To demonstrate pulsing with an SMU, we’re going to use the NI PXIe-4139 to characterize a high-power LED from CREE. Because of the IV requirements of the LED (37 Vf, 2.5 Imax), we need to operate the SMU in the extended pulsing boundary, which gives us the ability to pulse up to 500 W compared to the traditional 20 W DC boundary.
For the IV characterization of this LED, we’re going to sweep current into the LED from 0 to 2.5 A. Characterizing this LED with a traditional DC sequence presents two challenges. First, we would need to connect several SMUs in parallel to achieve the required current and voltage for the IV sweep. The additional SMUs not only complicate the setup from a wiring and programming perspective, but also increase the cost and size of the test system. Secondly, we would be sourcing up to 100 W of power into this small LED. Without installing a heat sink, as recommended in the figure below, we would damage this LED by applying a DC source for too long. Using the SMU in pulsed mode avoids these two challenges by giving us the ability to perform a full IV sweep on the LED with a single instrument and no external heat sink.
To execute the test as fast as possible while minimizing heat dissipation through the LED, we will use the minimum pulse width, which is 50 µs, for the instrument. Creating usable 50 µs pulses is challenging, so to ensure we get clean, stable pulses from the SMU, we’re going to use two unique features of the NI PXIe-4139. First, we’ll use the instrument as a digitizer to examine the detailed transient characteristics of the pulse. Secondly, we’re going to use NI SourceAdapt technology to customize the pulse for a fast rise time with no overshoot or oscillations.
Pulse Generation and Digitization
When generating high-power, narrow pulses, it’s important to ensure the SMU response is fast and stable. The SMU used in this example, NI PXIe-4139, has a built-in digitizer mode that can sample up to 1.8 MS/s, so we can use the measure function of the same SMU to digitize the output. Without this feature, you need an external oscilloscope that can measure both current and high voltage.
Digitizing the SMU pulse gives you the ability to examine the detailed pulse characteristics and verify the SMU can take an accurate measurement at each step in the sequence. In this case, we can see that the SMU doesn’t settle within the 50 µs window, so we cannot accurately acquire IV data with these settings. At this point, we need to either extend the pulse on-time or adjust the response of the SMU.
Shaping the Pulse With NI SourceAdapt
The NI PXIe-4139 is equipped with technology called NI SourceAdapt, which gives you the ability to customize the transient response of the SMU. In this example, we need to use this feature to improve the rise time of the pulse while maintaining a stable response with no overshoots.
The figure above shows the pulse characteristics after adjusting the SourceAdapt settings. By looking at the pulse above, we can determine the necessary settling and aperture time of the SMU, and feel confident that the final IV sweep will return accurate data. The graph below shows the SMU sweeping from 0 to 2.5 A, and measuring the voltage and current at each point of the sequence.