Using the Passthrough Command in IVI drivers

Hi everybody!

I was working on a customer question yesterday and I thought that it would make an enlightening blog post.  We have a feature for our IVI-COM and IVI-C drivers that allows you to directly send SCPI commands to your instrument.  This is really useful if you run into a situation where you think there is a function is missing from our driver or you run into something unexpected.  Overall, it is pretty easy to use once you know where to find it.

Let's look at the IVI-COM driver first.  You can find the passthrough in the Systems Interface under the IO property.  One you get to the IO level, you can just use the standard VISA-COM commands to send commands to the instrument and read responses back.  For my little example here I am just going to send a voltage measurement and read back the response:

In the code above, agDrvr is the name I gave to the instrument when I initialized it.

We also provide an IVI-C driver.  The IVI-C driver also has a passthrough command that is just a little more complicated than the IVI-COM version.. To send SCPI commands, you use the AgN67xx_SystemWrite function and to read data back you would use the AgN67xx_SystemRead function.  The same example as above would look like this:

In this IVI-C code:

status is where the IVI-C driver will report an error if there is an unsuccessful call 

session is the handle that I gave to this instrument when I initialized

32 was my best guess at the size of the response string.  I like to overestimate.

strResp is where I want the response to be stored

respSize is the actual size of the response and is returned by the program

All in all it's not too difficult and will definitely come in handy.  

That all I have for you this month.  As always, let us know if you have any questions.

Matt

New Keysight Power Analyzer called IntegraVision

Back on June 23, 2014, I posted about the last Agilent power products to ever be announced. At that time, we had not yet officially changed our name from Agilent to Keysight. So the AC6800 AC sources we released on that date were released under the Agilent name, soon to be rebranded to Keysight. Well, today, I am announcing the first new Keysight power product: the Keysight Technologies IntegraVision Power Analyzer Model PA2201A.

A press release went out about these products earlier today: click here to view. We here at the Power & Energy Division of Keysight have been involved in power products for decades, and of course, Keysight has an oscilloscope division with commensurate experience producing scopes. I consider the new IntegraVision power analyzers to be a combination of the vast experience of our engineers from these two disciplines combining a power analyzer and an oscilloscope. The power analyzer will enable you to accurately measure parameters such as watts, VA, VAR, power factor, crest factor, efficiency, watt-hours, amp-hours, and harmonics while the oscilloscope will allow you to visualize in real time the voltage, current, and power waveforms that are important in your design.

I am very exciting about this new line of power measurement instruments! I have been working for HP/Agilent/Keysight for nearly 35 years now and have always worked with power products during my career. One of my favorite product families to support has been the older sophisticated 6800 AC Power Source/Analyzers (not to be confused with the newer basic AC6800 series mentioned in the first paragraph above). The older AC sources can produce sine waves, square waves, and arbitrary waveforms (for tests such as cycle dropout tests) as well as measure most of the power analyzer parameters mentioned above since they have a power analyzer built into the AC source. But now the new IntegraVision power analyzer goes well beyond the capabilities of the power measurements built into our AC sources. Adding time-based measurements like watt-hours and amp-hours opens up many more energy measurement application areas for this new product and the visual waveform measurements are a huge benefit when doing things like characterizing AC inrush current or product response to AC line disturbances. I am delighted with the performance of the touch-screen on this product – it will help you gain faster insight into your designs plus it just makes using the product fun! With 0.05% basic accuracy, 5 MSample/second 16-bit digitization, and inputs isolated to 1000 V, the IntegraVision power analyzer really is a superb product for power consumption and power conversion applications. Click here for the IntegraVision web page with links to the individual products.

So the next time you need a power analyzer with great accuracy and you also want to see the power waveforms related to your application, be sure to look at Keysight’s new IntegraVision Power Analyzer Model PA2201A. I’m sure you will not be disappointed! And look here for future posts about some of the interesting applications for this product, such as AC power line disturbance measurements and micro-inverter efficiency measurements. Suggest some of your own power measurements for me to make and I’ll see what I can do for a future post for you!

A new current measurement methodology: It’s all about counting the electrons going by!

One thing near and dear to us here at the Power and Energy Division is making accurate current measurements. What exactly is current? It’s basically the flow of electric charge per unit of time. In a conductor it’s the flow of electrons through it per unit of time. 

The ampere is the fundamental unit of current in coulombs per second, which equates to 6.241x10¹⁸ electrons per second. Accurate current measurement is one of the core values of virtually all of our products. Some of the precision SMU products can measure down to femtoamp (fA) levels (10^-15 amps). This is where we tend to muse that we’re getting down to the levels where we’re virtually counting the individual electrons going by.

While there are a few different ways of measuring current, by far the most common is to measure the voltage drop across a resistive shunt. With careful design this provides the most accurate means of current measurement. There are a lot of non-obvious factors that can introduce unexpected errors that many are not aware of, leading them to believe they have better accuracy than what it really is. A good discussion of what it takes to truly make accurate current measurements was covered in a previous posting “How to make more accurate current measurements”(click here to review). We go through great pains in addressing these things in our products in order to provide accurate and repeatable measurements.

Unlike the volt and the ohm, which have quantum standards for their electrical units, the ampere instead relies on the standards for the volt and ohm for measurement, as a quantum standard for the ampere that directly relates it back to charge is still lacking. However, that may change in the not too distant future. A group of scientists were awarded the Helmholtz Prize in metrology for realization of the measurement of the ampere based on fundamental constants. Basically they’ve created an electron charge pump that moves a small, fixed quantity of electrons under control by a clock. You can say they’re literally “counting the electrons as they go by”. This could become the new SI standard reference for current measurement. To me this is very fascinating to find out about. More can be learned on this from the following link to the press release “Helmholtz Prize for the “new” ampere”(click here to review).  I am curious to see how this all plays out in the long run. Maybe it will lead to yet another, and better, way to make more accurate current measurements in products we all use today in our work in electronics!

Why is the Programming Resolution Supplemental Characteristic Listed as an Average?

Hello everyone!

Happy New Year!  This is our last post of 2014 so we wanted to wish all of our readers a Happy New Year.  Today I am going to talk about a question that I have been asked a few times lately.  In many of our power supplies, we list our Programming Resolution as an average number.  Many people want to know why we do it this way.

Look at the below snippet from our 664xA DC Power Supplies Supplemental Characteristics:

You can see that that it is clearly stated as an average.

The simple answer to this question is that this is because of calibration.

The more complex answer is that we use a DAC to control the output setting of the power supply.  A certain number of DAC counts is going to represent zero to full scale on the output of the supply.  For simplicity's sake, lets assume that we are using a 12 bit DAC for a power supply that goes to fifty volts.

In an ideal world where calibration is not necessary:
A 12 bit DAC gives us 2^12 or 4096 total counts.
The step size (programming resolution) of the 50 volt power supply would be 50/4096 or 0.0122 volts.

We do not live in an ideal world though so we have to disregard some DAC counts because of how the unit calibrates. We also generally let you program a little bit above the maximum settings (usually something like 2%).  Zero volts is not going to be zero DAC Counts and 50 V is not going to be 4096 DAC counts.  For our example, lets say that the minimum that we disregard 20 counts at the top and bottom (40 total counts) and the maximum we disregard is 120 counts (240 total counts) at the top and bottom.  In this scenario:

Minimum step size = 50/(4096-40) = 0.0123 V
Maximum step size = 50/(4096-120) = 0.0130 V

For our Supplemental Characteristic, we would take the average of those 2 numbers.   This gives us 0.01265 V.

The big question is how would I know what the programming resolution is for my particular unit.  I spent about half of yesterday trying to figure that out and I'm still working that out myself.  The best solution that I have right now is to hook a DMM to the output and slowly increment my output to see when it flips to a new setting.  I need to experiment on this though.  If any readers have a better idea, please let us know in the comments.  The fact of the matter is that the error is pretty small and to be safe, any error due to being in between DAC counts is included in our Programming Accuracy specification.

Well that is all for 2014. I hope that everyone has a safe and happy 2015.  See you next year!

Matt

Why does an electronic load draw a pulse of current when a voltage is initially applied?

We recently had a customer contact us about one of our electronic loads. He had a solid state switch in series with a fixed output voltage source (for example, 50 V) and set the load to a fixed current (for example, 1 A). He used a current probe and a scope to observe the current flowing into the electronic load. When the switch changed from open to closed he saw a pulse of current flowing into the load that was significantly higher than the load set value before the load settled to the set value of 1 A. He was wondering if this was normal. It is normal. Here’s why:

The electronic loads have a snubber network across their input terminals. The snubber typically consists of a resistor in series with a capacitor. For example, the Keysight N3304A electronic load has 2.2 uF in series with about 2 ohms. The snubber network is there to maintain stability on the load input for all settings and operating modes. When the customer’s switch was closed, the initially discharged capacitor in the snubber pulled a pulse of current to begin charging. If the dV/dt of the load input voltage waveform was infinitely fast, the cap would initially look like a short and the initial current pulse would be limited by the resistor as I = V/R. In this example, the current pulse would have been 50 V / 2 ohms = 25 A. But he was seeing a much smaller current pulse: around 2.4 A instead of 25 A, but still higher than the expected set value of 1 A. This means the dV/dt was not infinite (the solid state switch had a finite risetime). In this case, the current pulse would be limited by the dV/dt of the input voltage waveform.

As an example, see Figure 1 below showing the input voltage and input current for an N3304A load. The voltage rises from 0 V to 50 V over about 75 us and the fastest part of the risetime is about 1V/us. Since I = C dV/dt, and for this electronic load, C = 2.2 uF, the peak of the current is calculated to be 2.2 uF x 1V/us = 2.2 A. The plot shows it to be about 2.4 A, so this is close to the expected peak value. As the dV/dt of the input voltage slows down, the current drops from its peak and approaches zero amps as the dV/dt slows to zero (horizontal). (Note that in the plot below, the load was set for zero current.)

So you can see that the current flowing into the input of an electronic load may not be simply the DC setting you expect. If you apply a dynamic voltage waveform to the input, the RC snubber network will also draw some current for a short time until the voltage applied to the load input stabilizes. There is another factor involved here that is worth mentioning but I will not cover in detail in this post since it is a secondary effect in this case. In this customer’s situation, the load was set for 1 A and initially had no voltage on it (his solid state switch was open). The load was trying to draw current by turning on its input FETs, but there was no voltage applied, so the load went to an unregulated state. When the voltage finally appeared (the solid state switch was closed), the FETs that were turned on hard had to recover and take a finite amount of time to begin regulating the set current. This effect can also contribute to brief, temporary unexpected current draw by the load when a voltage is suddenly applied to the input.

Why Does Over Current Protect (OCP) have a Programmable Delay Value in the First Place?

Since I am on a roll about over current protect (OCP), having just completed a two-part posting “Why does the response time of OCP vary on the power supply I am using and what can I do about it?” (Review part 1) (Review part 2) there is yet another aspect about OCP that is worth bringing up at this time. And that is “why does OCP have a programmable delay value in the first place?” This actually came up in a discussion with a colleague here after having read my part posting.

It may seem a bit ironic that OCP has a programmable delay in that in my posting on OCP I shared ideas on how one can minimize the response time delay encountered. But this is not contradictory. One may very well want to minimize it, eliminating extra delay being encountered, but not necessarily eliminate it altogether. As can be seen in my previous postings, I had programmed the OCP delay time to 5 ms.

The programmable OCP delay does serve a purpose, and that is to prevent false OCP trips. Adding some delay time prevents these false trips.  For someone who knows the root cause of false OCP tripping they might be half right. There are actually been two main causes of false OCP trips which are prevented by adding some delay time.

The original problem with OCP was that it would be falsely tripped when output voltage settings were changed on the power supply, due to capacitive loading at the test fixture or within the DUT. This is especially prominent with inrush current when first bringing up the voltage to power the DUT. An OCP delay prevents false triggering under these conditions. To correct the false tripping the delay would be invoked when output programming changes were made. As one example, the OCP delay description in our manual for our 663x series power supplies states:

“This command sets the time between the programming of an output change that produces a constant

current condition (CC) and the recording of that condition by the Operation Status Condition register. The

delay prevents the momentary changes in status that can occur during reprogramming from being

registered as events by the status subsystem. Since the constant current condition is used to trigger

overcurrent protection (OCP), this command also delays OCP.”

Under this situation the momentary overcurrent is induced by the power supply. Although not nearly as much as in issue in practice, momentary overcurrents can also be DUT-induced as well. This is the second situation that can cause a false tripping of the OCP. The DUT may be independently turned on after the bias voltage has already been on and draw a surge of current. Or the DUT may change mode of operation and draw a temporary surge of current.  If the OCP delay is invoked only by an output programming change it does not have any effect in these situations.

On later generation products, such as our N6700, N6900, and N7900 series, the user also has the ability to programmatically select between having the OCP delay activate from either an output change, or from going into CC condition. This gives the user a way to remain consistent with original operation or have OCP delay effective for momentary DUT-induced overload currents as well!

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 2

In the first part of this posting (click here to review) I highlighted what kind of response time is important for effective over current protection of typical DUTs and what the actual response characteristic is for a typical over current protect (OCP) system in a test system DC power supply. For reference I am including the example of OCP response time from the first part again, shown in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.  As can be seen, for reasons previously explained, the response time clearly depends on the amount of overdrive beyond the current limit setting.

As the time to cause over current damage depends on the amount of current in excess of what the DUT can tolerate, with greater current causing damage more quickly, the slower response at lower overloads is generally not an issue.  If however you are still looking how you might further improve on OCP response speed for more effective protection, there are some things that you can do.

The first thing that can be done is to avoid using a power supply that has a full output current rating that is far greater than what the DUT actually draws. In this way the overdrive from an overload will be a greater percentage of the full output current rating. This will normally cause the current limit circuit to respond more quickly.

A second thing that can be done is to evaluate different models of power supplies to determine how quickly their various current limit circuits and OCP systems respond in based on your desired needs for protecting your DUT. For various reasons different models of power supplies will have different response times. As previously discussed in my first part, the slow response at low levels of overdrive is determined by the response of the current limit circuit.

One more alternative that can provide exceptionally fast response time is to have an OCP system that operates independently of a current limit circuit, much like how an over voltage protect (OVP) system works. Here the output level is simply compared against the protect level and, once exceeded, the power supply output is shut down to provide near-instantaneous protection. The problem here is this is not available on virtually any DC power supplies and would normally require building custom hardware that senses the fault condition and locally disconnects the output of the power supply from the DUT. However, one instance where it is possible to provide this kind of near-instantaneous over current protection is through the programmable signal routing system (i.e. programmable trigger system) in the Keysight N6900A and N7900A Advanced Power System (APS) DC power supplies. Configuring this triggering is illustrated in Figure 2.

Figure 2: Configuring a fast-acting OCP for the N6900A/N7900A Advanced Power System

In Figure 2 the N7909A software utility was used to graphically configure and download a fast-acting OCP level trigger into an N7951A Advanced Power System. Although this trigger is software defined it runs locally within the N7951A’s firmware at hardware speeds. The N7909A SW utility also generates the SCPI command set which can be incorporated into a test program.

Figure 3: Example custom-configured OCP system response time vs. overdrive level

Figure 3 captures the performance of this custom-configured OCP system running within the N7951A. As the OCP threshold and overdrive levels are the same this can be directly compared to the performance shown in Figure 1, using the conventional, current limit based OCP within the N7951A. A 5 millisecond OCP delay was included, as before. However, unlike before, there is now virtually no extra delay due to a current limit control circuit as the custom-configured OCP system is totally independent of it. Also, unlike before, it can now be seen the same fast response is achieved regardless of having just a small amount or a large amount of overdrive.

Because OCP systems rely on being initiated from the current limit control circuit, the OCP response time also includes the current limit response time. For most all over current protection needs this is usually plenty adequate.  If a faster-responding OCP is called for minimizing the size of the power supply and evaluating the performance of the OCP is beneficial. However, an OCP that operates independently of the current limit will ultimately be far faster responding, such as that which can be achieved either with custom hardware or making use of a programmable signal routing and triggering system like that found in the Keysight N6900A and N7900A Advanced Power Systems.

Why do I measure voltage to earth ground on a power supply with a floating output?

Occasionally, one of our power supply users contacts us with a question about voltages measured from one of the power supply output terminals to earth ground (same as chassis ground). All of our power supply outputs are floating with respect to earth ground. See my previous post about this here. In that post, I stated that neither output terminal is connected to earth ground. To be more specific, no output terminal is connected directly to earth ground. We do have internal components, mainly resistors and capacitors, connected from each output terminal to earth ground. These components, especially the caps to ground, help mitigate issues with RFI (radio-frequency interference) and ESD (electrostatic discharge). They help prevent our power supplies from being susceptible to externally generated RFI and ESD, and also help to reduce or eliminate any internally generated RFI from being conducted to wires connected to the output terminals thereby reducing RFI emissions.

So even though our outputs are considered floating with respect to earth ground, there frequently is a DC path from at least one of our output terminals to earth ground. It is typically a very high value resistor, such as several megohms, but could be as low as 0.5 MΩ. This resistor acts as a bleed resistor to discharge any RFI or ESD caps to earth ground that could be charged to a high float voltage.

As an example of a power supply with a resistor to earth ground, the Keysight N6743A has 511 kΩ (~0.5 M) from the minus output terminal to earth ground. This resistor was responsible for the voltage measurements to earth ground observed and questioned by one of our power supply users. He was using this power supply in the configuration shown in Figure 1 and measured 9.7 Vdc from his common reference point to earth ground (again, same as chassis ground).

He understandably did not expect to measure any stable voltage between these points given that the output terminals are floating from earth ground. But once we explained the high impedance DC path from the minus output terminal to earth ground inside each power supply (see Figure 2), and the 10 MΩ input impedance of his DMM, the measurement made sense. The input impedance of the voltmeter (DMM) must be considered to accurately calculate the measured voltage. This is especially true when high impedance resistors are in the circuit to be measured.

Figure 3 shows the equivalent circuit which is just a resistor divider accounting for the 9.7 V measurement. (The exact calculation results in 9.751 V.) Notice that the voltage of the 28 V power supply does not impact this particular voltage measurement (but its resistor to ground does). If the user had measured the voltage from the plus output of the 28 V power supply to earth ground, both the 28 V supply and 20 V supply would have contributed to his measurement which calculates out to be 37.05 V (if you check this yourself, don’t forget to move the 10 MΩ resistor accounting for the different placement of the DMM impedance).

So you can see that even with power supply output terminals that are considered floating, there can still be a DC path to earth ground inside the supply that will cause you to measure voltages from the floating terminals to ground. As one of my colleagues always said, “There are no mysteries in electronics!”

Prewired rack delivers up to 90 kW of single-output DC power

As I have mentioned before, I avoid posting product-only-focused material in this blog since our goal is to educate about power-related items rather than to directly promote Keysight products. But when something new comes out, I like to announcement it here.

A little over a year ago, Keysight (we were Agilent at the time) announced a new power supply family with high-power outputs up to 15 kW per output (see link here). These high-power autoranging DC power supplies with individual outputs up to 1500 V or 510 A can be paralleled for even more power. Paralleling outputs with these supplies is simplified since multiple outputs can be grouped to act as a single output and the current share bus enables multiple outputs to more equally share current. But there are other considerations when paralleling multiple units. Wiring the AC inputs and DC outputs together takes design time and assembly time. Integrating the units together physically in a rack also takes time and effort. Designing the system to ensure the safety of operators is also very important. So Keysight decided to offer a prewired rack to help you overcome the challenges associated with racking high-power supplies.

On November 11, 2014 (less than 2 weeks ago), we announced a prewired rack that combines up to six of the 15 kW N8900 series power supplies for total power up to 90 kW from a single output. You can choose from a variety of DC output combinations with voltages from 80 V to 1500 V and currents from 60 A to 3060 A. That’s a lot of current! The rack’s internal configuration makes the multiple outputs appear as a single output allowing you to communicate with just one power supply through LAN, GPIB, or USB (all standard in the system). Click here for the press release for this new system and here for additional system information.

Perhaps you are working in R&D or manufacturing on EV/HEV, alternative energy (fuel cells, solar, etc.), industrial DC motors, large UPS’s, electroplating, or any of the many other high-power application areas that need DC power up to 90 kW with voltages to 1500 V or currents to 3060 A. If so, and you want to spend your engineering resources on your core competencies instead of racking power supplies together, let Keysight help you with our new N8900 Series Rack System.

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 1

In a previous posting of mine “Providing effective protection of your DUT against over voltage damage during test”(click here to review), an important consideration for effective protection was to factor in the response time of the over voltage protect (OVP) system. Due to the nature of over voltage damage, the OVP must be reasonably fast. The response time can typically be just a few tens of microseconds for a reasonably fast OVP system on a higher performance system power supply to hundreds of microseconds on a more basic performance system power supply. This response time usually does not vary greatly with the amount of over voltage being experienced.

Just as with voltage, system power supplies usually incorporate over current protect (OCP) systems as well. But unlike over voltage damage, which is almost instantaneous once that threshold is reached, over current takes more time to cause damage. It also varies in some proportion to the current level; lower currents taking a lot longer to cause damage. The I²t rating of an electrical fuse is one example that illustrates this effect.

Correspondingly, like OVP, power supply OCP systems also have a response time. And also like OVP, the test engineer needs to take this response time into consideration for effective protection of the DUT.  However, unlike OVP, the response time of an OCP system is quite a bit different. The response time of an OCP system is illustrated in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.

This is quite different than the response time of an OVP system. Even if the OCP delay time was set to zero, the response is still on the order of milliseconds instead of microseconds for the OVP system. And when the amount of overdrive is small, as is the case for the 6A loading, providing just 1A of overdrive, the total response time is much greater. Why is that?

Unlike the OVP system, which operates totally independent of the voltage limit control system, the OCP system is triggered off the current limit control system. Thus the total response time includes the response time of the current limit as well. The behavior of a current limit is quite different than a simple “go/no go” threshold detector as well. A limit system, or circuit, needs to regulate the power supply’s output at a certain level, making it a feedback control system. Because of this stability of this system is important, both with crossing over from constant voltage operation as well as maintaining a stable output current after crossing over. This leads to the slower and overdrive dependent response characteristics that are typical of current limit systems.

So what can be done about the slower response of an OCP system? Well, early on in this posting I talked about the nature of over current damage. Generally over current damage is much slower by nature and the over drive dependent response time is in keeping with time dependent nature of over current damage. The important thing is understand what the OCP response characteristic is like and what amount of over current your DUT is able to sustain, and you should be able to make effective use of the over current protection capabilities of your system power supply.

If however you are still looking how you might further improve on OCP response speed, look for my follow up to this in my next posting!

Providing effective protection of your DUT against over voltage damage during test

The two most common ways DUTs can be electrically damaged during test are from current-related events or voltage-related events that mange to over-stress the DUT. Sometimes the cause can be an issue with the DUT itself. Other times it can be an issue stemming from the test system. The most common voltage-related damage to a DUT is an over voltage event, beyond a maximum level the DUT can safely tolerate. While there are a number of things that can cause this, most invariably it was an issue with the test system power supply, either from inadvertently being set too high or from an internal failure.

To protect against accidental over voltage damage, test system power supplies incorporate an over voltage protect (OVP) system that quickly shuts down the output upon detecting the voltage has gone above a preset threshold value. More details about OVP have been written about here in a previous posting “Overvoltage protection: some background and history”(click here to review).

The critical thing about over voltage damage is, in most all cases, that it is virtually immediate once the voltage threshold where damage to the DUT occurs is exceeded. It is therefore imperative that you optimize the test set up and settings in order to provide effective protection of your DUT against over voltage damage during test. To start with, the OVP trip threshold needs to be set at a reasonable amount below the threshold where DUT damage occurs and at the same time be set to a reasonable amount above the maximum expected DUT operating voltage. This is depicted in Figure 1.

Figure 1: OVP set point

However, to understand what are “reasonable amounts” above the maximum operating voltage and below the DUT damage voltage levels you need to take into account the dynamic response characteristics of the power supply output and OVP system, as depicted in Figure 2.

Figure 2: Power supply output and OVP dynamic response characteristics

It is important to have adequate margin above the maximum operating voltage to account for transient voltages due to the DUT drawing current from the power supply and resulting voltage response of the power supply in correcting for this loading, in order to prevent false OVP tripping. It is likewise important to adequate margin below the DUT damage threshold as it takes a small amount of time, in the range of 10’s to 100’s of microseconds, for the OVP system to start shutting down the power supply’s output once the OVP trip point has been crossed. At the same time the power supply typically has a maximum rate the output voltage can slew in. In practice these “reasonable amounts” typically need to be a few tenths to several tenths of a volt as a minimum.

Generally these margins are not difficult to manage, except when the DUT’s operating voltage is very small or the DUT operating current is very large producing a correspondingly large voltage drop in the power supply wiring. This is because the OVP is traditionally sensed on power supply’s output power terminals, so that it provides protection regardless of what the status and condition of the remote voltage sense wiring connection is. To improve on this we also provide OVP sensing on the remote sensing wires as an alternative to, or in addition to, the traditional sensing on the output power terminals. More details about this are described in another posting here “Protect your DUT: Use sense leads for over voltage protection (OVP)”(click here to review).

By following these suggestions you should be able to effectively protect your DUT against over voltage damage during test as well!

APS Paralleling Made Easier through Programming

Hi Everyone,

The Advanced Power Supply family has a very slick way to parallel units for higher current called current sharing.  This enables all of the paralleled units to be in Constant Voltage (CV) mode which is a change from most of our other power supplies that have one unit in CV mode and the rest of the units in Constant Current (CC) mode.  My colleague Ed did a very informative blog post about the different paralleling options that explains a bit more about paralleling units so I will not rehash any of that here.  Here is a link to that post: Paralleling Power Supplies.

The main drawback of the paralleling on the APS is that it can be a little difficult to get everything properly set to get the best performance.  You need to synchronize your current measurements and your voltage transients.  If you look in the manual, there are quite a few pages explaining how to set this all up.  I am happy to say that we have made this a little easier.  Our summer intern spent some time writing a VBA program in Excel that automatically does much of this.  The program uses Keysight VISA-COM so you need to have the Keysight IO Libraries installed to use it.  It will work with LAN, GPIB, and USB (all of which come standard on all APS units).

The first thing that we need to do is talk about the setup.  There are quite a few wire connections that need to be made.

First you need to connect the current sharing ports, the sense connections, and the outputs to the load:

After that, you also need to make some connections on the 8 pin  digital connector on the back of the APS units. You do not need to worry about setting up the pins if you plan on using the default pin assignments from the program.  The default pin assignments are:

Pin 6 on all units - On Couple

Pin 7 on all units - Off Couple

Pin 1 on the master unit - Trigger Out

Pin 1 on all other units - Trigger In

Pin 8 on all units - common

Here is a wiring diagram of the default assignments (for 3 units):

The On/Off Couple Pins make it so that when you enable or disable the output on any unit, all of the units enable or disable. The trigger line enable us to synchronize measurements as well as voltage changes.  

The Interface looks something like this:

It is divided into four boxes.  I will refer to them as boxes 1 to 4 with 1 being the left most box.  Box 1 is where you enter the VISA initialization string for each paralleled supply.  You can get this from the Keysight IO Libraries.  Box 2 is where you enter your settings  You can set the voltage limit, positive and negative current limit, change the output state, and change the voltage.  Most importantly, this is where you set the number of paralleled units.  This needs to be done or else the program will not work correctly.  You can parallel 1 kW and 2 kW units with each other, as long as they have the same maximum voltage so we also need to break out the number of 2 kW units in the scheme.  The third box will do a scalar measurement of the voltage and current.  This will report the total current of the paralleled units (it does a triggered measurement and adds all of the current measurements).  The fourth box will measure arrays of current and voltage (this function will not work on the N6900 APS units).      

I have posted this program on our Keysight Power Supply Forums at: Matt's Forum Post.  I have also opened a thread there where we can discuss this program.  It is still kind of preliminary so any feedback could possibly be incorporated into the program.  

That is all I have for this month.  Happy Halloween to all of our readers and please let me know any comments in the forum.

What is a reverse protection diode and what does it do?

A reverse protection diode is used on the output of a power supply to protect the power supply from damage due to an externally applied reverse voltage. Most power supplies have a polarized electrolytic capacitor (or several) across the output terminals. These caps help to filter ripple and noise on the output and provide a charge reservoir to reduce voltage sags and surges due to large load current changes. Electrolytic caps can withstand some reverse voltage, but not much. About 1 V to 1.5 V is the most they will tolerate without venting or worse…exploding! The reverse protection diode limits the reverse voltage to a diode drop thereby protecting the output caps. The diode is typically rated for the full output current of the power supply it is protecting. Adding to the diode drop, there can be some more small voltage drops due to current flowing in wires, tracks, current monitor resistors, output filter inductors, switching transformer windings, etc.

In a linearly regulated power supply, the reverse protection diode must be added to the design with the cathode connected to the plus output and anode connected to the minus output. See Figure 1. In a switching power supply, the reverse protection diode(s) is (are) an inherent part of the design. See Figure 2.

But where does reverse voltage come from? During normal operation, reverse voltage does not occur on the output of a power supply (unless it is a bipolar power supply which does not use polarized caps on its output…see this post). The power supply internal circuitry typically cannot produce reverse voltage on the output even if a failure occurs inside the power supply. So a reverse voltage has to be applied from an external source of power. For example, if you use two power supply outputs in parallel and inadvertently connect them to each other backwards, a reverse voltage would result. Another possibility can occur when two power supply outputs are connected in series. If the load across the series combination shorts, the two power supply outputs will be connected to each other backwards. See Figures 3 and 4. The reverse protection diode of one of the power supplies will conduct all available current from the other power supply forcing it into constant current (CC) operation and limiting the voltage to a diode drop (plus any additional small drops mentioned above).

So rest assured that your Keysight power supply is protected against reverse voltage if something unexpected happens!

Creating a "bumping" auto-restarting over current protect on the N6900A/N7900A Advanced Power System

The two main features in system power supplies that have traditionally protected DUTs from too much current are the current limit and the over current protect (OCP). When a device, for any of a number of reasons, attempts to draw too much current, the current limit takes control of the power supply’s output, limiting the level of current to a safe level. An example of current limit taking control of a power supply output is shown in Figure 1.

Figure 1: Current limit protecting a DUT against excess current.

For those devices that cannot tolerate a sustained current at the current limit level, the over current protect can be set and activated to work with the current limit and shut down the power supply output after a specified delay time. This will protect a DUT against sustained current at the limit.  An example of an OCP shutting down a power supply output for greater protection against excess current is shown in Figure 2.

Figure 2: OCP protecting a DUT against excess current

We have talked about the current limit and OCP in previous posts. For more details on how the OCP works, it is worth reviewing “What is a power supply’s over current protect (OCP) and how does it work?” (Click here to review)

Sometimes it is desirable to have something that is in between the two extremes of current limit and OCP.  One middle-ground is a fold-back current limit, which cuts back on the current as the overload increases. More details about a fold-back current limit are described in a previous posting “Types of current limits for over-current protection on DC power supplies” (Click here to review). One thing about a fold-back current limit is the DUT and power supply will not be able to recover back into constant voltage (CV) operation unless the DUT is able to cut way back on its current demand.

Another type of current limit behavior that operates between regular current limit and OCP is one that shuts down the output, like OCP, but only temporarily. After a set period of time it will power up the output of the power supply again. If the DUT is still in overload, the power supply will shut down again. However, if the DUT’s overload condition has gone away, it will be able to restart under full power. In this way the DUT is protected against continuous current and at the same time it the power supply is not shut down and requiring intervention from an operator.

While this type of current limit is not normally a feature of a system DC power supply, it is possible to implement this functionality in the N6900A/N7900A Advanced Power System (APS) using its expression signal routing feature. This is a programmable logic system that is used to configure custom controls and triggers that run within the APS. Here the expression signal routing was used to create an auto-restarting current shutdown protect in the example shown in Figure 3.

Figure 3: Custom auto-restarting current shutdown protect configured for N6900A/N7900A APS

A custom control was created in the expression signal routing that triggers the output transient system to run if the current limit is exceeded for longer than 0.3 seconds. A list transient was programmed into the APS unit to have its output go to zero volts for 10 seconds and then return to the original voltage setting each time it is triggered. In this way the output would pulse back on for 0.3 seconds and then shut back down for another 10 seconds if the overload was not cleared. The custom trigger signal was graphically created and downloaded into the APS unit using the N7906A software utility, as shown in Figure 4.

Figure 4: Creating custom trigger for auto-restarting current shutdown protect on APS

Current limit and over current protect (OCP) are fairly standard in most all system DC power supplies for protecting your DUT against excess current. There are not a lot of other choices beyond this without resorting to custom hardware. One more option now available is to make use of programmable signal routing like that in the N6900A/N7900A APS. With a little ingenuity specialized controls like a auto-restarting current shutdown protect can be created through some simple programming.

Simulating battery contact bounce, part 2

In part 1 of this posting on simulating battery contact bounce (click here to review) I discussed what battery contact bounce is about and why creating a voltage dropout may not be adequate for simulating battery contact bounce. The first answer to addressing this was provided; use a blocking diode and then a voltage dropout is certain to be suitable for simulating battery contact bounce.

Another approach for simulating battery contact bounce is to add a solid state switch between the DC source and the battery powered device. While this is a good approach it is complex to implement. A suitable solid state switch needs to be selected along with coming up with an appropriate way to power and drive the input of the switch need to be developed.

If for some reason using a blocking diode is not suitable, there is yet another fairly simple approach that can be taken to simulate high impedance battery contact bounce. Instead of programming a voltage dropout on the DC source, program a current dropout. Where the voltage going to zero during a voltage dropout is effectively a short circuit, as we saw in part 1, the current going to zero during a current dropout is effectively an open circuit. There are a couple of caveats for doing this. The main one is battery powered devices are powered from a battery, which is a voltage source, not a current source. In order for the DC source to act as a voltage source when delivering power, we need to rely on the DC source voltage limit being set to the level of the battery voltage. In order for this to happen we need to set the non-dropout current level to be in excess of the maximum level demanded by the device being powered and. Thus the DC source will normally be operating in voltage limit. Then when the current dropout drives the output current to zero, the DC source switches its operating mode from voltage limit to constant current, with a current value of zero. This operation is depicted in Figure 4, using a Keysight N6781A 2-quadrant SMU module designed for testing battery powered devices, operating within an N6705B DC Power Analyzer. In this example the current ARB for the dropout was both programmed and the results shown in Figure 1 captured using the companion 14585A software.

Figure 1: Current ARB creates a high impedance dropout to simulate battery contact bounce

Another caveat with using this approach for simulating battery contact bounce is paying careful attention to the behavior of the mode crossovers. For the first crossover, from voltage limit to constant current operation (at zero current) there is a small amount of lag time, typically just a fraction of a millisecond, before the transition happens. This becomes more significant only when trying to simulate extremely short contact bounce periods. More important is when crossing back over from constant zero current back to voltage limit operation. There is a short period when the current goes up to its high level before the voltage limit gains control, holding the voltage at the battery’s voltage level. Usually any capacitance at the input of the DUT will normally absorb any short spike of current. If this crossover is slow enough, and there is very little or no capacitance, the device could see a voltage spike. The N6781A has very fast responding circuits however, minimizing crossover time and inducing just 250 mV of overshoot, as is seen in Figure 1.

Hopefully, now armed with all of these details, you will be able to select an approach that works best for you for simulating battery contact bounce!

How Do I Properly Wire My Output?

Hi everyone,

September has been a hectic month here at Keysight’s Power Supply Headquarters (to give you an idea of the kind of month it has been, my dog literally ate my passport a week before I left on an international trip) but I am back with another blog post for your reading pleasure.  Today we are going to talk about how to properly wire your power supply.  This is a common question.  Wiring is something that on the surface seems like it should be really easy but when you dig a little deeper there are many layers to consider.  The repercussions can be pretty severe as well.  With improper wiring, you can make a high performance power supply seem like a low performance benchtop supply.

First, let's talk about the things repercussions of improper wiring.  The first and probably most undesired result is that your voltage will be unstable.  I have seen this in my own former career as a test engineer.  The inductance from our wiring coupled with some capacitance in our test equipment resulted in an oscillation that caused a test to fail.  We spent a Saturday chasing this down and fixed it by properly wiring our system.   

The second undesired result is that your voltage rise time and fall times could be much longer than specified.  This will negatively affect your test throughput which in high volume manufacturing test could cost money due to increased test time.   Properly wiring your power supply will enable you to get the maximum throughput from your power supply.  

The last repercussion that I'll discuss is voltage overshoots and undershoots.  You want these to be as small as possible.  A large overshoot can possibly damage your DUT especially if you do not have your over voltage properly set. A voltage undershoot could cause your DUT to shut down due to a low voltage condition. 

All of these are real pains when you are trying to get your test set up and running.  There are ways to properly wire your system so you can get the maximum specs out of your power supply.  

The first and most basic wiring tip is to keep the wiring as short as possible.  The longer the wiring the higher the impedance from the wiring will be.  The table below shows some specifications on some standard wire sizes:

The second tip is to use remote sensing.  This will sense around all of the wiring drops from the wiring.  This is good practice at all times.  Remote sensing is cool.

The third tip is to twist your wires together.  The key thing to remember her is that you twist the + and - output together and the + sense and - sense together.  This will reduce the mutual inductance in the wires.  Never, ever twist the sense and output leads together.  

This is a picture of the spool of wire that we use for our sense wires here.  You can see the the wires are very tightly twisted together here:

The other option is to use special low inductance wiring.  If you look at the below picture, you can see that there are two flat conductors separated by an insulator.  This reduces the mutual inductance even more than twisting the wires.:

Our N678xA SMU DC Power Modules are very sensitive to how they are wired.  Here is a diagram showing the proper wiring for the N678xA:

The top three items I mentioned should be standard practice when you set up your system.  These are just great wiring practices.  Sometimes you need to go the extra mile.  Back when I was in the test group we followed all of these tips as best I could but due to the test system, we could not minimize the wire length enough.  Our solution was to parallel more wires between the power supply and the load that we were using.  Instead of one twisted pair, we used three twisted pairs in parallel.  This also reduces the impedance of the wiring because you are paralleling the conductors (paralleled inductance and resistance reduces).

One of our design engineers wrote a very good article that touched on this subject a bit.  You can check that out here: Article Link.  

I hope that this is useful to everyone.  Please let us know if you have any questions or comments.

Properly sequence multiple power inputs to protect your DUT

As I mentioned in a previous post, we have devoted a lot of time writing about protecting your device under test (DUT) from the two main DUT-destroying forces available from a power supply: excessive voltage and excessive current. Click here for one of the latest posts including a list to the other posts.

Today I’d like to cover another topic that can cause DUT failure due to a power supply. Some DUTs have multiple DC inputs and some of these multiple-input DUTs are sensitive to the order in which the inputs turn on or turn off. Subjecting the device to an uncontrolled sequence could cause latch-up or excessive current to flow resulting in compromised reliability or even immediate catastrophic failure of the DUT. So properly sequencing the multiple voltages at turn on and off is essential. My colleague, Ed Brorein, wrote a very similar post last year (click here) but I thought this topic was worth repeating especially since we added another series of power products with higher power that has this capability.

Various methods have been used in an effort to address the potential problem associated with improperly sequenced power inputs. Diodes can be placed from one input to another to clamp the voltage thereby preventing one input voltage from going too far above or below another input voltage but this method has limited effectiveness and variable results. Relays can be put in series with each input and controlled with timing circuitry but the relays introduce variable series impedance and timing is imprecise. FETs with associated control circuitry can be placed in series with each input however this method requires significant design time and adds complexity to the setup. Multiple DC power supplies can be controlled through software, but once again, timing is imprecise and response times can be slow.

Several years ago, I wrote an application note on a closely related topic (click here). The method that is most precise and introduces the fewest complications is to use a power supply system that has output sequencing integrated into the system itself. Keysight has several power supply systems that can accommodate precise output sequencing: the N6700 Modular Power System, N6705 DC Power Analyzer, and the more recently released N6900/N7900 Advance Power System. Each system offers the ability to precisely control the turn-on and turn-off sequence of multiple outputs. Timing is set with sub-millisecond resolution. Synchronization across systems is also possible to facilitate timed shut downs of larger numbers of power supply outputs for your DUT inputs. The above mentioned application note specifically addressed the topic of how to configure the system to properly shut down your DC inputs in sequence upon a fault generated by any of the system power supplies.

Below is a simple example of a sequenced turn on of four outputs in an N6705B mainframe. The sequencing is facilitated by setting a different turn-on delay time for each of the outputs (turn-off delays can be set independently). When all outputs are told to turn on simultaneously, the delays are activated resulting in a precisely controlled sequenced turn on. Figure 1 shows how easy it is to implement the delays for a turn-on event. In this case, I used four power supply outputs in an N6705B mainframe with delays set to 5 ms, 10 ms, 15 ms, and 20 ms. I set the output voltages to 10 V, 7.5 V, 5 V, and 3.3 V. You can also set the output voltage rise time (slew rate) independently for each output. Figure 2 shows the results using the scope that is built into the N6705B mainframe.

So you can see that with the proper power supply system, sequencing your multiple DC power supply inputs on your device to protect it from damage is easy. Keysight provides you with the solution to do just that adding to our arsenal of features that protect your valuable DUT.

Simulating battery contact bounce, part 1

One test commonly done during design validation of handheld battery powered devices is to evaluate their ability to withstand a short loss of battery power due to being bumped and the contacts momentarily bouncing open, and either remain operating or have sufficient time to handle a shutdown gracefully. The duration of a contact bounce can typically range anywhere from under a millisecond to up to 100 milliseconds long.

To simulate battery contact bouncing one may consider programming a voltage drop out on a reasonably fast power supply with arbitrary waveform capabilities, like several of the N675xA, N676xA, or N678xA series modules used in the N6700 series Modular DC Power System or N6705B DC Power Analyzer mainframe, shown in Figure 1. It is a simple matter to program a voltage dropout of specified duration. As an example a voltage dropout was programmed in Figure 2 on an N6781A SMU module using the companion 14585A software.

 Figure 1: N6700 series and N6705B mainframes and modules

Figure 2: Programming a voltage drop out using the N6705B and N6781A SMU module

While a voltage dropout is fine for many applications, like automotive, in many situations it does not work well for simulating battery contact bounce. The reason for this is there is one key difference to note about a voltage dropout versus a battery contact bounce. During a voltage dropout the source impedance remains low. During a battery contact bounce the source impedance is an open circuit. However, a DC source having the ability to generate a fast voltage dropout is a result of it being able to pull its output voltage down quickly. This is due to its ability to sink current as well as source current. The problem with this is, for many battery powered devices, this effectively short-circuits the battery input terminals, more than likely causing the device to instantly shut down by discharging any carry-over storage and/or disrupting the battery power management system. As one example consider a mobile device having 50 microfarads of input capacitance and draws 4 milliamps of standby current. This capacitance would provide more than adequate carryover for a 20 millisecond battery contact bounce. However, if a voltage dropout is used to simulate battery contact bounce, it immediately discharges the mobile device’s input capacitance and pulls the battery input voltage down to zero, as shown by the red voltage trace in Figure 3. The yellow trace is the corresponding current drain. Note the large peaks of current drawn that discharge and recharge the DUT’s input capacitor.

 Figure 3: Voltage dropout applied to DUT immediately pulls voltage down to zero

One effective solution for preventing the DC source from shorting out the battery input is to add a DC blocking diode in series with the battery input, so that current cannot flow back out, creating high impedance during the dropout. This is illustrated in Figure 4.

Figure 4: Blocking diode added between SMU and DUT

One thing to note here is the diode’s forward voltage drop needs to be compensated for. Usually the best way to do this just program the DC source with the additional voltage needed to offset the diode’s voltage drop. The result of this is shown in Figure 5. As shown by the red trace the voltage holds up relatively well during the contact bounce period. Because the N6781A SMU has an auxiliary voltage measurement input it is able to directly measure the voltage at the DUT, on the other side of the blocking diode, instead of the output voltage of the N6781A. As seen by the yellow current trace there is no longer a large peak of current discharging the capacitor due to the action of the blocking diode.

 Figure 5: Blocking diode prevents voltage dropout from discharging DUT 

Now you should have a much better appreciation of the differences between creating a voltage dropout and simulating battery contact bounce! And as can be seen a blocking diode is a rather effective means of simulating battery contact bounce using a voltage dropout. Stay tuned for my second part on additional ways of simulating battery contact bounce on an upcoming posting.

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