EE 306 Sustainability Analysis

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This is an analysis of the sustainability issues of Experiment 5: Breakdown Diodes, from EE 346: Semiconductor Device Electronics Lab.

Experiment 5: Breakdown Diodes

Summary

The experiment consists of 3 parts. The first investigates the characteristics of a zener breakdown diode in order to measure its I-V characteristics in reverse bias. The second part investigates the ability of the zener diode to be used as a voltage regulator, supplying a load resistance with a relatively constant voltage over a varying supply range and varying load, using component values calculated in prelab. The last part of the experiment investigates the ability of the zener diode to be used in a clipping circuit.

Real world applications

This experiment serves as a first look into the idea of a voltage regulator, by exploiting the characteristics of a zener diode, namely its ability to hold a nearly constant voltage in breakdown over a varying current range. Voltage regulators are commonly found in nearly every consumer electronic product, which are used to keep circuits supplied with a known, stable voltage that does not vary with supply variations, such as that from a draining battery or ripple on an AC-DC converter.

While this experiment is only an introduction and a look at a specific device, the use of a zener diode as a regulator is a poor one from an efficiency standpoint.[1] This analysis will look into the energy, environmental, economical, and equity impacts of the real world applications of this experiment.

Energy

As stated earlier, the primary concern of this experiment is the issue of inefficiency in zener diode based voltage regulators. However, there are significant energy issues relating to the process of creating the diodes themselves. Industrial grade silicon is made through a process which reacts silica, SiO2 with carbon and carbon containing products, in order to produce raw silicon and Carbon Monoxide as byproducts.[2] For example, in photovoltaic cell production, one reference cites approximately 2000-4500 MJ per square meter of primary energy used in the production of silicon and wafer processing alone.[3] There are more environmentally friendly methods of producing silicon, but this is still the predominant method used today.

Environment

It is clear that the dominant method of producing silicon is not a sustainable one, and will have long term impacts on the environment as the volume of silicon production and consumer products increases. Silicon production requires the use of coal, charcoal, and wood chips, as well as a large amount of electrical energy to be produced. Once industrial grade silicon is produced, it must be further refined in another energy consuming process to create the pure silicon needed for semiconductors. Although the zener diode regulator is not the most efficient in use, alternatives still use as much if not more silicon, so the use of silicon itself is not an easily reduced factor. However, the processes that make silicon can be changed. As mentioned earlier, there are several more environmentally friendly options for producing silicon, namely electrowinning and electrolytic methods, which do not require as high of temperatures as other methods, and can produce higher purity silicon.[4][5]

Economics

There is a stronger argument toward using a zener diode regulator over a standard linear regulator or switching regulator IC, due to its simplicity and low cost. However, its inefficiency may actually have a greater cost over the lifetime of a product, over more efficient circuits. For example, thousands of devices such as cell phones, radios, gaming devices, and accessories require AC-DC converters to provide power for recharging their batteries. Most people do not consider the wasted energy that these "wall-warts" draw when plugged in and not providing power to an external device.[6] This is a clear example of the advantages of using a linear or switching regulator for voltage regulation. Both linear and switching regulators consume waste energy roughly proportional to the load. Linear regulators work by adjusting the current flow through a series pass transistor, and monitoring the voltage on the output. This method consumes the excess power from a higher voltage source by treating the transistor as a variable resistor, but the advantage comes through the much lower quiescent current drawn by this device when it is not supplying power to a load. Switching regulators provide a lower voltage to a load by utilizing the energy storage properties of an inductor, actually transforming the voltage to a lower amount, and ideally only losing waste energy through the parasitic resistances of the inductor, switch, capacitor, and voltage drop of the diode.

Equity

The environmental impacts of these devices also have far reaching effects on the economy and equity of people in foreign countries. There have been several undercover looks at the environmental impacts of electronic waste "recycling". Several recycling programs illegally export their e-waste for processing in countries like China and India, where laws and regulations regarding processing of electronic waste may be far less strict, or non-existent. A stunning example of the effects of electronic waste is Guiyu, China, called China's "electronic waste village". In the Time article, "E-Waste Not," by Brian Walsh, these effects have been shown to have a significant detrimental impact on those who work dismantling these electronics, "According to reports from nearby Shantou University, Guiyu has the highest level of cancer-causing dioxins in the world and elevated rates of miscarriages."[7] Clearly, there needs to be a stronger concern and stricter control over the processing of electronic waste, and by considering the design behind simple items like wall-plug chargers, for example making all chargers interchangeable and generic, we can reduce or eliminate one source of waste, and preserve the environment and the negative health effects on those who have to deal with it.

Suggestions for future improvement

The lab experiments themselves may not have a significant impact on the world around us, but their applications hold much greater importance and can affect a great deal of lives and the global environment. At the end of each lab or in the required or suggested reading, I propose the addition of a real world applications and improvements section, in which practical circuits utilizing the concepts shown in the lab experiment could be shown, with respective effects on efficiency and environmental impact. Like the documentary "Waste = Food" suggests, we must consider the lifetime of the product and through a few small changes in consumer products, a greater level of reuse can be achieved, truly turning what would have been waste into food for future devices.[8]

References

  1. Tony R. Kuphaldt, "Zener Diodes". Available: http://www.allaboutcircuits.com/vol_3/chpt_3/11.html [Accessed Nov. 18, 2010].
  2. "The Basics of Silicon Chemistry". Available: http://www.dowcorning.com/content/sitech/sitechbasics/siloxane_polymerization.asp [Accessed Nov. 18, 2010].
  3. Erik A. Alsema1, Mariska J. de Wild-Scholten, "Environmental Impacts of Crystalline Silicon Photovoltaic Module Production". Available: http://www.ecn.nl/docs/library/report/2006/rx06041.pdf [Accessed Nov. 18, 2010].
  4. Gopalakrishna M. Rao, Dennis Elwell, and Robert S. Feigelson, "Electrowinning of Silicon from K2SiFo-Molten Fluoride Systems". Available: http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JESOAN000127000009001940000001&idtype=cvips&prog=normal [Accessed Nov. 19, 2010].
  5. Monnier, R. et al. "Dual cell refining of silicon and germanium" U.S. Patent 3,219,561. Available: http://www.google.com/patents?id=m5xlAAAAEBAJ&printsec=abstract&zoom=4&source=gbs_overview_r&cad=0#v=onepage&q&f=false [Accessed Nov. 19, 2010].
  6. Jack Ganssle, "Wall Warts". Available: http://www.eetimes.com/discussion/embedded-pulse/4025540/Wall-warts [Accessed Nov. 19,2010].
  7. Brian Walsh, "E-Waste Not". Available: http://www.time.com/time/magazine/article/0,9171,1870485,00.html [Accessed Nov. 19, 2010]
  8. W. McDonough, Michael Braungart, “Waste = Food” Directed by R. van Hattum. Available: http://video.google.com/videoplay?docid=-3058533428492266222. [Accessed Nov. 19, 2010]