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ELC4345, Fall 2013Overview

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

You have no homework, because the circuits and reports are your “homework”

But to build and test 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 four 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

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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 3 times and at the final exam

• Read the lab document before starting to build!

• Build the circuits in two-person teams (except for the first circuit, where everybody builds their own circuit but then prepares a two-person team report)

• Compare theory to actual circuit performance

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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• Except for the first circuit, 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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• Be assigned a partner randomly for each circuit and be expected to work with your partner as a team. Solo requests will be considered.

• Receive the same report grade as your partner

• In some cases, be permitted to select a partner

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 graduate students will be requested to solo

• If you have a highly-constrained schedule and will be unable to coordinate work times with a partner, or if you have other good reasons to work alone, consider requesting solo status for the semester. Do this before the partners are assigned for the second lab (the DBR lab).

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

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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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The circuits are

• Light dimmer (AC to AC)

• Diode bridge rectifier (AC to DC)

• Photovoltaics (on ENS roof)

• MOSFET firing circuit

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

• PI controller for boost converter

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

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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

• Parts are provided – they 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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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?