Synchronize Your Measurements with Your List Transients

Hi everybody!

My blog post this month is the result of a recent customer question.  The question was: how do you synchronize measurements with list transients?  The short answer is that you use the built in digitizer to generate enough points to sample the measurements over the entire transient.  The rest of this blog will provide the long answer.  The program that I am using here was written for a N6762A DC Power Module but the technique will work with any power supply that has a built in digitizer such as the Advanced Power System or any N6700 module with option 054.

For simplicity’s sake, we are going to use a 5 point list.  The voltage steps are 1 V, 2 V, 3 V, 4 V, and 5 V and the dwell times are 0.1 s, 0.2 s, 0.3 s, 0.4 s, and 0.5 s.  Let’s first set the list up (please note that all programming is done in VB.net with VISA-COM):

The next thing to do is to set up the measurement system.  We need to figure out the total number of points that we need measure so that we can cover the entire transient.  The first thing that we need to do is to calculate the total time of the list transient (you can even do this in your program):

The total time of our transient is 1.5 s.  Now we need to use this to figure out the number of points. I am going to choose a measurement interval of 40.96 us.  This means that we want to take a measurement every 40.96 us for 15 s.  To get the total number of points, you need to divide the total transient time by the measurement time interval:

I’m going to round down and use 36,621 points.  I’m also going to tell the power supply to use the binary data format because as we know from my previous blog posts, this is the fastest way to read back data.   Here is the code to set up the digitizer:

We will set our trigger source to bus for both the transient system and the acquire system:

Next we initiate both systems:

Once the initiate is complete, we send a trigger:

This will start both the list transient and the digitizer.  After everything is completed, we can fetch our measured voltage array:

This array will have all of our measurements.

I hope that this has been useful, have a good month everyone.

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

In part 1 of my posting on testing the efficiency of DC to DC converters (click here to review) I went over the test set up, the requirements for load sweep synchronized to the measurements, and details of the choice of the type and set up of the current load sweep itself. In this second part I will be describing details of the measurement set up, setting up the efficiency calculation, and results of the testing. This is based on using the N6705B DC Power Analyzer, N6782A SMUs, and 14585A software as a platform but a number of ideas can be applicable regardless of the platform.

Figure 1: Synchronized measurement and efficiency calculation set up

The synchronized measurement and efficiency calculation set up, and display of results are shown in Figure 1, taking note of the following details corresponding to the numbers in Figure 1:

1. In the 14585A the data logging mode was selected to make and display the measurements. The oscilloscope mode could have just as easily been used but with a 10 second sweep the extra speed of sampling with the oscilloscope mode was not an advantage. A second thing about using the data logging mode is you can set the integration time period for each acquisition point. This can be used to advantage in averaging out noise and disturbances as needed for a smoother and more representative result. In this case an integration period of 50 milliseconds was used.
2. To synchronize the measurements the data log measurement was set to trigger off the start of the load current sweep.
3. Voltage, current, and power for both the input and output SMUs were selected to be measured and displayed. The input and output power are needed for the efficiency calculation.
4. The measurements were set to seamless ranging. In this way the appropriate measurement range for at any given point was used as the loading swept from zero to full load.
5. A formula trace was created to calculate and display the efficiency in %. Note that the negative of the ratio of output power to input power was used. This is because the SMU acting as a load is sinking current and so both its current and power readings are negative.

With all of this completed really all that is left to do is first start the data logging measurement with the start button. It will be “armed” and waiting from a trigger signal from the current load sweep ARB that had been set up. All that is now left to do is press the ARB start button. Figure 2 is a display of all the results after the sweep is completed.

Figure 2: DC to DC Converter efficiency test results

All the input and output voltage, current, and power measurements, and efficiency calculation (in pink) are display, but it can be uncluttered a bit by turning off the voltages and currents traces being displayed and just leave the power and efficiency traces displayed. This happened to be special DC to DC converter designed to give exceptionally high efficiency even down to near zero load, which can be seen from the graph. It’s interesting to note peak efficiency occurred at around 60% of full load and then ohmic losses start becoming more significant.

And that basically sums it all up for performing an efficiency test on a DC to DC converter!

Using Binary Data Transfers to Improve Your Test Throughput

From time to time I have shared here on “Watt’s Up?” a number of different ways the system DC power supply in your test set up impacts your test time, and recommendations on how to make significant improvements in the test throughput. Many of these previous posts are based on the first five of ten hints I’ve put together in a compendium entitled “10 Hints on Improving Throughput with your Power Supply” (click here for hints 1-5).

Oscilloscopes, data acquisition, and a variety of other test equipment are often used to capture and digitize waveforms and store large arrays of data during test, the data is then downloaded to a PC. These data arrays can be quite large, from thousands to millions of measurements. For long-term data logging the data files can be many gigabytes in size. These data files can take considerable time to transfer over an instrument bus, greatly impacting your test time.

Advanced system power supplies incorporating digitizing measurement systems to capture waveform measurements like inrush current are no different. This includes a number of system DC and AC power products we provide. Even though you usually have the choice of transferring data in ASCII format, one thing we recommend is instead transfer data in binary format. Binary data transmission requires fewer bytes reducing transfer time by a factor of two or more.

Further details about using binary mode data transfers can be found in hint 7 of another, earlier compendium we did, entitled “10 hints for using your power supply to decrease test time” (click here to access). Between these two compendiums of hints for improving your test throughput I expect you should be able find a few different ideas that will benefit your particular test situation!

How do I measure inrush current with an Agilent DC Power Supply?

Hello everybody! I want to build on my blog post from last month.  This month, we are going to discuss how to measure inrush current using the DC Power Analyzer’s scope function as well as the digitizer feature that is available on some of our system power supplies.

Measuring inrush current is a task that many customers that use DC Power Supplies want to accomplish.  When you are doing this test on the bench, the N6705B DC Power Analyzer (DCPA) is your best bet.  The DCPA has the scope feature which makes this a breeze.  One of the great things about Agilent power supplies is that they can measure current directly, without the need for a current probe. Some of our supplies have very high current measurement accuracy as well so you can get an accurate representation of your current.

In the below screenshot, I just had a capacitor connected to the output of the supply.  I set a voltage arbitrary waveform that went from 0 V to 20 V with the voltage slew set for the maximum.  I set the scope to trigger on the Arb run/stop key so that when I hit the key, both the arbitrary waveform and the scope triggered.  After I acquired the waveform, I used the markers to get the maximum current.  That number is our inrush current.   

As I said earlier, DCPA is geared towards bench use.   The graphical scope makes this task pretty easy.  Many of our system supplies (as well as the DCPA) have a digitizer feature that you can access using the SCPI programming interface.  The digitizer will sample the output using settings that you provide it.  These settings are: the number of points, the time interval between points, and the number of pretrigger points that you acquire.  In the N678xA SMU modules, the time interval is as low as 5.12 us and the number of points is as high as 512kpoints.  Here is a list of commands to set up the digitizer (written for the N67xx supplies) as well as some comments.

Set the digitizer to measure current:

 SENS:FUNC:CURR ON,(@1)

Set the number of pretrigger points, a negative value represents points taken before the trigger:

SENS:SWE:OFFS:POIN -100,(@1)

Set the total number of points to acquire:

SENS:SWE:POIN 5000,(@1)

Set the time interval between points:

SENS:SWE:TINT 0.000020,(@1)

Set the measurement trigger source to bus:

TRIG:ACQ:SOUR BUS,(@1)

 Initiate the measurement trigger system

INIT:ACQ (@1)

Send a trigger:

*TRG

Using this code, once the trigger is sent, the measurement system will acquire 5000 points at a time interval of 20 us while taking 100 pretrigger points. 

After the measurement occurs, you read the current back using:

FETC:ARR:CURR? (@1)

Once you have the array of current measurements, you can do any normal calculation that you can do on any array.  To measure inrush, you want to find the maximum current in the array.  This peak will be your inrush current.  I wrote a program that followed the exact same steps that I used on the scope above (setting up a step that went from 0 to 20 V and synchronizing triggers) and measured a maximum of 1.07748 A.  As you can see, I got a similar result from the two different approaches.

That is all that I have this month.  I hope that it is useful information.  If you have any questions at all please feel free to ask them in our comments.

Watt's Up with Datalogging and Digitizing?

All of our power supplies offer the ability to take an average measurement using either the front panel or the MEAS SCPI commands.  Some of our newer power supplies have some more advanced measurement capabilities.   The two capabilities that we are going to look at today are digitized measurements and datalogging.   Let’s take a short look at each one and then talk about when to use each one.

The digitizer has been in our products for a while now.  With the digitizer, you define three parameters and the measurement uses these parameters to return an array of measurements back to you.  The three parameters are: the number of points, the time interval, and the points offset.  The number of points is pretty simple.  It is the number of measurements that you want to take as well as the size of the array that you are going to read back.  The time interval is the pace of the measurements.  This is also the time between the points in the array.  The points offset is a way that you vary the starting point of the array.  This offset can be negative to return measured points before the trigger or positive to delay the start of the measurement.  The most points that we can measure and the fastest time interval is with our N678xA SMU modules.  These modules have a time interval of 5.12 us and a total number of points of 512 Kpoints (keep in mind that 1 Kpoint is 1,024 points).  This yields a total time of 5.12 us x 512 x 1,024 which yields a result of 2.68 seconds.  So the longest measurement that you can make is 2.68 seconds.  The largest time interval that we can measure is 40,000 seconds.  Setting this with the highest number of points would yield 40,000 s x 512 x 1,024 yields a total acquisition of 20,971,520,000 seconds.  That is 666.83 years! 

The other advanced measurement capability that we are going to talk about is our datalogger.  With the datalogger, you set a total acquisition time and an integration time.  The integration time is the amount of time that the power supply will average measurements.  The measurement system is still running at its maximum digitizing rate but it is averaging those measurements and returning that averaged measurement.  The digitizer on the N6705B DC Power Analyzer also will return the maximum measured value and the minimum measured value of each integration period.  The quickest integration time on the N6705B is 20.48 us.  The only limitation in the amount of data that you can log with the internal datalogger is the file size (the maximum file size is somewhere near 2 gB).  If you want to datalog huge files, you can use the external datalog feature (I wrote another blog post about this) or use our 14585A software where the only limitation is the free space on your hard drive.  The catch on the external datalogger is that that the quickest integration time is 102 us.

So when do you use one over the other?  It is pretty simple.  When you want to make a long term measurement (days, weeks, etc.) at a fast rate you should use the datalogger.  You would use this when you are looking to measure something like long term battery drain.  If you are looking for a more short term, faster measurement you would use the digitizer.  You would use the digitizer to measure something like inrush current. 

These are a few of the great features available in our power supplies.  Please let us know if you have any questions on these features or any of the features of our power supplies.