Two New Keysight Source Measure Units (SMUs) for Battery Powered Device and Functional Test

Over the past few years here on “Watt’s Up?” I have posted several articles and application pieces on performing battery drain analysis for optimizing run time on mobile wireless devices. The key product we provide for this application space is the N6781A 20V, +/-3A, 20W source measure module for battery drain analysis. A second related product we offer is the N6782A 20V, +/-3A, 20W source measure module for functional test. The N6782A has a few less key features used for battery drain analysis but is otherwise the same as the N6781A. As a result the N6782A is preferred product for testing many of the components used in mobile devices, where the extra battery drain analysis features are not needed. These products are pictured in Figure 1. While at first glance they may appear the same, one thing to note is the N6781A has an extra connector which is independent voltmeter input. This is used for performing a battery run-down test, one of a number of aspects of performing battery drain analysis. Details on these two SMUs can be found on by clicking on: N6781A product page.  N6782A product page,

Figure 1: Keysight N6781A SMU for battery drain analysis and N6782A for functional test

These products have greatly helped customers through their combination of very high performance specialized sourcing and measurement capabilities tailored for addressing the unique test challenges posed by mobile wireless devices and their components. However, things have continued to evolve (don’t they always!). Today’s mobile devices, like smart phones, tablets and phablets, have an amazing amount of capabilities to address all kinds of applications. However, their power consumption has grown considerably as a result. They are now utilizing much larger batteries to support this greater power consumption in order to maintain reasonably acceptable battery run-time. Optimizing battery life continues to be a critical need when developing these products. With their higher power however, there is in turn a greater need for higher power SMUs to power them during test and development. In response we have just added two new higher power SMUs to this family; the N6785A 20V, +/-8A, 80W source measure module for battery drain analysis and the N6785A 20V, +/-8A, 80W source measure module for functional test. These products are pictured in Figure 2. Details on these two new higher power SMUs can be found on by clicking on: N6785A product page.  N6786A product page.

Figure 2: Keysight N6785A SMU for battery drain analysis and N6786A for functional test

A press release went out about these two new SMUs yesterday; Click here to view. With their greater current and power capability, customers developing and producing these advanced mobile wireless devices and their components now have a way to test them to their fullest, not being encumbered by power limitations of lower power SMUs.

This is exciting to me having been working within the industry for quite some time now, helping customers increase battery life by improving how their devices make more efficient use of the battery’s energy. A key part of this has been by using our existing solutions for battery drain analysis to provide critical insights on how their devices are making use of the battery’s energy.  There is a lot of innovation in the industry to make mobile wireless devices operate with even greater efficiency at these higher power and current levels. There is no other choice if they are going to be successful. Likewise, it is great to see continuing to play a key role in this trend in making it a success!

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How to test the efficiency of DC to DC converters, part 1 of 2

I periodically get asked to provide recommendations and guidance on testing the efficiency of small DC to DC voltage converters. Regardless of the size of the converter, a DC source is needed to provide input power to the converter under constant voltage, while an electronic load is needed to draw power from the output, usually under constant current loading. The load current needs to be swept from zero to the full load current capability of the DC to DC converter while input power (input voltage times input current) and output power (output voltage times output current) are recorded. The efficiency is then the ratio of power out to power in, most often expressed in a percentage. An illustration of this is shown in Figure 1. In addition to sourcing and sinking power, precision current and voltage measurement on both the input and output, synchronized to the sweeping of the load current is needed.

Figure 1: DC to DC converter efficiency test set up

One challenge for small DC to DC voltage converters is finding a suitable electronic load that will operate at the low output voltages and down to zero load currents, needed for testing their efficiency over their range, from no load to full load output power. It turns out in practice many source measure units (SMUs) will serve well as a DC electronic load for testing, as they will sink current as well as source current.

Perhaps the most optimum choice from us is to use two of our N6782A 2-quadrant SMU modules installed in our N6705B DC Power Analyzer mainframe, using the 14585A software to control the set up and display the results.  This is a rather flexible platform intended for a variety of whatever application one can come up with for the most part. With a little ingenuity it can be quickly configured to perform an efficiency test of small DC to DC converters, swept from no load to full load operation. This is good for converters of 20 watts of power or less and within a certain range of voltage, as the N6782A can source or sink up to 6 V and 3 A or 20 V and 1 A, depending on which range it is set to. One of the N6782A operates as a DC voltage source to power the DUT and the second is operated as a DC current load to draw power from the DUT. A nice thing about the N6782A is it provides excellent performance operated either as a DC source or load, and operated either in constant voltage or constant current.

An excellent video of this set up testing a DC to DC converter was created by a colleague here, which you can review by clicking on the following link: “DC to DC converter efficiency test”.

The video does an excellent job covering a lot of the details. However, if you are interested in testing DC to DC converters using this set up I have a few more details to share here about it which should help you further along with setting it up and running it.

First, the two N6782A SMUs were set up for initial operating conditions. The N6782A providing DC power in was set up as a voltage source at the desired input voltage level and the second N6782A was set to constant current load operation with minimum (near zero) loading current.

Note that the 14585A software does not directly sweep the load current along the horizontal axis. The horizontal axis is time. That is why a time-based current sweep was created in the arbitrary waveform (ARB) section of the 14585A. In that way any point on the horizontal time axis correlates to a certain current load level being drawn from the output of the DUT. The ARB of course was set to run once, not repetitively. The 14585A ARB set up is shown in Figure 2.

Figure 2: Load current sweep ARB set up in 14585A software

This ARB sweep requires a little explanation.  While there are a number of pre-defined ARBs, and they can be used, an x³ power formula was chosen to be used instead. This provided a gradually increasing load sweep that allowed greater resolution of this data and display at light loads, where efficiency more quickly changes. As can be seen, the duration of the sweep, parameter x, was set to 10 seconds. As a full load current needed to be -1 A, using the actual formula (-x/10)³  gave us a gradually increasing load current sweep that topped out at -1A after 10 seconds of duration. The choice of 10 seconds was arbitrary. It only provided an easy way to watch the sweep on the 14585A graphing as it progressed. Finally, a short (0.1 second) pre-defined linear ramp ARB was added as a second part of the ARB sequence, to bring the load current back to initial, near zero, load conditions after the sweep was completed. This is shown in Figure 3.

Figure 3: Second part of ARB sweep to bring DUT load current back to initial conditions

I hope this gives you a number of insights about creative ways you can make use of the ARB. As there is a good amount of subtle details on how to go about making and displaying the measurements I’ll be sharing that in a second part coming up shortly, so keep on the outlook!

Consider the guard amplifier for making more accurate sub-µA current measurements with your DC source

As is the case with many sourcing and measurement challenges, when attempting to measure extreme values of most anything, factors that you can be blissfully unaware of, because they normally have an inconsequential impact on results, can become a dominant error to deal with. One example of this is when trying to make good low level leakage current measurements on devices and components and “phantom” leakages exceed that of the device you are attempting to test.

When measuring leakage currents of around a µA and lower, it is important to pay attention to your test set up as it is fairly easy to have leakage currents paths in the set up itself that range from adding error to totally obscuring the leakage current of the DUT itself you are trying to test. These leakage current paths can be modeled as a high value resistor in parallel to the DUT, as shown in Figure 1.

Figure 1: Leakage current path in DUT test fixture

• Many things can cause leakage currents on the fixture contributing to leakage current measurement error of the DUT:
• Is the PC fixture board made from appropriate high impedance material?
• Is the PC board truly clean?
• Was de-ionized water used to clean the PC board?
• If already in service for quite some time, have contaminants slowly built up over time?
• Any components associated with the connection path to the DUT are, or have become, unexpectedly leaky?
• Any standoffs and insulators associated with the connection path to the DUT are, or have become, unexpectedly leaky?

Even with all the above items in check there are still times when more needs to be done to further reduce leakage current inherent in the test set up. To help in this regard a guard amplifier is often added on high performance source-measure units (SMUs) to mitigate errors introduced from leakage current paths in the test set up. The Agilent N678xA and the B2900 series are examples of SMUs that include guard amplifiers. Application of a guard amplifier is illustrated in Figure 2.

Figure 2: Guard amplifier in a leakage current test set up

The guard amplifier is a unity gain buffer connected to the output of the SMU to provide a voltage that matches the SMU voltage. The guard amplifier can typically furnish 100’s of µA or more to offset any leakage currents. The test set up needs to be designed to incorporate a guard, which is a conductive path that surrounds, but is not connected to, the SMU’s output path. The guard and guard amplifier do not eliminate any leakage paths. Rather they “intercept” and furnish the leakage current. Because the guard surrounding the SMU output path maintains its potential at that of the SMU’s output potential, the net difference is zero. Because the potential difference is zero no current “leaks” from the SMU output to the guard. The only current now flowing from the SMU output is that which is flowing into the DUT itself. This is just one more tool to get accurate results when making measurements at an extreme value; in this case when making extremely low leakage currents!