ELC4345, Fall 2013
Overview
1
• 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%)
2
A switch
Rugged, reliable, efficient, long lived, but not very fast
3
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)
4
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.
Rseries
+
120Vac
–
Rbulb
Efficiency = Rbulb ÷ (Rbulb + Rseries)
Efficiency = 50% when light power is half
• Method 2 to dim the light – switch the voltage to the bulb onand-off, faster than the eye can detect, to reduce the rms
voltage at the bulb. This is lossless dimming.
+
120Vac
–
Rbulb
?
5
Another example – convert 39Vdc to 13Vdc
Stereo
voltage
Switch closed
Switch open
39
+
39Vdc
–
Rstereo
0
Switch state, Stereo voltage
Closed, 39Vdc
DT
T
Open, 0Vdc
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?
6
Convert 39Vdc to 13Vdc, cont.
+
39Vdc
–
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
C
L
+
39Vdc
–
C
Rstereo
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.
Rstereo
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.
lossless
L
+
39Vdc
–
C
A DC-DC Buck Converter
7
Another example - H-bridge inverter converts DC to AC
Vdc
Vdc
closed
closed
A+
A+
B+
+ Vload −
+ Vload −
closed
A–
B+
B–
closed
A–
B–
A+,B− closed; A−,B+ open
A+,B− open; A−,B+ closed
Vload = Vdc
Vload = −Vdc
8
The class is divided into four lab periods in ENS212
(the Power Lab)
But to build and test your circuits, you will also need
to work other times during the ECE 2nd floor
laboratory hours
You have no homework, because the circuits and
reports are your “homework”
9
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
10
Power labs are unlike other ECE labs – they can
be hazardous and require considerable caution
• 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.
11
Power labs are unlike other ECE labs, cont.
• 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.
12
This is a course where you will
• 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
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This is a course where you will, cont.
• 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
14
This is a course where you will, cont.
• 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)
15
This is a course where you will, cont.
• 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
16
Concerning Partners
• 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).
17
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
18
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
19
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)
20
The pace is fast, so don’t get behind
• 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
21
Team Tool Kit #1
22
Team Tool Kit #2
23
More Team Tools
Each student gets a
pair of safety
glasses and keeps
them
24
Lab Equipment
120 Vac Variac
120/120 Vac Isolation
Transformer
Panavises
120/25 Vac Transformer
Three-Series Headlight Load
Bank
25
Lab Equipment, cont.
Oscilloscope
Ground Fault Interrupter
(GFI)
SEL-421 Relay and Meter
26
Questions?
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