New performance options for the N6900A Advance Power System gives greater versatility for your test needs

Our N6900 and N7900 series Advanced Power System (APS) DC power supplies are some of our most sophisticated products, setting new levels of performance and capabilities on many fronts. They come in 1kW and 2kW power levels as shown in Figure 1 and can be grouped together to provide much greater power levels as needed.

Figure 1: N6900 and N7900 Advanced Power System 1kW and 2kW models

Most noteworthy is that these can be turned into full two-quadrant DC sources by connecting up the optional 1kW N7909A Power Dissipator (2 needed for 2kW units) providing 100% power sinking capability. This makes APS an excellent solution for battery, battery management and many alternative energy applications, where both sourcing and sinking power are needed.

• The N6900 series DC power supplies are designed for ATE applications where high test throughput and high performance is critical.
• The N7900 series dynamic DC power supplies are designed for ATE applications where high speed dynamic sourcing and measurement is needed, in additions to high performance.

A lot more about these products is covered in another post on our General Purpose Electronic Test Equipment (GEPTE) blog when they were first announced. This is a great resource for learning more about APS and can be accessed from the following link: “New Advanced Power System: Designed to Overcome Your Toughest Test Challenges”

If you are a regular visitor to the “Watt’s Up?” blog no doubt you have seen we have shared a lot about how to do things with the N6900 series and N7900 series APS to address a number of difficult test challenges. A lot of times it would have otherwise required additional equipment or custom hardware to accomplish these tasks. While many of these examples are suitable for the N6900 and N7900, a good number of times examples make use of the additional capabilities only available in the N7900 series.

In certain test situations the N6900 series APS would be a great solution and lower cost than the N7900 series, if only it also had a certain additional capability. To this end Keysight has recently announced four new performance options for the N6900 series APS to address a specific test need you may have, as follows:

1. Accuracy Package (option 301): Adds a second seamless measurement range for current
2. Measurement Enhancements (Option 302): Adds external data logging and voltage and current digitizers with programmable sample rates
3. Source and Speed Enhancements (Option 303): Adds constant dwell arbitrary waveforms and output list capability, and faster up and down programming speed
4. Disconnect and Polarity-Reversal Relays (Option 760 and 761): Provides galvanic isolation and allows output voltage to be switched between positive and negative values

 Additional details about the N6900 series APS and the four new performance options are available from the recent press release, available at the following link: “Keysight Technologies adds Versatile Performance Options to Industry’s Fastest Power Supplies”

With these new options you now have a spectrum of choices in the Advanced Power System product family to better address any test challenges you may be faced with!

How fundamental features of power supplies impact your test throughput – Part 2

In part 1 of” How fundamental features of DC power supplies impact your test throughput” (click here to access) I shared definitions of some of the fundamental power supply features that impact test throughput, including:

• Command processing time
• Up-programming response time
• Down-programming response time

Another fundamental DC power supply feature impacting test throughput is its measurement time. There are actually two aspects to a DC power supply’s measurement time as depicted in Figure 1:

• Measurement settling time
• Measurement integration time

Figure 1: DC power supply measurement time

A good indicator of a DC power supply having a high performance measurement system is having programmable measurement integration time, or aperture time, often programmed in power line cycles (PLCs).  One reason for having a programmable integration time is for minimizing any 50 or 60 Hz AC line ripple getting into the DC measurement, by setting the time one or more multiples of a PLC.  Setting the time to 1 PLC provides good ripple rejection with relatively good throughput. When AC line ripple is not an issue the integration time can be set even smaller than 1 PLC, further reducing measurement time. When the DC power supply has a programmable measurement integration time it will no doubt also have a fast-responding measurement system as well, typically just milliseconds, to complement the higher achievable throughput with programmable measurement integration time.

In comparison basic DC power supplies commonly use a 100 millisecond fixed integration time to support AC ripple rejection for both 50 and 60 Hz line frequencies. They also have low bandwidth, slow-responding measurement systems, which can long time to settle after any step change in loading, before a valid measurement can be taken.

We have just introduced our Advanced Power System (APS) DC power supplies. This is a family of high-performance, high power (1 and 2 kW) DC power supplies designed to address the most demanding test challenges. These fundamental throughput-related features for APS are typically more than two orders of magnitude faster compared to more basic-performance DC power supplies, providing much better throughput in manufacturing test. A colleague of mine recently posted details of their introduction on his “General Purpose Electronic Test Equipment (GEPETE)” blog (click here to access) which I believe you will find of interest. Included in this introduction is a link on throughput that takes you to a series of application briefs I have written that go into more detail on improving test throughput with the DC power supply, which you may find very useful.

So how much test throughput improvement might you expect to see by switching from a basic-performance DC source to a high-performance DC source? Well, it really depends on how much the testing makes use of the DC power supply. If it only uses the power supply to provide a fixed DC bias to the device under test (DUT) that never changes for the duration of the test then it will not make a significant difference. More often than not however, a DUT is tested at several bias voltages with several current drain measurements taken for the various bias voltage settings and DUT operating modes. This can add up to a considerable amount of test time. In this case a high-performance DC power supply can more than pay for itself many times over due to improved test throughput.  To get an idea of the kind of difference a high-performance DC power supply can make I set up a representative benchmark test It compares the throughput performance one of our new APS DC power supplies to that of a more basic-performance power supply.  If you are interested in finding out how much difference it made, I made a video of this benchmark testing, entitled “Increasing Test Throughput with Advanced Power System” (click here to access). All I am going to say here is it is an impressive difference but you will need to watch the video to see how much difference!

How fundamental features of power supplies impact your test throughput – Part 1

When it comes to manufacturing of electronic products, reducing test time to improve throughput is virtually always a top priority, because “time is money” as the old saying goes! Usually most all of the attention may be placed on reducing the test time of the banner aspects of the product, such as the RF performance of a wireless device, for example. However, the choice of the DC system power supply can also have a huge impact on your test time and throughput during manufacturing. You may find the lowest cost, more basic-performance DC power supply that meets your immediate needs end up costing you the difference in price many, many times over of that of a higher-performance DC power supply having better throughput performance in the long run!

The DC power supply can incorporate a number of advanced features, such as elaborate triggering and sequencing systems, which will allow you restructure your testing to optimize throughput. However, even fundamental throughput-related features of the power supply can also have a large impact on your test time, including:

• Command processing time
• Output up-programming time
• Output down-programming time
• Measurement time

Figure 1 illustrates what the command processing and up-programming times are for a DC power supply. The command processing time is the time from when the command is first received to the point where the power supply starts acting on it. In this case it is when power supply’s output starts to change. The up-programming response time is the time the power supply takes for the output to rise and settle within a small band around the final output level, after processing the command instructing it to change its output level.

Figure 1: Power supply command processing and up-programming response times

The down-programming response time is like the up-programming response time except that the power supply is instead being programmed to a lower level. However, you need to look at down-programming independently as short up-programming time does not necessarily guarantee comparably short down-programming time. More basic performance DC power supplies usually lack an active down-programmer circuit that quickly brings down the output. In this case the down-programming response time can be very dependent on how much load the DUT presents to the power supply’s output.

How much difference is there in performance between more basic performance and higher performance DC power supplies on these throughput-related features? It can be considerable; over several orders of magnitude difference. As one example, command processing time can range from up to 100’s of milliseconds for entry-level power supplies to under 1 millisecond for high performance power supplies.

Another fundamental throughput-related feature of a DC power supply is its measurement time. There are a couple of aspects to consider here as well, which I will elaborate on in part 2 of this series on how fundamental features of power supplies impact your test throughput, in an upcoming posting here on “Watt’s Up?” along with tying it all together to show how they affect actual test throughput!

How to choose a Power Supply Part 1: Student Edition

Note from GaryR: We were fortunate to have an intern, Patrick, in our department for the summer. He has since returned to college, but during his time here at Agilent Technologies, he successfully completed several projects for us. During his last week, he wrote a power supply blog post! Here it is, unedited!!

Hello, I am Patrick, an intern here at Agilent Technologies. This is my first blog post but you might recognize me as “new intern… named Patrick” from the Matthew Carolan insta-classic, “What is Command Processing Time?” And as “Uncredited Waveform Capture Author” from GaryR’s visually-stunning, “Current Limit Setting Affects Voltage Response Time” I might not be able to provide the same engineering life lessons Mr. Brorein, Mr. GaryR, and Mr. Carolan provide you monthly but hopefully you will enjoy this post nonetheless.

As you can probably tell from the title, this blog post is the first in a series written to help readers find the right DC power supply for them. For starters, there are a lot of other power supply companies out there; their products are not mentioned here. Actually, I better give you a disclaimer: this is an Agilent blog post about Agilent products written by an Agilent employee. However, my intentions in this post are to inform our readers on how to find a reliable and dependable power supply to the best of my abilities. I simply do not know enough about non-Agilent DC power supplies to be able to confidently recommend you any. With that out of the way, today’s post is written for college students, like myself, but can help give some insight to anyone looking for a power supply.

Agilent Technologies offers over 200 models of DC power supplies. Finding the perfect power supply for your needs is possible but can take some time. Fortunately, you have found your way to this blog post where I have already done all the work.

The first step in finding your power supply to buy is to determine your budget. If you are financially disabled like me, you are really lucky if you can save up $300 - $500 dollars between buying textbooks for a personal power supply. If you need convincing on why you make the expense, Matt’s “The Joys of Owning a DC Power Supply” brings up some good points. Personally, I like being able to work where I want and when I want, including at my home during the summer where I would be lucky to find working batteries let alone a power supply. Also with the lifespan these power supplies have, this is a piece of equipment you will be using for decades. So first, figure out your budget!

The next thing to determine is what are you using this power supply for? Are you a hobbyist who only has low power needs? Do you need to power multiple DUTs at once for a senior project? Maybe you need a very precisely regulated and accurate output for a research project? If I wanted to work on labs outside of school I would need a supply with multiple outputs. If I wanted to work on side projects in my spare time my requirements are a lot simpler. Figure out what your performance requirements are, and try to anticipate future projects if you can.

Now, how much space do you have to work with? Power supplies come in many different sizes. In a test rack, a smaller power supply means more room for other test equipment. As a student you probably are not going to have a test rack, but you might have a test bench. If your test bench is anything like mine, it is also your laptop table, study space, dining area, and entertainment center. As you do before you buy a new couch, make sure you measure before you buy your new power supply.

For a student, here would be my priorities when shopping for a power supply: inexpensive, compact, and capable. All Agilent power supplies already provide low noise and well regulated outputs, so you do not have to worry about that. But keep in mind what functions and features you would like. For example, it would be nice to find one with an autoranging output characteristic, which would essentially allow one to perform the job of multiple, and/or a computer interface.

For those with similar priorities as above, shown in Table 1 are the power supplies that you might like. The table was populated with power supplies listed in our “Agilent Power Products Selection Guide”. The row on the top represents the base (no options) price range of the power supply if ordered from Agilent. The column on the left side represents your best choice in that price range. Keep in mind that the only difference in some budget ranges is the voltage/current ratings, which is important for your application but does not necessarily make one power supply better than the other. Within the table, each square states the model number of the power supply, voltage/current/power ratings, number of outputs, and number of ranges, available computer interfaces, and dimensions. The output ratings are sometimes separated by commas or there is another set listed below the first. If the power supply has multiple outputs, then each output is separated by a comma. If the power supply has multiple ranges, then each additional range is listed below the first.

To use Table 1, simply find your budget range and then match your application needs to the available models. Models in the same budget range but have increased functionality are placed higher.

The two families of power supplies that fall into the sub $1,000 price range are the E3600 and U8000 series basic power supplies. The E3600 gives you a lot of voltage/current choices, a number of outputs, as well as the ability to program with it in four of the options listed above. The U8000 is more limited in all these aspects but is more affordable and has features typically found only in programmable supplies such as OVP and OCP protection, recall states, and more. See for yourself how the E3600 and U8000 stack up against other power supplies.

All the power supplies listed above come in a ½ RU w x 2 RU h, with 1 RU being equivalent to 19” width and 1.75” height. This size is compact and will have no trouble fitting on a desk, and when not being used can easily be moved and stored.

It is unfortunate that there is no relatively inexpensive Agilent power supply with autoranging output characteristics. At least there are choices (within the E3600 series) with multiple ranges, giving you more output flexibility for more applications (learn more about rectangular vs. autoranging output characteristics).

Also there is a three output power supply on the list (the E3630A) which is in the middle of the pack in terms of budget. The E3630A is actually the same power supply available at my school laboratories, able to handle every exercise I have done so far.

As a student, these power supplies might be outside your budget, but the least expensive choice, the U8001A, is about the same price as a Play Station 3. Would you rather choose: “fun” (the PS3) or “FUN!” (a power supply)? Please note that the distinguishing lowercase and uppercase variations of the word fun is not a reference to the level of fun but rather how loud I would yell it at you if we were face-to-face.

So, I have listed out your choices. All you have to do is figure out which one fits your applications. If you do not know what your applications are then you already missed a step in this blog post, so start over from the beginning. If you do not have an application then you probably do not need a power supply, but thank you for taking the time to read this. If you cannot find the right power supply for your application in this post then keep saving your money and look out for the next part on, “How to choose a DC power supply”, that is where the real fun starts. If you read this post and succeeded in choosing a power supply then awesome. Thanks for reading!

How to read your DC power supply’s data sheet

When you have to select a programmable DC power supply to power your device under test (DUT), you will have many power supply vendors to choose from. To narrow your selection, you will likely read the data sheets associated with the power supplies you are interested in. While some of the basic information about the power supplies presented on the data sheets will be similar, you will also find different specification descriptions from different power supply vendors. To ensure you are considering the right specs, it is best to start with a good understanding of your DUT’s power requirements. Start with your DUT’s required maximum input voltage, current, and power. Think about whether or not you need to supply dynamic input voltages to your DUT and about how accurate the DUT input voltage needs to be.  Then consider what power-related measurements you will need to make.

Once you know your DUT’s power requirements, you can begin to peruse the specs in various power supply data sheets to find one that meets your needs. Most likely, you will be considering specs such as:

• DC output ratings
• These show the maximum voltage, current, and power available from the power supply. Make sure your DC input requirements are within these values. 

• Output noise
• This specification describes the deviations in the DC output voltage, typically expressed as peak-to-peak volts and rms volts. If your DUT is sensitive to noise, be sure to choose a low noise power supply.

• Load regulation (also called load effect)
• This spec shows how much the steady-state output voltage can change when the load current changes. If you want the input voltage to your DUT to vary very little when your DUT current changes, choose a power supply with low load regulation.

• Load transient recovery time
• This is the time for the output voltage to recover to within a settling band around the steady-state value when the load current changes. If your DUT is sensitive to large short-term input voltage changes, choose a power supply with a short load transient recovery time.

• Line regulation (also called line effect, source regulation, or source effect)
• This spec shows how much the steady-state output voltage can change when the power supply’s AC input line voltage changes. If you want the input voltage to your DUT to vary very little when your AC line voltage changes, choose a power supply with low line regulation.

• Programming accuracy
• This spec shows how much the steady-state output voltage can vary from its programmed (set) value. If you want the input voltage to your DUT to be very precisely controlled, choose a power supply with a low programming accuracy. (Most “programming accuracy” specs are really describing the maximum possible “programming error”, so you want this number to be low.)

• Measurement accuracy
• This spec shows how much the steady-state measurements (voltage or current) can vary from the actual output value. If you are counting on measuring your DUT’s input voltage or current with high accuracy, choose a power supply with low measurement accuracy. (Most “measurement accuracy” specs are really describing the maximum possible “measurement error”, so you want this number to be low.)

One of my colleagues, Kevin Cavell, wrote an article entitled “How to Read Your DC Power Supply’s Data Sheet” that appeared in the March, 2013 issue of Design World addressing in more detail many of these considerations. Here is a link to the on-line version:
http://www.designworld-digital.com/designworld/201303#pg71

The same article can be accessed here:
http://www.powersupplytips.com/how-to-read-your-dc-power-supplys-data-sheet

Finally, here is a link to Kevin’s application note – the article was based on this app note:
http://cp.literature.agilent.com/litweb/pdf/5991-2293EN.pdf

Kevin’s app note uses examples from the data sheets for a Sorenson power supply and an Agilent power supply. You will notice that the magazine articles refer to these as Power Supply A and Power Supply B.

Experiences with Power Supply Common Mode Noise Current Measurements

I wrote an earlier posting “DC Power Supply Common Mode Noise Current Considerations” (click here to review) as common mode noise current can be an issue in electronic test applications we face. This is not so much of an issue with all-linear based power supply designs as it can be for ones incorporating switching based topologies. High performance DC power supplies designed for test applications should have relatively low common mode current by design. I thought this would be a good opportunity to get some more first-hand experience validating common mode noise current. The exercise proved to be a bit of an eye-opener. I tried different approaches and, no surprise; I got back seemingly conflicting results. Murphy was busy working overtime here!

I settled on a high performance, switching-based DC source on having a low common mode noise characteristic of 10 mA p-p and 1 mA RMS over a 20 Hz to 20 MHz measurement bandwidth. To properly make this measurement the general consensus here is a wide band current probe and oscilloscope is the preferred solution for peak to peak noise, and a wide band current probe and wide band RMS voltmeter is the preferred solution for RMS noise. As the wide band RMS voltmeters are pretty scarce here I relied on the oscilloscope for both values for the time being. The advantages of current probes for this testing are they provide isolation and have very low insertion impedance.

I located group’s trusty active current probe and oscilloscope. The low signal level I intended to measure dictated using the most sensitive range providing 10 mA/div (with oscilloscope set to 10 mV/div).
One area of difficulty to anticipate with modern digital oscilloscopes is there are a lot of acquisition settings to contend with, all having a major impact on the actual reading. After sorting all of these out I finally got a base line reading with my DC source turned off, shown in Figure 1.

Figure 1: Common mode noise current base-line reading

My base-line reading presented a bit of a problem. With 1 mV corresponding to 1 mA my 2.5 mA p-p / 0.782 mA RMS base-line values were a bit high in comparison to my expected target values. It would be nicer for this noise floor to be at about 10X smaller so that I don’t have to really factor it out. Resorting to the old trick of looping the wire through the current probe 5 times gave me a 5X larger signal without changing the base-line noise floor. The oscilloscope was now displaying 2 mA /div, with 1 mV corresponding to 0.2 mA. In other words my base-line is now 0.5 mA p-p / 0.156 mA RMS. The penalty for doing this is of course more insertion impedance. Now I was all set to measure the actual common mode noise current. Figure 2 shows the common mode noise current measurement with the DC source on.

Figure 2: Common mode noise current measurement

Things to pay attention to include checking the current on both + and – leads individually to earth ground and load the output with an isolated load (i.e. a power resistor). Full load most often brings on worst case values. Based on the 0.2 conversion ratio I’m now seeing 8 mA p-p and 1.12 mA RMS, including the baseline noise. I am reasonably in the range of the expected values and having a credible measurement!

I decided to compare this approach to making a 50 ohm terminated direct connection. This set up is depicted in Figure 3 below.

Figure 3: 50 ohm terminated directly connected common mode noise current measurement

I knew insertion impedance was considerably more with this approach so I tried both 10 ohm and 100 ohm shunt values to see what kind of readings I would end up with. Table 1 summarizes the results for the directly connected measurement approach.

Table 1: 50 ohm terminated directly connected common mode noise current results

Clearly the common mode noise current results were nowhere near what I obtained with using a current probe, being much lower, and also highly dependent on the shunt resistor value. Why is that? Looking more closely at the results, the voltage values are relatively constant for both shunt resistor cases. Beyond a certain level of increasing shunt resistance the common mode noise behaves more as a voltage than a current. For this particular DC source the common mode voltage level is extremely low, just a few millivolts.

Not entirely content with the results I was getting I located a different high performance DC source that also incorporated switching topology. No actual specifications or supplemental characteristics had been given for it. When tested it exhibited considerably higher common mode noise than the first DC source. The results are shown in table 2 below.

Table 2: 50 ohm terminated directly connected common mode noise current results, 2nd DC source

With both voltage and current results changing for these two test conditions the common mode noise is exhibiting somewhere between being a noise current versus being a noise voltage. I had hoped to see what the results would be using the current probe but it seemed to have walked away when I needed it!

In Summary:
Making good common mode current noise measurements requires paying a lot of attention to the choice of equipment, equipment settings, test set up, and DUT operating conditions. I still have bit more to investigate but at least I have a much better understanding as to what matters. Maybe in a future posting I can provide what could be deemed as the “golden set up”! To get results that correlate reasonably with any stated values will likely require a set up that exhibits minimal insertion impedance across the entire frequency spectrum. Making directly coupled measurements without the use of a current probe will prove challenging except maybe for DC sources having rather high levels of common mode noise currents

The underlying concern here of course is what is what will be the impact to the DUT due to any common mode noise current from the test system’s DC source. Generally that is any common mode noise current ends up becoming differential mode noise voltage on the DUT’s power input due to impedance imbalances. But one thing I found from my testing is that the common mode noise is not purely a current with relatively unlimited compliance voltage but somewhere between being a noise voltage and noise current, depending on loading conditions. For the first DC source, with what appears to be only a few millivolts behind the current it is unlikely that it would create any issues for even the most sensitive DUTs. For the second DC source however, having 100’s of millivolts behind its current, could potentially lead to unwanted differential voltage noise on the DUT. Further investigation is in order!

Watts and volt-amperes ratings – what’s the difference and how do I choose an inverter based on them?

At the end of September, I posted about hurricane Irene and inverters. In that post (click here to read), I talked about the power ratings for inverters and just skimmed the surface about the differences between ratings in watts (W) and volt-amperes (VA). In this post, I want to go further into detail about these differences. Both watts and VA are units of measure for power (in this case, electrical). Watts refer to “real power” while VA refer to “apparent power”.

Inverters take DC power in (like from a car battery) and convert it to AC power out (like from your wall sockets) so you can power your electrical devices that run off of AC (like refrigerators, TVs, hair dryers, light bulbs, etc.) from a DC source during a blackout or when away from home (like when you are camping). Note that this power discussion is centered on AC electrical power and is a relatively short discussion about W, VA, and inverters. Look for a future post with more details about the differences between W and VA.

Watts: real power (W)
Watts do work (like run a motor) or generate heat or light. The watt ratings of inverters and of the electronic devices you want to power from your inverter will help you choose a properly sized inverter. Watt ratings are also useful for you to know if you have to get rid of the heat that is generated by your device that is consuming the watts or if you want to know how much you will pay your utility company to use your device when it is plugged in a wall socket since you pay for kilowatt-hours (power used for a period of time).

The circuitry inside all electronic devices (TVs, laptops, cell phones, light bulbs, etc.) consumes real power in watts and typically dissipates it as heat. To properly power these devices from an inverter, you must know the amount of power (number of watts, abbreviated W) each device will consume. Each device should show a power rating in W on it somewhere (390 W in the picture below) and you can just add the W ratings of each device together to get the total expected power that will be consumed. Most inverters are rated to provide a maximum amount of power also shown in watts (W) – they can provide any number of watts less than or equal to the rating. So, choose an inverter that has a W rating that is larger than the total number of watts expected to be consumed by all of your devices that will be powered by the inverter.


Volt-Amperes: apparent power (VA)
VA ratings are useful to get the amount of current that your device will draw. Knowing the current helps you properly size wires and circuit breakers or fuses that supply electricity to your device. A VA rating can also be used to infer information about a W rating if the W rating is not shown on a device, which can help size an inverter. Volt-amperes (abbreviated VA) are calculated simply by multiplying the AC voltage by the AC current (technically, the rms voltage and rms current). Since VA = Vac x Aac, you can divide the VA rating by your AC voltage (usually a known, fixed number, like 120 Vac in the United States, or 230 Vac in Europe) to get the AC current the device will draw. To combine the apparent power (or current) of multiple devices, there is no straightforward way to get an exact total because the currents for each device are not necessarily in phase with each other, so they don’t add linearly. But if you do simply add the individual VA ratings (or currents) together, the total will be a conservative estimate to use since this VA (or current) total will be greater than or equal to the actual total.


What if your device does not show a W rating?
Some electrical devices will show a VA rating and not a W rating. The number of watts (W) that a device will consume is always less than or equal to the number of volt-amperes (VA) it will consume. So if you need to size an inverter based on a VA rating when no W rating is shown, you will always be safe if you assume the W rating is equal to the VA rating. For example, assume 300 W for the 300 VA device shown in the picture above. This assumption may cause you to choose an oversized inverter, but it is better to have an inverter will too much capacity than one with too little capacity. An inverter with too little capacity will make it necessary for you to unplug some of your devices; otherwise, the inverter will simply turn itself off to protect its own circuitry each time you try to start it up, so it won’t work at all if you try to pull too many watts from it.

Some electrical devices will show a current rating (shown in amps, or A) and not a VA rating or W rating. Usually, this current rating is a maximum expected current. Maximum current usually occurs at the lowest input voltage, so calculate the VA by multiplying the current rating (A) times the lowest voltage shown on the device. Then, assume the device consumes an equal number of W as mentioned in the previous paragraph. For example, the picture below shows an input voltage range of 100 to 240 V and 2 A (all are AC). The VA would be the current, 2 A, times the lowest voltage, 100, which yields 200 VA. You could then assume this device consumes 200 W.

When powering multiple DUTs, should I use multiple small power supplies or one big power supply?

If you have to provide DC power to multiple devices under test (DUTs) at the same time, you will have to choose between using multiple smaller power supplies to provide power to each individual DUT (Figure 1) or one big power supply to power all of the DUTs at once (Figure 2). As will most choices, each has advantages and disadvantages. However, in this case, the advantages of choosing multiple smaller power supplies seem to outnumber those for the single bigger supply.




One of my colleagues, Bob Zollo, wrote an article on this topic that appeared in Electronic Design on October 12, 2011. Here is a link to the article:

http://electronicdesign.com/test-amp-measurement/powering-multiple-duts-parallel-consider-individual-supplies

Below is my summary of the contents:

Advantages of choosing multiple smaller power supplies
• Enables individual DUT current measurements without additional hardware
• Enables individual DUT voltage control
• Enables individual DUT shutdown upon DUT failure
• Enables individual DUT galvanic disconnect with relays inside power supply
• Prevents one DUT inrush current from disturbing other DUT’s voltage
• Prevents one DUT failure from affecting other DUT testing
• Isolates power supply failure to one DUT instead of affecting all DUTs

Advantages of choosing one big power supply
• Power supply hardware is less expensive
• Less power supply hardware to calibrate

The disadvantages of the smaller power supply choice are that the total power supply hardware is more expensive and is a larger quantity of hardware to calibrate. The disadvantages of the one big power supply are that it does not provide any of the advantages listed for the smaller supplies.

So you can see that the multiple smaller power supply choice has more advantages over the one big power supply choice. For the one big power supply choice, current monitoring and relays can be added in series with each DUT; however, this will contribute significantly to the cost of the system. If your application does not require you to monitor or control the power to each of your DUTs individually, you may be able to use the less capable one big power supply approach. Otherwise, use multiple smaller power supplies to get all of the performance, measurement, and control you need to test your DUTs.

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 4 of 4)

Part 4 of 4: Making the Comparison and Choice
In the first three parts of this post we looked at the topologies and merits of linear DC power supplies, traditional and high-performance switching DC power supplies, and common mode noise current considerations of each. So now in this final part we have reached a point where we can hopefully make an informed comparison and choice. Tables 1 and 2 summarize several key qualitative and quantitative aspects of all three DC power supply types, based on what we have learned.
Table 1: Qualitative comparison of DC power supply topologies

Table 2: Quantitative comparison of DC power supply topologies

So what DC power supply topology is the best choice for your next test system? In the past it usually ended up having to be a linear topology to meet performance requirements in most all but very high power, lower performance test situations. However, high-performance switching DC power supplies have nowadays for the most part closed the performance gap with linear DC power supplies. And, at higher power, the favorable choice may come down to selecting between several different switching DC power supplies only, due to their cost, size, and availability. So the answer is you need to make a choice based on how well the power supply meets your performance, space, and cost requirements, rather than basing the choice on its topology. Except for the most demanding low power test applications, like those needing the performance of a source measure unit (SMU), chances are much higher these days that the next DC power supply you select for your next test system you will be a switcher (and you possibly may not even realize it). What has been your experience?

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 3 of 4)

Part 3 of 4: DC Power Supply Common Mode Noise Current Considerations
Common mode noise current is a fact of life that manifests itself in many ways in test systems. There are several mechanisms that couple unwanted common mode noise currents into ground loops. An excellent overview on this is given in a two part post on the General Purpose Test Equipment (GPETE) blog “Ground Loops and Other Spurious Coupling Mechanisms and How to Prevent Them” (click here). However this is also an important consideration with our choice of a DC system power supply for testing as they are a source of common mode noise current. This is one area where linear DC power supplies still outperform switching DC power supplies. This can become a concern in some highly noise-sensitive test applications. As shown in Figure 1 the common mode noise current ICM is a noise signal that flows out of both output leads and returns through earth. By nature it is considered to be a current signal due to its relatively high associated impedance, ZCM.

Figure 1: Common Mode Noise Current and Path

Common mode noise current is often much greater in traditional switching DC power supplies. High voltage slewing (dv/dt) of the switching transistors capacitively couples through to the output, in extreme cases generating up to hundreds of milliamps pk-pk of high frequency current. In comparison, properly designed linear DC power supplies usually generate only microamps pk-pk of common mode noise current. It is worth noting even a linear DC power supply is still capable of generating several milliamps pk-pk of common mode noise current, if not properly designed. High-performance switching DC power supplies are much closer to the performance of a linear. They are designed to have low common mode noise current, typically just a few milliamps.

Common mode noise current can become a problem when it shows up as high frequency voltage spikes superimposed on the DC output voltage. This depends on the magnitude of current and imbalance in impedances in the path to the DUT. If large enough, this can become more troublesome than the differential mode noise voltage present. Generally, the microamp level of a linear DC power supply is negligible, while hundreds of milliamps from a traditional switching DC power supply may be cause for concern. Because common mode noise current is often misunderstood or overlooked, one may be left with a false impression that all switching DC power supplies are simply unsuitable for test, based on a bad experience with using one, not being aware that its high common mode noise current was actually the underlying cause.

In practice, at typical levels, common mode noise current often turns out not to be an issue. First, many applications are relatively insensitive to this noise. For example, equipment in telecommunications and digital information systems are powered by traditional switching DC power supplies in actual use and are reasonable immune to it. Second, where common mode noise current is more critical, the much lower levels from today’s high-performance switching DC power supplies makes it a non-issue in all but the most noise sensitive applications.

In those cases where common mode noise current proves to be a problem, as with some extremely sensitive analog circuitry, adding filtering can be a good solution. You can then take advantage of the benefits a switching DC power supply has to offer. A high-performance switching DC power supply having reasonably low common mode current can usually be made to work without much effort in extremely noise-sensitive applications, using appropriate filtering, capable of attenuating the high frequency content present in the common mode noise current. Such filtering can also prove effective on other high frequency noises, including AC line EMI and ground loop pickup. These other noises may be present regardless of the power supply topology.

Coming up next is the fourth and final part where we make our overall comparison and come to a conclusion on which power supply topology is best suited for test.

References:
1. Taking The Mystery Out Of Switching-Power-Supply Noise Understanding the source of unspecified noise currents and how to measure them can save your sanity
By Craig Maier, Hewlett Packard Co. © 1991 Penton Publishing, Inc.

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 2 of 4)

Part 2 of 4: Switching DC system power supply attributes
In part 1 we looked at the topology and merits of a linear DC power supply. To be fair we now have to give equal time to discuss the topology and merits of a switching DC system power supply, to make a more informed choice of what will better suit our needs for powering up and testing our devices.

Traditional switching DC power supply topology
The basic traditional switching power supply depicted in Figure 2 is a bit more complex compared to a linear power supply:
1. The AC line voltage is rectified and then filtered to provide an unregulated high voltage DC rail to power the following DC-to-DC inverter circuit.
2. Power transistors switching at 10’s to 100’s of kHz impose a high voltage, high frequency AC pulse waveform on the transformer primary (input).
3. The AC pulse voltage is scaled by the transformer turns ratio to a value consistent with the required DC output voltage.
4. This transformer secondary (output) AC voltage is rectified into a pulsed DC voltage.
5. An LC (inductor-capacitor) output filter averages the pulsed voltage into a continuous DC voltage at the power supply’s output.
6. As with a linear power supply, an error amplifier compares the DC output voltage against a reference to regulate the output at the desired setting.
7. A modulator circuit converts the error amplifier signal into a high frequency, pulse width modulated waveform to drive the switching power transistors.



Figure 2: Basic switching DC power supply circuit

In spite of being more complex the key thing is its much higher operating frequency, several orders of magnitude over that of a linear power supply, greatly reduces the size of the magnetic and filtering components. As a result traditional switching DC power supplies have some inherent advantages:
• High power conversion efficiency of typically 85%, relatively independent of output voltage setting.
• Small size and lightweight, especially at higher power.
• Cost effective, especially at higher power.

Traditional switching DC power supplies also have some typical disadvantages:
• High output noise and ripple voltage
• High common mode noise current
• Slow transient response to AC line and DC output load changes.


High-performance switching DC power supplies lessen the gap
Traditional switching DC power supply performance is largely a result of optimizing well established switching topologies for cost, efficiency and size, exactly the areas where linear DC power supplies suffer. Performance generally had been a secondary consideration for switching DC power supplies. However, things have now improved to better address the high-performance needs for electronics testing. Incorporating more advanced switching topologies, careful design, and better filtering, high-performance switching DC power supplies compare favorably with linear DC power supplies on most aspects, while still retaining most of the advantages of switchers.

So our choice on whether to use a linear or switching power supply has now gotten a bit more difficult! One area that still differentiates these DC power supply topologies is common mode current noise, worthy of its own discussion, which is exactly what I will do in part 3, coming up next!

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 1 of 4)

Part 1 of 4: Linear System DC Power Supply Attributes
To kick things off I thought it would be helpful to start with a short series of posts discussing something fundamental we’re often faced with; that is making the choice of whether to use a switching or linear DC power supply to power up our devices under test. In part 1 here I’ll begin my discussion with the topology and merits of linear DC power supplies, as I have heard countless times from others that only a linear power supply will do for their testing, principally due to its low output noise. Of course we do not want to take the chance of having power supply noise affect our devices’ test results. While I agree a linear DC power supply is bound to have very low noise, a well-designed switching DC power supply can have surprisingly good performance. So the choice may not be as simple anymore. The good thing here however is this may give us a lot more to choose from, something that may better meet our overall needs, including size and cost, among other things.

Linear DC Power Supply Topology
A linear DC power supply as depicted in Figure 1 is relatively simple in concept and in basic implementation:

1. A transformer scales the AC line voltage to a value consistent with the required maximum DC output voltage level.
2. The AC voltage is then rectified into DC voltage.
3. Large electrolytic capacitors filter much of the AC ripple voltage superimposed on the unregulated DC voltage.
4. Series-pass power transistors control the difference between the unregulated DC rail voltage and the regulated DC output voltage. There always needs to be some voltage across the series pass transistors for proper regulation.
5. An error amplifier compares the output voltage to a reference voltage to regulate the output at the desired setting.
6. Finally, an output filter capacitor further reduces AC output noise and ripple, and lowers output impedance, for a more ideal voltage source characteristic.

Figure 1: Basic Linear DC Power Supply Topology

Linear DC power supply design is well established with only incremental gains now being made in efficiency and thermal management, for the most part. Its straightforward configuration, properly implemented, has some inherent advantages:

• Fast output transient response to AC line and output load changes
• Low output noise and ripple voltage, and primarily having low frequency spectral content
• Very low common mode noise current
• Cost competitive at lower output power levels (under about 500 watts)

It also has a few inherent disadvantages:

• Low power efficiency, typically no better than 60% at full output voltage and decreases with lower output voltage settings
• Relatively large physical size and weight
• High cost at higher power (above about 500 watts)

So it sounds like a linear power supply has to be the hands-down winner especially for low power applications. Or not? To make a more-informed choice we need to look at the topology and merits of a switching power supply, which I will be doing in part 2!

Ideal Versus Real: Understanding Some Fundamentals When Selecting Power Supplies

Introduction
In college, we learned about electronics using ideal components: pure resistors without series inductance, pure capacitors without ESR, op amps with infinite gain and zero offset voltage. For power supplies, the situation was no different: a constant voltage source with zero output impedance, unlimited current compliance, and infinite bandwidth. With components like these, how difficult could it be to design electronic circuits and systems?

Then, we got jobs as engineers in the real world and discovered things like temperature coefficients in resistors, dielectric absorption in capacitors, and phase shifts in the gain of amplifiers. Power supplies did not escape the omnipotent forces determined to destroy our idealistic view of electronics. Non-zero output impedance, output current limitations, and finite bandwidth have all conspired to make our lives a little more complicated when applying power supplies. The effects of these non-ideal power supply attributes and others are discussed in this post.

The Ideal Voltage Source
An ideal voltage source would maintain its output voltage constant irrespective of the loading conditions. For example, if the source were a 5 V DC source, the output would measure exactly 5.0 V with no current flowing, or with 1 A flowing, or 10 A, or 500 A, and so on. Additionally, when the load current changed from one value to another, such as from 5 A to 10 A, the output voltage would be maintained at exactly 5.0 V, unperturbed throughout the change. See Figure 1a.

The Real Voltage Source
Unfortunately, power supplies like the ideal one described above do not exist in real life. Real power supplies try to maintain a constant voltage on their outputs by employing a feedback loop that monitors the output voltage, compares that voltage to a reference, and continuously makes adjustments based on the difference. They also have to be designed to fit in a limited space, with limited input power, and limited ability to dissipate the inevitable heat generated internally. Consequently, real power supplies have limited current compliance, finite output impedance, and finite bandwidth. The effects of these attributes become apparent when drawing current from the power supply, whether that is a static current or dynamic current. For example, a 5.0 V output at no load with 10 milliohms of output impedance will drop to 4.9 V with a 10 A static load. The output voltage will continue to decrease as the current increases. See Figure 1b.

With dynamic loads, the non-ideal nature of the real power supply output becomes more evident. Consider the output voltage behavior shown in Figure 1b immediately following the load current changes. The voltage overshoots and undershoots of the real source are a result of its non-zero output impedance which is a function of frequency (Zo(f)) and is determined by the internal feedback loop used to maintain the output voltage.

Power Supply Output – Deviant Behavior?
When selecting a power supply to meet your needs, first make sure you know what output voltage deviations you can tolerate. Evaluate your needs with respect to both static and dynamic conditions. For example, some devices, such as cell phones, have a low voltage detection circuit built-in. Make sure you are aware of the voltage level at which this circuit takes effect and how long the voltage must be below that level for the circuit to trip. The power supply used for testing should be selected to maintain its output voltage to meet your needs under changing load current conditions, especially to avoid nuisance tripping of a low voltage detection circuit. The load regulation (or load effect) specification tells you how well the power supply will maintain its output voltage when subjected to static load changes. The transient response specification will tell you how long it will take for the output voltage to recover to within a voltage band around the output voltage following a current change. Power supplies with different performance levels have correspondingly different specifications as shown in the table.

Other Non-ideal Attributes to Consider
In addition to the output voltage response to static and dynamic load changes, real power supplies also exhibit many other non-ideal behaviors. Line regulation, output noise, and cross regulation for multiple output power supplies are some examples.

• Line regulation is a measure of how the output voltage responds statically to input line voltage changes. It is primarily caused by finite loop gain, with some secondary effects from internal bias supply line regulation effects.
• Output noise is usually specified in either peak-to-peak volts, or rms volts, or both, and within a specified bandwidth such as 20 Hz to 20 MHz. Output noise has many sources, including residual effects from rectification circuits, internal digital circuits, and even the op amps themselves that are used for output voltage regulation.
• On a multiple output power supply, cross regulation is a measure how one output voltage responds to load current changes on the other output(s).

Clearly, for all of these attributes, the lower the specified behavior, the more “ideal” the power supply. While it may be tempting to look for a power supply with the lowest specifications in all of these areas, it is always prudent to evaluate your true needs, and make your selection based on those needs. Since tradeoffs must often be made, knowing your requirements will always make the selection process easier by broadening the choices when compared to just looking for the best specifications in all areas.

Other, more subtle behavior of non-ideal power supply outputs can also be important, depending on your application:

• Overshoots at AC (or DC) input turn-on and turn-off should be considered.
• Output voltage behavior when the power supply enters or leaves a current limit condition (mode crossover overshoots) can sometimes cause problems.

These behaviors are often unspecified by the power supply manufacturer. However, relying on reputable power supply vendors helps avoid problems since these vendors frequently take steps during the design process to minimize these effects.

Wrap-up
Obviously, real power supplies don’t behave like ideal power supplies. Sometimes this non-ideal behavior makes a difference in your application, and sometimes it does not. When selecting a power supply, it is important for you to know your true requirements in order to make the selection process go as smoothly as possible and to avoid overspending. A power supply’s specifications outline its non-ideal behavior, so look for specifications that meet your needs. Also realize that there are unspecified performance issues that could be important in your application as well. If you don’t see the specification for which you have interest, ask your power supply vendor about parameters you feel are important in your application.