Tuesday, November 29, 2011

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.

Wednesday, November 23, 2011

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!

Tuesday, November 15, 2011

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 1 of 4)

Part 1 of 4: Linear System DC Power Supply Attributes
To kick things off I thought it would be helpful to start with a short series of posts discussing something fundamental we’re often faced with; that is making the choice of whether to use a switching or linear DC power supply to power up our devices under test. In part 1 here I’ll begin my discussion with the topology and merits of linear DC power supplies, as I have heard countless times from others that only a linear power supply will do for their testing, principally due to its low output noise. Of course we do not want to take the chance of having power supply noise affect our devices’ test results. While I agree a linear DC power supply is bound to have very low noise, a well-designed switching DC power supply can have surprisingly good performance. So the choice may not be as simple anymore. The good thing here however is this may give us a lot more to choose from, something that may better meet our overall needs, including size and cost, among other things.

Linear DC Power Supply Topology
A linear DC power supply as depicted in Figure 1 is relatively simple in concept and in basic implementation:
  1. A transformer scales the AC line voltage to a value consistent with the required maximum DC output voltage level.
  2. The AC voltage is then rectified into DC voltage.
  3. Large electrolytic capacitors filter much of the AC ripple voltage superimposed on the unregulated DC voltage.
  4. Series-pass power transistors control the difference between the unregulated DC rail voltage and the regulated DC output voltage. There always needs to be some voltage across the series pass transistors for proper regulation.
  5. An error amplifier compares the output voltage to a reference voltage to regulate the output at the desired setting.
  6. Finally, an output filter capacitor further reduces AC output noise and ripple, and lowers output impedance, for a more ideal voltage source characteristic.

Figure 1: Basic Linear DC Power Supply Topology

Linear DC power supply design is well established with only incremental gains now being made in efficiency and thermal management, for the most part. Its straightforward configuration, properly implemented, has some inherent advantages:

  • Fast output transient response to AC line and output load changes
  • Low output noise and ripple voltage, and primarily having low frequency spectral content
  • Very low common mode noise current
  • Cost competitive at lower output power levels (under about 500 watts)

It also has a few inherent disadvantages:

  • Low power efficiency, typically no better than 60% at full output voltage and decreases with lower output voltage settings
  • Relatively large physical size and weight
  • High cost at higher power (above about 500 watts)

So it sounds like a linear power supply has to be the hands-down winner especially for low power applications. Or not? To make a more-informed choice we need to look at the topology and merits of a switching power supply, which I will be doing in part 2!

Friday, November 4, 2011

Wirelessly communicate with your power product

I recently had to do an evaluation of the interaction of one of our solar array simulator (SAS) power products with a customer’s device under test (DUT – in this case, a DC/DC optimizer). This required me to set up a variety of pieces of electronic test equipment in our lab area, located about 100 feet from my office cube. As part of the evaluation, I needed to change the internal firmware revision of the SAS to see if the output behavior was different with different firmware revisions. The SAS is LAN accessible and has downloadable firmware, so I planned to simply connect the SAS to LAN in our lab area and change the firmware using our firmware update utility located on my PC in my cube. However, I assembled the equipment in a location in our lab area that did not have a LAN port located nearby. So, I was ready to disconnect my laptop from its docking station in my cube and carry it over to the equipment in the lab area when I realized there was an easier way to do this: wirelessly!

A few months ago, I wrote an application note describing how to use a mobile router to wirelessly access one of our data acquisition/switch units. While the app note focused on controlling a data acquisition instrument, the process to connect wirelessly is identical for any well-behaved LAN-enabled product. So I grabbed one of the mobile routers I used to prove out the method described in the app note and connected it to the SAS. Within just a few minutes, I was able to change the firmware in the SAS from my cube located 100 feet away, without connecting the SAS to a wired LAN – I simply used my laptop’s built-in wireless.


In this case, I used the Sapido RB-1632 mobile router, shown in the photo below.


If you want to read the details about how to wirelessly connect to an instrument, refer to one of the following application notes. Once again, these were written to control the 34972A, but the same process can be applied to any well-behaved LAN-enabled instrument (LXI compliance is recommended).

“Access Your 34972A Wirelessly with a Sapido Mobile Router”:
http://cp.literature.agilent.com/litweb/pdf/5990-8734EN.pdf

“Access Your 34972A Wirelessly with a TRENDnet Travel Router”:
http://cp.literature.agilent.com/litweb/pdf/5990-8286EN.pdf

Friday, October 7, 2011

What is line effect and how does it affect my testing?

Line effect is a power supply specification (also known as line regulation or source effect) that describes how well the power supply can maintain its steady-state output setting when the AC input line voltage changes. More formally, it specifies the maximum change in steady-state DC output voltage (or current) resulting from a specified change in the AC input line 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 AC input voltage changes. Here is an example:

Let’s say the voltage line effect specification for a 20 V, 5 A power supply is 1 mV and is specified for any line change within ratings. And let’s say that the AC input line voltage range for this power supply for a nominal 120 Vac line is -13% to +6% (104.4 Vac to 127.2 Vac). This means for any AC input line voltage change within the rating of the supply, the output voltage will not change by more than 1 mV. For example, if the power supply is set to 10 V, the actual output may measure 9.999 V at low line (104.4 Vac). (Note that the difference between the setting and the actual output voltage is a different specification called programming accuracy.) If you then increase the AC input line voltage from low line (104.4 Vac) to high line (127.2 Vac), the line effect specification guarantees that the output voltage will not change by more than 1 mV, so it will be somewhere between 9.998 V and 10.000 V. So if the actual output voltage started at 9.999 V at low line and measured 9.9994 V at high line, the line effect for this output when set for 10 V measures 0.4 mV (9.9994 – 9.999), well within the 1 mV specification. You must make the second voltage measurement immediately following the line voltage change to avoid capturing any short-term drift effects.


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

Most performance power supplies have line effect specifications of about 1 mV or less. A lower performance model may have a line effect specification of up to 10 mV or more. Power supplies with higher maximum voltage ratings and higher maximum power ratings typically have higher line effect specifications.

If you have an application where maintaining an exact voltage at your DUT is critical and your AC input line can vary throughout the day, you will want to use a power supply with a low line effect specification. If changes in the voltage at your DUT are less critical to you, most power supplies will perform well for your application regardless of line voltage behavior.

Friday, September 30, 2011

Hurricane Irene and inverters

During the weekend of August 27-28, 2011, hurricane Irene wreaked havoc along the east coast of the United States. I live in northern New Jersey where we got more than 10 inches of rain in a short time! Flooding, downed trees, and power outages were rampant! My mother called me during the storm to tell me her basement was flooded. She still lives in the house where I grew up, and I know that basement had not flooded in decades. But she lost power disabling her sump pump, so the heavy rain resulted in several inches of water in the basement saturating the carpet and ruining furniture and other personal items. What a mess! And my brother, who lives in another NJ town, has a restaurant that ended up with 4 feet of water in it!! Fresh fish, anyone?

So when my mother called me for help, I gathered up various tools, buckets, hoses, extension cords, flashlights, my wet/dry vac, and stopped at a friend’s house to borrow an inverter he used when camping (thanks, Andy!). An inverter takes DC in and puts out AC. My hope was to power the inverter from my car battery and plug in my mom’s sump pump to empty out the water in her basement. Luckily, as I was driving to her house with my friend who was coming to help (thanks, Nyla!), my mom called my cell phone to let me know the power was back on, so the sump pump kicked in and pumped out the bulk of the water. Of course, a soggy mess was left behind (7 hours of wet vacuuming made only a small dent in the cleanup, but it was a start). So, it turns out I did not use the inverter at her house (it would not have provided enough power anyway), but when I went to work the next week, I figured I’d play around with it in our lab area. Here are some of the things I found…

This inverter is a Coleman Powermate (model PMP400) 400 W inverter. It takes 12 V DC in and has a 40 A fuse on the input side, and two outlets with an on/off switch on the output side.


The output is a modified sine wave (looks more like a modified square wave to me, but OK, I’ll call it by its rightful name), at nominally 120 Vrms and 60 Hz, which are the standard AC mains voltage and frequency in the US. The waveform below was captured with a scope (an Agilent MSO7054A) and shows the actual output of the inverter with 12V DC in (from an Agilent N6754A installed in an N6705A) and a light load (~32 W) on the output.



Below is what the standard AC line looks like in the US, so you can see that the inverter's output (shown above) is only an approximation of the waveshape, although the inverter does maintain the correct rms voltage and frequency:


As a load on the inverter, I powered up another one of our DC power supplies (an Agilent 66332A) by plugging it into the inverter output. I could then program the output of the 66332A power supply to a voltage (20 V), connect it to one of our DC electronic loads (an Agilent 6063B) and vary the load current (up to nearly 5 A), thereby changing the loading on the 66332A, which in turn, changed the load on the inverter.


The inverter output frequency remained very close to 60 Hz for all loading conditions, and the output voltage dropped slightly (just a few volts) as I increased the loading on the inverter. The maximum power I drew from the inverter was limited by my input power source, the N6754A, which is a 300 W, 60 V, 20 A power supply. Since I was using it at 12 V, I set the current limit on it to the maximum of 20 A providing a maximum of about 240 W to the inverter input. So I was able to exercise the inverter up to only a little over one half of its 400 W capability.

The 66332A power supply I used as my load for the inverter has a standard AC input and seemed to operate just fine when powered by the modified sine wave coming from the inverter output. Regarding other loads you might plug into the output of an inverter, I think most AC motors would operate when supplied by a modified sine wave, however other devices such as audio equipment, fluorescent lighting, and some laser printers might not work properly or at all. Inverters are available with pure sine wave outputs to more closely mimic the power supplied by your utility company, however, these tend to be much more costly – sometimes several times the cost of an equally powered modified sine wave inverter.

I looked up a few numbers about waveforms and found that a pure square wave has a THD of about 45% while a modified sine wave has a THD of about 24%. Here is an interesting article on this topic:
http://powerelectronics.com/mag/608PET21.pdf

So if you ever lose AC mains power and need to run one or more AC powered devices, you could temporarily use an inverter powered from your car battery. Just be sure to get an inverter with enough power to handle the load you will put on it, and make sure the type of inverter you choose (modified or pure sine wave output) is appropriate for the load you want to power. Although it turned out I did not need it for my mom’s sump pump, the 400 W inverter I borrowed would not have been powerful enough for the pump. The current rating on the pump was about 6 A, so at 120 V, that is 720 VA (120 V x 6 A) which is more than the 400 W inverter could provide. But how do you compare VA (volt-amperes) to W (watts), you ask? The power that a device consumes expressed in W will always be less than or equal to the power in VA, but I’ll leave that discussion for another post! For now, if you think you’ll need an inverter, get one with a W rating higher than the total VA you require. This approach may be a bit overkill, but you will definitely have enough power.

Wednesday, September 21, 2011

What is load effect and how does it affect my testing?

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.