Skip navigation
All Places > Keysight Blogs > General Electronics Measurement > Blog > 2016 > November
2016
StevenLee

Where's the CC button?

Posted by StevenLee Employee Nov 16, 2016

One thing has been consistent over that period of time: we are asked about how to put a power supply in constant current (CC) mode. The question takes on many forms, one of which is "Where's the CC button?". Given that this is an important fundmental concept about power supplies, I figured it was a good place to start this power blog. Well, the simple answer is: there is no CC button, but continue reading to find out how to "put" a power supply in CC mode.... There are two primary output operating modes for most power supplies: constant voltage (CV) mode and constant current (CC) mode. While you don't set the mode, you do set the output voltage setting and the output current setting. Then, the output operating mode is determined by what you connect to the output (the load). The output operating mode is detemined by three things:

 

  1. Output voltage setting (Vset)

 

  1. Output current setting (Iset)

 

  1. Load value (Rload)

 

 

If the load current is low enough such that the current that is drawn is LESS than the current setting, the power supply will operate in CV mode regulating the voltage at a constant value with the current determined by the load. If the load current is high enough such that the load is trying to draw MORE current than the current setting, the power supply will limit the current at the current setting value and operate in CC mode regulating the current with the voltage determined by the load.

Consider a simple resistive load, Rload:

If Rload > Vset / Iset, the power supply will be operating in CV mode.

If Rload < Vset / Iset, the power supply will be operating in CC mode.





The two extreme examples of the above are with Rload open (near infinite ohms) and Rload shorted (near zero ohms). When a power supply output is open (Rload = infinite, a vertical line from the origin on the graph), it should be obvious that the output will be in CV mode with no current flowing. When a power supply output is shorted (Rload = zero, a horizontal line from the origin on the graph), it should be obvious that the output will be in CC mode with near zero voltage. Note that Agilent power supplies typically show the dynamic operating mode on the front panel. If the power supply is unable to regulate either the voltage or the current, the indicator will show UNR (unregulated) since neither the voltage nor the current is being regulated. This condition is rare, but can happen sometimes if Rload = Vset/Iset, or if there is a problem with the internal circuitry.

Load effect is a power supply specification (also known as load regulation) that describes how well the power supply can maintain its steady-state output setting when the load changes. More formally, it specifies the maximum change in steady-state DC output voltage (or current) resulting from a specified change in the load current (or voltage), with all other influence quantities maintained constant. So, when a power supply is regulating its output voltage in CV (constant voltage) mode, this specification tells you how much the voltage can change when the current changes. Here is an example:

Let’s say the voltage load effect specification for a 20 V, 5 A power supply is 2 mV and is specified for any load change. This means for any current change within the rating of the supply (in this case, up to 5 A), the output voltage will not change by more than 2 mV. For example, if the power supply is set to 10 V, the actual output may measure 9.999 V with no load (0 A). (Note that the difference between the setting and the actual output voltage is a different specification called programming accuracy.) If you then increase the current from 0 A to a full load condition of 5 A, the load effect specification guarantees that the output voltage will not change by more than 2 mV, so it will be somewhere between 9.997 V and 10.001 V. So if the actual output voltage started at 9.999 V with a 0 A load and measured 9.9982 V with a 5 A load, the load effect for this output when set for 10 V measures 0.8 mV (9.999 – 9.9982), well within the 2 mV specification. You must make the second voltage measurement immediately following the load current change to avoid capturing any short-term drift effects.



In the above example, the specified change in load current was “any load change”. Of course, it is implied that the load change is within the output ratings of the supply. You cannot change the output current from 0 A to 100 A on a 5 A power supply. Some load effect specifications state that the load change is a 50% change (e.g., 2.5 A to 5 A) while others may say 10% to 90% of full load (e.g., 0.5 A to 4.5 A).

And what does “with all other influence quantities maintained constant” mean? Things like temperature and the AC line input voltage can affect the output parameter, so these things must be held constant in order to see only the effect of the load change. The effects on the power supply output of changes in each of these influencing quantities (temperature, AC line input voltage) are described in different specifications.

Most performance power supplies have load effect specifications in the range of just a few hundred uV up to a few mV. A lower performance model may have a load effect specification of between 10 mV and 100 mV. Power supplies with higher maximum voltage ratings and higher maximum power ratings typically have higher load effect specifications.

If you have an application where maintaining an exact voltage at your DUT is critical and your DUT draws different amounts of current at different times, you will want to use a power supply with a low load effect specification. If changes in the voltage at your DUT with changes in DUT current are less critical to you, most power supplies will perform well for your application.

The quick answer to this question is, yes, most standard DC power supplies can be used as current sources. However, this question deserves more attention, so what follows is the longer answer.

 

Most DC power supplies can operate in constant voltage (CV) or constant current (CC) mode. CV mode means the power supply is regulating the output voltage and the output current is determined by the load connected across the output terminals. CC mode means the power supply is regulating the output current and the output voltage is determined by the load connected across the output terminals. When operating in CC mode, the power supply is acting like a current source. So any power supply that can operate in CC mode can be used as a current source (click here for more info about CV/CC operation).

 

Is a standard power supply a good current source? An ideal current source would have infinite output impedance (an ideal voltage source would have zero output impedance). No power supply has infinite output impedance (or zero output impedance) regardless of the mode in which it is operating. In fact, most power supply designs are optimized for CV mode since most power supply applications require a constant voltage. The optimization includes putting an output capacitor across the output terminals of the power supply to help lower output voltage noise and also to lower the output impedance with frequency. So the effectiveness of a standard power supply as a current source will depend on your needs with frequency.

 

At DC, a power supply in CC mode does make a good current source. Typical CC load regulation specifications support this notion (click here for more info about load regulation). For example, an Agilent N6752A power supply (maximum ratings of 50 V, 10 A, 100 W) has a CC load regulation specification of 2 mA. This means that the output current will change by less than 2 mA for any load voltage change. So when operating in CC mode, a 50 V output load change will produce a current change of less than 2 mA. If we take the delta V over worst case delta I, we have 50 V / 2 mA = 25 kΩ. This means that the DC output impedance will always be 25 kΩ or more for this power supply. In fact, the current will likely change much less than 2 mA with a 50 V load change making the DC output impedance in CC mode much greater than 25 kΩ.

 

Of course, a power supply’s effectiveness as a current source should be judged by the output impedance beyond the DC impedance. See the figure below for a graph of the N6752A CC output impedance with frequency:

 

If the graph continued in the low frequency direction, the output impedance would continue to rise as a “good” current source should. At higher frequencies, the CC loop gain inside the product begins to fall. As the loop gain moves through unity and beyond, the output capacitor in the supply dominates the behavior of the output impedance, so at high frequencies, the output impedance is lower. So how good the power supply is as a current source depends on your needs with frequency. The higher the output impedance, the better the current source. The output impedance also correlates to the CC transient response (and to a much lesser extent, the output programming response time).

 

The bottom line here is that in most applications, a standard DC power supply can be used in CC mode as a current source.

One feature we include in our Agilent system DC power supplies for providing additional safeguard for overload-sensitive DUTs is over current protect, or OCP. While some may think this is something separate and independent of current limiting, OCP actually works in concert with current limiting.

 

Current limiting protects overload-sensitive DUTs by limiting the maximum current that can be drawn by the DUT to a safe level. There are actually a variety of current limit schemes, depending on the level of protection required to safeguard the DUT during overload. Often the current limit is relatively constant, but sometimes it is not, depending on what is best suited for the particular DUT. Additional insights on current limits are provided in an earlier posting, entitled “Types of current limits for over-current protection on DC power supplies“.

 

By limiting the current to a set level may DUTs are adequately protect from too much current and potential damage. When in current limit, if the overload goes away the power supply automatically goes back to constant voltage (CV) operation. However, current limit may not be quite enough for some DUTs that are very sensitive to overloads. This is where OCP works together with the current limit to provide an additional level of protection. With OCP turned on, when the DC power supply enters into current limit OCP takes over after a specified time delay and shuts down the output of the DC power supply. The delay time is programmable. This prevents OCP from shutting down the DC power supply from short current spikes and other acceptably short overloads that are not considered harmful. Like over voltage protect or OVP, after tripping the output needs to be disabled and an Output Protect Clear needs to be exercised in order to reset the power supply so that its output can be re-enabled.  Unlike OVP, OCP can be turned on and off and its default is usually off. In comparison, OVP is usually always enabled and cannot be turned off. A typical OCP event is illustrated in Figure 1.

 

 

 

Figure 1: OCP operation

 

When powering DUTs, either on the bench or in a production test system, it is always imperative that adequate safeguards are taken to protect both the DUT as well as the test equipment from inadvertent damage. Over current protect or OCP is yet another of many features incorporated in system DC power supplies you can take advantage of to protect overload-sensitive DUTs from damage during test!

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

 

Download the free "4 Ways to Build Your Power Supply Skill Set" eBook.
 

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.

 

 

In a previous posting; “How Does a Power Supply regulate It’s Output Voltage and Current?” I showed how feedback loops are used to control a DC power supply’s output voltage and current.  Feedback is phenomenally helpful in providing a DC power supply with near-ideal performance. It is the reason why load regulation is measured in 100ths of a percent. A major reason for this is it bestows the power supply, if a voltage source, with near zero impedance, or as a current source, with high output impedance. How does it do this?

 

Download the free "4 Ways to Build Your Power Supply Skill Set" eBook.

 

The impedance of a typical DC power supply’s output stage (like the conceptual one illustrated in the above referenced posting) is usually on the order of an ohm to a couple of ohms. This is the open-loop output impedance; i.e. the output impedance before any feedback is applied around the output.   If no feedback were applied we would not have anywhere near the load regulation we actually get. However, when the control amplifier provides negative feedback to correct for changes in output when a load is applied, the performance is transformed by the ratio of 1 + T, where T is loop gain of the feedback system. As an example, the output impedance of the DC power supply operating in constant voltage becomes:

 

Zout (closed loop) = Zout (open loop) / (1+T)

 

The loop gain T is approximately the gain of the operational amplifier times the attenuation of the voltage divider network. In practical feedback control systems the gain of the amplifier is quite large at and near DC, possibly as high as 90 dB of gain. This reduces the power supply’s DC and low frequency output to just milliohms or less, providing near ideal load regulation performance. Another factor in practical feedback control systems is the loop gain is rolled off in a controlled manner with increasing frequency in order to maintain stability. Thus at higher frequency the output impedance of a DC power supply operating as a voltage source increases towards its open loop impedance value as the loop gain decreases. This is illustrated in the output impedance plots in Figure 1, for the Agilent 6643A DC power supply.

 

 

Figure 1: Agilent 6643A 35V, 6A system DC power supply output impedance

 

As can be seen in Figure 1, for constant voltage operation, the 6643A DC power supply is just about 1 milliohm at 100 Hz, and exhibits an inductive output characteristic with increasing frequency as the loop gain decreases.

 

As also can be seen in Figure 1, feedback control works in a similar fashion for constant current operation. While a voltage source ideally has zero output impedance, a current source ideally has infinite impedance.  For constant current operation the 6643A DC power supply exhibits 10 ohms impedance at 100 Hz and rolls off in a capacitive fashion as frequency increases. However, for the 6643A, it is not so much the constant current control loop gain dropping off with frequency but the output filter capacitance dominating the output impedance. While the 6643A can be used as an excellent, well-regulated current source (see posting: “Can a standard DC power supply be used as current source?”) it is first and foremost optimized for being a voltage source. Some output capacitance serves towards that end.

 

 

An example of one use for the output impedance plots of a DC power supply is to estimate what the amount of load-induced AC ripple might be, based on the frequency and amplitude of the current being drawn by the load, when powered by power supply operating in constant voltage.

 

Two years ago I added a post here to “Watt’s Up?” titled:  “Zero-burden ammeter improves battery run-down and charge management testing of battery-powered devices” (click here to review). In this post I talk about how our N6781A 20V, 3A 20W SMU (and now our N6785A 20V, 8A, 80W as well) can be used in a zero-burden ammeter mode to provide accurate current measurement without introducing any voltage drop. Together with the independent DVM voltage measurement input they can be used to simultaneously log the voltage and current when performing a battery run-down test on a battery powered device. This is a very useful test to perform for gaining valuable insights on evaluating and optimizing battery life. This can also be used to evaluate the charging process as well, when using rechargeable batteries. The key thing is zero-burden current measurement is critical for obtaining accurate results as impedance and corresponding voltage drop when using a current shunt influences test results. For reference the N678xA SMUs are used in either the N6705B DC Power Analyzer mainframe or N6700 series Modular Power System mainframe.

There are a few considerations for getting optimum performance when using the N678xA SMU’s in zero-burden current measurement mode. The primary one is the way the wiring is set up between the DUT, its battery, and the N678xA SMU. In Figure 1 below I rearranged the diagram depicting the setup in my original blog posting to better illustrate the actual physical setup for optimum performance.

 

Figure 1: Battery run-down setup for optimum performance

Note that this makes things practical from the perspective that the DUT and its battery do not have to be located right at the N678xA SMU.  However it is important that the DUT and battery need to be kept close together in order to minimize wiring length and associated impedance between them. Not only does the wiring contribute resistance, but its inductance can prevent operating the N678xA at a higher bandwidth setting for improved transient voltage response. The reason for this is illustrated in Figure 2.

 

 

Figure 2: Load impedance seen across N678xA SMU output for battery run-down setup

The load impedance the N678xA SMU sees across its output is the summation of the series connection of the DUT’s battery input port (primarily capacitive), the battery (series resistance and capacitance), and the jumper wire between the DUT and battery (inductive). The N678xA SMUs have multiple bandwidth compensation modes. They can be operated in their default low bandwidth mode, which provides stable operation for most any load impedance condition. However to get the most optimum voltage transient response it is better to operate N678xA SMUs in one of its higher bandwidth settings. In order to operate in one of the higher bandwidth settings, the N678xA SMUs need to see primarily capacitive loading across its remote sense point for fast and stable operation. This means the jumper wire between the DUT and battery must be kept short to minimize its inductance. Often this is all that is needed. If this is not enough then adding a small capacitor of around 10 microfarads, across the remote sense point, will provide sufficient capacitive loading for fast and stable operation. Additional things that should be done include:

  • Place remote sense connections as close to the DUT and battery as practical
  • Use twisted pair wiring; one pair for the force leads and a second pair for the remote sense leads, for the connections from the N678xA SMU to the DUT and its battery 
  • By following these best practices you will get the optimum performance from your battery run-down test setup!
  •  

 

Last month, on June 2, 2015, I celebrated working for Hewlett-Packard/Agilent Technologies/Keysight Technologies for 35 years. During the earlier times of my career, on significant anniversaries such as 10 years or 20 years, employees could choose from a catalog of gifts to have their contributions to the company recognized. That tradition has been discontinued, but I did select a couple of nice gifts over the years. During my HP days, one gift I selected was a clock with a stand shown here:

I have had that clock for decades and it uses a silver oxide button cell battery (number 371). I have to replace the battery about once per year and wondered if that made sense based on the battery capacity and the current drain the clock presents to the battery. I expected the battery to last longer so I wanted to know if I was purchasing inferior batteries. These 1.5 V batteries are rated for about 34 mA-hours. So I set out to measure the current drain using our N6705B DC Power Analyzer with an N6781A 2-Quadrant Source/Measure Unit for Battery Drain Analysis power module installed. Making the measurement was simple…..making the connections to the tiny, delicate battery connection points was the challenging part. After one or two failed attempts (I was being very careful because I did not want to damage the connections), I solicited the help of my colleague, Paul, who handily came up with a solution (thanks, Paul!). Here is the final setup and a close-up of the connections:

 

I set the N6781A voltage to 1.5 V and used the N6705B built-in data logger to capture current drawn by the clock for 5 minutes, sampling voltage and current about every 40 us. The clock has a second hand and as expected, the current showed pulses once per second when the second hand moved (see Figure 1). Each current pulse looks like the one shown in Figure 2. There was an underlying 200 nA being drawn in between second-hand movements. All of this data is captured and shown below in Figure 3 showing the full 5 minute datalog along with the amp-hour measurement (0.28 uA-hours) and average current measurement (3.430 uA) between the markers.

 

Given the average current draw, I can calculate how long I would expect a 34 mA-hour battery to last:                  34 mAh / 3.430 uA average current = 9912.54 hours = about 1.13 years This is consistent with me changing the battery about every year, so once again, all makes sense in the world of energy and electronics (whew)! Thanks to the capabilities of the N6705B DC Power Analyzer, I now know the batteries I’m purchasing are lasting the expected time given the current drawn by the clock. How much current is your product drawing from its battery?

Over the past few years here on “Watt’s Up?” I have posted several articles and application pieces on performing battery drain analysis for optimizing run time on mobile wireless devices. The key product we provide for this application space is the N6781A 20V, +/-3A, 20W source measure module for battery drain analysis. A second related product we offer is the N6782A 20V, +/-3A, 20W source measure module for functional test. The N6782A has a few less key features used for battery drain analysis but is otherwise the same as the N6781A. As a result the N6782A is preferred product for testing many of the components used in mobile devices, where the extra battery drain analysis features are not needed. These products are pictured in Figure 1. While at first glance they may appear the same, one thing to note is the N6781A has an extra connector which is independent voltmeter input. This is used for performing a battery run-down test, one of a number of aspects of performing battery drain analysis. Details on these two SMUs can be found on by clicking on: N6781A product page.  N6782A product page,

 

 

 

Figure 1: Keysight N6781A SMU for battery drain analysis and N6782A for functional test

 

These products have greatly helped customers through their combination of very high performance specialized sourcing and measurement capabilities tailored for addressing the unique test challenges posed by mobile wireless devices and their components. However, things have continued to evolve (don’t they always!). Today’s mobile devices, like smart phones, tablets and phablets, have an amazing amount of capabilities to address all kinds of applications. However, their power consumption has grown considerably as a result. They are now utilizing much larger batteries to support this greater power consumption in order to maintain reasonably acceptable battery run-time. Optimizing battery life continues to be a critical need when developing these products. With their higher power however, there is in turn a greater need for higher power SMUs to power them during test and development. In response we have just added two new higher power SMUs to this family; the N6785A 20V, +/-8A, 80W source measure module for battery drain analysis and the N6785A 20V, +/-8A, 80W source measure module for functional test. These products are pictured in Figure 2. Details on these two new higher power SMUs can be found on by clicking on: N6785A product page.  N6786A product page.

 

 

 

Figure 2: Keysight N6785A SMU for battery drain analysis and N6786A for functional test

 

A press release went out about these two new SMUs yesterday; Click here to view. With their greater current and power capability, customers developing and producing these advanced mobile wireless devices and their components now have a way to test them to their fullest, not being encumbered by power limitations of lower power SMUs.

 

This is exciting to me having been working within the industry for quite some time now, helping customers increase battery life by improving how their devices make more efficient use of the battery’s energy. A key part of this has been by using our existing solutions for battery drain analysis to provide critical insights on how their devices are making use of the battery’s energy.  There is a lot of innovation in the industry to make mobile wireless devices operate with even greater efficiency at these higher power and current levels. There is no other choice if they are going to be successful. Likewise, it is great to see continuing to play a key role in this trend in making it a success!