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EE462L/EE394-7, Spring 2014Overview

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•Question: What are power electronic devices?

•Answer: Fast switches that can handle high voltages and currents

•Question: Why do we need these fast switches?

•Answer: To efficiently convert AC to DC, DC to DC, or DC to AC, or to efficiently control average power flow. (Efficiently usually means greater than 80% – 90%)

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A switch

Rugged, reliable, efficient, long lived, but not very fast

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The ideal power electronic device is a perfect switch that

• is fast − can open and close instantly (thus no switching losses), and at a high rate (i.e., operating frequency)

• when closed, can conduct any amount of current with no internal voltage drop (thus no conduction losses)

• when open, will conduct no current and can withstand any voltage without breakdown

• will be unidirectional or asymmetric (that is an inherent property of power electronic devices, and we can always place two switches in antiparallel and use blocking diodes to prevent backward conduction)

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An everyday example – a light dimmer

• Method 1 to dim the light – insert a series resistor between the 120Vac source and the bulb. This method has high insertion loss and low efficiency.

• Method 2 to dim the light – switch the voltage to the bulb on-and-off, faster than the eye can detect, to reduce the rms voltage at the bulb. This is lossless dimming.

Rbulb

Rseries

+120Vac

–

Efficiency = Rbulb ÷ (Rbulb + Rseries)

Efficiency = 50% when light power is half

Rbulb

+120Vac

–

?

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Another example – convert 39Vdc to 13Vdc

If the duty cycle D of the switch is 1/3, then the average voltage to the expensive car stereo is 39 ÷ 3 = 13Vdc. This is lossless conversion. However, is this acceptable?

Rstereo

+39Vdc

–

Switch state, Stereo voltage

Closed, 39Vdc

Open, 0Vdc

Switch open

Stereo voltage

39

0

Switch closed

DT

T

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Convert 39Vdc to 13Vdc, cont.

Try adding a large C in parallel with the load to control ripple. But if the C has 13Vdc, then when the switch closes, the source current spikes to a huge value

Rstereo

+39Vdc

–C

Try adding an L to prevent the huge current spike. But now, if the L has current when the switch attempts to open, the inductor’s current momentum and resulting Ldi/dt will burn out the switch.

By adding a “free wheeling” diode, the switch can open and the inductor current can continue to flow. With high-frequency switching, the load voltage ripple can be reduced to a small value.

Rstereo

+39Vdc

–C

L

Rstereo

+39Vdc

–C

L

A DC-DC Buck Converter

lossless

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Another example - H-bridge inverter converts DC to AC

Vdc

A+ B+

A– B–

+ Vload −

A+,B− closed; A−,B+ open

Vload = Vdc

closed

closed

Vdc

A+ B+

A– B–

+ Vload −

A+,B− open; A−,B+ closed

Vload = −Vdc

closed

closed

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The H-bridge inverter that you will build to convert DC (e.g. from a solar panel) to AC

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The class is divided into three lab periods in ENS212 (the Power Lab)

•Sections 17060 and 17374: Thur. from 3:30 pm to 6:30 pm

•Sections 17065 and 17370: Wed. from 6:30 pm to 9:30 pm (some projects, such as the PV project, may require going to the lab at different times)

This is not a theoretical course. This is a practical course.

Circuits and reports are your primary form of “homework”

But to build and test some of your circuits, you will also need to work other times during the ECE 2nd floor

laboratory hours

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The class is divided into three lab periods, cont.

The optimum situation is:

• Each lab period has the same number of students

• If you have a strong case for permanently and informally switching lab sections, it will be considered as long as the numbers are not too unbalanced or your request leads from even number of students to odd number of students.

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• Unlike 5V digital labs, power experiments can burn, go “bang,” blow fuses, melt components, and scare or hurt you.

• Never test a power circuit by “let’s power it up and see if it works!”

• It is important that you and your partner triple-check your wiring before energizing a circuit for the first time. For the first few experiments, let a TA or me check it with you.

Power labs are unlike other ECE labs – they can be hazardous and require considerable caution

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• Never dangle wires or oscilloscope leads over an energized circuit

• You will have several different “grounds” – so be careful when taking measurements. Never attach two oscilloscope ground clips to nodes with different potentials.

Power labs are unlike other ECE labs, cont.

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• Learn how to convert AC to AC, AC to DC, DC to DC, and DC to AC (i.e., the AC – DC – AC “round trip”)

• Learn the theory related to the circuits, and be tested over it 2 times and at the final exam

• Read the lab document before starting to build!

• Build the circuits in two-person teams. Each team then prepares a single two-person team report.

• Compare theory to actual circuit performance. In order to do well in this class you need to have a critical approach when working with your circuits. If you just try to do your projects by “following a recipe” chances are that you will not do well in this course.

This is a course where you will

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• Sometimes work with voltages over 100V, and where short circuit currents are high, so be careful! Remove hand jewelry and dangling neckchains.

• Use safety glasses 1. when soldering to avoid accidental hot solder “splashback” in your eyes, or 2. when getting “up close and personal” to observe an energized power circuit.

• Wash your hands after soldering (because solder contains lead)

• Use knowledge of circuit operation, plus your senses of sight, sound, smell, and touch (carefully), to observe and debug your circuits

This is a course where you will, cont.

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• Build your circuit together with your partner

• Taking turns, one partner builds, while the other partner double-checks with the schematic and verifies the connections

• Not simply build your circuit by blindly copying the sample circuit or lab document photograph, wire for wire! (if so, you cheat yourself and you will not understand the circuit)

This is a course where you will, cont.

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• Write 11 reports that describe 8 circuits plus photovoltaics (tentative) and 2 additional applications of the inverter

• Be assigned a partner and be expected to work with your partner as a team. Solo/a requests will be considered only and justified only in well justified circumstances.

• Receive the same report grade as your partner.

• Be permitted to select a partner. Please do so by next Tuesday. If you don’t indicate a partner preference, your partner will be selected randomly.

This is a course where you will, cont.

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• Bring any partner problems to my attention right away

• If a section has an odd number of partners, then a randomly chosen graduate student will be requested to solo.

• In some well justified cases partners may be changed during the course. However, not liking your partner is not a justified case. Like in real jobs you will need to learn how to work with your partner even when you don’t like each other.

Concerning Partners

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This is a course where you will, cont.

• At the start of the due date lab period, and as called in team order, turn in your report (hardcopy only – no emails), and turn in your circuit

• We will inspect your circuit for construction quality, and decide if it meets the quality threshold

• Reports and circuits not turned in when a team is called forward receive

– 1. fractional +/− letter grade reduction if turned in later during that lab period, or

– 2. full letter grade reduction if turned in during the following Tuesday’s class. Later reports will not be accepted.

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This is a course where you will, cont.

• Become proficient in soldering and in the use of oscilloscopes and meters

• Learn the resistor color code to make your life easier

• Understand and appreciate why resistors have power ratings

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This is a course where you will, cont.

• Learn about solar panels, and how to use the electricity they produce, with the panels atop ENS

• Learn about sun position in the sky throughout the year, components of solar radiation, and the effect that panel angles have on daily energy produced

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The circuits are

• Light dimmer (AC to AC)

• Diode bridge rectifier (AC to DC)

• Photovoltaics (on ENS roof) (actually it is not a circuit).

• MOSFET firing circuit

• Buck, boost, and buck/boost converters (DC to DC)

• Inverter (DC to AC) and its three component circuits (PWM control, isolated firing circuit, and H-bridge)

• Optional – PI controller for boost converter

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• Because you will build many circuits in a relatively short time, it isn’t possible for you to design your own circuits

• Most parts are not available at the regular ECE checkout counter

• Recommendations for improving the circuits, construction, testing techniques, lab documents, as well as ways to improve the overall lab experience are always appreciated

• Regarding design opportunities, many students use these circuits as starting points for Senior Lab 464

The pace is fast, so don’t get behind

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If you do not want to

• build circuits,• be cooperative,• be courteous,• be a good lab citizen,• be careful, and• carry your weight, then …

you are in the wrong course!

Is EE462L for you?

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Is EE462L for you?

• Some comments about the course’s concept:• The focus of the course is on lab work and learning through practical work in the lab.

• This course is NOT a theoretical-based course.

• Although most of the lectures include an overview of the theoretical background, you are expected to gain further insights from the lab work and from your own work.

• Lectures are mostly based on Powerpoint presentations. Many concepts and circuits require time consuming circuit drawings and text explanations. Without Powerpoint it will be impossible to cover the course material.

• If you have trouble following Powerpoint presentations, you are encouraged to consult the recommended books or the lab manual, or to ask the TAs or the professor.

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Tuesday and Thursday lectures

• The supporting theory and explanations of the circuits are taught during the lectures.

• There are usually three lectures each week.

• Many times there will be an attendance roster as you enter the room. You are expected to sign it at the beginning of the class and no later than 5 minutes after class starts

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Lectures, cont.• Please be punctual – having many late arrivals for the first 5-

10 minutes is disruptive and, in effect, delays the start of class

• Cell phones and laptops – turn off (unless you clear it with Dr. Kwasinski). Students not complying with this rule may be assigned an “absent” for the class in which they are using their laptops or cell phones without asking Dr. Kwasinski first.

• A few times I will need to be away for lectures to attend conferences or to participate in meetings. These trips are intended to benefit the power electronics program at UT so they also benefit you, students in this class. These trips are not pleasure trips at all. Most of the time they involve being the entire time at a hotel conference room. Trips to disaster areas may not even involve a hotel at all.

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The course grading formula is• Lab circuits and reports – 50%

• Two tests. Tentative dates are on Mon. (Feb. 25, April 22) – 20%

• Comprehensive final exam (everybody takes it at the UT scheduled day and time. However, the final is not planned for the whole 3-hour slot) – 20%

• Attendance (taken at all lab sessions, and at most lectures) – 10% (scaled so that perfect attendance gets full credit, 50% attendance gets zero credit).

• Fractional letter grade (A, A-, B+….) will be used for the final grade. A is equivalent to 100 % and according to http://registrar.utexas.edu/students/grades, B is equivalent to 75% and so on.

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Concerning tests and the final exam

• One 8.5” x 11” sheet of notes is permitted at each test and at the final exam

• The questions are sometimes multiple choice, with several possible answers given, plus an “Other” box. The correct answer is supposed to be among the given answers. Adequate work must be shown to justify the chosen answer.

• Tests and the final exam may be individually curved so that they have approximately the same average grade

• If it works to your advantage, your lowest (or missing) test grade will be replaced by your final exam grade

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Concerning tests and the final exam, cont.

Regrade requests are handled as follows:

• Neatly write down on a piece of paper your reasons for requesting a regrade,

• Staple the paper to your test,

• Return it to me for consideration

Within a week of getting your graded test back (but not the same day),

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Good citizenship in the power lab

• Be courteous to your fellow students – we all share the same power lab

• When finished in the lab, clean up your mess

• When finished in the lab, return all equipment items to their proper place

• When crowded, do not “hog” the lab benches by doing unnecessary or unrelated work there

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Absences and Other Extenuating Circumstances

• Please do not ask for excused absences or late attendance roster sign-ins

• Instead, handle absences, missed attendance roster sign-ins, and all other extenuating circumstances as follows:

• Keep a record book with details

• At the end of the semester, prepare a written statement that itemizes each and gives reasons

• Staple the statement to your final examination

• Your statements will be taken consideration, all at once, when assigning the final course grades

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Team Tool Kit #1

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Team Tool Kit #2

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More Team Tools

Each student gets a pair of safety

glasses and keeps them

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Lab Equipment

120 Vac Variac 120/120 Vac Isolation Transformer

120/25 Vac Transformer

Panavises Three-Series Headlight Load Bank

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Lab Equipment, cont.

Oscilloscope

SEL-421 Relay and Meter

Ground Fault Interrupter (GFI)

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Questions?