26 Oct 2011

HBLED High Current Testing Techniques

David Wyban of Keithley Instruments looks at the various testing techniques used for cost effective testing of High Brightness LEDs, HBLEDs

High brightness light-emitting diodes, HBLEDs are being used increasingly now that LED technology has developed sufficiently. Accordingly they are used in many high brightness lighting applications.

This has brought with it, many challenges for testing of these HBLEDS in a cost effective an accurate manner. Engineers encounter many issues when testing these HBLEDs, often routed in the fact that high currents are required.

Some typical applications are the characterization of high power LED modules and high brightness light-emitting diodes (HBLEDs). The first problem is finding instruments that can provide both the current needed and efficient test routines. The second problem is preventing self-heating in the device under test (DUT), which usually requires pulse testing. A third problem is avoiding measurement errors due to test lead resistance that causes excessive voltage drop at high currents.

Figure 1. Wiring diagram for connecting two SMUs for high current measurements

Today the first problem can be solved with the appropriate source-measure units (SMUs), which are often used for LED testing. Modern SMUs, including Keithley’s Model 2651A High Power System SourceMeter® instrument, combine source and measure operations normally performed by two separate instruments into a single simultaneous operation done by a single instrument. With the right kind of test setup, two of these instruments can be combined in a parallel arrangement capable of supplying up to 40A DC or 100A pulsed current to a DUT (Figure 1). Pulsed operation solves the second problem mentioned above – self-heating.

Still, certain precautions are required when combining two SMUs to achieve higher test current:

1. Use two identical high power SMUs – for example, capable of sourcing pulsed currents up to ±50A – using the same current range for both instruments. Configuring both SMUs to source on the same current range ensures both SMUs will respond similarly to changes in current levels. This reduces the chances for overshoots, ringing, and other undesired SMU-to- SMU interactions.

Figure 2. Power envelope capabilities of the Keithley Model 2651A

2. Restrict both SMUs to the same region of the power envelope. This can prevent damage under certain test conditions that might force one to sink all the current from the other. When SMUs are configured as current sources, the sinking SMU must be operating in an equivalent region of the power envelope as the sourcing SMU, which is determined by the source current range and the voltage limit value. (See Figure 2.) Since both SMUs should be set to the same source current range, the determining factor for the operating region is the voltage limit. As an example of how this works, if one SMU’s voltage limit is set to 20V, then the other SMU’s voltage limit should be set to a value that is less than 20V and greater than 10V in order to keep both SMUs in the same operating region.

3. Set the appropriate compliance levels. When SMUs operate as parallel current sources, the voltage limit of one SMU should be set 10% lower than the voltage limit of the other SMU. This allows only that SMU to go into compliance and become a low impedance voltage source that sinks current from the other SMU, which continues to source its programmed current level without raising its output voltage or ever going into compliance. If both SMUs were to go into compliance and become voltage sources, the system would have two voltage sources in parallel, which could allow an uncontrolled amount of current to flow between the SMUs, possibly causing unexpected results and/or damage to the DUT.

4. Select the output off-mode of each SMU. When two SMUs are functioning in parallel as current sources, the SMU whose voltage compliance is 10% less should be configured as a voltage source when its output is off (0V). The SMU that has its voltage compliance set higher should be configured as a current source when its output is off (0A).

5. Always use test cabling capable of supporting the high levels of current that high power SMUs can produce. The cable used must be designed for both low resistance and low inductance. Generally, 10 AWG or heavier gauge wire should be used. Otherwise SMU performance and measurement accuracy could be affected, and there could be a potential fire hazard.

6. Use 4-wire mode to eliminate measurement errors due to voltage drops in the test leads (Figure1). SMUs allow both 2-wire and 4-wire test modes. Even low resistance test leads can cause significant voltage drops at high currents and substantial measurement error in the 2-wire mode. Two-wire mode uses a single set of test leads to both source and measure.

Although two-wire mode simplifies cabling, it creates measurement errors because voltage measurements will not only include the voltage across the device under test (DUT) but across the test leads as well. To eliminate measurement error due to lead resistance (the third problem in high current testing), use the four-wire connections shown in Figure 1 for both SMUs. In four-wire mode, one set of leads, known as the source leads, is used to source the test current while a second set of leads, known as the sense leads, is used to measure voltage across the DUT. Because the measurement circuitry the sense leads are connected to has very high impedance, very little current will flow through these leads; therefore, the voltage drop across them will be insignificant and the voltage measured will be the same as the voltage across the DUT.

Note: The voltage-sensing leads should be connected as close to the DUT as possible.

7. Select test cables with minimum inductance for pulsed testing. Minimizing cable inductance is crucial when pulse testing LEDs with high currents. Essentially, inductance resists changes in current by creating a voltage drop when the test current is changing.

Keithley Model 2651A

Although inductance in the test leads will have no effect at the top and bottom of the pulse, where the current is constant, it will cause an additional voltage drop during the rising and falling edges for which the SMU must compensate. Minimizing lead inductance is important for obtaining the fastest rise time. Some SMUs have characteristics (enough voltage overhead) that compensates to a certain degree for voltage drops caused by inductance in the source leads.

8. Recognize the critical nature of lead resistance in pulsed testing. Although four-wire mode is designed to compensate for the voltage drops in the test leads caused by high currents, excessive lead resistance can still create measurement problems. When testing an LED using pulsed rather than DC currents, fast rise and settle times are critical to obtaining the shortest pulse widths to minimize device self-heating. Minimizing lead resistance makes it possible to obtain the fastest rise times.

Applying these precautions should enable the HBLED testing to be undertaken quickly, accurately and with confidence.

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