Dave Wilson, Motion Products Evangelist, Texas Instruments

 We are now on the eight part of a ten part series about teaching your PI controller to behave like you want it to.  At the heart of this discussion is a proposed tuning technique which allows you to be more proactive with the tuning process and reduce the amount of empirical guesswork at the back-end of your design process.  TI has already used this tuning technique on several motor control designs with good success!  But as with any “parameter based” design process, the results are only as good as how well you know your parameters.  In summary, for a permanent magnet synchronous machine, we need to know the following parameters to tune the PI loops:

Rs (motor stator resistance)

Ls (motor stator inductance)

P (# of motor poles)

l_r (rotor flux)

J (system inertia) 

If you’re lucky, the motor data sheet may have some of this information.  But in many cases you’re on your own trying to figure out what they are.  But don’t despair!  TI has just released a new sensorless Field Oriented Control algorithm called InstaSPIN-FOC which can measure most of these parameters for you automatically!  Figure 1 is a high level diagram of InstaSPIN-FOC, which shows its three distinct components:

1.        Motor ID:  This algorithm interrogates the motor to gather most of the parameters above.  It is usually run just once during the commissioning process for a particular motor.

2.        FAST:  This is the observer which calculates the real-time motor parameters needed for Field Oriented Control.  The neat part is that it doesn’t require a shaft angle or speed sensor to do it!  F.A.S.T. stands for Flux, Angle, Speed and Torque, which represent the main outputs of the FAST observer.  A diagram of how the FAST observer can be used in a Field Oriented Controller is shown in figure 2.

3.        PowerWarp:  This is a special energy savings operating mode used to reduce the copper losses in AC Induction Machines, especially during light-load conditions.

Figure 1.  Components of InstaSPIN-FOC 

There is actually a fourth component of InstaSPIN-FOC which is part of the motor ID function, and runs in real time to provide continuous estimates of the stator resistance.  This is the parameter that typically changes the most as the motor heats up during operation.  Monitoring it in real time significantly improves the performance of the FAST observer, and also allows you to dynamically change Kb in the current PI controllers to maintain pole/zero cancellation.  I have a lot more to say about this feature, as well as the other components of InstaSPIN-FOC in an upcoming blog series.

Figure 2.  FAST Embedded in a Field Oriented Controller

So we see that InstaSPIN-FOC provides measurements for Rs, Ls, and the rotor flux.  Fortunately, the number of motor poles is often listed on most data sheets, or can easily be determined empirically.  This leaves only system inertia (J) that is unknown.  But the entire velocity loop tuning procedure hangs on whether we accurately know what this parameter is.  Even though InstaSPIN-FOC doesn’t directly measure inertia for you, the following is a proposed procedure for how to use the outputs from FAST to measure inertia:

1.        Design the current controller by setting Ka = Ls/t, or p Ls/(10T_s), whichever is lower.  Recall that t equals the velocity feedback filter time constant, Ls = the stator winding inductance, and T_s = sampling period.  Also set Kb = Rs/Ls.

2.        Disable the velocity loop and use only the current loop.  Set Id =0, and apply a fixed value of q-axis current which is sufficient to cause the motor and load to accelerate through a good portion of the rated speed range.

3.        As the motor is ramping up to speed, periodically sample and record the values for motor mechanical speed (w(n) in rad/sec) and also the motor torque (T₁(n) in Newton-meters).  Both of these measurements are outputs from the FAST observer.  The motor torque should be pretty consistent since Iq is constant, and the motor speed should monotonically increase for a well behaved load.  For example, figure 3 shows the simulated samples of the speed output from the FAST observer for a permanent magnet motor driving a fan load with fixed inertia.  The sampling frequency is 200 mS over a 4 second span, resulting in a total of 20 samples.

4.        Now enable the velocity loop and set the PI coefficients to very low values that will barely allow you to spin the motor up to speed.  Since we don’t know inertia yet, we can’t accurately specify the velocity loop coefficients anyway.  Don’t worry about the dynamic response of the system at this point, since we are only interested in steady state performance.

5.        Command the motor to go to each of the recorded speeds from step 3 by monitoring the speed output of the FAST observer.  After the system response has completely settled out for each commanded velocity, again record the torque values from FAST (T₂(n) in Newton-meters).  This torque will correspond to the static load torque apart from any acceleration artifacts.

6.        We now have all the data required to solve for inertia as a function of time (J(n) in kg-meters^2) by using the following difference equation:

                                        Equ. 1

where:     J(n) is the inertia at sample time "n" (kg-m²)

                T_s is the periodic sampling period (sec)

                T₁ is the total torque consisting of static friction and acceleration torque (Newton-meters) from the FAST observer

                T₂ is the static friction torque only (Newton-meters) from the FAST observer

                w is the mechanical speed measurement (rad/sec) from the FAST observer

Figure 3.  Simulated Speed Samples of a Fan Controller with Fixed Inertia

Figure 4 shows an example inertia plot using equation 1 applied to the data from figure 3.  The estimated values are plotted in red at each time step along with the actual load inertia in green.  In this particular scenario, the inertia is constant and the torque ripple is zero, making it the best condition to estimate inertia.  As you can see, the results are most accurate for the range in figure 3 where the acceleration is fairly constant.  In this case you would simply time-average those inertia estimates to come up with a single value for system inertia.  But what if you have excessive torque ripple, such as a compressor load?  In that case, you can think of the torque ripple as unwanted noise in the system.  To maximize the signal-to-noise ratio, you should set the q-axis current as high as possible to increase the ratio between the T₁ and T₂ torque readings.  It would also help to do a time average of the steady-state torque readings at each velocity point.

Figure 4.  Estimated vs. Actual Values of System Inertia as a Function of Time

 For applications where inertia is a function of velocity (like a centrifuge load), this technique can provide a distinct advantage over other techniques.  Since equation 1 calculates inertia as a function of time, and you also have data for motor speed as a function of time from step 3, you can combine the two sets of data to correlate inertia as a function of speed.  Having this data in a Look Up Table in memory allows you to dynamically update the Kc term in the velocity PI controller as the speed changes to optimize the response at any speed.

 So far, we have discussed PI tuning in generic terms which are mostly independent of the control topology.  In my next blog, I want to focus specifically on some of the more subtle points to consider when designing PI controllers for Field Oriented Control systems.  Until then…

Keep Those Motors Spinning,

www.ti.com/motorblog

It's getting exciting at Beagleboard.org this week preparing for the launch of the next generation. (I just love the ritzy new graphics!)
You can sign up to receive notice when order entry will be open.  If you are going to be in San Jose this week, stop by the TI DESIGN West Booth #1607 
and see demos or attend one of the sessions with founders Jason Kridner and Gerald Coley.

Reply to this post and tell us what you want to create...

Stay tuned at the University Program blog for updated information this week about BeagleBone, project opportunities, teaching materials and more.

~ Look forward to hearing about your project ideas! ~ Cathy Wicks, TI University Program  www.ti.com/university 

He’s an engineer – turned university campus inspirer. TI worldwide university marketing manager Brad Ruzicka’s path to his current position at TI follows an unlikely route.

Imagine a car that can fully function alone. It can take an elderly family member to the pharmacy for a prescription. It can drive a blind person to the park. It’s intelligent enough – and capable enough – to take you wherever you need to go without human input. All you have to do is enter your address in the navigation system.

We’re not talking about the fictional worlds of Batman or Total Recall. We’re talking about a technology that is closer to coming to fruition than most people realize. It’s technology that TI is already working to help develop. “People had thought that the idea of a self-driving car was 20 years away. Now, they are believing it’s 10 years away, so it’s really exciting,” says Bill Krenik, TI’s chief technologist of high-volume linear products. “It’s an idea that has really taken off now and we’re seeing some really cool things happen.”

Krenik recently joined Robert Atkinson, president of the Information Technology and Innovation Foundation (ITIF); Jason Schulz, a Toyota Motor sales partnership manager; Chris Urmson, a leader from Google’s self-driving car program; and Washington, D.C. city councilmember Mary Cheh in an ITIF panel discussion on Capitol Hill.

The “The Social and Economic Case for Autonomous Vehicles” discussion, which you can watch here, covered the social and economic benefits of automated vehicles. ITIF’s Robert Atkinson opened the talk, pointing out that human error is the cause of 93 percent of today’s accidents.

Replacing a human “with a machine that doesn’t really cause errors,” could significantly decrease the 36,500 traffic fatalities – and $450 billion in economic losses -- that occur yearly nationwide, Atkinson says. These vehicles also could help reduce the $200 billion in economic losses and environmental damage from traffic congestion.

That’s a big impact. What’s more, Krenik says, these cars can be used with the same infrastructure -- or with minimal road enhancements. And, with autonomous cars, the distance between each car on the road could decrease – which would increase the number of cars able to quickly move from one location to another. “It can save tens of thousands of lives, reduce environmental impact, improve economic efficiency, and improve quality of life,” Krenik says. “For many people, this technology will mean a lot.”

From research at the university level to corporate, TI is among those working to pioneer technology that can be used in these vehicles of the future. Krenik says that TI is currently supplying components for brake and control systems – such as computer vision, advanced braking, traction and stability control, and electrical power systems – that are being used in experimental vehicles being created today.

TI’s technologies (see the full range of TI’s automotive progress here) will be the enablers in the autonomous system of the future, he says. “These are the pieces upon which autonomous transportation will be built.”

Autonomous transportation in general isn’t new – for instance, many airport trains and certain planes are already automated. “It’s much harder to build an autonomous car than a train, but it’s not as foreign of a concept as people think,” Krenik says.

In fact, Google has already created several autonomous cars using the Toyota Prius, and launched a program in 2009 that began testing the vehicles – in real traffic. The cars are “equipped with sensing that allows them to see comparable or better than a person, they can see traffic lights, pedestrians in the road, they can see vehicles as they move around them,” and even drive carefully through toll booths, Google’s Urmson said in the discussion.

 It’s proof that the technology can work. And proof that you might be driving in a robotic vehicle sooner than later. “It’s really exciting,” Krenik says. We’re ready for a ride.

Everyone knows that op amps should have power supply bypass capacitors located near the IC’s terminals, right? But why? Why, for example, is an amplifier more apt to oscillate without proper bypassing? The reasons will increase your understanding and awareness.

Power supply rejection is an amplifier’s ability to reject variations in the power supply voltage. Figure 1, an example, shows that this rejection capability is very good at low frequency but diminishes as frequency increases. Hummm... poorer rejection at high frequency where oscillations occur?

We often think of external power supply-borne noise interfering with an amplifier. But op amps can create their own problems. For example, output load current must come from the power supply terminals. Without proper bypassing the impedance at a supply terminal can be high. This allows AC load current to produce an AC voltage on the supply pin. This creates an unintended, uncontrolled feedback path. Inductance in this power supply connection can magnify the resulting AC voltage at the supply pin. At high frequency, where power supply rejection is poor, this unintended feedback can cause oscillation.

There are internal forces at work, too. Without a solid power supply, internal circuit nodes may talk to one another creating unwanted feedback paths. Internal circuitry is designed to operate with firm, low impedance on the power supply terminals. An amplifier may behave quite differently and unpredictably without the solid base of low impedance supplies.

With a clean sine wave input, the unintended feedback due to poor bypassing may not be a tidy sine wave. The signal currents in the supply terminals, figure 2, are often highly distorted because they only represent one half of sine wave current. With different power supply rejection characteristics on the positive and negative supplies, the net effect will distort the output waveform.

The issues are magnified with high load current. Reactive loads create phase-shifted load currents that may exacerbate issues. Capacitive loads are already at higher risk of oscillations due to additional phase shift in the feedback path (more detail here). These higher risk cases may need higher value tantalum bypass capacitors and extra care in circuit layout, compact and direct.

Of course, not all poorly bypassed amplifiers oscillate. There may not be sufficient positive feedback, or the phase not quite right (or wrong!) to sustain an oscillation. Nevertheless, performance may be compromised. Frequency and pulse response may be affected with excessive overshoot and poor settling time.

As discussed in a previous blog, these behaviors are not well-modeled in TINA-TI or other SPICE programs. Voltage sources in SPICE are perfectly solid, unperturbed by load currents. Modeling the actual source impedance of your supply and board layout with additional components is tricky and imprecise. Power supply rejection magnitude is modeled in our best macro-models, but the phase relationship of this feedback path is unlikely to match reality. Simulation can be tremendously useful but won’t accurately predict this behavior.

All this should not cause you to be paranoid—no need to go crazy with bypassing. Just be alert to particularly sensitive situations and signs of potential problems. Good analog design thrives with a healthy dose of understanding and awareness.  :-)

Thanks for reading and comments are welcome below.

Bruce       email:  thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Look here… 55+ other interesting technical topics  The Signal blogs.

For those of you waiting for the much anticipated & coveted next-generation canine, your wait is over!

Powered by the 1GHz Sitara™ AM335x ARM® Cortex-A8 processor, BeagleBone Black is a community-supported development platform for developers and hobbyists. Boot Linux in under 10 seconds and get started on development in less than 5 minutes with just a single USB cable.

Features include:

• High performance with TI’s 1GHz Sitara™ AM335x ARM® Cortex-A8 processor
• On-board HDMI to connect directly to TVs and monitors
• More and faster memory now with DDR3
• On-board flash storage frees up the microSD card slot
• Support for existing Cape plug-in boards

And it's priced at only $45. To learn more and to order yours today, visit the BeagleBone Black page: BEAGLEBK.

Alejandro Erives, Sitara ARM Processors brand manager, has been told that his bark is bigger than his bite. Grrrr!

Previously, I posted part 1 and 2 of a 3 part series on how to test your power supply design: measuring efficiency (part 1) and measuring noise (part 2). I covered various noise sources and how to properly measure them with an oscilloscope. I also discussed output errors created by line and load transients

Today, I'll touch on the third and final metric when testing power supply: measuring stability.

A power supply is a closed loop amplifier; it takes in electrical energy and converts it to electrical energy in another form, at a specific regulated voltage and/or current. Power supplies regulate by sensing the output and comparing a portion of it to a reference voltage. The difference between the sense signal and the reference is amplified and then used to control the power stage of the regulator to keep the voltage (or current) constant (Figure 1).

Power supplies employ negative feedback from the output back to an error amplifier to ensure proper regulation over various operating conditions (load changes, temp changes, input voltage changes, etc.). As with any stable closed-loop system, one must ensure the closed-loop gain is less than one at frequencies of operation or risk oscillation and/or other non-desirable characteristics.  The negative feedback term of a power supply must be sufficiently out of phase with the input or establish a gain of less than one to ensure proper operation.

A typical regulator IC provides the necessary phase margin within the device to ensure stable operation.  Like any engineer, an IC designer works with operating limit assumptions and often provides control mechanisms to adjust the internal phase delay to accommodate for various load extremes.  A regulator might be designed to provide 90 degrees of phase margin with a nominal output impedance yet if the impedance is more capacitive than anticipated the phase delay might grow to a point to where the phase being fed back from the power supply output is in phase with the internal feedback point.  The reversal in phase creates positive feedback with a gain of more than one, the formula for an oscillator. We all know that this is not desirable within a voltage regulator circuit. 

Many regulators provide a mechanism to adjust the internal phase delay, usually accomplished with an external compensation network employing a few passive components.  In some cases the regulator does not provide such hooks and must be employed within defined operating extremes (min/max output impedance over load extremes).  In any case one must be able to properly analyze a circuit to determine if design adjustments are necessary.   While loop characteristics can be simulated, real world system level characteristics such as PCB and connector impedances are difficult to accurately model, especially with lower cost simulation tools. So an actual stability measurement is necessary to understand actual loop stability. 

Yes,  I have seen many situations where a system was released to production and later in production became unstable based on environmental changes and/or operating extremes.  In these situations the prototype likely worked fine yet phase and gain margins within the power supply where were not testing during prototype test.  If stability of a power supply is tested a designer identifies the problem and corrects the issue before it causes more costly production issues.

Read in more detail how to measure noise in a power supply in my full article on EDN.

Walk through how to measure noise in your power supply with me in this Engineer It video.

If you have any questions or comments on how to measure stability, please leave them in the comments section below.

Good luck!

There are usually only 24 hours in a day – but yesterday, there happened to be 26.

A huge team from TI flew from Dallas to San Jose to kick off DESIGN West 2013, and let me tell you – the two hour time change really made a difference. It felt like we pulled a college-style all-nighter to get our booth together in time – but it was well worth it and those extra two hours came in handy!

If you haven’t had a chance to see our booth (#1607) yet – please make a point to stop by. I promise you’ll feel right at home when you see our ‘TI Tech Cave’. It’s decked out with the latest technology like an LED coffee table, home automation system and a throwback Nintendo console to name a few – which has made it the most comfortable place to rest at the show this week.

Another cool technology you’ll have to check out in our ‘Tech Cave’ is the wireless charging station. Just drop your phone on the desk and poke around our booth while it recharges. It’s been a lifesaver for media and customers who have drained their phones while walking around the show floor.

If you’ve conserved your phone battery – you’ll still have a reason to stop by our booth. Snap a photo of the new BeagleBone Black with our Sitara AM335x processor team. It’s been the most talked-about (and desired) product at the show.

A few reminders for tomorrow:

-          TI’s “Make the Switch” Tool Swap is still happening. Bring an old competitor tool and we’ll swap it out for a new MCU LaunchPad or Hercules kit.

-          In-booth training happens every hour, on the hour. We’ve received great feedback from today’s sessions – so don’t miss out.

-          Follow TI on Facebook  or Twitter for all the latest updates and exclusive insights from the show.

We hope to see you all in the morning!

More and more electronics are needed for very harsh environments, which then results in the need for multiple electronic components.  For instance, oil and gas drilling, where temperatures reach up to 200°C require complete solutions for intelligent down hole tools to find and recover resources for energy production.

We’ve been working on high temperature integrated circuits (IC) for this market for several years and have developed a complete signal chain solution of microcontrollers, digital signal processer (DSP), amplifiers, data converters, power, and interface functions, but the one area missing has been non-volatile memory. Now, thanks to the SM28VLT32-HT, there’s a high temperature flash memory device capable of reading and writing up to 210°C.  This device eliminates the need for costly up-screening and qualification testing of industrial-grade components for use outside their standard operating range.

The SM28VLT32-HT has a capacity of 4-MBytes and is the industry's first high-temperature, nonvolatile Flash memory device designed for harsh environments.  It is qualified and tested across the entire temperature range to provide robust read/write operation over the device's operating life.  Designed utilizing our 180nm CMOS flash process, it is built on the same process technology that allows our ARM7 and motor control DSPs to operate at 210°C.

Since size is a big concern in many of these applications, the SM28VLT32-HT was designed from the ground up to support extreme temperatures.  It is also available as a Known Good Die (KGD) to support higher levels of integration into multi-chip modules. This use of a serial SPI interface simplifies design and packaging, and reduces pin count.

The availability of high temperature non-volatile memory will enable more capabilities in many harsh environment applications.  The HT Flash offers designers more options for reading and writing to memory at temperatures greater than 210°C and provides reliable solutions for logging data or storing programs.

After blogging last night about all the cool demos in our DESIGN West booth, I didn’t realize you would ALL come see us today! I’m not complaining, but my feet may be.

From the time the doors opened at 11:30, which was highlighted by people literally sprinting to our WEBENCH speed training session with Jeff Perry, to the doors closing at 5:30 – it was a busy day.

There were very few open seats at each of the in-booth training sessions and we anticipate even more people attending tomorrow. We didn’t plan it this way, but the trend in the TI booth seems to be flashing LEDs. We’ve got a great ZigBee Light Link (ZLL) demo where we can control the color of the lights via a tablet. One of the media guys who stopped by to see it was amazed – his favorite part was when we took a digital postcard of a sunset and matched each of the light bulbs to a different shade of red, yellow and orange from the postcard. We’ve also got blinking LEDs on our “Tech Cave” coffee table, MSP430 LaunchPad LED Cube, the SafeTI rotary saw demo, not to mention all the flashes from the cameras near the BeagleBone Black pod.

I really encourage you to check out our booth tomorrow. It’ll just take you a few minutes – but you’ll be amazed at all the cool projects that we have on display. This isn’t just about TI demos – we’ve invited several of our partners and hobbyist community members to show off their projects as well.

And just because it’s the last day of the show, we’re not wrapping up early!

See you tomorrow!

The University Technology Day took place on March 21th, 2013 at the National Library of Science and Technology of the university: Instituto Politecnico Nacional, Campus ESIME Zacatenco in Mexico City.

The main purpose of the event was to demonstrate the benefits of using Texas Instruments high performance technology as a teaching tool at the main engineering universities of the country.

Conferences and workshops

The event consisted of conferences and workshops that offered updates on microcontrollers (MCU) technology such as MSP430, Stellaris, C2000, and analog design using TI’s WEBENCH® Design Center tools. Brad Ruzicka, Worldwide University Program Manager gave a speech about the benefits the universities get with the University Program. The agenda contained presentations like:

• “University Program and MSP430” The conference was about University Program, and MSP430 Applications. It was an explanation of the benefits of this TI program. It focused on the TI portfolio and Value Line MSP430 MCUs.
• “WEBENCH® Workshop” It was a hands on workshop to learn how to use the WEBENCH® on line tool. WEBENCH® is a tool to help analog designers implementing TI sockets in their design. The workshop consisted on designing a buck power source for automotive applications.
• “MSP430 LaunchPad Workshop” This workshop was an introduction to the value line. The presenter taught about initialization of LaunchPad, GPIOs and ADC uses. There were many examples like a temperature sensor using the LaunchPad.
• “Stellaris LaunchPad Workshop” This workshop focused on the use of GPIOs, watch dog timer, interruptions and UART communication.
• “C2000 LaunchPad Workshop”This new LaunchPad Workshop focused on the hardware and the controlSUITE tool for C2000. The presenter taught how to initialize the C2000 LaunchPad and the use of the GPIOs. He showed some lighting DEMOS controlled by this LaunchPad.

ShowRoom

Applications and evaluation modules were showed and explained. The showroom was divided in three settings:

• MSP430 BoosterPacks, focused on MSP430 LaunchPad BoosterPacks. There were capacitive touch, audio, OLED Display, RF, and other applications.
• MCU & microprocessors (MPU), focused on TI MCUs in general and MPUs like the BeagleBoard and Sitara touchscreen DEMO. There was also a DSP intelligent vision application with a DaVinci EVM.
• Analog Tools focused on analog applications, such as dimming LED Lighting, power sources, and amplifiers. There was also shown a new university kit: ASLK Pro University Kit, for helping students to learn about analog design and control applications.

The main purpose of these activities is to show university professors the tools from Texas Instruments and to prepare them in order to implement laboratories at their universities and teach their classes with the best technology. Professors from the most important Mexican engineering universities attended to this event. The University Technology Day let Texas Instruments create new relationships with professors.

In the past two weeks, Jennifer gave you a deeper view of our MSP430 LCD portfolio and introduced you to the applications and benefits of using these integrated devices in your application. Now, if you’re like me, you are ready to get started!

Today I’m going to explore some of the great development tools available from the MSP430 team. Specifically, I will highlight those tools featuring our 100+ devices with integrated LCD so that you can better decide which MSP430 is right for your LCD based applications.

Let’s start by taking a closer look at our 4 series. As you read previously, this series offers CPU speeds of up to 16MHz with 120kB Flash and has integrated features like a 16-bit Sigma Delta ADC for precise measurement. This family is a strong fit for applications including portable medical and metering. Intrigued? Check out the MSP-EXP430FG4618. This Experimenter Board is extremely versatile with both a MSP430F2013 and a MSP430FG4618 on board. You can use the included LCD screen, buttons, capacitive touch pad, audio output and even add a TI Wireless Connectivity evaluation module to implement a full solution.

Do you need higher speeds or more analog integration? Then the 6 series is the answer. These devices run at up to 25MHz and feature more memory and USB connectivity in addition to the LCD driver you need. The devices in this family are more powerful without compromising the ultra-low power you have come to expect from MSP430. If that isn’t enough we have a tool designed specifically for three-phase electricity metering coming soon. The EVM, based on the MSP430F6779, can be connected to the main power lines and has inputs for voltage and current, as well as a third connection to setup anti-tampering. If you throw in our Energy Library and available GUI, you can get started in no time!

Looking for an even more integrated solution? Then look no further! Our RF SoC series offers up to 20MHz speeds and 32kB Flash and includes LCD and an on chip sub 1-GHz radio. We even have a highly integrated and wearable development tool, the EZ430-CHRONOS. This tool is fully programmable and comes in a watch form factor. Check out some of the applications a tool like this can enable!

Still not exactly what you need? Looking to integrate an MSP430 directly into your system? We have tools for that too! Pick out an MSP430 Design Kits for the MSP430 that meets your needs.

If you are interested in LCD applications the MSP430 team is here to help! Don’t hesitate to ask questions below and head on over to www.ti.com/msp430 to explore our LCD portfolio and choose the right tool today! To help you get started we will disount several tools including the MSP-FET430U128 and the EZ430-CHRONOS on 4/30! So follow us on Facebook (https://www.facebook.com/timicrocontrollers) or on twitter (@TXInstruments) to learn more.

Rub the lamp and make a wish!

C.P. Ravikumar, Texas Instruments

I remember the winter morning on which I visited NSIT, Delhi. With help from Prof.  Dhananjay Gadre, who is on the board of faculty of Electronics and Communication Engineering at NSIT, the TI India University Program was hosting four hands-on workshops in January.  The event was technically co-sponsored by the IEEE Student Chapter of NSIT.

“Shall I make some coffee for you?” offered Gadre, and to my surprise, proceeded to make two cups of instant coffee using a microwave. In answer to my puzzled expression, he explained – “We keep the condiments for making tea and coffee in this office. My students use my office to do their projects. Some of them even sleep here if they have stayed up all night!” and pointed at the sofa.

I took a sip of hot coffee and looked around the office. Wires, soldering iron, printed circuit boards in various stages of construction, boxes, electronic components, text books. But there is a method in this madness, because Prof. Gadre is able to pull out what he is looking for without much searching.

I broached the subject of TI India Educators’ Conference, planned in April 2013. “Can you make an electronic lamp for the inaugural ceremony, Prof. Gadre?”

Like the coffee, his consent was also instant.  I had seen his “birthday candles” project and wanted something similar, but being a free-lance designer, he likes to out-do his own prior designs.

“You may have seen the traditional brass lamp that is used in inaugural ceremonies. Perhaps you could build five small lamps that can be placed as five wicks for the lamp …”

“We will work on it,” he said, and smiled.

*  * *

Here comes the lamp!

I told my team members about this conversation. Then we got busy with the preparations for the conference. At some stage, we received a list of semiconductors that Prof. Gadre needed for the lamp project and Vaibhav took care of that request.  When we were about three weeks away from TIIEC, Prof. Gadre sent us a mail. The lamp project was going slow, but steady. He expects to have a prototype soon.  We were prepared for the eventuality of using the conventional lamp as a backup.

The day before the conference,  Sagar, Vaibhav, and I reached the venue at 5.30pm and got busy with some of the preparations we had to do.  Prof. Gadre arrived at 8.00pm with a large suitcase. Very carefully, he took out the parts of the lamp and assembled them. He pulled out a matchbox from his pocket and an electronic matchstick. He rubbed the matchstick against the matchbox and the tip of the matchstick glowed milky white. He brought the matchstick in contact with one of the five lamps. The electronic wick caught on the light and began to flicker.

We were all smiles. “Hurrah!” we cried. “Bravo, Prof. Gadre!”

“But wait! The electronics works great. But we have this aesthetic lampstand that we have made for the lamp, which we need to assemble. That will take time.”

At this time, the electricity failed and we were covered with darkness. There was no backup power and we decided on the next action plan.

“There is no point in waiting here. Let’s go. I will be here by 6.00am and start assembling the lampstand. Your inauguration is at 9.00am. I will need at least 2 hours.”

*  * *

The stage is all set

Apr 4. 8.45am. The “Silicon Hall” was beginning to fill up.  The dignitaries who were going to “light” the lamp were already inside the hall, chatting, waiting for me (the host of the inaugural ceremony) to make an announcement. On the stage, Prof. Gadre and two of his students were kneeling on the floor, assembling the lampstand. Prof. Gadre had a soldering iron in his hand.  He was pairing up wires of the same color coming from the power adapter and from the lamp and was going to solder them together. I looked at all this helplessly. What are the odds? I thought. Beads of perspiration formed on my forehead. Prof. Gadre’s steady hands continued to solder.

9.00am. The dignitaries in the front row occasionally looked up at the stage. The suitcase and plastic bubblewrap and boxes containing electronic components were still lying on the stage. Prof. Gadre was soldering. Should I act on the backup plan? The conventional lamp was ready, decorated with flowers and all. But the earnestness with which the students and Prof. Gadre were working did not permit me to say a word. 

9.05am. I broke my silence. “Should we begin to clear the stage from the debris?” I said to one of the students. He obeyed. Just then, Prof. Gadre got up. The soldering was completed. Now for the acid test. He repeated the matchbox trick again – this time on all the five lamps. The first four wicks caught the light. The fourth one played hard to get. Wrong wiring? I thought. But Prof. Gadre held the matchstick for a little longer. The fifth wick obeyed. I rushed to the podium and began my welcome speech.  It was 9.08am.

A successful lamp-lighting ceremony drew  a thunderous applause from the audience. Then I invited our President and Managing Director Dr. Biswadip Mitra to say a few words, he chose to speak about the importance of building electronic systems. “My advice to students is to soil their hands. Choose a system, a sub-system, or even a sub-sub-system for implementation. Build it out of integrated circuits …”

I was thinking about Prof. Gadre kneeling on the floor, soldering the wires. “Surely his hands must be soiled,” I thought.

* * *

The lamp was made in many stages

Many days after the conclusion of the conference, I decided to ask Prof. Gadre to share the story of the lamp so that our readers from the Universities will be benefited. In particular, I am hoping that this story will help all those students who are starting out on projects using Texas Instruments integrated circuits.

“There were many stages during the development of the lamp,” said Prof. Gadre.  The first stage was to form a team and brainstorm. A team of four worked with Prof. Gadre on this project – Rohit Gupta, Prateek, Piyush, and Rohit Dureja.  More about the team later.

There are so many ways in which we could build an electronic lamp. We wanted a lamp that can be lit like a conventional lamp; simply pressing a switch will be breaking the tradition too much, although it will simplify things considerably.  We also talked about several ways to trigger the lamp with the electronic matchstick. Finally, we settled on using infrared (IR) transmitter and receiver used in TV remotes. We tested out the idea by building a quick prototype – that was in the end of Jan.”

The second stage involved building the LED lamp with an integrated IR remote receiver. Several versions were built in February 2013. A final version was tested in early March 2013.  The third stage was to build and test the electronic matchstick. This was done in March, about 3 weeks before the date of deployment.  The matchstick uses an 8-pin MSP430, an infrared LED, a TPS61070 DC-DC boost converter and a 4.7F/2.7V super-capacitor.

After we got the lamp prototype to work, the next stage was decide on the structure of the lamp.  “After much brainstorming, we decided to build a complete lampstand from acrylic sheets.  Earlier, our idea was to build the lamp as an add-on attachment for the traditional brass lamp. In hind sight, building an entire lampstand was not a good idea – the stand was fragile and presented challenges in transport. It was also expensive to build.”

So there is a lesson in system design. Reuse what you can. 

The final stage was to do “mass production.” Of course, in this case, the team only produced 10 lamps. This was done in the last week of March. “We clicked the image of the prototype, shot a small video demonstration, and sent them off to the TI team,” said Prof. Gadre.   "A lot of care was taken to pack the lampstand and the lamps into a suitcase. I took an afternoon flight to Bangalore and when I checked in the luggage, I asked the airline to place a ‘Fragile’ label on my sturdy suitcase!”

* * *

Throwing some light on the lamp

The lamp uses warm white High Brightness (HB) LED which is powered with 12V, as the main source of light.  In addition, two lower power (100 mW) 5-mm warm white LEDs are used as additional sources of light to create a perception of flickering. The lamp is powered from a 220V AC mains supply; an adapter converts the AC voltage into 12V DC, which drives the LEDs. An LM117 Low Dropout (LDO) voltage regulator chip from Texas Instruments steps down the 12V DC supply to 3.3V required by the lamp controller circuit.   The TV remote IC receiver is also powered with a 3.3V supply from the LM117 regulator. The lamp controller ccircuit also includes suitable drivers for the LEDs. To make debugging easier, the controller card includes a reset switch and an override switch, a power ON indicator LED and a status indicator LED.

Figure 1 - Block Diagram of the Lamp

A block diagram of the lamp controller appears on Figure 1. An MSP430 microcontroller has been used in the lamp controller circuit; in particular, a 14-pin DIP package was selected to simplify manufacturing. When the reset switch is pressed or when power is first applied to the lamp, a status indicator LED begins to blink, indicating that the controller is waiting for a trigger. The MSP430 microcontroller continuously waits for a trigger from the IR receiver or the press of the override switch. When either of these events happens, the microcontroller enters a loop where it shifts out bits from a 32-bit LFSR to the three output pins driving the three LEDs, creating a flickering light pattern.  Providing the “override switch” allows the designer of the lamp controller card to independently test the card; it is also a valuable diagnosis tool for later use.

The electronic matchstick is powered by a 4.7F supercapacitor, charged to 2.5V using a pair of external AA batteries prior to use. The supercap drives a DC-DC converter that provides a DC supply voltage of 3.6V to an 8-pin (SOIC package) MSP430 microcontroller as illustrated in Figure 2.

Figure 2 – The Electronic Matchstick

The microcontroller in the matchstick drives two LEDs, a white LED and an IR LED. When the microcontroller is powered, it waits for a trigger signal from a 100mH sense coil. The trigger is provided by ‘rubbing’ the matchstick against conventional matchbox fitted with a couple of powerful neodymium magnets inside. When the matchstick is sharply struck against the matchbox (which side of the matchbox the matchstick is struck, is also important), a short pulse is induced across the sense coil and that triggers the microcontroller to light up the white LED using a random sequence of bits, thus creating an illusion of a real matchstick being lit. Also, the IR LED is fed with a sequence of ‘1’s and ‘0’s modulating a 40 kHz train of pulses. The 40 kHz carrier frequency is required because the IR receiver in the lamp has that requirement. The modulation of a carrier frequency with the required bit sequence offers immunity against unwanted ambient optical noise. The bit sequence transmitted by the IR LED is what triggers the LED lamp when the matchstick is held close to the lamp. The lamp waits for a sequence of five ‘1’s before lighting up. The white LED on the matchstick does not really have a role in lighting the LED lamp; it is only provided for creating an illusion of a burning matchstick!

* * * 

The Team

Given that there were only three months to build the whole thing, four students were involved in the project. Rohit Gupta from Delhi Technological University helped in programming the MSP430 microcontrollers used in the lamp and the matchstick. “Piyush Guddu and Prateek Jha, alumni of NSIT, implemented the LED lamp based on a structural design I suggested. Piyush is an artist and Prateek transferred the design to Corel Draw, got the design fabricated, and raised the structure. He also did several designs of the lamp cover using the 3D printer we have in the TI Center of Excellence in Product Design. Rohit Dureja, a final-year undergraduate student of NSIT, helped in mass production. “Rohit Dureja is more experienced, can do better soldering and has some idea about testing,” says Prof. Gadre. “Since the volume was low, only 10 pieces, we managed everything in the lab itself, including the PCBs, soldering and testing.”

No project is without hiccups

To achieve the “flickering” effect, a random sequence of 0s and 1s are sent to a port pin that drives the LED.  Rohit Gupta set out to program this on MSP430. Prof. Gadre told him about the Linear Feedback Shift Register (LFSR), which can be implemented easily on MSP430. The LFSR can generate a pseudorandom sequence of 0s and 1s. If these data bits are shifted at a suitable rate, the flickering effect is easy to create.  Being an ambitious student, Rohit decided to try a different approach – use a pulse width modulation (PWM) generator to drive the LED.   “This was an overkill,” said Prof. Gadre. “Moreover, there was something in the implementation which was causing the LED intensity to show a pattern in the short term.” In other words, the flickering was not random.  Prof. Gadre sat down with Rohit for a debug session, when he realized that PWM has made inroads into the code. He advised Rohit to revert to the planned LFSR scheme, which, of course, worked! Once again, the lesson for the students is to “Keep things simple.” The lesson for mentors is to watch out for feature creeps in an ambitious student’s code!

There were other hiccups. “We noticed some minor errors in the lamp during its field trial,” says Prof. Gadre. “For example, we had not imagined that the flash light of the photographer’s camera could also trigger our lamp! Not sure how many people noticed this! Our plan is to correct this in the next version.”

System-level Design

Prof. Gadre had to think about other “system-level” issues as his students proceeded to carry out the design and implementation of the sub-systems. For example, he thought of adding a feature that can trigger the lamp without the matchstick. “There are two sub-systems – the lamp and the matchstick. Having an independent way to test the lamp will allow the design and debugging of the two sub-systems to proceed concurrently.”

Providing power supply to the lamp without adversely impacting the aesthetics was another concern. The lampstand was made of transparent acrylic material. Having a lot of wires running up the lampstand would be far from desirable. “We used copper tape for attacking this problem,” explained Prof. Gadre. “A vinyl cutter was used to cut the tape in required shapes and sizes.  The copper tape was made part of the aesthetic feature!”

Transporting the lamp safely from New Delhi to Bangalore was no mean task. In the book “Soul of a new machine,” Tracy Kidder describes an incident that happened in a computer manufacturing company. The computer was assembled in a room on a higher floor. When the time came to transport the machine out of the office, the workers realized that the computer would not fit into the elevator!

“We were naturally worried about the acrylic sheets breaking. For transporting the fragile acrylic sheets, I had to think of a suitable scheme and I ended up using 6 mm plywood sheets of a size that would fit a large suitcase,” said Prof. Gadre. “I taped the acrylic cut sheets on these plywood sheets and then the entire structure could be transported safely.  This has also been a lesson for us. Our next version will overcome these problems, so that the lamp has a higher acceptance in the market.”

Test, test, and test some more!

When the final prototype was ready, it worked just fine. The team then started the mass production, but continued to test the prototype lamp installed on the lampstand. One evening, when testing the lamp without the 3D printed covers, the lab assistant walked in and turned the tube light on. The team stared in disbelief – the lamp had been unintentionally triggered! The humble tube light has significant IR component and could trigger the lamp without the matchstick!

“There were only two days before the deadline. Fortunately, covering the lamp with the 3D printed covers protected the sensor from false triggering,” explained Prof. Gadre. “We heaved a sigh of relief.  In hindsight, as we discovered during the conference, even this was not enough! The photographer was using a strong flashlight, which also has a very strong IR component, thereby triggering the lamps without the matchstick!”

The lesson here for students is to test the prototype as much as possible. Having it tested by someone who does not know the working principles of your project will also help!

I asked Prof. Gadre if their team has a solution to the false trigger problem. “The solution is simple – the matchstick and the lamp must trigger on a unique code of IR radiation and not just count 5 pulses of IR radiation, which happens in the prototype. Another lesson we learned at the conference was not to depend on AC power as the source of power for the lamps. Even during the conference, brief power outages happened and that turned the lamp off. In the new design, we are using batteries to power the lamps.”

What next?

I congratulated Prof. Gadre on the first product that officially rolled out from TI Center for Embedded Product Design at NSIT. To be precise, the lamp is the second product. “The Stellaris Guru” educational kit was the first product to be developed; but the Center had then not yet come into existence formally.  I asked Prof. Gadre the questions that most people would be tempted to ask – will the design be available to someone who wishes to productize it? How does one approach the Center for this purpose? What support would Prof. Gadre and his team of students be willing to provide?

Prof. Gadre is willing to be a consultant for a company that wishes to bring out the electronic lamp as a product, either in its current form or with customized modifications. “The existing design as well as the new version that we are working on, will be made open source,” says Prof. Gadre. “If there is enough traction, we can think of making it available as a complete product or a DIY kit.  I am available on e-mail to take questions about the productizing the lamp.”

Pictures!

Here is an attempt to capture the various stages of creation of the lamp! 

The National Math and Science Initiative (NMSI) recently awarded $12 million in challenge grants to endow the highly successful UTeach programs at 12 American universities, including two supported by the Texas Instruments Foundation.

NMSI awarded $1 million to each of 12 universities that have raised matching funds and met performance benchmarks for implementing the UTeach program to recruit and train college students to become math and science teachers.

"We're proud to support UTeach, which is making a real difference in addressing the critical shortage of math and science teachers in our schools," said Ann Pomykal, Texas Instruments Foundation executive director. "The Texas Instruments Foundation has been involved from the beginning with the North Texas universities in this program, and we're glad to see the matching grants that extend our investments for even greater results in the future."

The TI Foundation has supported two universities receiving the NMSI awards, the University of North Texas (UNT) and the University of Texas at Dallas, with grants to expand existing UTeach centers.

To read the full story, please visit our Corporate Citizenship news site.

I recently returned from a trip to Germany where I spent the week visiting customers in and around the Stuttgart region.  Outside the lack of sun (apparently it was the darkest winter season in 40 years), the area was pretty amazing with a high concentration of automotive and industrial industries.  One of the things I most enjoy about these trips is talking motors with the local sales and application engineering teams and one of the most common questions I get is “how do I select the right motor driver?”

Start with the basics.  What type of motor is the customer trying to spin? Luckily, 90% of the 12 billion or so motors produced each year are either brushed, brushless, or stepper.

Next question to ask is what is the operating voltage range of the motor? Common motor rails include +12 V, +24 V, +48 V and the various battery generated voltages. The trick here is to size the driver’s operating range with enough margin to handle the inductive voltage spikes and supply pumping that comes along with spinning a motor. For example, say a customer is running a 3A brushed DC motor off a +24 V rail, I would probably recommend a driver like the DRV8842 whose maximum operating range is +45 V in order to guarantee the long term reliability of the system.  As a general rule of thumb, pick a motor driver whose operating voltage range is around 1.5x to 2x that of the supply you are running the motor off.  You always have the option of placing snubbers on the outputs to reduce the PWM edge ringing and TVS diodes on the supply to prevent supply pumping from overdriving the driver, but this adds unnecessary cost and complexity to the system.  Do yourself a favor and size the driver right at the beginning of the design in order to avoid future re-spins.

With respect to the current rating, consider the continuous or RMS current as well as the peak current requirements.  With brushed and brushless DC motors, during startup and stall events the back EMF is non-existent and the driver only sees the motor’s winding resistance.  Therefore you get an inrush of current before the motor spins up and generates a back EMF which counters the supply voltage, bringing the current draw down.  So even though your application may only need a max of say 1A continuous current during normal operation, it may need to handle 2 to 3 times or even higher the continuous current during start up and stall events.

For stepper motors, it’s a little more straight forward as they are primarily driven by sine waves, so the peak and RMS current are just related by the square root of two. For example, if you look at the current ratings on the DRV8825 stepper driver, you’ll see it rated for 1.75A RMS and 2.5A peak operation. For all types of motors, make sure your driver can handle both the peak and continuous/RMS current needs of the system.

For more info on motor drivers, check out my Engineer It videos on “how to select a pre driver vs. an integrated motor driver” or “thermal considerations when selecting an integrated motor driver.” Or, you can always go to TI’s Motor Driver Forum and ask the experts or share your experience.   

Current feedback (CFB) amplifiers mostly belong in the realm of high speed amplifiers. There are lot of good application notes developed over the years that describe the operation and the main issues encountered when applying current feedback amplifiers to a problem.  Here we’ll try to summarize them in a few good words.

A CFB amplifier has one high impedance input (the non-inverting input), one low impedance input (the inverting input), and one output low impedance, as is represented below.  Note that for the purpose of this discussion, I will be ignoring the power supply pin and disable functions.

Figure 1: CFB internal elements

The voltage on the non-inverting input sees a high input impedance so as not to load the input.  The voltage on the non-inverting input appears on the inverting input as it passes through a buffer.  As the buffer is non-ideal, it will have a gain a(s) that varies with frequency with DC magnitude very close to 1V/V but typically 0.996V/V.  The buffer also ideally has output impedance equal to 0W.  In practice, the output impedance varies between a few ohms to a few tens of ohms.  I will also ignore the inductive component of that resistance as well for now.

 The intent for the buffer is two-fold:

1)      It forces the inverting node voltage to follow the non-inverting input.

2)      It provides a low impedance path for the error current to flow.

As the error current passes through the buffer it is sent to the output through a high-transimpedance gain stage.  Closing the feedback loop will drive the error current to almost zero in a fashion similar to the error voltage being driven to zero in a voltage feedback amplifier.

The only action left is to write the equation and interpret it.

 is the noise gain, and in the case of the non-inverting configuration shown, the signal gain as well.

The loop gain can be expressed as:

This is a very important equation for an ideal CFB  as it expresses the loop gain is proportional to the feedback resistance hence the feedback resistance is acting as the main compensation for CFB.  In effect, increase the feedback resistance and the bandwidth (BW) will decrease the feedback resistance, while increasing the BW.  In practice, it is not possible to reduce the feedback resistance below a certain value otherwise the amplifier will oscillate.

As long as , the BW, is not proportional to the gain, the CFB is considered fain-bandwidth product independent.  In practice this is true to the first order as  .

CFB will also have a naturally high slew rate and low bias current.  The input stage is a buffer and provides as much current as it can until the internal transistors saturate.  This saturation happens much later than traditional differential pair input voltage feedback amplifiers (VFB).  That characteristic is very important and translates to much higher full power BW.

To conclude, CFB is not meant for every application.  They fit best in applications that are most affected by increase in noise gain and where limited BW (a few 100MHz) but where high gain is needed.  The CFB most likely is not used as the front-end amplifier as the VFB tends to do better due to lower noise. But as a second stage, they do offer a much better BW to quiescent current ratio than any VFB.  CFB also does better in summing application where several inputs are required.  In such applications, VFB’s BW will be limited by the noise gain.  The last application in which CFB is most useful is line driver, where typically high gain and high BW are required simultaneously but also have high output current and high slew rate.

Anyone who works in power conversion and battery management electronics knows that efficiency is important. Engineers measureefficiency on the lab bench, but an end user of a portable product experiences battery life and the surface temperature of the device. While someone may not notice a 5% or 10% difference in battery life over a period of several hours, he or she could certainly sense an additional 5 or 10 degrees of heat on the outer surface of their handheld device. 

 As smartphones, tablets and portable instruments add more sophisticated features and bigger, brighter displays, they require higher capacity batteries – and correspondingly more power to charge quickly. The tight packaging of handheld products means that the heat generated on a PC board often finds its way to the exterior case of the device. Furthermore, many studies have shown that Lithium-Ion batteries suffer capacity degradation over time when exposed to high temperatures. So low efficiency is not just bad for the battery run-time on an individual cycle, but bad for long term service life of the battery, as well as an end user’s perception of the product quality. Using a high efficiency battery charger with low-resistance power FETs is one of the best ways to minimize heat generation in a portable system and keep your battery cool.

 For the rest of the post, see Upal's Fully Charged blog on Battery Power Online here:http://www.batterypoweronline.com/main/blogs/the-path-of-least-resistance/#more-4324.

Today, the smallest feature sizes on integrated circuits are approximately 20 nanometers.  It’s now fairly clear that CMOS will be further scaled,at least by about another factor of ½.  However, we are already well into the region of diminishing returns from this historical scaling trend.  For example, microprocessor clock speeds essentially saturated almost 10 years ago.  Note that this is largely due to the challenge of heat-removal, which, of course, also reflects the fact that high-speed scaled CMOS now uses relatively more power. 

Texas Instruments has been a member of the Semiconductor Research Corporation (SRC), supporting university research for over 25 years.  Just this year, the SRC Focus Center Research Program, now called “STARnet,” and the SRC Nanoelectronics Research Initiative (NRI), both working on “beyond CMOS,” have been successfully recompeted for the next five years.  In addition, some of the STARnet centers will also address new circuit and system techniques for better exploiting CMOS. 

I’m looking forward to the discovery of new devices by NRI and STARnet that might be further developed by the semiconductor industry into practical technologies for supplementing and/or replacing CMOS in many applications.  Of course, CMOS is a tough act to follow.  It’s even possible that there isn’t anything that could be developed over the next few decades into a practical technology that would be significantly superior to CMOS in speed, power efficiency, and cost per function.  This is largely because the field-effect transistors (FETs) that serve as the “switches” in CMOS logic are so elegantly simple and effective for controlling the flow of electric current.  However, as the great breadth of research in NRI and STARnet indicates, we are definitely not idea-limited!

For example, one of the research threads aims to improve on the turn-off characteristics of the switch.  Below the gate threshold voltage, FET current drops by an order of magnitude for at least 60 millivolts of drop in voltage on the gate.  This turn-off rate is limited by the mechanism of thermionic emission for the flow of electrons from the source to the channel of the device.  Research is underway on devices in which quantum tunneling through the source-channel barrier replaces thermionic emission over the barrier.  The main challenge for such “tunneling” devices is to conduct sufficient current when the device is “on” (i.e., gate voltage above the threshold).  In part, this is being addressed through using new channel materials – for example, graphene as a replacement for silicon.  Graphene also plays a role in research on other potential “beyond CMOS” devices, and you can read more about it in the blog by Luigi Colombo.

Another promising area of research on superior electronic switches involves materials that are called “multiferroics.”  This means that they exhibit more than one of the properties of ferromagnetism, ferroelectricity, and ferroelasticity.  Many of these materials are perovskite transition-metal oxides.  Those combining ferromagnetism and ferroelectricity offer the promise of controlling the flow of spin/magnetization with electric charge rather than with current-generated magnetic fields, which is a much less efficient process.  Thus, for example, we can envision devices in which the flow of information is via waves of electron-spin-polarization rather than simply electric current.  Research is also underway on devices in which the orientation of nanomagnets represents a logic state.  Such devices represent a type of nonvolatile logic that retains its state even when no power is supplied.  In this case, the main challenge is in making circuits that operate at high speed.  Of course, speed versus power efficiency is almost always a major trade-off, even in non-electronic technologies!

When you think of engineering for industrial applications, my first thought goes to environmental conditions of applications such as steel mills.  There are motor controllers working right next to (or attached to) giant electric furnaces and smelters, huge overhead cranes and massive electric fields.  It’s just another day at the office, huh? You may think it’s fairly straight forward to design for this environment until you realize that your electronics have passive cooling – no fan.  Fans fail and if the circuit is part of a system that needs to be shut down to service, the financial impact to the company could be enormous. 

So, here are some simple rules to get you in the ball park.  First, heat is not your friend – even in Minnesota in the middle of winter.  Consider how semiconductors are fabricated.  There are processes (not unlike metallurgy) to anneal crystal defects or diffuse impurities… all are accomplished by the application of heat.  So as a component heats up, this process continues.  There are other creeping problems aided by elevated temperatures and large current which can cause metal interconnects within components to migrate and short.  Designers of industrial control systems often de-rate operating junction temperatures based on their models – mostly from years of experience. 

These systems stay in operation for over 20 years … working every day without failure until replaced or retired.  So designers must take into consideration how to keep the temperature of the die well below the maximum operating point.  To do this a thermal model is used.  It can be extremely complicated using computers to calculate the temperature rise and heat flow.  Or it can be simple to see if the circuit has a chance of working at all.  To calculate the temperature rise of the die, the thermal flow can be modeled pretty much the same way we model current flow. 

The thermal “impedance” or resistance to heat flow is given in data sheets relative to Point A and Point B.  For instance Ө_JC (pronounced “theta” sub “J” “C”) is the junction (die) to case thermal impedance.  It is given in degrees centigrade per watt (°C/W).  It means that for every watt of power dissipated by the device, the junction temperature will rise so many degrees above the case temperature.  So if the Ө_JC is 3°C/W and the device dissipates 10 watts, then the junction temperature will be 30 degrees C higher than the case temperature.  There is also a junction to ambient version called Ө_JA which is the thermal impedance to the ambient air surrounding the part (assuming no heat sink).  Any device dissipating more than 100mW will probably need a method to carry the heat away.

The worst case problem with this scenario is having large thermal impedances with high power dissipation.  This happens often in small linear regulators such as the LM340A in a SOT-223.  Even with unlimited copper, the best Ө_JA will be no lower than 50°C/W which is a limitation of the package.  So if the ambient air in the box is 85°C, the input voltage is 12V, the output voltage is 5V (7V drop) with a load current of 100mA (700mW power dissipation), then the temperature of the die will be 120°C.  You can see that package selection is EXTREMELY important.  Take the same part and put it in a DPAK (32°C/W) and the die temperature (same conditions) drops to a bit over 107°C.

So next time you’re designing for industrial strength, take out the hand calculator for your sanity check to make sure you’re devices won’t end up in thermal shutdown… or worse!  Next time I’ll cover some more on industrial strength design ideas for keeping your circuits alive in harsh environments… Till next time!