The Joys of Owning a DC Power Supply

Back in 2000, when I was a young man and had just begun working for Agilent., Gary came up to me and told me that there was a brand new power supply that was going to be scrapped and that I should take it.  The power supply was a 6024A 200 W autoranger that was a very old design, even at that time.  I took a look and it had analog meters and no keypad!  I questioned Gary on why I would want it and he said, “Trust me it will come in handy”.

Here it is:

Gary, being older and wiser than I was at the time, was right.  I had just started working at Agilent and I was still living with my parents so I put it in the basement.  When I moved, I took it with me to my apartment.  Since then there have been several occasions where it came in very handy. 

The first is that there is always an occasional battery that needs charging so I can hook up the 6024A and charge my batteries (we highly recommend putting a blocking diode in series with the output of the supply to protect your battery). It is controlled so there is no danger to the batteries and it totally works like a champ.

The biggest use I have had for it has been troubleshooting the various electronic gadgets that I own.  The first time I used it for this purpose was to troubleshoot my wireless router.  One day it just stopped working.  Instead of throwing it out (wireless G routers were pretty expensive at that point) I figured that I would use my power supply to determine whether it was a problem with the wall adaptor for the router or the router itself.  I bought some cables from an electronics store, grabbed my soldering iron, and hooked my router up to my DC Power Supply.  I set the voltage and the current limit and the router came to life.  All I need to do was to get a new adapter and I was good to go. 

A similar thing happened with an internet streaming box that I had connected to my TV.  All of a sudden it stopped working.  I called their tech support (a terrible experience) they told me that I had to purchase a new box.  Since I had nothing to lose, I chopped up the DC adaptor and hooked it up to my 6024A.  I wish that I took a picture.  I had the box connected to my TV being powered by my 6024A.  The box came to life and I did not need to spend 100 bucks on a new one

Not everyone is as lucky as I am to work for Agilent and get a free HP supply but if you can get your hands on any kind of programmable DC power supply I recommend taking it.  I am glad that I have my power supply and I keep it on my bench, always ready to use.

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Types of current limits for over-current protection on DC power supplies

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”

The difference between constant current and current limit in DC power supplies

Constant Voltage/Constant Current (CC/CV) Power Supplies

In most of our discussions in “Watt’s Up?” on current limiting we have primarily talked about power supplies as having a constant current (CC) output characteristic. This is what is found in many lab and industrial system power supplies, including most of the power supplies provided by us. Even though the terms often get used interchangeably, there is actually a distinction between constant current and current limit. To help explain this distinction, Figure 1 illustrates the output characteristics of a constant voltage/constant current (CV/CC) power supply.

Figure 1: Operating locus of a CC/CV power supply

Five operating points are depicted in Figure 1:

1. With no load (i.e. infinite load resistance): Iout = 0 and Vout = Vset
2. With a load resistance of RL > Vset/Iset: Iout = Vset/RL and Vout = Vset
3. With a load resistance of RL = Vset/Iset: Iout = Iset and Vout = Vset
4. With a load resistance of RL < Vset/Iset: Iout = Iset and Vout = Iset*RL
5. With a short circuit (i.e. zero load resistance): Iout = Iset and Vout = 0

The advantage of a CV/CC power supply is it can be used as either a voltage source or a current source, providing reasonable performance in either mode. The point at which RL = Vset/Iset is the mode crossover point where the power supply transitions between CV and CC operation. For a CV/CC power supply there is a sharp transition between CV and CC operation. Note that for an ideal CV/CC power supply the CV slope is zero (horizontal), indicating zero output resistance for CV operation while the CC slope is infinite (vertical), indicating infinite output resistance for CC operation. Note that this is at DC. How close the slope of each mode is to ideal is what determines quality of load regulation for each.  To achieve good performance for both CV and CC modes requires carefully designed and more complex control loops for each mode. More details about using a power supply as a current source is provided in an earlier posting here, entitled: “Can a standard DC power supply be used as a current source?”

Constant Voltage/Current Limiting Power Supplies

In comparison a constant voltage/current limiting (CV/CL) power supplies are intended to be used only as a voltage source while providing over-current protection for the DUT, as well as protection for the power supply itself. Figure 2 depicts typical output characteristics of a CV/CL power supply.

Figure 2: Operating locus of a CV/CL power supply

In CV/CL power supplies the current limit may be a fixed maximum value or it may be settable. In comparison to Figure 1 CV operation is still the same. However, what is found at the current limit cross-over point there is loss of voltage regulation where the voltage starts falling off. Unlike true CC operation in a CV/CC power supply, CL operation does not typically have as sharply a defined cross-over point and once in CL it may not be tightly regulated between the cross-over and short circuit points. The reason for this is CL control circuits are usually more basic in nature in comparison to a true CC control loop. CL is meant for over-current protection only, not CC operation.  For this reason the correct use of CL is to set its value a bit higher than the maximum current required by the DUT. This assures good voltage regulation for the full range of normal loading. You may find many of the more basic bench power supplies have CV/CL operation and may not be useful as current sources as a result.

Reference: Agilent Technologies DC Power Supply Handbook, application note AN-90B, part number 5952-4020

Happy New Year! May it be a “powerful” one!!

I just want to take this opportunity to thank all of our readers for taking an interest in the Watt’s Up? blog posts. Power-related topics have been a part of our professional careers (and personal lives) for decades and we are both thrilled and honored to be able to share some of what we have learned over the years with you, our readers.

During 2012, we have seen our readership grow by more than 5 times that of 2011! And we hope to see that growth continue during 2013. To make that happen, we would like to hear from you, our readers, about what power-related topics are of greatest interest to you for 2013. Please comment below and we will be happy to post about any power-related issues you bring up!  

Finally, on behalf of Ed Brorein, Matt Carolan, and myself, and all of us at Agilent Technologies, Happy New Year! May 2013 be a “powerful” year for you all!!

Two-quadrant power supplies are better than one!

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.

More on power supply current source-to-sink crossover characteristics

On my earlier posting “Power supply current source-to-sink crossover characteristics” I showed what the effects on the output voltage of a unipolar two-quadrant-power supply were, resulting from the output current on the power supply transitioning between sourcing and sinking. In that example scenario, the power supply was maintaining a constant output voltage and the transitioning between sourcing and sinking current was dictated by the external device connected to and being powered by the power supply. This is perhaps the most common scenario one will encounter that will drive the power supply between sourcing current and sinking current.

Other scenarios do exist that will drive a unipolar two-quadrant power supply to transition between sourcing and sinking output current. One scenario is nearly identical to the earlier posting. However, instead of the device transitioning its voltage between being less and greater than the power supply powering it, the power supply instead transitions its voltage between being less and greater than the active device being normally powered.  A set up for evaluating this scenario on an Agilent N6781A two-quadrant DC source is depicted in Figure 1.

Figure 1: Evaluating current source-to-sink crossover on an N6781A operating in constant voltage

In this scenario having the DC source operating as a voltage source and transitioning between 1.5 and 4.5 volts causes the current to transition between -0.75 and +0.75A.  The voltage and current waveforms captured on an oscilloscope are shown in Figure 2.

Figure 2: Voltage and current waveforms for the set up in Figure 1

The waveforms in Figure 2 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 3.

Figure 3: Expanded voltage and current waveforms for the set up in Figure 1

As can be seen the voltage ramp transitions smoothly at the threshold point, or zero crossing point, of the current waveform. The reason being is that the DC is maintaining its operation as a voltage source. Its voltage feedback loop is always in control.

Yet one more scenario that will drive a unipolar two-quadrant source to transition between sourcing and sinking current is operate it as a current source and program is current setting between positive and negative values. In this case the device under test that was used is a voltage source.  One real-world example is cycling a rechargeable battery by alternately applying charging and discharging currents to it. The set up for evaluating this scenario, again using an N6781A two-quadrant DC source is depicted in Figure 4.

Figure 4: Evaluating current source-to-sink crossover on an N6781A operating in constant current

For Figure 4 the N6781A was set to operate in constant current and programmed to alternately transition between -0.75A and +0.75A current settings. The resulting voltage and current waveforms are shown in Figure 5.

Figure 5: Voltage and current waveforms for the set up in Figure 4

The waveforms in Figure 5 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 6.

Figure 6: Expanded voltage and current waveforms for the set up in Figure 4

As the N6781A is operating in current priority the interest is in how well it controls its current while transitioning through the zero-crossing point. As observed in Figure 6 it transitions smoothly through the zero-crossing point. The voltage performance is determined by the DUT, not the N6781A, as the N6781A is operating in constant current.

So what was found here is, for a unipolar two-quadrant DC source, transitioning between sourcing and sinking current should generally be virtually seamless as, under normal circumstances, should remain in either constant voltage or constant current during the entire transition.

Power supply current source-to-sink crossover characteristics

A two-quadrant power supply is traditionally one that outputs unipolar voltage but is able to both source as well as sink current. For a positive polarity power source, when sourcing current it is operating in quadrant 1 as a conventional power source. When sinking current it is operating in quadrant 2 as an electronic load. Conversely, a negative polarity two-quadrant  power source operates in quadrants three and four. Further details on power supply operating quadrants are provided in a recent posting here in ‘Watt’s Up?”, “What is a bipolar (four-quadrant) power supply?” Often a number of questions come up when explaining two-quadrant power supply operation, including:

• What does it take to get the power supply operating as a voltage source to cross over from sourcing to sinking current?
• What effect does crossing over from sourcing to sinking current have on the power supply’s output?

For a two-quadrant voltage source to be able to operate in the second quadrant as an electronic load, the device it is normally powering must also be able to source current and power as well as normally draw current and power. Such an arrangement is depicted in Figure 1, where the device is normally a load, represented by a resistance, but also has a charging circuit, represented by a switch and a voltage source with current-limiting series resistance.

Figure 1: Voltage source and example load device arrangement for two-quadrant operation.

There is no particular control on a two-quadrant power supply that one has to change to get it to transition from sourcing current and power to sinking current and power from the device it is normally powering. It is simply when the source voltage is greater than the device’s voltage then the voltage source will be operating in quadrant one sourcing power and when the source voltage is less than the device’s voltage the voltage source will be operating in quadrant two as an electronic load. In figure 1, during charging the load device can source current back out of its input power terminals as long as the charger’s current-limited voltage is greater than the source voltage.

It is assumed that load device’s load and charge currents are lower than the positive and negative current limits of the voltage source so that the voltage source always remains in constant voltage (CV) operation. A step change in current is the most demanding from a transient standpoint, but as the voltage source is always in its constant voltage mode it handle the transition well as its voltage control amplifier is always in control. This is in stark contrast to a mode cross over between voltage and current where different control amplifiers need to exchange control of the power supply’s output. In this later case there can be a large transient while changing modes. See another posting, “Why Does My Power Supply Overshoot at Current Limit? Insights on Mode Crossover” for further information on this.  There is a specification given on voltage sources which quantifies the impact one should expect to see from a step change in current going from sourcing current to sinking current, which is its transient voltage response.  A transient voltage response measurement was taken on an N6781A two-quadrant DC source, stepping the load from 0.1 amps to 1.5 amps, roughly 50% of its rated output current.

Figure 2: Agilent N6781A transient voltage response measurement for 0.1A to 1.5A load step

However, the transient voltage response shown in Figure 2 was just for sourcing current. With a well-designed two-quadrant voltage source the transient voltage response should be virtually unchanged for any step change in current load, as long as it falls within the voltage source’s current range.  The transient voltage response for an N6781A was again capture in Figure 3, but now for stepping the load between -0.7A and +0.7A.

Figure 3: Agilent N6781A transient voltage response measurement for -0.7A to +0.7A load step

As can be seen in Figures 2 and 3 the voltage transient response for the N6781A remained unchanged regardless of whether the stepped load current was all positive or swung between positive and negative (sourcing and sinking).

While the transient voltage response addresses the dynamic current loading on the voltage source there is another specification that addresses the static current loading characteristic, which is the DC load regulation or load effect.  This is a very small effect on the order of 0.01% output change for many voltage sources. For example, for the N6781A the load effect in its 6 volt range is 400 microvolts for any load change. In the case of the N6781A being tested here the DC change was the same for both the 0.1 to 1.5 amp step and the -0.7 to +0.7 amp step change.

There are two more scenarios which will cause a two-quadrant power supply transition between current sourcing and sinking.  The first is very similar to above with the two-quadrant power supply operating in constant voltage (CV) mode, but instead of the DUT changing, the power supply changes its voltage level instead.  The final scenario is having the two-quadrant power supply operating in constant current with the DUT being a suitable voltage source that is able to source and sink power as well, like a battery for example. Here the two-quadrant power supply can be programmed to change from a positive current setting to a negative current setting, thus transitioning between sourcing and sinking current again, and its current regulating performance is now a consideration.  Both good topics for future postings!