IEEE Power Electronics Magazine - March 2020 - 92

White Hot

by Robert V. White

You Need More
Than Cookbooks
"

ou don't learn how to cook
from a cookbook." This
came to me while planning
the teaching of this past semester's
power electronics project lab. In a
previous column, I talked about
teaching a project lab at the University of Colorado-Boulder for master's students who were going into
industry and not continuing for a
Ph.D. degree. The goal of the project
lab is for the students to do an original design project under conditions
as similar to industry as possible.
Rather than a weekly "build this,
take data, write a report" of the typical university lab class, these students are given a specification and a
schedule. The students are told
"what," they are not told "how." They
are also given a standard parts library,
component derating guidelines, and
product safety requirements. Some of
the instructional objectives are learning to design with ac mains power,
learning to design for a wide range
of operation of both input voltage
and output load, and learning how to
debug an original design. The major
goal of the class is to demonstrate
working hardware at the end of
the semester.
The project is a 25-W constantcurrent output LED driver that must
work from a universal ac input (85-
264 Vac) a nd va r y the LED curDigital Object Identifier 10.1109/MPEL.2019.2959357
Date of current version: 19 February 2020

IEEE POWER ELECTRONICS MAGAZINE

rent from 0 to 1,200 mA, as the dimming control voltage varies from 0 to
10 Vdc.
For most of the students, this is
their first original design. Almost all
of them come to the class with no
industry experience. This is the first
time they have to create (synthesize)
a complete power converter rather
than just analyze one circuit function
as part of a homework problem.
This past fall was the third time I
have taught this class with the assistance of Roger Bell, another consultant here in the Denver area. Roger
and I have known each other for many
years and have collaborated on a few
consulting projects along the way.
The first time we taught the class,
we naïvely assumed that since these
were second year master's students,
they would know pretty much all they
needed to know about the power, control, and analog circuits needed for
this design. We prepared a few lectures
on industry-oriented topics like the
concepts of creepage and clearance
distances in product safety requirements. I also prepared a lecture on the
flyback converter, as I knew this had
not been covered in any detail in the
main power electronics class.
We were quite surprised to find
that these students were actually very
poorly prepared. We found that they
had no real knowledge of analog
design, including the characteristics
of op-amps, common op-amp circuits,
or how to read a data sheet to under-

z March 2020

stand the various operation modes of
a control integrated circuit (IC). They
also were very weak in understanding
power MOSFETs and the design of a
magnetic device (transformer or flyback inductor). We found that most of
the students had no idea of how to
design their LED driver, even at a
block-diagram level. Nevertheless, all
of the students were able to design
and build a driver that illuminated the
LED module, although the students
had to put in way too many hours
under too much stress. At the time of
the demonstration, many did not comply with the specifications very well
(such as being able to dim) and most
designs were not very robust.
For the second year, we prepared
more lectures but still provided only a
schedule with major deliverables
(such as a concept report and a design
review package). We gave the students
about four weeks from presenting
their concept to meeting with Roger
and me for a one hour design review.
This did not work well. The students had no idea of the amount of
work and how to plan the design tasks,
such as designing the magnetics or
control loop. Instead, they left everything to the last few days. Only then did
they realize how much work they had
to do and many did not have a workable design to present for the design
review. Again, despite these challenges, all of the students managed to get
(continued on page 89)

IEEE Power Electronics Magazine - March 2020

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