Monday, June 30, 2014

Doing Inrush Current Testing with the New AC6800 AC Sources

Hi everybody,

It is the last day of the month and therefore time for me to get my blog post out.  I want to build on Gary's latest post concerning the new Agilent/Keysight AC6800 AC Sources (Click Here).  One of the key features that Gary mentioned is inrush current testing.

My colleague Russ did a video on inrush current testing for the launch.  This gives you a little bit of perspective on why you want to do the testing and gives some good tips.

When you do inrush current testing, you typically want the highest value that the current has reached when the power is enabled.  The AC6800 has a peak hold current value that will store this value for you.  The unit stores the highest current value it has measured since that value was last cleared (either manually or from power on).  One key thing to remember is to always clear out the peak hold value before doing your measurement so that you know that your measurement is up to date.

The AC6800 can synchronize the enabling of the output to a user defined phase.  When you specify the phase, it will enable the output at that phase in the sine wave (anything from 0 to 360 degrees).  The combination of the peak hold measurement and this phase synchronization are what make this testing possible.  

I  did a video for the launch where I did a tour of the front panel, including a short description of how to do inrush current testing:


I also have a programming example on this topic.  Below is a snippet of a program that I wrote in VB.NET using Agilent VISA-COM:

That's about it for me this month.  Please let us know if you have any questions in the comments.  

Monday, June 23, 2014

New Agilent Basic AC Power Sources

I have mentioned several times before that I avoid posting product-only-focused material in this blog, but when we announce something new, it is appropriate for me to mention it here. Today, a press release went out about our new AC sources (click here to view). You may not realize it, but this press release marks the end of an era; these are the last power products Agilent Technologies will ever announce! Now don’t go all non-linear on me…..I’m sure we will continue to design and release new power products for decades to come. But as I mentioned in a previous post (click here), as of August 1, 2014, our products will be Keysight Technologies products and not Agilent Technologies products. So these new AC sources will be rebranded to Keysight in a few weeks, but because we are releasing them before the company name change is official, we have to release them as Agilent and not Keysight. Go figure….

Anyway, what are these new Agilent (soon to be Keysight) AC sources? Well, the model numbers will remain the same through the company name change and they are:

  • AC6801A (500 VA)
  • AC6802A (1000 VA)
  • AC6803A (2000 VA)
  • AC6804A (4000 VA)

This new AC6800 Series of basic AC sources compliments our previous line of more sophisticated AC sources (click here for those) by adding lower cost models and higher power. Here is what the new series looks like (of course, the big one is the 4 kVA model):
All four new AC6800 models share these features:
  • Output capabilities
    • Single-phase output
    • 2 ranges: 0 to 135 Vrms; 0  to 270 Vrms
    • 40 Hz to 500 Hz and DC
    • Sine wave (other waveforms with analog interface)
  • Measurement capabilities
    • Vac, Vdc, Vrms
    • Iac, Idc, Irms, Ipeak, Ipeak&hold, crest factor
    • Watts, VA, VAR, power factor
  • Other
    • Universal AC input
    • LAN (LXI-Core), USB, optional GPIB
    • Optional analog programming interface
The differences in the models are due to the output power ratings and can be summarized by looking at the output characteristics when producing an AC output or a DC output:

For a DC output, the graph above shows only the positive voltage and current quadrant (first quadrant). The output is equally capable of putting out negative voltage and negative current (the third quadrant) and the ratings are the same (except negative). These AC sources only source power; they cannot sink (absorb) power.

These AC sources do have one advanced feature: you can set the phase angle at which the output turns on. Coupled with the ability to measure peak current (and hold the peak current measurement), this is good for AC inrush current measurements.
To view the data sheet, click here.

So that’s the new line of basic AC power sources from Agilent and the last power products to be announced by Agilent. I wonder when the first Keysight power product announcement will be…..wouldn’t you like to know!?!?

Safeguarding your power-sensitive DUTs from an over power condition

Today’s system DC power supplies incorporate quite a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most all system DC power supplies to help protect against power-related damage.

OVP helps assure the DUT is protected against power-related damage in the event voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous the OVP level is set at reasonable margin below this level to be effective, yet is suitably higher than maximum expected DUT operating voltage so that any transient voltages do not cause false tripping. Causes of OV conditions are often external to the DUT.

OCP helps assure the DUT is protected against power-related damage in the event it fails in some fashion causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current from consuming excess power due to overloading or internal problem causing inefficient operation and excessive internal power dissipation.

OVP and OCP are depicted in Figure 1 below for an example DUT that operates at a set voltage level of 48V, within a few percent, and uses about 450W of power. In this case the OVP and OCP levels are set at about 10% higher to safeguard the DUT.

Figure 1: OVP and OCP settings to safeguard an example DUT

However, not all DUTs operate over as limited a range as depicted in Figure 1. Consider for example many, if not most all DC to DC converters operate over a wide range of voltage while using relatively constant power. Similarly many devices incorporate DC to DC converters to give them an extended range of input voltage operation. To illustrate with an example, consider a DC to DC converter that operates from 24 to 48 volts and runs at 225W is shown in Figure 2. DC to DC converters operate very efficiency so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC to DC converter causing it to run inefficiently it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, as can be seen it does not work well to protect the DUT for over power at higher voltage levels.

Figure 2: Example DC to DC converter input V and I operating range

A preferable alternative would instead be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of input voltage setting.

Figure 3: Example DC to DC converter input V and I operating range with over power protect

As an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient or desirable, or in some cases practical to do. However, in the Agilent N6900A and N7900A Advance Power System DC power supplies it is possible to continually sense the output power level in the configurable smart triggering system. This can in turn be used to create a logical expression to use the output power level to trigger an output protect shutdown. This is depicted in Figure 4, using the N7906A software utility to graphically configure this logical expression and then download it into the Advance Power System DC power supply. As the smart triggering system operates at hardware speeds within the instrument it is fast-responding, an important consideration for implementing protection mechanisms.

Figure 4: N7906A Software utility graphically configuring an over power protect shutdown

A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown it could have been just as easily used to trigger a variety of other actions as well.

Tuesday, June 3, 2014

Upcoming Webinar on High Power Source/Sink Solutions for Testing Bidirectional Energy Devices

Bidirectional and regenerative energy devices both source and sink power and energy. Correspondingly, a solution that can both source and sink power and energy is needed for properly testing them. In the past here on “Watt’s up?” we have talked about what two and four quadrant operation is in our posting “What is bipolar four quadrant power? (Click here to review). We have also talked about cross over behavior between sourcing and sinking current with a DC source that will operate in two quadrants in a two-part posting  “Power supply current source-to-sink crossover characteristics” (Click here to review pt. 1) and (Click here to review pt. 2). These give useful insights about the nature of multi-quadrant solutions for bi-directional test applications.

Figure 1: The four operating quadrants

Bidirectional and regenerative energy devices that are used in many applications, such as satellite power systems, alternative energy, automotive, and many other areas, operate at kilowatt and higher power levels. These higher power levels have a significant impact on solutions and approaches taken to address their testing.  Also, the nature of these bidirectional and regenerative energy devices are not all the same. This also has an impact in that the capabilities of the test solutions need to be different to address these different types of devices.

In my upcoming webinar on June 18th, titled “Conquering the High Power Source/Sink Test Challenge” I will be exploring the test needs of key bidirectional and regenerative energy devices and then go into the details of various test solutions and approaches for sourcing and sinking power and energy, along with their associated advantages and disadvantages. This is just a couple of weeks away. So if you are involved in this kind of work and are interested, or would just like to learn more, you can register online at the following (click here).  In case you cannot join the live event you will still be able to register and listen to seminar afterward instead, as it will be recorded.  I hope you can join in!