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!

Techniques for using the Agilent N6781A and N6782A and their seamless measurement ranging when currents exceed 3 amps

In an earlier posting “Zero-burden ammeter improves battery run-down and charge management testing of battery-powered devices” (click here to access) I had talked about how the Agilent N6781A 2-quadrant SMU can alternately be used as a zero-burden ammeter. When placed in the current path as a zero-burden ammeter, due to its extended seamless measurement ranging, it can measure currents from nanoamps, up to +/-3 amps, which is the maximum limit of the N6781A. The N6782A 2-quadrant SMU can also be used as a zero burden ammeter. It is basically the same as the N6781A but with a few less features.

One customer liked everything about the N6782A’s capabilities, but he had a battery-powered device that drew well over 3 amps when it was active. When in standby operation its current drain ranged back and forth between just microamps of sleep current to 6 or greater amps of current during periodic wake ups. The N6782A’s +/- 3 amps of current was not sufficient to meet their needs.

An alternate approach was taken that worked out well for this customer, which was made possible only because of the N6782A’s zero-burden ammeter capability. The set up is shown in Figure 1.

Figure 1: Setup for measuring micro-amps in combination with large active-state currents

The N6752A 50V, 10A, 100W autoranging DC power module provides all the power. The N6782A is set up as a zero-burden ammeter and is connected in series with the N6752A’s output. When current ranges from microamps up to +/- 3 amps the N6782A maintains its zero-burden ammeter operation, holding its output voltage at zero. Once +/- 3 amps is exceeded, the N6782A goes into current limit and the voltage increases across its output, at which point one of the back-to-back clamp diodes turns on, conducting current in excess of 3 amps through it. This all can be observed in the screen image of the 14585A software in Figure 2. The blue trace is the N752A’s output current. The middle yellow trace is the N6781A’s current and the top yellow trace is the N6781A’s voltage.

Figure 2: Current and voltage signals for Figure 1 setup captured with 14585A software

In Figure 2 measurement markers have been placed across a portion of the sleep current and we find from the N6782A’s measurement readback it is just 1.458 microamps average. The reason why this works is because of zero burden operation. Because the N6782A is maintaining zero volts across its output, there is no current flowing through either diode. If this same thing was attempted using a conventional ammeter or current shunt, the voltage would increase and current would flow through a diode, corrupting the measurement.

Now the customer was able to get the microamp sleep current readings from the N6782A and at the same time get the high level wake up current readings from the N6752A!

In a similar fashion another customer wanted to perform battery run down testing. Everything was excellent about using the N6781A in its zero-burden ammeter mode, along with using its independent DVM input for simultaneously logging the battery’s run down voltage in conjunction with the current. The only problem was they wanted to test a higher power device. At device turn-on, it would draw in excess of 3 amps, which is the current limit of the N6781A. Current limit would cause the N6781A to drop out of its zero-burden ammeter operation and in turn the device would shut back down due to low voltage. The solution was simple; add the back-to-back diodes across the N6781A acting as a zero-burden ammeter, as shown in Figure 3.  Any currents in excess of 3 amps would then pass through a diode. Schottky diodes were used so the device would momentarily see just a few tenths of a volt drop in the battery voltage, during the short peak current in excess of 3 amps. Now the customer was able to perform battery run-down testing using the N6781A along with the 14585A software to log all the results!

Figure 3: Agilent N6781A battery run-down test set up, with diode clamps for peak currents above 3A

New Firmware Available for the N6700B, N6701A, and N6702A Modular Power System Frames.

Hello everybody!

I recently posted a new firmware file for the N6700B, N6701A, and N6702A mainframes to the Agilent website.  You can access the new firmware at: https://www.agilent.com/find/N6700firmware.  The latest firmware revision is D.01.09.  There are two new measurement features that I wanted to highlight. s These two new features are External Datalogging and Power Measurements. 

External Datalogging (ELOG for short) is a feature that we have had in the N6705 DC Power Analyzer for some time now that we have just added to the modular power mainframes.  This feature allows you to take averaged measurements at a specified time interval for however long you want (be careful though, you can fill your hard drive).  The time interval can be anywhere from 102.4 us to 60 s depending on the number of parameters being logged.  You can take those measurements and store them in whatever format you want (I usually store everything in a CSV file).  You can only access ELOG using a SCPI program (you cannot ELOG from the front panel).  I plan on writing more about ELOG in the future but here is a quick peek of what the SCPI commands to set up and execute an ELOG look like:

SENS:ELOG:FUNC:CURR ON,(@2)                     !Turn current logging on

SENS:ELOG:FUNC:CURR:MINM ON,(@2)        !Turn current min/max logging on

SENS:ELOG:FUNC:VOLT ON,(@2)                     !Turn voltage logging on

SENS:ELOG:FUNC:VOLT:MINM ON,(@2)        !Turn voltage min/max logging on

SENS:ELOG:PER 0.0007,(@2)                            !Sets an integration time of 700 us

TRIG:ELOG:SOUR BUS,(@2)                              !The ELOG will start when there is a bus trigger

INIT:ELOG (@2)                                                   !Tell the unit to wait for a trigger

*TRG                                                                      !Trigger

This is an example of how you would read back the logged data:

while current time<the time you want to log for

FETC:ELOG? 4096 ,(@2)                                    !Read back a maximum of 4096 ELOG records

store into a file

loop

This command kills the ELOG:

ABOR:ELOG (@2)                                              !Return the unit to its normal state

Stay tuned to this blog for more information.

The other feature that we added to a few of our modules was the ability to measure power.  We can now measure power on the N676xA, N6781A, N6782A, and N6784A modules.  Why is it only on these few modules you ask?  That is because these modules have two measurement digitizers that allow it to measure both voltage and current at the same time.  Since power = voltage * current, you need to have a simultaneous voltage and current measurement to get an accurate power measurement. 

That is all I have for today.  If you have any questions, please just let us know.