What is a bipolar (four-quadrant) power supply?

To answer this question, I have to start with a basic definition of polarity conventions. Figure 1 shows a simple diagram of a power supply (a two-terminal device) with the standard polarity for voltage and current. A standard power supply typically is a source of power. To source power, current must flow out of the positive voltage terminal. Most power supplies source energy in this way by providing a positive output voltage and positive output current. This is known as a uni-polar power supply because it provides voltage with only one polarity. By convention, the “polarity” nomenclature typically refers to the polarity of the voltage (not the direction of current flow).

If current flows into the positive voltage terminal, the power supply is sinking current and is acting like an electronic load – it is absorbing and dissipating power instead of sourcing power. Most power supplies do not do this although many Agilent power supplies can sink some current to quickly pull down their output voltage when needed – this is known as a down-programmer capability – see this post for more info: http://powersupplyblog.tm.agilent.com/2012/03/if-you-need-fast-rise-and-fall-times.html.

To fully define power supply output voltage and current conventions, a Cartesian coordinate system is used. The Cartesian coordinate system simply shows two parameters on perpendicular axes. See Figure 2.  By convention, the four quadrants of the coordinate system are defined as shown. Roman numerals are typically used to refer to the quadrants. For power supplies, voltage is normally shown on the vertical axis and current on the horizontal axis. This coordinate system is used to define the valid operating points for a given power supply. A graph of the boundary surrounding these valid operating points on the coordinate system is known as the power supply’s output characteristic.

As mentioned earlier, some power supplies are uni-polar (produce only a single polarity output voltage), but can source and sink current. These power supplies can operate in quadrants 1 and 2 and can therefore be called two-quadrant supplies. In quadrant 1, the power supply would be sourcing power with current flowing out of the more positive voltage terminal. In quadrant 2, the power supply would be consuming power (sinking current) with current flowing into the more positive voltage terminal.

Some power supplies can provide positive or negative voltages across their output terminals without having to switch the external wiring to the terminals. These supplies can typically operate in all four quadrants and are therefore known as four-quadrant power supplies. Another name for these is bipolar since they are able to produce either positive or negative voltage on their output terminals. In quadrants 1 and 3, a bipolar supply is sourcing power: current flows out of the more positive voltage terminal. In quadrants 2 and 4, a bipolar supply is consuming power: current flows into the more positive voltage terminal. See Figure 3.

Agilent’s N6784A is an example of a bipolar power supply. It can source or sink current and the output voltage across its output terminals can be set positive or negative. It is a 20 W Source/Measure Unit (SMU) with multiple output ranges. See Figure 4 for the output characteristic of the N6784A.

To summarize, a bipolar or four-quadrant power supply is a supply that can provide positive or negative output voltage, and can source or sink current. It can operate in any of the four quadrants of the voltage-current coordinate system.

Flyback Inverter for Fluorescent Lamp: Part 2, A Little Theory of Operation

In part 1 of this posting “Flyback Inverter for Fluorescent Lamp: Part 1, Making Repairs” a little careful and straightforward troubleshooting and repair brought my friend’s fluorescent lamp assembly back to life again. But a fluorescent lamp has quite a few unique requirements to get it to start up and stay illuminated. How does this flyback converter manage to do these things?

I had first looked around to see if I could find a schematic for this fluorescent lamp assembly, but nothing turned up for me. However, the parts count was low enough, and circuit board large enough, that it was a fairly simple matter to trace out and sketch the inverter’s schematic in fairly short order, as shown in Figure 1.

Figure 1: Fluorescent lamp single-ended flyback inverter circuit

When first powered up the switching transistor is biased on by the 812 ohm resistor, energizing transformer winding W1. This in turn applies positive feedback to the transistor through winding W2, driving it into saturation. There are two mechanisms in the flyback transformer that are critical for making this inverter work:

• First it has a gapped core. This allows it to store a substantial amount of energy in its magnetic field which in turn gets dumped over to the fluorescent tube through the secondary winding W3 when the transistor turns off and the transformer’s magnetic field collapses.  During this period the winding voltage continues to climb as the magnetic field collapses until the energy can find a place to discharge to, in this case into the fluorescent tube. The voltage is also further increased by the turns ratio of the transformer. This is the “flyback” effect that creates sufficiently high enough voltage to get the fluorescent tube to “strike” or ionize its gas to get it to start conducting and give off illumination, typically many hundreds of volts.
• As can be seen this inverter is a very simple circuit with a minimum of parts. A second mechanism in the transformer is it is designed to saturate in order to make the inverter oscillate. At the end of the transistor’s “on” period the transformer reaches its maximum magnetic flux at which point the transformer saturates. Winding voltage W2 drops to zero and then reverses driving the switching transistor into cutoff.  After the magnetic field has collapsed and energy discharged to the fluorescent tube the process repeats itself.

The switching transistor’s collector and base voltages during turn on are captured in the oscilloscope diagram shown in Figure 2.

Figure 2: Inverter switching transistor collector and base voltage waveforms

A number of interesting things can be observed in Figure 2.  The oscillation period is roughly 50 microseconds, or oscillation frequency of 20 kHz. It takes about 10 cycles, 500 microseconds, for the fluorescent tube to strike. During this initial phase the peak collector voltage is flying up to nearly 100 volts or about 8 times the DC input voltage being applied. Again, this voltage is being multiplied up by the turns ratio of windings W1 and W2 to bring this up in the vicinity of 600 volts or so needed to make the fluorescent tube to strike. Once the tube does strike and starts conducting its impedance drops. This causes the collector voltage to drop down to about 35 volts which is consistent with the proportion of drop in voltage needed for the fluorescent tube once it’s gas is ionized and is conducting. Note also the collector voltage pulse also widens as it takes a longer time for the energy in the transformer to be dumped when it’s at a lower voltage.

Although this inverter at first glance is a rather simple and minimum viable, minimum parts count circuit, with careful design it can be made to be very efficient. This is where the design of the transformer becomes as much art as science, knowing how the subtle characteristics of the magnetic material and inductive and capacitive parasitics can be used to advantage in contributing to and improving the overall performance of the design.

Anyway, what my friend really cared about is the lamp now works and he is able to put it to good use in his camper!

Flyback Inverter for Fluorescent Lamp: Part 1, Making Repairs

A friend of mine approached me a while ago asking for some help. The fluorescent lamp assembly for his VW Westfalia camper was dead and, knowing I knew more about electronic devices than he did, figured it was worth challenging me with it.  I was actually happy to do so. Being involved with DC power conversion of a variety of forms I was always a bit curious to learn about how fluorescent lamp assemblies that were powered from low voltage DC worked anyway.

“My lamp does not work; can you look at it for me?”

“I suppose. Did it just stop working? Did you try anything to get it working again?”

“Well, it really never worked for me. I messed around with it a little but it did not help. I may have hooked it up backwards.”

“Why do you think you hooked it up backwards?”

“Well, it did not work so I tried reversing the power connections. That didn’t make it work however.”

“You really should not do that with electronic things!”

I took the lamp home and later when I had chance to look at it carefully I visually identified several problems. Like many other things I have repaired, a lot of the times it is not the device itself but rather a previous owner unintentionally inflicts unnecessary damage on it when attempting to make repairs.  In my friend’s partial defense, someone previously had already made unsuccessful attempts at trying to make it work again, unwittingly making things worse.

Referring to Figure 1 I unanchored the inverter circuit board from the back of the lamp assembly for closer inspection. It was immediately obvious there were problems that would keep it from working:

• The connectors for the wiring to the fluorescent tube were not making contact.
• A portion of a circuit board trace where the power feeds in was blown away.

Figure 1: Fluorescent lamp inverter board had obvious problems

Clearly someone had let the smoke out of it that made it work!  After making repairs to these problems I then tried powering it up using a power supply with a current limit to keep things safe. As I expected I was not going to get off that easy. The power supply went right up to its current limit setting. The lamp still did not work. 

The next step was to probe around the circuit board with a DMM.  With the abuse this lamp assembly has been subjected to I suspected the switching transistor would be damaged and sure enough it was measuring shorted. However, after removing it, it seemed to check out good. Probing around on the board again, a diode adjacent to the transistor measured shorted as well. Upon its removal it fell in half as a result of being overheated. I found where the rest of the smoke that makes it work had come out!  I replaced the diode, reinstalled the transistor and remounted the circuit board. Upon applying power again the result was a bit different as shown in Figure 2. I managed to reinstall all the smoke back into it again!

Figure 2: Fluorescent lamp assembly back in working order

While I had a general idea of how it works, now that I had the fluorescent lamp assembly working again I had take the opportunity to make some measurements and study the finer aspects of how it works, which I will cover, coming up in part 2. Stay tuned!

Power Supply Programming Examples

Hello everyone!

One of my responsibilities at Agilent is to oversee programming example generation for our new product introductions.  Programming examples are an area that we wish to improve upon.  Our goal is to provide a selection of programming examples that allow our customers to use the exciting new features of our products at introduction.

The first thing that we want to do is to make sure that we provide examples that our customers can use so I researched the most popular programming languages that customers use with power supplies.  The list that we came up with is: VB.NET, Labview, C#, and in some cases Matlab.  In terms of IO Libraries, we will use direct IO for everything.  We will not be including any driver examples in this plan.  We will also be providing a text file with the SCPI programs and an Agilent Command Expert sequence file.

 

I would like to solicit some feedback on this plan.  What do you think?  Can we improve this plan?  Are any programming languages missing? What do you look for in a programming example?  

 Please leave comments.

Battery-killing cell phone apps? – Part 2

Back on May 25, 2012, I posted about mobile device users avoiding security apps because they think the apps run down their batteries too quickly (read that post here). I also mentioned that a member of the Anti-Malware Testing Standards Organization (AMTSO) is using Agilent’s N6705B DC Power Analyzer to evaluate just how much the security apps affect battery run time and that the results would not be available for a few months. Well, the results are in and guess what? Which security app you choose does not make much difference in your battery run time.

On average, they reported that the effect of using a security app on reducing battery run time is only about 2% which translates into less than 30 minutes of lost battery life per day. And the study went on to explain that the differences in performance of one mobile security product to another were small (they tested 13 products each from a different vendor). I was amused by the author’s comment that they were “not providing a ranking” because it “could get misused by marketing departments”. Indeed!

Here is a link to the report:
http://www.av-comparatives.org/images/docs/avc_mob_201209_en.pdf

The report shows a picture of Agilent’s N6705B DC Power Analyzer as the measuring device. They used this product because “This high-precision instrument can measure battery drain exactly”. A screen shot of Agilent’s 14585A Control and Analysis Software for the DC Power Analyzer was also shown in the report. The software allowed them to evaluate power consumption while performing various mobile phone tasks, such as making phone calls, viewing pictures, browsing websites, watching YouTube (I wonder if they watched any of the DC Power Analyzer videos we have posted!), watching locally stored videos, receiving and sending mails, and opening documents.

If the N6705B DC Power Analyzer and 14585A Control and Analysis Software can evaluate power consumption for all of those things, just think of what it could do for you! Check out Ed’s post from earlier this week for some of those things: http://powersupplyblog.tm.agilent.com/2012/09/optimizing-mobile-device-battery-run.html

Optimizing Mobile Device Battery Run-time Seminars

On many occasions in the past here both I, and my colleague, Gary, have written about measuring, evaluating, and optimizing battery life of mobile wireless battery powered devices. There is no question that, as all kinds of new and innovative capabilities and devices are introduced; battery life continues to become an even greater challenge.

I recently gave a two-part webcast entitled “Optimize Wireless Device Battery Run-time”. In the first part “Innovative Measurements for Greater Insights” a variety of measurement techniques are employed on a number of different wireless devices to illustrate the nature of how these devices operate and draw power from their batteries over time, and in turn how to go about making and analyzing the measurements to improve the device’s battery run-time. Some key points brought out in this first part include:

• Mobile devices operate in short bursts of activities to conserve power. The resulting current drain is pulsed, spanning a wide dynamic range. This can be challenging for a lot of traditional equipment to accurately measure.
• Not only is a high level of dynamic range of measurement needed for amplitude, but it is also needed on the time axis as well, for gaining deeper insights on optimizing a device’s battery run-time.
• Over long periods of time a wireless device’s activity tends to be random in nature. Displaying and analyzing long term current drain in distribution plots can quickly and concisely display and quantify currents relating to specific activities and sub-circuits that would otherwise be difficult to directly observe in a data log.
• The battery’s characteristics influence the current and power drawn by the device. When powering the device by other than its battery, it can be a significant source of error in testing if it does not provide results like that of when using the battery.

Going beyond evaluating and optimizing the way the device makes efficient use of its battery power, the second part, “The Battery, its End Use, and Its Management” brings out the importance of, and how to go about making certain you are getting the most of the limited amount of battery power you have available to you. Some key points for this second part include:

• Validating the battery’s stated capacity is a crucial first step both for being certain you are getting what is expected from the battery and serve as a starting reference point that you can correlate back to the manufacturer’s data.
• Evaluating the battery under actual end-use conditions is important as the dynamic loading a wireless device places on the battery often adversely affects the capacity obtained from the battery.
• Charging, for rechargeable batteries, must be carefully performed under stated conditions in order to be certain of in turn getting the correct amount of capacity back out of the battery. Even very small differences in charging conditions can lead to significant differences in charge delivered during the discharge of the battery.
• The wireless device’s battery management system (or BMS) needs to be validated for proper charging of the battery as well as suitability for addressing the particular performance needs of the device.

In Figure 1 the actual charging regiment was captured on a mobile phone battery being charged by its BMS. There turned out to be a number of notable differences in comparison to when the battery was charged using a standard charging regiment.

Figure 1: Validating BMS charge regiment on a GSM/GPRS mobile phone

If you are interested in learning more about optimizing wireless device battery run-time this two part seminar is now available on-demand at:

Optimize Wireless Device Battery Run-time: Two Part Webcast Series

I think you will enjoy them!

Early Power Transistor Evolution, Part 2, Silicon

As discussed in part 1 of this two-part posting on early power transistor evolution, by the early 1960’s germanium power transistors were in widespread use in DC power supplies, audio amplifiers, and other relatively low frequency power applications. Although fairly expensive at that time the manufacturers had processes establish to reliably produce them in volume. To learn more about early germanium power transistors click here to review part 1.

As with most all things manufacturers continued to investigate ways of making things better, faster, and cheaper. Transistors were still relatively new and ready for further innovation. Next to germanium silicon was the other semiconductor in widespread use and with new and different processes developed for transistor manufacturing, silicon quickly displaced germanium as the semiconductor of choice for power transistors. One real workhorse of a power transistor that has truly stood the “Test of Time” is the 2N3055, pictured in Figure 1. Also pictured is his smaller brother, the 2N3054.

Figure 1: 2N3055 and 2N3054 power transistors

Following are some key maximum ratings on the 2N3055 power transistor:

• V_CEO = 60V
• V_CBO = 100V
• V_EBO= 7V
• I_C = 15A
• P_D = 115W
• hfe= 45 typical
• f_T = 1.5 MHz
• Thermal resistance = 1.5 ^oC/W
• T_J= 200 ^oC
• Package: TO-3 (now TO-204AA)
• Polarity: NPN
• Material/process: Silicon diffused junction hometaxial-base structure

Diffused junction silicon transistors made major inroads in the early 1960’s ultimately making the germanium power transistors obsolete.  One huge improvement using silicon, especially for power transistors, is the junction temperature, which is generally rated for 200 ^oC.  This allowed operating at much higher ambient temperatures and at higher power levels when compared to germanium. 

While the alloy junction process being used for the early germanium transistors favored making PNP transistors, the diffused junction process on silicon favored making NPN transistors somewhat more. Silicon diffused junction NPN transistors are much more prevalent than PNP devices, and the PNP complements to NPN devices, where available, are more costly.  

The diffusion process made a giant leap in transistor mass production possible. Many transistors could now be made at once on a larger silicon wafer, greatly reducing the cost. The more precise nature of the diffusion junction over the alloy junction also improved performance. As one example, tor the 2N3055 the transition frequency increased roughly another order of magnitude over the 2N1532 germanium alloy junction transistor in part 1, to 1.5 MHz.  

The hometaxial-base structure is a single simultaneous diffusion into both sides of a homogenously-doped base wafer, one side forming the collector and the other side the emitter. A pattern on the emitter side is etched away around the emitter, down to the P-type layer, to form the base. The emitter is left standing as a plateau or “mesa” above the base.

The 2N3054 was electrically identical to the 2N3055 except for its lower current and power capabilities. It’s smaller TO-66 package however was never very popular and was quietly phase out in the early 1980’s, sometimes along with some of the devices that were packaged in it!

Process improvements beyond the single diffused hometaxial-base structure continued through the 1960s with silicon transistors, including double diffused, double- and triple diffused planar and epitaxial structures. The epitaxial structure is a departure from the diffused structures in that features are grown onto the top of the base wafer. With greater control of doping levels and gradients, and more precise and complex geometries, the performance silicon power transistors continued to improve in most all aspects.

Plastic-packaged power transistors have for the most part come to displace hermetic metal packages like the TO-3 (TO-204AA), first due to the lower cost of the part and second, with simpler mounting, reducing the cost and labor of the products they are incorporated into. One drawback of most of the plastic-packaged power devices is their maximum temperature rating has been reduced to typically 150 ^oC, taking back quite a bit of temperature headroom provided by the same devices in hermetic metal packages. Sometimes there is a price to be paid for progress! Pictured in figure 2 are two (of many) popular power device packages, the smaller TO-220AB and the larger TO-247.

Figure 2: TO-220AB and TO-247 power device plastic packages

It’s pretty fascinating to see how transistors and the various processes used to manufacture them evolved over time. In these two posts I’ve hardly scratched the surface of the world of power transistors and power devices. For one there is a variety of other transistor types not touched upon, including a variety of power FETs. Power FETs have made major inroads in all kinds of applications in power supplies. Also work continues to provide higher power devices in surface mount packages. These are just a couple of numerous examples, possibly something to write about at a future date!

References: “RCA Transistor Thyristor & Diode Manual” Technical Series SC-14, RCA Electronic Components, Harrison, NJ