Can a standard DC power supply be used as a current source?

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.

Experiences with Power Supply Common Mode Noise Current Measurements

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

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

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

Figure 1: Common mode noise current base-line reading

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

Figure 2: Common mode noise current measurement

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

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

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

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

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

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

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

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

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

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

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

Pay Attention to the Impact of the Bypass Capacitor on Leakage Current Value and Test Time

It is no secret there is big demand for all kinds of wireless battery powered devices and, likewise, the components that go into these devices. These devices and their components need to be very efficient in order to get the most operating and standby time out of the limited amount of power they have available from the battery. Off-mode and leakage currents of these devices and components need to be kept to a minimum as an important part of maximizing battery run and standby time. Levels are typically in the range of tens of microamps for devices and just a microamp or less for a component. Off-mode and leakage currents are routinely tested in production to assure they meet specified requirements. The markets for wireless battery powered devices and their components are intensely competitive. Test times need to be kept to a minimum, especially for the components. It turns out the choice of the input power bypass capacitor being used, either within the DUT on the DUT’s test fixture, can have a large impact on the leakage current value and especially the test time for making an accurate leakage current measurement.

Good things come in small packages?
A lot has been done to provide greater capacitance in smaller packages for ceramic and electrolytic capacitors, for use in bypass applications. It is worth noting that electrolytic and ceramic capacitors exhibit appreciable dielectric absorption, or DA. This is a non-linear behavior causing the capacitor to have a large time-dependent charge or discharge factor, when a voltage or short is applied. It is usually modeled as a number of different value series R-C pairs connected in parallel with the main capacitor. This causes the capacitor to take considerable time to reach its final steady state near-zero current when a voltage is applied or changed. When trying to test the true leakage current on a DUT it may be necessary to wait until the current on any bypass capacitors has reached steady state before a current measurement is taken. Depending on the test time and capacitor being used this could result in an unacceptably long wait time.

So how do they compare?
In Figure 1 I captured the time-dependent current response waveform for a 5.1 megohm resistor, a 5.1 megohm resistor in parallel with 100 microfarad electrolytic capacitor, and finally a 5.1 megohm resistor in parallel with 100 microfarad film capacitor, when a 5 volt step stimulus was applied.

Figure 1: Current response of different R-C loads to 5 volt step

The 5.1 megohm resistor (i.e. “no capacitor”) serves as a base line to compare the affect the two different bypass capacitors have on leakage current measurement. The film capacitor has relatively ideal electrical characteristics in comparison to an equivalent electrolytic or ceramic capacitor. It settles down to near steady state conditions within 0.5 to 1 second. At 3 to 3.5 seconds out (marker placement in Figure 1) the film capacitor is contributing a fairly negligible 42 nanoamps of additional leakage. In comparison the electrolytic capacitor current is still four times as great as the resistor current and nowhere near being settled out. If you ever wondered why audio equipment producers insist on high performance film capacitors in critical applications, DA is one of those reasons!

So how long did it take for the electrolytic capacitor to reach steady state? I set up a longer term capture in Figure 2 for the electrolytic capacitor. After about a whopping 40 seconds later it seemed to be fully settled out, but still contributing a substantial 893 nanoamps of additional steady state leakage current.

Figure 2: 100 microfarad electrolytic capacitor settling time

Where do I go from here?
So what should one do when needing to test leakage current? When testing a wireless device be aware of what kind and value of bypass capacitor has been incorporated into it. Most likely it is a ceramic capacitor nowadays. Film capacitors are too large and cost prohibitive here. Find out how long it takes to settle to its steady state value. Also, off-state current measurements are generally left until the end of the testing to not waste time waiting for the capacitor to reach steady state. If testing a component, if a bypass capacitor is being used on the test fixture, consider using a film capacitor. With test times of just seconds and microamp level leakage currents the wrong bypass capacitor can be a huge problem!

What’s the difference between a fixed output power supply and a programmable output power supply?

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!

Protect your DUT with power supply features including a watchdog timer

The two biggest threats of damage to your device under test (DUT) from a power supply perspective are excessive voltage and excessive current. There are various protection features built into quality power supplies that will protect your DUT from exposure to these destructive forces. There are also some other not-so-common features that can prove to be invaluable in certain applications.

Soft limits
The first line of defense against too much voltage or current can be using soft limits (when available). These are maximum values for voltage and current you can set that later prevent someone from setting output voltage or current values that exceed your soft limit settings. If someone attempts to set a higher value (either from the front panel or over the programming interface), the power supply will ignore the request and generate an error. While this feature is useful to prevent accidentally setting voltages or currents that are too high, it cannot protect the DUT if the voltage or current actually exceeds a value due to another reason. Over-voltage protection and over-current protection must be used for these cases.

Over-voltage protection
Over-voltage protection (OVP) is a feature that uses an OVP setting (separate from the output voltage setting). If the actual output voltage reaches or exceeds the OVP setting, the power supply shuts down its output, protecting the DUT from excessive voltage. The figure below shows a power supply output voltage heading toward 20 V with an OVP setting of 15 V. The output shuts down when the voltage reaches 15 V.

Some power supplies have an SCR (silicon-controlled rectifier) across their output that gets turned on when the OVP trips essentially shorting the output as quickly as possible. Again, the idea here is to protect the DUT from excessive voltage by limiting the voltage magnitude and exposure time as much as possible. The SCR circuit is sometimes called a “crowbar” circuit since it acts like taking a large piece of metal, such as a crowbar, and placing it across the power supply output terminals.

Over-current protection
Over-current protection (OCP) is a feature that uses the constant current (CC) setting. If the actual output current reaches or exceeds the constant current setting causing the power supply to go into CC mode, the power supply shuts down its output, protecting the DUT from excessive current. The figure below shows a power supply output current heading toward 3 A with a CC setting of 1 A and OCP turned on. The power supply takes just a few hundred microseconds to register the over-current condition and then shut down the output. The CC and OCP circuits are not perfect, so you can see the current exceed the CC setting of 1 A, but it does so for only a brief time.

The OCP feature can be turned on or off and works in conjunction with the CC setting. The CC setting prevents the output current from exceeding the setting, but it does not shut down the output if the CC value is reached. If OCP is turned off and CC occurs, the power supply will continue producing current at the CC value basically forever. This could damage some DUTs as the undesired current flows continuously through the DUT. If OCP is turned on and CC occurs, the power supply will shut down its output, eliminating the current flowing to the DUT.

Note that there are times when briefly entering CC mode is expected and an OCP shutdown would be a problem. For example, if the load on the power supply has a large input capacitor, and the output voltage is set to go from zero to the programmed value, the cap will draw a large inrush current that could temporarily cause the power supply to go into CC mode while charging the cap. This short time in CC mode may be expected and considered acceptable, so there is another feature associated with the OCP setting that is a delay time. Upon a programmed voltage change (such as from zero to the programmed value as mentioned above), the OCP circuit will temporarily ignore the CC status just for the delay time, therefore avoiding nuisance OCP tripping.

Remote inhibit
Remote inhibit (or remote shutdown) is a feature that allows an external signal, such as a switch opening or closing, to shutdown the output of the power supply. This can be used for protection in a variety of ways. For example, you might wire this input to an emergency shutdown switch in your test system that an operator would use if a dangerous condition was observed such as smoke coming from your DUT. Or, the remote inhibit could be used to protect the test system operator by being connected to a micro switch on a safety cover for the DUT. If dangerous voltages are present on the DUT when operating, the micro switch could disable DUT power when the cover is open.

Watchdog timer
The watchdog timer is a unique feature on some Agilent power supplies, such as the N6700 series. This feature looks for any interface bus activity (LAN, GPIB, or USB) and if no bus activity is detected by the power supply for a time that you set, the power supply output shuts down. This feature was inspired by one of our customers testing new chip designs. The engineer was running long-term reliability testing including heating and cooling of the chips. These tests would run for weeks or even months. A computer program was used to control the N6700 power supplies that were responsible for heating and cooling the chips. If the program hung up, it was possible to burn up the chips. So the engineer expressed an interest in having the power supply shut down its own outputs if no commands were received by the power supply for a length of time indicating that the program has stopped working properly. The watchdog timer allows you to set delay times from 1 to 3600 seconds.

Other protection features that protect the power supply itself
There are some protection features that indirectly protect your DUT by protecting the power supply itself, such as over-temperature (OT) protection. If the power supply detects an internal temperature that exceeds a predetermined limit, it will shut down its output. The temperature may rise due to an unusually high ambient temperature, or perhaps due to a blocked or incapacitated cooling fan. Shutting down the output in response to high temperature will prevent other power supply components from failing that could lead to a more catastrophic condition.

One other way in which a power supply protects itself is with an internal reverse protection diode across its output terminals. As part of the internal design, there is often a polarized electrolytic capacitor across the output terminals of a power supply. If a reverse voltage from an external power source was applied across the output terminals, the cap (or other internal circuitry) could easily be damaged. The design includes a diode across the output terminals with its cathode connected to the positive terminal and its anode connected to the negative terminal. The diode will conduct if a reverse voltage from an external source is applied across the output terminals, thereby preventing the reverse voltage from rising above a diode drop and damaging other internal components.

What Is Going On When My Power Supply Displays “UNR”?

Most everyone is familiar with the very traditional Constant Voltage (CV) and Constant Current (CC) operating modes incorporated in most any lab bench or system power supply. All but the most very basic power supplies provide display indicators or annunciators to indicate whether it is in CV or CC mode. However, moderately more sophisticated power supplies provide additional indicators or annunciators to provide increased insight and more information about their operating status. One annunciator you may encounter is seeing “UNR” flash on, either momentarily or continuously. It’s fairly obvious that this means that the power supply is unregulated; it is failing to maintain a Constant Voltage or Constant Current. But what is really going on when the power supply displays UNR and what things might cause this?
To gain better insight about CV, CC and UNR operating modes it is helpful to visualize what is going on with an IV graph of the power supply output in combination with the load line of the external device being powered. I wrote a two part post about voltage and current levels and limits which you may find useful to review. If you like you can access it from these links levels and limits part 1 and levels and limits part 2. This posting builds nicely on these earlier postings. A conventional single quadrant power supply IV graph with resistive load line is depicted in Figure 1. As the load resistance varies from infinity to zero the power supply’s output goes through the full range of CV mode through CC mode operation. With a passive load like a resistor you are unlikely to encounter UNREG mode, unless perhaps something goes wrong in the power supply itself.
Figure 1: Single quadrant power supply IV characteristic with a resistive load

However, with active load devices you have a pretty high chance of encountering UNR mode operation, depending where the actual voltage and current values end up at in comparison to the power supply’s voltage and current settings. One common application where UNR can be easily encountered is charging a battery (our external active load device) with a power supply. Two different scenarios are depicted in Figure 2. For scenario 1, when the battery voltage is less than the power supply’s output, the point where the power supply’s IV characteristic curve and the battery’s load line (a CV characteristic) intersect, the power supply is in CC mode, happily supplying a regulated charge current into the battery. However, for scenario 2 the battery’s voltage is greater than the power supply’s CV setting (for example, you have your automobile battery charger set to 6 volts when you connect it to a 12 volt battery). Providing the power supply is not able to sink current the battery forces the power supply’s output voltage up along the graph’s voltage axis to the battery’s voltage level. Operating along this whole range of voltage greater than the power supply’s output voltage setting puts the power supply into its UNR mode of operation.
Figure 2: Single quadrant power supply IV characteristic with a battery load

A danger here is more sophisticated power supplies usually incorporate Over Voltage Protection (OVP). One kind of OVP is a crowbar which is an SCR designed to short the output to quickly bring down the output voltage to protect the (possibly expensive) device being powered. When connected to a battery if an OVP crowbar is tripped, damage to the power supply or battery could occur due to batteries being able to deliver a fairly unlimited level of current. It is worth knowing what kind of OVP there is in a power supply before attempting to charge a battery with it. Better yet is to use a power supply or charger specifically designed to properly monitor and charge a given type of battery. The designers take these things into consideration so you don’t have to!
I have digressed here a little on yet another mode, OVP, but it’s all worth knowing when working with power supplies! Can you think of other scenarios that might drive a power supply into UNR? (Hint: How about the other end of the power supply IV characteristic, where it meets the horizontal current axis?)

If you need fast rise and fall times for your DUT power, use a power supply with a downprogrammer

If you have to provide DC power to a device under test (DUT) and you want the voltage fall time to be just as fast as the rise time, use a power supply with a downprogrammer. A downprogrammer is a circuit built into the output of a power supply that actively pulls the output voltage down when the power supply is moving from a higher setting to a lower setting. Power supplies are good at forcing their output voltage up since that is what their internal circuitry is designed to do. This design results in fast rise times. However, when the supply’s output is changed to move down in voltage, the power supply’s output capacitor (and any additional external DUT capacitance) will need to be discharged. Without a downprogrammer, if there is a light load or no load on the output of the power supply, there is nowhere for the current from the output cap to flow to discharge it. This scenario causes the voltage to take a long time to come down resulting in slow fall times. And this behavior leads to longer test times since you will have to wait for the output voltage to settle to the lower value before you can proceed with your test.

The figures below show an example of the output voltage rise and fall times of a power supply without a downprogrammer under light load conditions. You can see the short rise time (tens of milliseconds) and longer fall time (several seconds).



One of my colleagues, Bob Zollo, wrote an article on this topic that appeared in Electronic Design on February 7, 2012. Here is a link to the article:

https://electronicdesign.com/article/test-and-measurement/If-Your-Power-Supply-Needs-Fast-Rise-And-Fall-Times-Try-A-Down-Programmer-64725

A power supply without an active downprogrammer can have fall times that are tens to hundreds of times longer than a power supply with a downprogrammer. If your test requires you to have fast fall times for your DUT power, or your test requires you to frequently change the voltage on your DUT (both up and down) and throughput is an issue for you, make sure the power supply you choose has a downprogrammer – you won’t have to wait as long for the voltage to move from a higher value to a lower value.

Using Current Drain Measurements to Optimize Battery Run-time of Mobile Devices

One power-related application area I do a great deal of work on is current drain measurements and analysis for optimizing the battery run-time of mobile devices. In the past the most of the focus has been primarily mobile phones. Currently 3G, 4G and many other wireless technologies like ZigBee continue to make major inroads, spurring a plethora of new smart phones, wireless appliances, and all kinds of ubiquitous wireless sensors and devices. Regardless of whether the device is overly power-hungry due to running data-intensive applications or power-constrained due to its ubiquitous nature, there is a need to optimize its thirst for power in order to get the most run-time from its battery. The right kind of measurements and analysis on the device’s current drain can yield a lot of insight on the device’s operation and efficiency of its activities that are useful for the designer in optimizing its battery run-time. I recently completed an article that appeared in Test & Measurement World, on-line back in November and then in print in their Dec 2011- Jan 2012 issue. Here is a link to the article:
https://www.tmworld.com/article/520045-Measurements_optimize_battery_run_time.php

A key factor in getting current drain measurements to yield the deeper insights that really help optimize battery run-time is the dynamic range of measurement, both in amplitude and in time, and then having the ability to analyze the details of these measurements. The need for a great dynamic range of measurement stems from the power-savings nature of today’s wireless battery powered devices. For power-savings it is much more efficient for the device to operate in short bursts of activities, getting as much done as possible in the shortest period of time, and then go into a low power idle or sleep state for an extended period of time between these bursts of activities. Of course the challenge for the designer to get his device to quickly wake up, stabilize, do its thing, and then just as quickly go back to sleep again is no small feat! As one example the current drain of a wireless temperature transmitter for its power-savings type of operation is shown in Figure 1.


Figure 1: Wireless temperature transmitter power-savings current drain

The resulting current drain is pulsed. The amplitude scale has been increased to 20 µA/div to show details of the signal’s base. This particular device’s current drain has the following characteristics:
• Period of ~4 seconds
• Duty cycle of 0.17%
• Currents of 21.8 mA peak and 53.7 µA average for a crest factor of ~400
• Sleep current of 7 µA
This extremely wide dynamic range of amplitude is challenging to measure as it spans about 3 ½ decades. Both DC offset error and noise floors of the measurement equipment must be extremely low as to not limit needed accuracy and obscure details.

Likewise being able to examine details of the current drain during the bursts of activities provides insights about the duration and current drain level of specific operations within the burst. From this you can make determinations about efficiencies of the operations and if there is opportunity to further optimize them. As an example, in standby operation a mobile phone receives in short bursts about every 0.25 to 1 seconds to check for incoming pages and drops back into a sleep state in between the receive (RX) bursts. An expanded view of one of the RX current drain bursts is shown in figure 2.


Figure 2: GPRS mobile phone RX burst details

There are a number of activities taking place during the RX burst. Having sufficient measurement bandwidth and sampling time resolution down to 10’s of µsec provides the deeper insight needed for optimizing these activities. The basic time period for the mobile phone standby operation is on the order of a second but it is usually important to look at the current drain signal over an extended period of time due to variance of activities that can occur during each of the RX bursts. Having either a very deep memory, or even better, high speed data logging, provides the dynamic range in time to get 10’s of µsec of resolution over an extended period of time, so that you can determine overall average current drain while also being able to “count the coulombs” it takes for individual, minute operations, and optimize their efficiencies.

Anticipate seeing more here in future posts about mobile wireless battery-powered devices, as it relates to the “DC” end of the spectrum. In the meantime, while you are using your smart phone or tablet and battery life isn’t quite meeting your expectation (or maybe it is!), you should also marvel at how capable and compact your device is and how far it has come along in contrast to what was the state-of-the-art 5 and 10 years ago!

On DC Source Voltage and Current Levels and (Compliance) Limits Part 2: When levels and limits are not the same

In part 1 my colleague made a good argument for current and voltage level and limit settings actually being one and the same thing and it was really just a case of semantics whether your power supply was operating in constant voltage or in constant current mode. I disagreed and I was not ready to admit defeat on this yet. Now is my chance to explain why I believe they’re not one and the same thing.

I have been doing quite a bit of work with source measure units (SMUs) that support multi quadrant output operation. They in fact feature (constant) voltage sourcing and current sourcing modes of operation. This tailors the operation of the SMU for operating as a voltage source with a set current compliance range or conversely as a current source with a set voltage compliance range. Right at the start one difference is the set up conditions. The output voltage or current level is set to zero while the corresponding current or voltage limit is set to some value, often maximum, so that the DC source accordingly starts out in either constant voltage or constant current for normal operating conditions.

Some products feature a programmable or fixed power limits. In one product I know of, the programmable power limit acts accordingly to override and cut back the either the voltage limit when set for current sourcing, or the current limit when set for voltage sourcing. It does not do this in true real-time however. It cuts back the limit based on the level setting, as a convenient means as to help prevent the user from accidently over-powering the DUT. Alternately many auto-ranging output DC power sources exist that provide an extended range of output and voltage for a given output power capacity. They incorporate a fixed power limit to protect the power supply itself from being inadvertently overloaded, as shown in Figure 1. Usually the idea is for the user to stay below the limit, not operate in power limit. The point here on these examples is that the power parameter is an example of being a limit but not really a level.

Figure 1: Auto-ranging DC power supply power limit

More to the point is some SMUs may incorporate two limits to provide a bounded compliance range with specified positive and negative limits. Not all DUTs are passive, non-reactive devices. As one illustrative example a DUT may be the output of 2-quadrant DC voltage source which you want to force up or down, within limits, or a battery you want to charge and discharge at a fixed rate, with your test system DC source. This set up is illustrated in Figure 2.

Figure 2: Test system DC source driving the output of a DUT source

Figure 3 shows the constant voltage or voltage priority output characteristic for one particular SMU having two programmable current limits. Clearly both limits cannot also be the current level setting as you can only have one level setting. For the case of the external voltage source load line #1 (not all load lines are resistances!), when SMU voltage is less than the DUT source voltage (VEXT1 load line), the current is –ILIM. Conversely when SMU voltage is greater than the DUT source voltage (VEXT2 load line), the current is then +ILIM. In the case of the battery as a DUT this can be used to charge and discharge the battery to specified voltage levels. This desired behavior is achieved using voltage priority operation. Current priority operation would yield very different results. Understanding the nuances of voltage priority, current priority, levels, and limits is useful for getting more utility from your DC sources for more unusual and challenging power test challenges.

Figure 3: Example of a current priority output characteristic driving a DUT voltage source

In closing I’ll concur with my colleague, in many test situations using most DC sources the voltage and current levels and limits may not have a meaningful difference. However, in many more complex cases, especially when dealing with active DUTs and using more capable DC sources and SMUs, there is a clear need for voltage and current level and limit controls that are clearly differentiated and not one and the same! What do you believe?

On DC Source Voltage and Current Levels and (Compliance) Limits Part 1: When levels and limits are one and the same

I was having a discussion with a colleague about constant current operation versus constant voltage operation and the distinction between level settings and limit settings the other day. “The level and limit settings are really the same thing!” he claimed. I disagreed. We each then made ensuing arguments in defense of our positions.

He based his argument on the case of a DC power supply that has both constant voltage and constant current operation. I’ll agree that is a reasonable starting point. As a side note there is a general consensus here that if it isn’t a true, well regulated constant voltage or constant current, whether settable or fixed, then it is simply a limit, not a level setting, end of story. He continued “if the load on the power supply is such that it is operating in constant voltage, then the voltage setting is the level setting and the current setting is the limit setting. If the load increases such that the power supply changes over from constant voltage operation into constant current operation then the voltage setting is becomes the limit setting and the current setting becomes the level setting!” (See figure 1.) He certainly has a good point! For your more basic DC power supply that only operates in quadrant 1 capable of sourcing power only, the current and voltage settings usually interchangeably serve as both the level and compliance limit setting, depending on whether the DC power supply is operating in constant voltage or constant current. The level and compliance limit regulating circuits are one and the same. Likewise with the programming, there are only commands to set the voltage and current levels. There are not separate commands for the limits. I might be starting to lose grounds on this discussion!
Figure 1: Unipolar single quadrant DC source operation

However, all is not lost yet. The DC power supply world is often more complicated than just this unipolar single quadrant operation just presented. Watch for my second part on when the levels and limits are not necessarily one and the same.

The economics of recharging your toy helicopter

While on a business trip visiting customers in Taiwan back in December, I got a toy helicopter as a thank-you gift from one of my coworkers (thanks, Sharon!). This toy helicopter is fun to fly and is surprisingly stable in the air.


Flight time is about 7 minutes, and the battery recharge time is about 40 minutes. It can be recharged from a powered USB port by using a wire that came with the toy that has a USB connector on one end and the helicopter charging connector on the other end. Or, it can be recharged from the six AA alkaline batteries inside the handheld controller via a wire that exits from the controller. Thinking I did not want to prematurely drain the controller batteries, I typically used the USB charging method by using my iPad’s 10 W USB power adapter plugged into a wall outlet. So I got to thinking about which charging method was more economical: charging from a wall outlet or from the batteries. Luckily, I have test equipment at my disposal that can help me answer that question!


Recharging using AC power via USB power adapter
Using one of our Agilent 6812B AC sources, I captured the AC power used during a recharge cycle. I used the AC source GUI to take readings of power every second for the charge period and plotted it in a spreadsheet (graph shown below). I found that the power consumed started at about 2.2 W and ended at about 1.2 W roughly 41 minutes later. The energy used during this time was 1.1 W-hours. Where I live in New Jersey, the utility company charges about 15 cents per kilowatt-hour, so 1.1 W-hours of energy used to charge the helicopter costs fractions of a penny (0.0165 cents = US$ 0.000165). This is basically nothing!


Recharging using controller battery power
To analyze the current drawn from the controller batteries, I used one of our Agilent N6705B DC power analyzers with an N6781A SMU module installed. I ran the battery current path through the SMU set for Current Measure mode and used our 14585A Control and Analysis software. I captured the current drawn from the six AA batteries in the controller during the helicopter recharge cycle. These batteries are in series, so the same current flows through each of the six batteries and also through the SMU for my test.



For the recharge period (about 43 minutes using this method), the software shows the batteries provided 173 mA-hours of charge to the helicopter. A typical AA alkaline battery is rated for 2500 mA-hours, so that means I would get about 14 (= 2500/173) charge cycles from these six batteries. If you shop around for high-quality AA batteries, you might find them for as low as 25 cents per battery. Since the controller takes six of these, the battery cost for the controller is $1.50. If I can recharge the helicopter 14 times with $1.50 worth of batteries, each recharge cycle costs about 10.7 cents (= US$ 0.107). This is 650 times more expensive than using the AC power method, so I will continue using the wall outlet to recharge my toy helicopter! How about you?
Note that with the AC power recharge method, you pay for the kilowatt-hours you consume from your utility company. With the controller battery power method, you pay for the mA-hours you consume from your batteries. Consider this: if you choose the AC power method, you will save US$ 0.106835 per recharge cycle. That means after just 2.81 million recharge cycles, you will have saved enough money to buy yourself a real helicopter worth US$ 300,000, so you better get started now!

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

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

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

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

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


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


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

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

Six of seven new Agilent power supplies are autorangers, but what is an autoranger, anyway?

In this blog, I avoid writing posts that are heavily product focused since my intention is generally to provide education and interesting information about power products instead of simply promoting our products. However, when we (Agilent) come out with new power products, I think it is appropriate for me to announce them here. So I will tell you about the latest products announced last week, but I also can’t resist writing about some technical aspect related to these products, so I chose to write about autorangers. But first…..a word from our sponsor….

From last week’s press release, Agilent Technologies “introduced seven high-power modules for its popular N6700 modular power system. The new modules expand the ability of test-system integrators and R&D engineers to deliver multiple channels of high power (up to 500 watts) to devices under test.” Here is a link to the entire press release:

https://www.agilent.com/about/newsroom/presrel/2012/17jan-em12002.html

I honestly think these new power modules are really great additions to the family of N6700 power products we continue to build upon. We have several mainframes in which these power modules can be installed and now offer 34 different power modules that address applications in R&D and in integrated test systems. Oooooppps, I slipped into product promotion mode there for just a short time, but it was because I really believe in this family of products….I hope you will forgive me!

OK, now on to the more fun stuff! Since six of these seven new power modules are autorangers, let’s explore what an autoranger is. Agilent has been designing and selling autorangers since the 1970s (we were Hewlett-Packard back then) starting with the HP 6002A. To understand what an autoranger is, it will be useful to start with an understanding of what a power supply output characteristic is.

Power supply output characteristic
A power supply output characteristic shows the borders of an area containing all valid voltage and current combinations for that particular output. Any voltage-current combination that is inside the output characteristic is a valid operating point for that power supply.

There are three main types of power supply output characteristics: rectangular, multiple-range, and autoranging. The rectangular output characteristic is the most common.

Rectangular output characteristic
When shown on a voltage-current graph, it should be no surprise that a rectangular output characteristic is shaped like a rectangle. See Figure 1. Maximum power is produced at a single point coincident with the maximum voltage and maximum current values. For example, a 20 V, 5 A, 100 W power supply has a rectangular output characteristic. The voltage can be set to any value from 0 to 20 V, and the current can be set to any value from 0 to 5 A. Since 20 V x 5 A = 100 W, there is a singular maximum power point that occurs at the maximum voltage and current settings.

Multiple-range output characteristic
When shown on a voltage-current graph, a multiple-range output characteristic looks like several overlapping rectangular output characteristics. Consequently, its maximum power point occurs at multiple voltage-current combinations. Figure 2 shows an example of a multiple-range output characteristic with two ranges also known as a dual-range output characteristic. A power supply with this type of output characteristic has extended output range capabilities when compared to a power supply with a rectangular output characteristic; it can cover more voltage-current combinations without the additional expense, size, and weight of a power supply of higher power. So, even though you can set voltages up to Vmax and currents up to Imax, the combination Vmax/Imax is not a valid operating point. That point is beyond the power capability of the power supply and it is outside the operating characteristic.

Autoranging output characteristic
When shown on a voltage-current graph, an autoranging output characteristic looks like an infinite number of overlapping rectangular output characteristics. A constant power curve (V = P / I = K / I, a hyperbola) connects Pmax occurring at (I1, Vmax) with Pmax occurring at (Imax, V1). See Figure 3.

An autoranger is a power supply that has an autoranging output characteristic. While an autoranger can produce voltage Vmax and current Imax, it cannot produce them at the same time. For example, one of the new power supplies just released by Agilent is the N6755A with maximum ratings of 20 V, 50 A, 500 W. You can tell it does not have a rectangular output characteristic since Vmax x Imax (= 1000 W) is not equal to Pmax (500 W). So you can’t get 20 V and 50 A out at the same time. You can’t tell just from the ratings if the output characteristic is multiple-range or autoranging, but a quick look at the documentation reveals that the N6755A is an autoranger. Figure 4 shows its output characteristic.

Autoranger application advantages
For applications that require a large range of output voltages and currents without a corresponding increase in power, an autoranger is a great choice. Here are some example applications where using an autorangers provides an advantage:
• The device under test (DUT) requires a wide range of input voltages and currents, all at roughly the same power level. For example, at maximum power out, a DC/DC converter with a nominal input voltage of 24 V consumes a relatively constant power even though its input voltage can vary from 14 V to 40 V. During testing, this wide range of input voltages creates a correspondingly wide range of input currents even though the power is not changing much.
• There are a variety of different DUTs of similar power consumption, but different voltage and current requirements. Again, different DC/DC converters in the same power family can have nominal input voltages of 12 V, 24 V, or 48 V, resulting in input voltages as low as 9 V (requires a large current), and as high as 72 V (requires a small current). The large voltage and current are both needed, but not at the same time.
• A known change is coming for the DC input requirements without a corresponding change in input power. For example, the input voltage on automotive accessories could be changing from 12 V nominal to 42 V nominal, but the input power requirements will not necessarily change.
• Extra margin on input voltage and current is needed, especially if future test changes are anticipated, but the details are not presently known.

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

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




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

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

Below is my summary of the contents:

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

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

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

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

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

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

Table 2: Quantitative comparison of DC power supply topologies

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

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

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

Figure 1: Common Mode Noise Current and Path

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

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

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

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

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

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

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

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

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



Figure 2: Basic switching DC power supply circuit

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

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


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

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