Monday, March 31, 2014

Use the FETCH Command to Minimize Your Measurement Time

Hi everyone,

I am back again with another programming tip for you.  A neat feature on some of our products that many people many not know about is the ability to fetch measurements from a previous acquisition.  Quite a few of our power supplies and loads (the N6700 modules, the N7900 APS power supplies, the 681xB AC Sources, the N3300 loads, and probably some others that I am probably forgetting) have the ability to acquire voltage and current measurements at the same time.  This is done using the FETCH command (in my little example snippets I use the SCPI short form of FETC).  In a previous blog post I used this command to read back an array of current measurements (see Inrush Current Measurements).  In that command, I use a FETCH command to retrieve a triggered measurement.  

There is another, very useful way to use the FETCH commands.  I am not really sure what the best way to phrase it so I am going to take a shot and then illustrate with an example.  When you send a measure command (say for voltage), the measurement system will also acquire the other measurement (in this case current) and you can send a FETCH command to retrieve that acquired data.   Here is a very small example with some comments (all these commands tested on a N7952A Advanced Power System):

Example Snippet 1:
MEAS:VOLT? -> This will start a new acquisition and take the measurements 
<read back the voltage measurement data>
FETC:CURR? -> This will return the current measured during the voltage measurement above
<read back fetched current measurements>

Since we have voltage and current measurements, the instrument can calculate power:
<read back calculated power>

Please note that you can do this with arrays as well. 

How can this save me time in my program you ask?  Well these power supplies all have built in digitizers that you can access with some programming commands.  The default measurement (at 60 Hz line frequency) is 3255 points measured at 5.12 us per point.  That is a total measurement time of  16.67 ms.  You have the ability to change this to fit your needs though.  You can measure up to 512 Kpoints at up to 40,000 s per point.  Every time you send a measure command you need to wait for the measurement to complete.  For instance:

Example Snippet 2:
<read back the voltage measurement data> 
<read back the current measurement data>

You will need to wait for two acquisition periods because you are initiating two separate measurements.  In the first example snippet, only the MEAS:VOLT? is initiating a measurement, the FETC:CURR is just reading data out of the instrument.    The downside is that the data that you fetch is going to be of the same age as the last measurement you did so if you need something newer, you need to do a new measurement.  Overall though I think that FETCH is a very useful command.  

I hope people find this useful.  Let us know if you have any questions by using the comments.  

Friday, March 28, 2014

What is a floating power supply output?

First let me tell you that a floating power supply output is NOT what is shown below in Figure 1 (haha).

Now some background: earth ground is the voltage potential of the earth and to greatly reduce the risk of subjecting a person to an electrical shock, the outer covering (chassis) of most electrical devices is internally connected to a wire that is connected to earth ground usually through the power cord. The idea here is to ensure that all surfaces a person can touch are at the same voltage potential; namely, the one that he is standing on: earth ground. As long as that is true, the person can freely touch things without the risk of getting shocked due to two of the things he touches at the same time being at different voltage potentials, or one of the things being at a high voltage potential with respect to the earth. If the voltage difference is high enough, the person could be shocked. Earth grounding the chassis also protects the user if there is an internal problem with an electrical device causing its chassis to inadvertently come in contact with an internal high voltage wire. Since the chassis is earth grounded, an internal short to the chassis is really a short to ground and will blow a fuse or trip a circuit breaker to protect the user instead of putting the chassis at the high voltage. If you touched a chassis that had a high voltage with respect to ground on it, your body completes the path to ground and you get shocked!

So to protect the user (and for some other reasons), the chassis of Agilent power supplies are grounded internally through the ground wire (the third wire) in the AC input line cord. Additionally, most if not all of our Agilent power supplies have isolated (floating) outputs. That means that neither the positive output terminal nor the negative output terminal is connected to earth (chassis) ground. See Figure 2.

Figure 3 shows an example of non-floating outputs with the negative output terminal grounded.

For floating DC power supplies, the voltage potential appears from the positive output terminal to the negative output terminal. There is no voltage potential (at least, none with any power behind it) from either the positive terminal to earth ground or from the negative output terminal to earth ground. A power supply with a floating output is more flexible since, if desired, either the positive or negative terminal (or neither) can be connected to earth ground. Some devices under test (DUT) have a DC input with either the positive or negative input terminal connected to earth ground. If one of the power supply outputs was also internally connected to earth ground, when connected to the DUT, it could short out the power supply output. So power supplies with floating output terminals (no connections to earth ground) are more versatile.

If the outputs are floating from earth ground, we need to specify how far above or below earth ground you can float the output terminals. Our power supply documentation provides this information. For example, most Agilent power supply output terminals can float to +/-240 Vdc off of ground. You will frequently see the following in our documentation:

Also, some power supplies have different float ratings for the positive and negative output terminals. For example, for Agilent N5700 models rated for more than 60 Vdc, the following note in the manual means you can float the positive output terminal up to +/-600 Vdc from ground or the negative output terminal up to +/-400 Vdc from ground:

The output characteristic table may list this as “Output Terminal Isolation” as shown below which means the same thing as maximum float voltage:

Figure 4 shows an example of floating a power supply to 200 V above ground. The power supply output is set to 40 V.

You can see from the last example that you have to take the power supply output voltage into consideration when ensuring you are not violating the float voltage rating. If you exceed the float voltage rating of the power supply, you are potentially exceeding the voltage rating of internal parts that could cause the internal parts to fail or break down and present a shock hazard, so don’t violate the float voltage rating!

Monday, March 10, 2014

Upcoming software release unleashes the N7900 APS’s potential without any programming

Our N7900A Advanced Power System (APS) is well named, being the most advanced power product we’ve introduced to date. In many ways it is based on our N6700 series modular DC power system and N6705B DC power analyzer, incorporating their capabilities, including:
  • High precision programming and measurement
  • Seamless measurement ranging
  • High speed measurement digitization of voltage, current, and power
  • Long term data logging of voltage, current, and power
  • Output ARB and List capabilities
  • And quite a bit more

In addition the N7900A APS brings quite a few new and unique capabilities as well, including:
  • Much greater output power
  • Logic-configurable expression signal routing for advanced custom triggering and control
  • Optional external dissipater unit for full two quadrant operation
  • Optional black box recorder for post-test diagnostics when needed
  • And quite a bit more

To take advantage of these advance capabilities does require a bit of programming, which is to generally be expected for an automated test environment, but in low volume design validation and R&D this can slow down the desired quick time-to-result. The N6705B DC Power Analyzer, in Figure 1, has a full-featured front panel menu and graphical display that lets design validation and R&D users quickly configure and run complex power-related tests on their devices. In comparison, the N6700 series, pictured in Figure 2, does not have all the front panel capabilities of the N6705B and can be looked on as the ATE version of this product platform, requiring programming to take advantage of its advanced capabilities. The N6705B shares all the same DC power modules that the N6700 series uses.

Figure 1: The N6705B DC Power Analyzer, primarily for bench use

Figure 2: The N6700 series Modular DC Power System, primarily for ATE

The N7900A APS is very similar in form and function to the N6700 series, not having all the advanced front panel capabilities that the N6705B has for bench-friendly use of its advanced features. I am really pleased to be able to share with you that this is now changing! While we are not creating a bench version of the N7900 APS, we are upgrading our 14585A Control and Analysis software, which emulates the front panel of an N6705B and more, to work with the N7900 APS as well. The 14585A will soon let you quickly and easily create and configure complex power-related tests based on using the N7900 APS.  I am fortunate enough to be working with a beta version of the software. Some examples of things I was able to do in just a few minutes were to capture the inrush current of an automotive headlight, shown in Figure 3, and superimpose an AC sine wave disturbance on top of the DC output, shown in Figure 4.

Figure 3: Auto headlamp inrush current captured with 14585A software and N7951A APS

Figure 4: Sine wave voltage disturbance on top of DC generated by 14585A software and N7951A APS

The updated release of the 14585A Control and Analysis software is just a few weeks away. More about the 14585A software can be found by clicking on the following link <14585A>With the 14585A being a great way to implement ideas and tests quickly, using the N7900 APS, look for me and others coming up with some interesting applications in future posts here on “Watt’s Up?”!

Friday, February 28, 2014

R, L, C measurements with an AC source

I recently had a customer ask if there was a way to use one of our AC sources to determine whether the load connected to its output was capacitive or inductive. Agilent’s 6811B, 6812B, and 6813B AC power source/analyzers are all capable of making many different measurements, several of which can be used to calculate the impedance of the connected load. To determine the impedance, the simplest measurements to use are the amplitude and phase measurements of the applied sine wave voltage and resultant current. Agilent’s AC sources can measure harmonic content of the voltage and current, both amplitude and phase, up to the 50th harmonic. From these measurements, you can calculate the impedance of the R, L, or C connected to the AC source output.

I grabbed a few sample parts from our lab area to demonstrate these measurements. First, I used a power resistor that was about 49 ohms. I applied a sine wave of about 20 Vac at 1000 Hz (0 phase) and used the built-in measurement capability to measure about 0.4 Aac at a phase (angle) of very near 0 degrees (the measurement was -7.01E-2 = -0.071 degrees). Of course, 0 degrees of phase between the voltage and current means the sine waves are in phase and the load is resistive as expected. See Figure 1.

The next test I did was with an inductor. Connecting it to the output of the AC source and once again applying about 20 Vac at 1000 Hz from the AC source, using the built-in measurement capability, I measured about 0.129 Aac at a phase of -88.66 degrees. The phase measurement of nearly -90 degrees confirmed that the load on the AC source was an inductor and the magnitude could be calculated to be about 25 mH which is what I expected (I measured it using an Agilent LCR meter). The series resistance in the line cord, clip leads, and inductor wire itself (I did not use remote sense) accounted for the non-ideal phase of -88.66 degrees instead of the ideal phase of -90 degrees. And the R was calculated from the AC source measurements to be about 3.6 ohms and agreed with external verification. See Figure 2.

Finally, I connected a capacitor to the AC source output and applied about 10 Vac at 1000 Hz. The AC source measurement system showed 0.633 Aac at an angle of 87.47 degrees indicating a capacitor was across its output. From these measurements, the capacitance and series R were calculated to be 9.88 uF and 0.711 ohms, consistent with externally verified measurements. See Figure 3.
So you can see that it is possible to determine the impedance (resistance, capacitance, or inductance) of a device connected across the output of an AC source when the right measurement capabilities are built into the AC source, as they are with the Agilent AC sources. These highly capable products not only make measurements like this easy, they also can easily create a large variety of AC output stimulus waveforms.

Use Agilent's BenchVue Software to Save Time

Hi everyone!

I wanted to take the time I have in this month’s blog post to tel you about a great new tool that we have for you here at Agilent.  It is a new software package called BenchVue.  BenchVue offers users the ability to control their power supplies, oscilloscopes, spectrum analyzers,  DMMs, and function generators using their computer without the need to write a program.  It is a free download.  You can download BenchVue at:

Agilent BenchVue Software

Naturally I am going to talk about the power supply potion of BenchVue (this is a power supply blog after all).  Presently the software supports the Agilent N6700 Modular Power Supplies and the Agilent E36xxA Basic Power Supplies.  I do not know if many people remember but we used to have a free software package for the E36xxA power supplies called Intuiilink.   BenchVue is  the modern successor to Intuiilink.  I recently checked BenchVue out on my E3646A DC Power Supply.  

The E3646A is a basic two output DC Power Supply.  BenchVue communicates with it using GPIB (in my case my Agilent 82357 USB/GPIB converter).  Take a look at the below picture:

You can program all of the basic settings (even for multiple channels) on your power supply on one screen.  If you used the front panel to do all of these entries it would take quite a while to navigate through all of the menus.  BenchVue saves you time by putting all of this in one place.  The other neat thing that I noticed is that you can get the voltage and current readback from both channels on the screen at the same time.  You can't see both channels on the front panel, you have to manually switch.

The other main feature of BenchVue that can save you some time is the built in datalogger.  For those of you that are unfamiliar with the term datalog, this is when an instrument takes measurements at a predetermined time interval and stores them in a file that you can look at later.  With BenchVue, you can set a datalog up to take a measurement as fast as every second.  You can then take the stored values and export them to Matlab, Microsoft Excel, Microsoft Word, or to a CSV file.  

You could write your own datalog program but the beauty of BenchVue is that you can set the log up by entering a few values and then just press the "Run" button to make it go.  You also do not have to worry about formatting the data or creating files since BenchVue does all of that for you.  You basically just run it, then export the data and then you can view it in your chosen format.

Here is a sample thirty second datalog where I logged the voltage on Output 1:

After it was complete, I just hit the "Export" button, chose Microsoft Excel and I got the following:

Pretty cool!

That's my short intro to the new Agilent BenchVue software.  Go download it.  Let us know any thoughts you have in the comments section.

Monday, February 24, 2014

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!

Thursday, February 20, 2014

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!