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Identify electrically overstressed LEDs (Part 2)

Posted: 27 May 2014 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? electrical overstress? EOS? Structure 2? Structure 3?

Structure 2 LEDs use both vias and metal contacts. Figure 1 shows the structure of these LEDs.

These LEDs follow closely behind the Structure 3 components in terms of maximum withstand power, and this is most likely due to the use of vias. Multiple vias increase current spreading, which in turn reduces the current density. Since EOS is caused by over-current or over-voltage events, reducing current density helps the LED withstand high current. Figure 2 highlights the EOS robustness of the Structure 2 LEDs.


Figure 1: Cross-section of Structure 2 LEDs.

Figure 2: Pulse-power-to-failure vs time-to-failure of Structure 2 LEDs.

Single-chip high-power LED Structure 3
The Structure 3 LEDs have only vias for contact, as shown in figure 3.

Figure 3: Cross-section of Structure 3 LEDs.

These LEDs showed the highest EOS robustness of any singlechip component. This direct attachment architecture using vias shows a great increase in the LEDs ability to withstand an EOS event. Figure 4 highlights EOS robustness of the Structure 3 LEDs.

Figure 4: Pulse-power-to-failure vs time-to-failure of Structure 3 LEDs.

EOS susceptibility of high-power COB LEDs
A COB LED typically consists of several parallel strings of multiple LED chips in series in each string. The maximum EOS withstand power of this type of device not only depends on LED chip structure, but also on package construction, such as the number of parallel strings, and to some extent, the package dimensions. Figure 5 illustrates EOS robustness of the COB LEDs.

Figure 5: Pulse-current-to-failure vs time-to-failure of COB LEDs.

When comparing EOS susceptibility of COB LEDs, if the package and LED chip structure are similar, the number of parallel strings determines the component with the higher maximum withstand power. More strings provide more paths for current flow, resulting in greater maximum withstand power. The 1600lm LEDs have an additional parallel string compared to the 700-lm LED, resulting in an increase in EOS robustness. The effect of the number of strings is greater than the effect of package size.

The rated voltage and power for a COB LED determines its typical operation and applicability in a lighting application. These factors can also determine an LEDs EOS susceptibility. The following design elements not only produce an LED with a higher maximum power rating but also increase heat transfer and spread current, improving EOS robustness.
???COB LEDs with more chips or more parallel strings are more robust than LEDs with less. More parallel strings help split the current.
???COB LEDs with a larger chip size are more robust than LEDs with a smaller chip size. Increased chip size reduces the current density and improves EOS robustness.
???COB LEDs with larger package dimensions are more robust than smaller LEDs. Increased package dimensions improve heat dissipation.

About the author
Yankun Fu contributed this article.

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