Tuesday, November 29, 2011
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.
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
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
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:
- A transformer scales the AC line voltage to a value consistent with the required maximum DC output voltage level.
- The AC voltage is then rectified into DC voltage.
- Large electrolytic capacitors filter much of the AC ripple voltage superimposed on the unregulated DC voltage.
- 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.
- An error amplifier compares the output voltage to a reference voltage to regulate the output at the desired setting.
- 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
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”:
“Access Your 34972A Wirelessly with a TRENDnet Travel Router”: