Wednesday, July 31, 2013

What is Dynamic Current Correction?

Gary and I were talking to one of the design engineers here yesterday about what he worked on recently that might make a good blog post.  We wound up talking about dynamic current correction.  This is an option for the current measurement systems of some of our power supplies.  In order to explain its purpose, let us start with a simplified picture of one of our power supplies:


If you look at the above figure, the current monitor resister is inboard of the output capacitor.   This means that our current measurement system is going to measure both Iout and Ic when we take a current measurement.  Ic is not in any way being sent to the output of the power supply and the DUT will never see this current, the DUT will only see Iout.    We wanted to provide a way that you can see the actual current that is going through the DUT so we offered the Dynamic Current Correction option in our current ranges.  

Since we are talking about a capacitor here, remember that the current through a capacitor equals the capacitance multiplied by the change in voltage over time (I = C * dv/dt).  If you are making a measurement at a DC voltage level, then there is no current through your capacitor since your dv/dt is near zero.  When you have a rapidly changing voltage waveform you can have a large dv/dt and your Ic will be a non-zero number.    A good rule of thumb would be that you want to use the dynamic current correction when you have a changing voltage and you want to turn dynamic current correction off when you have a DC voltage due to reasons that we will get into later.

In the below screenshot from my DC Power Analyzer I am operating an N6762A module set to go from 0 to 50 V with nothing connected to the output.  I do not have the Dynamic Current Correction range selected.


You can see here that the measured current goes up to 1 A even though the output is completely open therefore limiting any current flow.  That current is all flowing through the output capacitor due to the dv/dt of going from 0 to 50 V.  In this screenshot, you are seeing all Ic from the diagram above since Iout is 0.  This is not representative of the DUT current.  In this case we are going to want to use Dynamic Current Correction. 

Keeping everything set the same on the supply I turned the Dynamic Current Correction on and I measured the following waveforms:


As you can see, with Dynamic Current Correction turned on, the effect that the capacitor current has is much less noticeable. With a changing voltage, you definitely want to have this enabled.  

When Dynamic Current Correction is on, the power supply is using the capacitor equation (I= C* dv/dt) to calculate what the capacitor current is and then subtracting the calculated value out of the measured current.  This is a more accurate representation of the output current flowing through the DUT (Iout in the first picture).  There are tradeoffs though.  In some models dynamic current correction will increase the peak to peak current measurement noise and it can also limit the output measurement bandwidth.  These factors are the reason why you should turn it off when you are operating at DC voltages. 

The moral of this blog post is that you want to use the Dynamic Current Correction when you have a rapidly changing voltage and not use it when you have a static voltage.  Please let us know if you have any questions.

Tuesday, July 30, 2013

Power analysis of automobile self-charging emergency tool

I was recently given a “Swiss+Tech BodyGard Survivor 8-in-1 Automobile Self-Charging Emergency Tool”. How’s that for a compact name? This device does have many features, so I imagine the company had some difficulty devising a name for it. It is meant to be carried in your car and kept close enough to the driver to be used in an emergency. It contains a glass breaker, a seatbelt cutter, a flashlight, an emergency flasher and siren, an AM/FM radio, and rechargeable NiCad batteries that charge by using the self-charging hand crank. See Figure 1.
Since this device contains rechargeable batteries and Agilent makes instrumentation that can do battery drain analysis, I figured I would test the device using our equipment. I used an Agilent N6705B DC Power Analyzer loaded with an N6781A 2-Quadrant Source/Measure Unit (SMU) for Battery Drain Analysis. See Figure 2.

The product’s instruction sheet includes information about the batteries (700 mAH) and the expected battery run time when using the various features. With fully charged batteries, the expected battery run time for each of the features listed below is:

  • Flashlight: 12 to 16 hours
  • Flasher: 10 to 12 hours
  • Radio (low volume): 35 to 40 hours
  • Flasher/siren: 6 to 9 hours

Given the battery amp-hour rating (700 mAH) and the expected run time in hours, we can calculate the approximate expected average current draw for each of the various features:

  • Flashlight: 700 mAH / 14 hours = 50 mA
  • Flasher: 700 mAH / 11 hours = 63.6 mA
  • Radio: 700 mAH / 37.5 hours = 18.7 mA
  • Flasher/siren: 700 mAH / 7.5 hours = 93 mA

Using the N6781A SMU and the built-in front panel features of the N6705B DC Power Analyzer, I was able to analyzer the current drawn from the batteries when using each of the features. Each feature was used by itself with the other features turned off.

The flashlight draws a steady-state current that I read right from the front panel meter as shown in Figure 3: 50 mA. This agrees perfectly with the expected current draw I calculated. For this measurement and all subsequent current measurements, I connected the N6781A in series with the batteries and set it to Current Measure mode where it acts likes a zero-burden shunt. The measured current is negative in my setup because positive current is current flowing into the battery and with the flashlight on, current is flowing out of the battery.
For the flasher, since the current is not constant, I used the N6705B/N6781A built-in data logger feature and captured 30 seconds of data while the device was flashing. I then used the markers to measure the average current. Since the flasher flashes for a very short period of time (low duty cycle), I expected the average current to be low. When using the flasher, the expected battery run time seemed unusually short to me. At 10 to 12 hours, it is shorter than the flashlight or radio run time, which seems odd. In reality, as shown in Figure 4, the flasher drew very little current (5.6 mA), so it appears that the instruction sheet run time for the flasher is too low. With the device flashing, the battery will last much longer than indicated. In fact, the expected battery run time, when flashing, is about 700 mAH / 5.6 mA = 125 hours, 10 times longer than the time shown on the instruction sheet!

With the radio on, tuned to a station, and set to a low but audible volume, I once again used the data logger to capture the current. The markers show an average current of about 10 mA, which is less than the calculated value of 18.7 mA, but within reason. See Figure 5.

Using the flasher and siren, the data logger shows a current draw of 93 mA, in exact agreement with the expected current draw calculated from the numbers on the instruction sheet. See Figure 6.

The last current analysis I did was to capture 30 seconds of data logging when turning the self-charging crank to recharge the batteries. I purposely varied my cranking rate to see what would happen. Figure 7 shows an average of about 350 mA when turning the crank at what I considered to be a typical rate (highest average numbers on the captured data log). To fully charge 700 mAH batteries, it would take about 2 hours at that rate, which is in agreement with the instruction sheet (it says 2 to 3 hours). I don’t know about you, but I don’t want to turn that crank for 2 hours straight! Let’s hope I never have to use the tool for real, but I’m glad I have it just in case!




Wednesday, July 17, 2013

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!

Friday, July 12, 2013

Why have multiple output range DC power supplies?

Most often DC power supplies have a rectangular output characteristic, as depicted in figure 1. With an increasing load they output a fixed output voltage up to the current limit, at which point the voltage drops in order to maintain the current fixed at its limit.



Figure 1: DC power supply rectangular output characteristic.

There is however DC power supplies that offer multiple output ranges. One example of a multiple (dual in this case) output range DC power supply is our N678xA series DC source measure modules. Their output characteristics are depicted in Figure 2.



Figure 2: Agilent N678xA series source measure modules output characteristics

Unlike the output characteristic of a single output range DC power supply, you cannot get both the maximum current and maximum voltage of a multiple output range DC power supply at the same time.

What is the purpose of having multiple output ranges on a DC power supply?
There are times, especially when having to test a variety of devices, the need for greater current or voltage, but not necessarily needing both maximum voltage and current at the same time.  In these situations many times these test power needs are better served by a DC power supply having multiple output ranges. The advantages of a multiple output range DC power supply are smaller size, less power dissipation, and less input power required, in comparison to a single output range DC power supply of comparable voltage and current capability. If the N678xA series DC source measure modules had a single output range they would need to have a 60 watt output to cover the span of voltage they now provide with 20 watts of output power.  An even more extreme example is our B2900 series source measure units. They output up to 31.8 watts continuously, but can provide up to 210 volts and up to 3.03 amps over three output ranges.

The downside of having multiple output ranges is somewhat greater complexity. Figure 3 depicts a conceptual design for a dual output range DC power supply. 



Figure 3: Conceptual dual output range DC power supply

Because the transformer efficiently converts AC power by square of its turn ratio there is very little impact on its size to accommodate secondary windings with multiple taps or multiple secondary windings that can be alternately connected in series or parallel, in order to accommodate multiple output power ranges. Similarly, the linear series pass element dissipates about the same maximum power whether it is operating at a higher voltage with lower current, or at a lower voltage with a higher current.  

The end result is a multiple range DC power supply can provide a greater range of voltages and currents for a given output power at the expense of a little greater complexity. Often this is far preferable to the alternative of a much higher power, and larger single output range DC power supply!

Sunday, June 30, 2013

What is Command Processing Time?

Hello everybody,

We have a new intern here at Agilent Power & Energy HQ named Patrick.  Gary, Patrick, and I have been having a philosophical debate on what the term command processing time means.  This is a very important number for many of our customers since it tells them what kind of throughput they can get out of our test equipment.  A fast command processing time allows you to reduce your test times and therefore increase your throughput.  The question that we have been debating is:  what is command processing time and how can we measure it?  We have been discussing three scenarios.   Let’s go through them.

The first option is the amount of time that it takes the processor to take one command off the bus so that it can get to the next command.  This tells you how quickly you can send commands to the instrument.  The only issue with this is that some instruments have a buffer so it is not actually “processing” the command, just bringing it into the buffer and letting you send the next command.  Obviously this is useful but it really does not address the throughput question.  This is pretty easy to test by sending a command in a loop and timing it.  You record the time before the command is sent and the time after the loop and then divide by the number of loops you executed.  This would yield a pretty good approximation of the time.

The second option is the amount of time from when the instrument receives a command until it starts performing the action.  I believe that this is what we list in our manuals for the Command Processing Time Supplemental Characteristic.  This does address the throughput issue.  This is also easy for us at Agilent to measure.  We have a breakout for GPIB that allows us to monitor the attention line.  The test that we did was send a VOLT 5 to the instrument.  We looked at the GPIB attention line.  The time from when the attention line toggles until the power supply starts slewing the voltage up would be our command processing time (measured with an always awesome Agilent Oscilloscope).  This is what I consider to be the command processing time.

The third option includes what I spoke about in the last paragraph but also includes the slewing of the voltage.  The processing time would be the time that it takes to take the command and complete all the actions associated with it (for example settling at 5 volts after being sent a VOLT 5 command).  I do not think that this is a bad option but we have a Supplemental Characteristic for voltage rise time that addresses the slewing of the voltage.   The test method would be the same as above using an oscilloscope but watching for where the voltage settles at five volts.


What do you, our readers and customers think the correct interpretation of command processing time is?  Also, please stay tuned for a future installment where we try to figure out what the quickest interface is: LAN, USB, or GPIB.  

Tuesday, June 25, 2013

Current limit setting affects voltage response time

The current limit setting in a power supply is primarily used to protect the device under test (DUT) from excessive current. You should set your current limit setting higher than the maximum amount of current you expect your DUT to draw, but low enough so that if your DUT fails as a short or low impedance, it does not draw an amount of current that can damage wires, connectors, or the DUT itself due to excessive current. The power supply will limit the current at the current limit setting and reduce the voltage accordingly. If you want, you can turn on over-current protection (OCP) and then the power supply output will turn off if the output transitions into constant current (CC) mode. For previous posts on this topic, click here and here.

Current limit plays an important role in protecting your DUT. But you should also know that the current limit setting can affect the voltage response time, specifically the up-programming speed. Voltage up-programming speed is the time it takes the output voltage to go from a lower voltage to a higher voltage. For example, the up-programming output response time for an Agilent N5768A power supply (rated for 80 V, 19 A, 1520 W) is specified to be no more than 150 ms with a full load (settling band is 1% of the rated output voltage). This spec assumes the current limit is set high enough to not limit the current. The output capacitor of this power supply will draw current as the voltage on the cap rises (Ic = C * dVc/dt). The output current and the cap current flow through the current monitoring resistor which is where the current is measured and compared to the current limit setting. See Figure 1. Therefore, the output cap current adds to the output current and can cause the power supply to momentarily go into CC mode as the output cap charges. If this happens, the output voltage will rise more slowly than if the power supply stayed in constant voltage (CV) mode the entire time the output voltage was rising and charging the output cap.

So, the current limit setting can slow down the voltage response time if set too low causing the power supply to momentarily go into CC mode as the output voltage is rising and the output cap is charging. This effect is shown in Figure 2 for various current limit settings on the N5768A power supply. As you can see, the lower the current limit setting (Iset), the longer it takes for the voltage to reach its final value.


If fast up-programming response time is important to you in your power supply application, make sure you set your current limit high enough to provide current to your DUT and to charge the power supply’s output capacitor without going into CC mode. Once the output voltage reaches its final value, you can always lower the current limit again to properly protect your DUT.

Thursday, June 20, 2013

How can I measure output impedance of a DC power supply?

In my last posting “DC power supply output impedance characteristics”, I explained what the output impedance characteristics of a DC power supply were like for both its constant voltage (CV) and constant current (CC) modes of operation. I also shared an example of what power supply output impedance is useful for. But how does one go about measuring the output impedance of a DC power supply over frequency, if and when needed?

There are a number of different approaches that can be taken, but these days perhaps the most practical is to use a good network analyzer that will operate at low frequencies, ranging from 10 Hz up to 1 MHz, or greater, depending on your needs. Even when using a network analyzer as your starting point there are still quite a few different variations that can be taken.

Measuring the output impedance requires injecting a disturbance at the particular frequency the network analyzer is measuring at. This signal is furnished by the network analyzer but virtually always needs some amount of transformation to be useful. Measuring the output impedance of a voltage source favors driving a current signal disturbance into the output. Conversely, measuring the output impedance of a current source favors driving a voltage signal disturbance into the output. The two set up examples later on here use two different methods for injecting the disturbance.

The reference input “R” of the network analyzer is then used to measure the current while the second input “A” or “T” is used to measure the voltage on the output of the power supply being characterized. Thus the relative gain being measured by the network analyzer is the impedance, based on:
zout = vout/iout = (A or T)/R
The output voltage and current signals need to be compatible with the measurement inputs on the network analyzer. This means a voltage divider probe may be needed for the voltage measurement, depending on the voltage level, and a resistor or current probe will be needed to convert the current into an appropriate voltage signal. A key consideration here is appropriate scaling constants need to be factored in, based on the gain or attenuation of the voltage and current probes being used, so that the impedance reading is correct.



Figure 1: DC power supply output impedance measurement with the Agilent E5061B

One example set up using the Agilent E5061B network analyzer is shown in Figure 1, taken from page 15 of an Agilent E5061B application note on testing DC-DC converters, referenced below. Here the disturbance is injected in through an isolation transformer coupled across the power supply output through a DC blocking capacitor and a 1 ohm resistor. The 1 ohm resistor is doing double duty in that it is changing the voltage disturbance into a current disturbance and it is also providing a means for the “R” input to measure the current. The “T” input then directly measures the DC/DC converter’s (or power supply’s) output voltage.

A second, somewhat more elaborate, variation of this arrangement, based on using a 4395A network analyzer (now discontinued) has been posted by a colleague here on our Agilent Power Supply forum: “Output Impedance Measurement on Agilent Power Supplies”. In this set up the disturbance signal from the network analyzer is instead fed into the analog input of an Agilent N3306A electronic load. The N3306A in turn creates the current disturbance on the output of the DC power supply under test as well as provide any desired DC loading on the power supply’s output. The N3306A can be used to further boost the level of disturbance if needed. Finally, an N278xB active current probe and matching N2779A probe amplifier are used to easily measure the current signal.

Hopefully this will get you on your way if the need for making power supply output impedance ever arises!


Reference: “Evaluating DC-DC Converters and PDN with the E5061B LF-RF Network Analyzer” Application Note, publication number 5990-5902EN (click here to access)