IEEE Power Electronics Magazine - December 2019 - 86

White Hot

(continued from page 88)

in the design, the chip cannot be re-
programmed to correct your mistake,
you need to use a fresh chip.
The GreenPAK devices can be
used for some basic power supply
control functions as one of the inter-
nal building blocks in a pulsewidth
modulator (PWM). Dialog gives some
example circuits of using this PWM
block to make LED dimming circuits.
However, I don't see any indication
that these have been used to make
more complicated power supply con-
trol chips, although it seems possible.
This article was inspired by a
recent consulting project. The appli-
cation was a dc-dc converter for an
industrial application. This was gen-
erally low voltage (24 V) but moder-
ate power. Due to the high operating
ambient temperature and lack of air
flow, I decided to split the power stage.
I designed an interleaved power stage
to spread the heat among multiple
power switches.
The control had some unusual
requirements. The output voltage,
current limit, and power limit had to
be programmable via a serial bus.
The duty cycle had to be controllable
from fully off (0% duty cycle) to fully
on (100% duty cycle). In addition, the
output voltage and current had to be
monitored and reported to the sys-
tem controller upon demand. The
communications requirements meant
a microcontroller was required, but
this was not a big challenge.
However, finding a power supply
control IC that met the requirements
was a challenge. I was reminded of
the time I was working with Gedy, and
we could not find a chip that met our
needs. Aside from the interleaving and
full range of duty cycle requirement, I
was looking for an IC that would
accept the output of a digital-analog
converter (DAC) as its reference, so
that I could control the output voltage.
After some hours of searching and
poring over data sheets, I concluded
no suitable controller was available.
I then recalled that it might be possi-
ble to make my own controller after all.
No, not from discrete op-amps and com-

86

IEEE POWER ELECTRONICS MAGAZINE

parators (although that is possible), but
by using programmable analog blocks
inside a microcontroller. I recalled the
PIC16F1769 and PIC16F1779 microcon-
trollers from Microchip Technology.
These microcontrollers have a variety of
analog building blocks included-
building blocks like op-amps, compar-
ators, programmable PWMs, program-
mable ramp generators, complementa-
ry output generators, and configurable
logic cells for logic functions like
latches and flip flops. These chips had
all of the building blocks that I needed
to make my own custom PWM con-
troller. It seemed that Gedy's vision
had been realized.
I took a few hours of studying the
data sheets and sketching out block
diagrams of my desired controller.
After that, the coding was straightfor-
ward using Microchip's configuration
tool. Once I got to the bench, I pro-
grammed the code into a PIC16F1769
on one of Microchip's demo boards.
It did not take long to get the basic
PWM operation working correctly.
And then the fun began. I was using
one of the DACs to create a reference
voltage for the current limit circuit. I
just could not get the proper DAC out-
put voltage on the pin where it was
supposed to be. After several frustrat-
ing hours, it took a discussion with my
client to realize that an undocumented
connection between the onboard pro-
grammer and the chip socket was
interfering with the DAC output. Once
that was resolved, I passed my code
and design off to the client.
My client now has a compact solu-
tion for their industrial power convert-
er. The control circuit consists of the
PIC and one quad op-amp. With the key
functions in the PIC any competitor
will have a bit of a challenge to decode
how the circuit is working. And if my
client really wants to make it almost
impossible for a competitor to reverse
engineer, this design they could erase
the markings from the top of the PIC.
So why are there not lots of pro-
grammable analog power supply con-
trol ICs on the market? The answer
has to be simple economics-a lack of

z	December 2019

demand. There are so many control
ICs available, and so many finely tuned
to support particular applications. If a
fully programmable solution is needed
today, there is also the fully digital
option; although, that takes a different
skill set to implement well. Still, I am
happy I had the option of a full custom
"make my own" programmable analog
controller available from Microchip. It
allowed me to make a simple, com-
pact, and inexpensive solution for my
client, and now we are both happy.

Disclosure and Disclaimer
I have a relationship with Microchip
Technology in that I have been teach-
ing introduction to switch mode power
classes at their annual customer train-
ing event (MASTERs Conference).
However, I have received no compen-
sation or consideration for mentioning
their products, or any other products,
in this column. In fact, the companies
mentioned here won't know that I
have written about their products until
this issue of IEEE Power Electronics
Magazine is published.
Further, the mention of these com-
panies and their products is not an
endorsement by the IEEE or by the
IEEE Power Electronics Society.

About the Author
Robert V. White (bob.white@ieee
.org) has more than 30 years of industry
experience as a power electronics engi-
neer. He has worked in product design,
systems and applications engineering,
and technology development. He has
been an active volunteer with the IEEE
Power Electronics Society, serving sev-
eral years on the Administrative Com-
mittee, two terms as technical vice
president, and as a Chapter chair. He
earned a B.S.E.E. degree from the Mas-
sachusetts Institute of Technology and
an M.S.E.E. degree from Worcester
Polytechnic Institute. He is currently
pursuing a Ph.D. degree in power elec-
tronics at the University of Colorado,
Boulder. Presently, he is the chief engi-
neer of Embedded Power Labs, a
power electronics consulting company.
He is a Fellow of the IEEE.
IEEE Power Electronics Magazine - December 2019

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