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!