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2017

On a previous posting “The difference between constant current and current limit in DC power supplies”, I discussed what differentiates a DC power supply having a constant current operation in comparison to having strictly a current limit for over-current protection. In that post I had depicted one very conventional current limit behavior. However there is actually quite a variety of current limits incorporated in different DC power supplies, depending on the intended end-use of the power supply.

 

Fold-back Current Limit

The output characteristic of a constant voltage (CV) power supply utilizing fold-back current limiting is depicted in Figure 1. Fold-back current limiting is sometimes used to provide a higher level of protection for DUTs where excess current and power dissipation can cause damage to a DUT that has gone into an overload condition. This is accomplished by reducing both the current and voltage as the DUT goes further into overload. The short circuit current will typically be 20% to 50% of the maximum current level. A reasonable margin between the crossover current point and required maximum rated DUT current needs to be established in order to prevent false over-current tripping conditions. Due to the fold-back nature, and depending on the loading nature of the DUT, the operating point could drop down towards the short-circuit operating point once the crossover point is reached/exceeded. This would require powering the DUT down and up again in order to get back to the CV operating region.

 

 

 

 

Figure 1: Output characteristic of a CV power supply with fold-back current limiting

 

In addition to providing over-current protection for the DUT, fold-back current limiting is often employed in fixed output linear DC power supplies as a means for reducing worst case dissipation in the power supply itself. Under short circuit conditions the voltage normally appearing across the DUT instead appears across the power supply’s internal series linear regulator, requiring it to dissipate considerably more power than it has to under normal operating conditions. By employing fold-back current limiting the power dissipation on the series-linear regulator is greatly reduced under overload conditions, reducing the size and cost of the series-linear regulator for a given output power rating of the DC linear power supply.

 

 

Fold-forward Current Limit

A variety of loading devices, such as electric motors, DC-DC converters, and large capacitive loads can draw large peak currents at startup. Because of this they can often be better suited for being powered by a DC power supply that has a fold-forward current limit characteristic, as depicted in Figure 2. With fold-forward current limiting after exceeding the crossover current limit the current level instead continues to increase while the voltage drops while the loading increases.

 

 

 

Figure 2: Output characteristic of a CV power supply with fold-forward current limiting

 

As one example of where fold-forward current limiting is a benefit, it can help a motor start under load which otherwise would not start under other current-limits. Indeed, with fold-back current limiting, a motor may not and then it would remain stalled, due to the reduced current.

 

Special Purpose Current Limits

Unlike the previous current limit schemes which are widely standard practice, there is a number of other current limit circuits used, often tailored for more application-specific purposes. One example of this is the current limiting employed in our 66300 series DC sources for powering mobile phones and other battery powered mobile wireless devices. Its output characteristic is depicted in Figure 3.

 

 

 

Figure 3: Agilent 66300 Series DC source output characteristics

 

We refer to this power supply series as battery emulator DC sources. One reason why is they are 2-quadrant DC sources.  Like a rechargeable battery, they need to be able to source current when powering the mobile device and then sink current when the mobile device is in its charging mode.  In Figure 3 there are actually two separate current limits; one for sourcing current and another for sinking current. Each has different and distinctive characteristics for specific purposes.

 

Many battery powered mobile wireless devices draw power and current in short, high peak bursts, especially when transmitting. To better accommodate these short, high peaks, the 66300 series DC sources have a time-limited peak current limit that is of sufficient duration to support these high peaks. They also have a programmable constant current level that will over-ride the peak current limit when the average current value of the pulsed current drain reaches this programmed level. With this approach a higher peak power mobile device can be powered from a smaller DC power source.

 

Just like an electronic load, when the 66300 series DC source is sinking current the limiting factor is how much power it is able to dissipate. Instead of using a fixed current limit, it uses a fold-forward characteristic current limit (although folding forward in the negative direction!). This is not done for reasons that a fold-forward current limit that was just discussed is used; it is done so higher charging currents at lower voltage levels can be accommodated, taking advantage of the available power that can be dissipated. Again, this provides the user with greater capability in comparison to using a fixed-value limit.

 

Other types of current limits exist for other specific reasons so it is helpful to be aware that not all current limits are the same when selecting a DC power supply for a particular application!

 

Reference: Agilent Technologies DC Power Supply Handbook, application note AN-90B, part number 5952-4020 “Click here to access”

One significant drawback of a linear DC power supply is its efficiency for most applications. You can generally design a linear DC power supply with reasonable efficiency when both the output and input voltage values are fixed. However, when either or both of these vary over a wide range, after assuring the DC power supply will properly regulate at low input voltage and/or high output voltage, it then has to dissipate considerable power the other extremes.

 

For DC power supplies running off an AC line, having to accommodate a fairly wide range of AC input voltage is a given. A 35% increase in line voltage from the minimum to the maximum value is not uncommon. Today’s high frequency switching based power supplies have resolved the issue of efficiency as a function of input line voltage variance. However, prior to widespread adaptation of high frequency switching DC power supplies, variety of different types of low-frequency pre-regulators were developed for linear DC power supplies

 

What is a pre-regulator? A pre-regulator is a circuit that provides a regulated voltage to the linear output stage from an unregulated voltage derived from the AC line voltage, with little loss of power. Although not nearly as commonly used as other pre-regulator schemes, on rare occasion ferroresonant transformers were used as an effective and efficient pre-regulator in DC power supplies.

 

What is a ferroresonant transformer? It is similar to a regular transformer in that it transforms AC voltage through primary and secondary windings. Unlike a regular transformer however, once it reaches a certain AC input voltage level it starts regulating its AC output voltage at a fixed level even as the AC input voltage continues to rise, as depicted in Figure 1. Ferroresonant transformers are also commonly called constant voltage transformers, or CVTs.

 

 

Figure 1: Ferroresonant transformer input-output transfer characteristic

 

The ferroresonant transformer employs a rather unique magnetic structure that places a magnetic shunt leakage path between the primary and secondary windings. This structure is illustrated in Figure 2. This way only part of the transformer structure saturates at a higher fixed peak voltage level during each AC half cycle. When part of the core magnetically saturates, the primary and secondary windings are effectively decoupled. The AC capacitor on the secondary side resonates with existing inductance. This provides the carry-over energy to the load during this magnetically saturated phase, holding up the voltage level. The resulting waveform is a clipped sine wave with a fairly high level of harmonic distortion as a result. Some more modern designs include additional filtering that can bring the harmonic distortion down to just a few percent however.

 

 

Figure 2: Ferroresonant transformer structure

 

A ferroresonant transformer has some very appealing characteristics in addition to output voltage regulation:

  • Provides isolation from line spikes and noise that is normally coupled through on conventional transformers
  • Provides protection from AC line voltage surges
  • Provides carry over during momentary AC line drop outs that are of a fraction of a line cycle
  • Limits its output current if short-circuited
  • Extremely robust and reliable

 

 

Because of a number of other tradeoffs it is unlikely that you will find them in a DC power supply today. High frequency switching designs pretty much totally dominate in performance and cost. Ferroresonant transformer design tradeoffs include:

  • Large physical size
  • Relatively expensive and specialized
  • Limited to a specific line frequency as it resonates at that frequency

 

 

So, even though you are very unlikely to encounter a ferroresonant transformer in a DC power supply today, it’s interesting to see there still appears to be a healthy demand for ferroresonant transformers as AC line conditioners in a wide range of sizes, up to AC line power utility sizes.  Their inherent simplicity and robustness is hard to beat when long term, maintenance-free, reliable service is paramount, and AC line regulation in many regions around the world cannot be counted on to be well controlled.

I am very fortunate to work with a lot of very smart, talented, and knowledgeable engineers with vast technical backgrounds. I also work with some very smart, talented, and knowledgeable non-technical individuals, some of whom are involved in our sales process. Last month, during a sales training session, one of these individuals identified a competitor’s power supply product that looked very similar to one of our Agilent power supply products: a mainframe with plug-in modules. Upon further investigation, it turned out that the competitor’s product really consisted of modules that were virtually fixed output power supplies while our Agilent product provides programmable output power supplies. So, in fact, these two products do not compete against each other despite the initial appearance. This experience inspired me to post about the differences between a fixed output DC power supply and a programmable output DC power supply.

Fixed output power supplies
A fixed output power supply has, well, a fixed output voltage. This means that when the power supply is plugged in and the output is on, the output voltage is a single voltage that is not expected to change – it is fixed at that voltage. These power supplies are typically used to provide simple bias for a circuit. Some are embedded on a printed circuit board or mounted inside a larger chassis with other circuits, and others may be rack mounted. Fixed output power supplies come in many forms as shown below. Some have a single output voltage while others provide multiple output voltages. One example of a fixed output power supply with multiple outputs is a PC supply (upper left in the figure) – it typically has the following DC output voltages: +3.3 V, +5 V, and +/- 12 V. These voltages provide power to the chips on the PC’s motherboard, including the microprocessor, and to the peripherals installed in the PC, such as the disk drive.



Fixed output power supplies normally have a fixed current limit setting. They typically regulate their output voltages to an accuracy of a few percent (for example, 5%). Many have output noise specifications of 50 to 150 mV peak-to-peak and typically have no measurement capability (such as output voltage or output current measurement).

Programmable output power supplies
A programmable output power supply’s output voltage can be set (programmed) by the user. This means that you can set the voltage to any value between zero and the maximum rated voltage (plus and/or minus) of the supply and change it whenever necessary. The set values are normally controlled either from the front panel of the supply with knobs or buttons, or through the built-in interface connected to a computer. Commands are sent from the computer to the supply to change its output voltage. These power supplies are typically used in test and measurement applications. They might be found on a design engineer’s bench or mounted in a rack of automated test equipment. They come in many forms as shown below. Some have a single output voltage while others provide multiple output voltages. The ability to change the output voltage is required in a circuit test environment. For example, to test a PC’s disk drive, you will need +5 V and + 12 V to power the drive. When installed in a PC, the disk drive will get power from a fixed output power supply in the PC. But when testing the disk drive outside of the PC, you should use a programmable power supply. Since the output voltage of a fixed output supply has an accuracy of a few percent, the voltage could be higher or lower than the nominal. For example, if the +5 V fixed supply has an accuracy of 5%, it could be any value from +4.75 V to +5.25 V. When installed in the PC, the disk drive has to work over this entire range of possible voltages applied to it. So to test it outside of the PC, a programmable power supply should be used and set to various voltages in this range to ensure the drive will always work.


Programmable output power supplies normally have a programmable current limit setting to help protect the device under test from exposure to excessive current. They typically regulate their output voltages to an accuracy of a few tenths of a percent or even better (for example, 0.06%). They have output noise specifications of 1 to 50 mV peak-to-peak and typically have built-in output voltage and output current measurement capability.

Summary
So the main differences between fixed output power supplies and programmable output power supplies are the ability to change the output voltage and the specifications. You can change the output voltage of a programmable supply while that of a fixed supply cannot be changed. Programmable supplies have much more accurate output voltages and much lower noise. They also can typically measure their own output voltage and current while a fixed output supply cannot. Of course, the extra capabilities of the programmable supplies add to their price, but you get what you pay for!

Back in October, I posted an explanation about what was a bipolar (four-quadrant) power supply (see post here: http://powersupplyblog.tm.agilent.com/2012/10/what-is-bipolar-four-quadrant-power.html). That post covered two-quadrant supplies as well. Last week, while in Lorton, Virginia, I had an opportunity to meet with some of our U.S. Army customers  - engineers working at Fort Belvoir. Many of the engineers worked in the Counter Measures Research Laboratory (CMRL). While they are very careful to not reveal any details about the specifics of the work they do, one of the engineers shared a story with me about two-quadrant operation that is worth repeating.

 

The story was told while I was providing a demonstration of one of our power supplies, the N6705B DC Power Analyzer (see Figure 1). I was explaining to a group of engineers that some of the 34 power modules that can be installed in the N6705B are two-quadrant power supplies: they can source current and also sink current at one voltage polarity. Other power modules are four-quadrant power supplies: they can source and sink current, and provide positive or negative voltage. This explanation inspired one of the engineers to tell the group that the N6705B helped him solve a problem!

 

A battery operated device (he did not mention what it was) came into his lab because it was not functioning properly: it had some type of intermittent problem. In an attempt to reproduce the problem, he removed the battery and connected the device’s power input terminals to a power supply on his lab bench. But even after running the device for long periods of time and through all of its operating modes, he was unable to reproduce the intermittent problem.

 

One of his colleagues suggested he try connecting the device to a two-quadrant power supply installed in the N6705B they owned. The original power supply he was using was a one-quadrant supply – it could source power, but could not absorb power. The battery that normally powers the device can source and sink (absorb) power, so perhaps a power supply that more closely mimicked the behavior of the battery could help uncover the problem. Well, this worked! With the device connected to the two-quadrant power supply in the N6705B, the intermittent problem showed up again proving that it was related to the battery being able to source and sink power – a power supply with similar characteristics was needed. Apparently, the device has a mode in which it momentarily forces current back out of the battery input terminals. That current is normally absorbed by the battery. And during that time, this intermittent problem must show up. During test, a single-quadrant power supply is unable to absorb the power and therefore does not reveal the problem. A two-quadrant power supply can sink the momentary current, and the problem was back, enabling the engineer to track it down and eliminate it! See Figure 2 for an example of the output characteristic of a two-quadrant power supply.

This example demonstrates the importance of choosing a power supply with the right output characteristics for your test. When testing a device or circuit with a power supply, the closer that power supply’s behavior is to the actual power used with the device or circuit, the more you will reveal about the actual performance of your device or circuit.  There are applications in which a two-quadrant power supply will better replicate a battery’s behavior than a single-quadrant power supply, even if you don’t expect the battery to absorb power during test. One CMRL engineer experienced this firsthand.

Trueform waveform generation technology is an exclusive technology found in Keysight’s 33500B / 33600A Series waveform generators. Trueform technology provides sizable advantages over direct digital synthesis (DDS), the incumbent technology used in waveform generators. These advantages include significantly lower waveform jitter for less test uncertainty and a true representation of the selected waveform, not an approximation. In this blog post, we will look at Trueform and compare it to DDS.

 

Conceptually, the simplest way to generate a waveform is to store its points in memory and then read those points out one after another and clock them into a DAC. After the last point has been read, the generator jumps back to the first point again to begin the next cycle. This is sometimes called “point per clock” (PPC) generation. Even though this method seems like the most intuitive way to create waveforms, it has two big drawbacks. First, to change the waveform’s frequency or sample rate, the clock frequency has to change, and making a good low-noise variable-frequency clock adds cost and complexity to the instrument. Second, since the stepwise output of the DAC is undesirable in most applications, complex analog filtering is needed to smooth the steps out. Because of its complexity and cost, this technology is used mainly in high-end waveform generators.

 

DDS uses a fixed-frequency clock and a simpler filtering scheme, so it’s less expensive than the PPC method. In DDS, a phase accumulator adds an increment to its output in every clock cycle, and the accumulator’s output represents the phase of the waveform. The output frequency is proportional to the increment, so it’s easy to change frequency even though the clock frequency is fixed. The output of the accumulator is converted from phase data into amplitude data typically by passing it through some type of look-up table. The phase accumulator design allows DDS to use a fixed clock, but still execute waveforms at a perceived faster sample rate than the clock. So with DDS, not every individual point is being expressed in the resulting output waveform. In other words, DDS is not using every point in waveform memory, but it creates a really good approximation. But since it is an approximation, waveform data is changed in some way. DDS can skip and/or repeat aspects of the waveform in an unpredictable way. In best-case scenarios, this leads to added jitter; in worst-case scenarios, severe distortion can result. Small features in the waveform can be partly or completely skipped over.

 

Keysight’s Trueform technology represents the next leap in waveform generation technology. Trueform provides the best of both worlds. It gives you a predictable low-noise waveform with no skipped waveform points like PPC technology, but at the price point of DDS technology. Trueform works by employing an exclusive virtual variable clock with advanced filtering techniques that track the sample rate of the waveform. In the following sections, we will look at some of the waveform generation advantages Trueform provides over DDS.

 

Improved signal quality

One of the key advantages Trueform provides over DDS is better overall signal quality. One of the best ways to show this is by doing a jitter measurement comparison with DDS. The following figures show a jitter measurement made on a 10-MHz pulse signal using a high-performance oscilloscope. The scope view is zoomed in on the rising edge of the pulse signal with the persistence setting of the scope turned on. The histogram function of the scope is used to measure the period jitter of the signals. The standard deviation measurement in each figure is circled in red and represents the signal’s RMS jitter. The Trueform pulse signal jitter measurement is shown in the below figure and the DDS pulse signal jitter measurement is shown in the next figure.

 Trueform signal with < 5ps of RMS jitter

Trueform signal with < 5ps of RMS jitter

DDS signal with > 50ps of RMS jitter

DDS signal with > 50ps of RMS jitter

In the above figures both the amplitude and time scales for the scope are the same. The Trueform pulse waveform has more than 10 times less jitter compared to the DDS pulse waveform. The substantially lower jitter that Trueform offers over DDS means less uncertainty in your tests. This is especially true when you consider edge-based timing applications like generating a clock signal, trigger signal or communication signal.

 

The waveform you create is the waveform you get

As we mentioned earlier, DDS uses a fixed clock and a phase accumulator so it cannot guarantee that every point or feature in a waveform will be played. The higher the frequency, the more gaps you will see in the output waveform compared to the ideal waveform. Trueform, on the other hand, plays every waveform point regardless of the set frequency or sample rate. This becomes critical when you are dealing with a waveform that may have a small detail that is critical to the test you are performing. As an example, we created an arbitrary waveform that consisted of a pulse with seven descending amplitude spikes on top of the pulse. The waveform was then loaded into a Trueform waveform generator and a DDS waveform generator. First the waveform was played at a 50-KHz frequency on each generator. The result was captured on a scope, shown in the below figure. The yellow trace is the Trueform waveform and the green trace is the DDS waveform.

Trueform on top in yellow and DDS in green

Trueform on top in yellow and DDS in green

At 50 KHz, each generator was able to reproduce the waveform with seven spikes on top of the pulse. You can see that the Trueform spikes reached higher amplitude. In the below figure scope screen shot, the waveforms were played again, but this time at 100 KHz.

 Trueform on top in yellow and DDS in green. Note that all the DDS spike points were skipped

Trueform on top in yellow and DDS in green. Note that all the DDS spike points were skipped

At 100 KHz, the Trueform waveform generator played all seven spikes and the DDS generator did not play any of the spikes. In the below figure scope screen shot, the waveforms were played again, but the frequency was doubled again to 200 KHz.

Trueform on top in yellow and DDS in green. Note that DDS only shows 3 of the seven spikes

Trueform on top in yellow and DDS in green. Note that DDS only shows 3 of the seven spikes

At 200 KHz, once again, the Trueform waveform generator shows all seven spikes in the waveform. The DDS generator went from playing no spikes at 100 KHz to playing three spikes at 200 KHz. Notice that the three spikes played in the 200 KHz waveform do not match the correct time location of any of the seven spikes that are in the actual waveform points. These waveform examples demonstrate that when working with waveforms that have fine detail, DDS cannot be trusted.

 

Keysight’s Trueform technology offers a new alternative that blends the best of DDS and PPC architectures, giving you the benefits of both without the limitations of either. Trueform technology uses an exclusive digital sampling technique that delivers unmatched performance at the same low price you are accustomed to with DDS.

 

For more info on the 33500B / 33600A series with Trueform technology click here