current crowding.
Current crowding can lead to localized overheating and formation of thermal hot spots, which in catastrophic cases can lead to thermal runaway, and can also aggravate electromigration effects and formation of voids, causing localized, unevenly distributed current density. 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. In a similar self-propagating cycle, 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.
Figure 1 depicts two examples of metal migration. The picture on the left shows a device subjected to repeated transient currents below the maximum threshold; the picture on the right shows a device subjected to roughly 20 times the normal forward voltage, resulting in a dramatic, instantaneous failure.
These effects can always be mitigated with proper power-supply design, which prevents electrical transients from reaching the LED component.
Figure 1: Two examples of metal migration (photo taken by camera through microscope).
Another theoretically limiting factor is the current-carrying capacity of the bond wire(s) to the LED chip. If the current-carrying capacity is exceeded (as in a massive EOS), the conductor(s) fuse and cause an open circuit. While the exact physics involved in the fusing of a metal wire or conductive pad are beyond the scope of this document, the factors include the wire length and diameter, the type of bonds (ball or wedge), and physical material properties of the metal, including melting point, thermal conductivity, electrical resistivity, etc. Fusing of the bond wire is a highly uncommon, secondary failure that occurs only after the chip has already failed and continues to be subjected to excessive current, which in turn leads to overheating of the bond wire(s).
The second type of over-current condition, high-current repetitive pulsing, may or may not result in 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 lifetime, on the order of tens or hundreds of thousands of hours. A particular device subjected to repeated transients at an amplitude some percentage above the data-sheet limits but below the threshold required for single-pulse failure will still eventually fail. The failure mechanism will most likely be due to electromigration as enough metal ions are eventually shifted away from their original lattice positions. The other factor that can lead to a reduced lifetime is excessive heating of the p-n junction, which causes the LED’s output to degrade below 70 percent of its original luminous flux.
Cree’s Application Engineering group tested four 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 Cree does not recommend that customers operate LEDs at such extreme levels. The following charts show the relationship between current amplitude and luminous efficiency typical of white XP-C, XP-E, XP-G and MX-6 LED devices operating at three different duty cycles (5 percent, 20 percent, and 50 percent), at a frequency of 1000 Hz.² The chart for the XM-L LED shows the relationship typical of the LED operating at the 20 percent duty cycle. The dashed vertical line in each graph represents the maximum-rated continuous current for each XLamp LED product type.
Figure 2: XLamp XP-C luminous efficiency versus input current [Q3 bin] (dashed vertical line is maximum rated continuous current, 500 mA).
Figure 3: XLamp XP-E luminous efficiency versus input current [R4 bin] (dashed vertical line is maximum rated continuous current, 1000 mA).
Figure 4: XLamp XP-G luminous efficiency versus input current [R5 bin] (dashed vertical line is maximum rated continuous current, 1500 mA).
Figure 5: XLamp MX-6 luminous efficiency versus input current [Q3 bin] (dashed vertical line is maximum rated continuous current, 1000 mA).
Figure 6: XLamp XM-L luminous efficacy versus input current [T4 bin] (dashed vertical line is maximum rated continuous current, 3000 mA).
The relationship between light output and forward current is non-linear, as is the relationship between forward voltage and current. For example, 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 as shown in Table 1 (derived from the data in Figure 4). The same holds true for light output. Doubling the current does not double the output, and in fact, above a certain point, light output may even decrease as the internal temperature of the LED rises.
Table 1: Typical XLamp XP-G, 20 percent duty cycle, 1 Khz pulse (maximum rated continuous current at 1500 mA).In addition to reduced efficiency, there are other potential problems caused by driving LEDs at high currents, including chromaticity shift. As the LED forward current increases, the x and y color coordinates begin to shift to the left and downwards on the International Commission on Illumination (CIE) 1931 color space, resulting in an increase in correlated color temperature (CCT). Table 2 shows data typical for a Cool White and a Warm White XP-E LED.
Table 2: XLamp XP-E chromatic shift under various pulsed-current conditions (1000 mA is the maximum rated continuous current for the XP-E).
Figure 7 and Figure 8 show information from the same data set plotted in the CIE color space, with the averaged results of five XP-E LEDs showing the chromaticity shift within the maximum specified driving current (the green data sets) and the continuing chromaticity shift when driven at over-current specifications (the red data sets).
Figure 7: Warm white XLamp XP-E, driven at 1 KHz, 20 percent duty cycle, with under- and over-current settings.
Figure 8: Cool white XLamp XP-E, driven at 1 KHz, 20 percent duty cycle, with under- and over-current settings.
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 percent. To determine if the maximum junction temperature (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 is proportional to the duty cycle (D); therefore, the following formula can be used to calculate Tj.
Tj = Tc + D * If * Vf * Rthj-c
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.³
Based on the 1 KHz pulse testing we have reviewed in this application note, Cree suggests the following guidelines for pulsed current operations: