High-Power LED Modules Demand Specialized Electrical Test Tools and Techniques

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Fig 1. Fortunately, for OEMs of HPLED modules, the latest generation of source-and-measure instrumentation reflects the requisite capabilities for test of these modules. For example, Keithley’s Model 2651A high-power System SourceMeter instrument provides 200 W of dc power and up to 2000 W of pulsed power output.

Fig 2. Minimizing lead resistance is not the only cabling challenge to take into consideration when working with large currents; minimizing inductance is equally important. Inductance is essentially a resistance to changes in current resulting from voltage drops created across the test leads during the rising and falling edges of the pulse when the test current is changing.

We now find high-brightness light-emitting diodes (HBLEDs) in a growing array of applications, including automotive displays and exterior lights, backlighting for televisions and video monitors, streetlights, outdoor signs, and interior lighting. Characterizing these devices demands a somewhat different set of test tools and techniques than do ordinary LED devices. In this article, we will examine some of these tools and techniques.

High-brightness LEDs vs. high-power LED modules

What are HBLEDs exactly, and how do they differ from the LEDs with which we are all familiar? Essentially, HBLEDs are LEDs that operate at 1W of power or higher, and usually in the range of from 1 to 3 W. The current draw of typical LEDs ranges from 10 to 30 mA, but HBLEDs generally draw from 300 mA to 1 A. Typical LEDs are packaged in a small 3-mm or 5-mm plastic dome with a pair of leads. In many instances, manufacturers mount HBLEDs on a small, thermally conductive board designed to draw heat away from the device’s junction. The leads themselves must be much thicker to handle the higher levels of current associated with HBLEDs.

As bright as individual HBLEDs are, they are still not bright enough for many lighting applications. Often, multiple HBLEDs combine to create luminaires, such as in an LED light bulb for a retrofit application or a complete lighting fixture. This is a good solution for applications in which it is desirable to have the light fan out in multiple directions and where the luminaire’s size provides sufficient space for multiple devices. However, in applications where space is very limited and/or the light must be directional, this approach is unworkable.

In contrast with HBLEDs, high-power LED modules (HPLEDs) offer far more compact form factors and can deliver lots of directional light. These characteristics have made HPLEDs extremely popular in applications such as automotive headlamps, projectors, and even some high-power flashlights.

HPLED modules consist of one or more large-die LEDs assembled in a single package and can handle higher currents than typical HBLEDs. It is common for a single die to be required to withstand currents of 10A or higher. These high currents produce a significant amount of heat, which must be dissipated and/or monitored during the characterization process. Modules are typically built on a heavy copper-core board or other thermally conductive material in order to pull the heat out of the LED junctions. It is also simple to mate these heavy boards with a heat sink of appropriate size to keep the LEDs cool. Modules usually have a thermistor built into the board to simplify monitoring the junction temperature of the LEDs. Despite manufacturers’ efforts to dissipate the heat produced by the LEDs, operating junction temperatures for these modules commonly go as high as 140ºC.