Planar magnetics are finding their way into every
kind of power electronics application today, in high-frequency
designs where power levels can range from 2
Watts all the way up to 150 kilowatts! The two photos
in Figure 4 show these two extremes in power levels
from a physical perspective. In Figure 4B, this particular
unit was custom designed to provide a 150V output
with load capability up to 1,000A and to operate at a frequency
of 40kHz! The approximate height of this unit is
approximately 4 inches and its volume on the order of
252 cubic inches, giving a power-handling density figure
close to 600 watts/cubic inch! The copper thickness
of the planar windings here was 10 oz, with trace widths
on the order ot 0.5 inches. The unit shown in Figure 4B
was built by UPC, Inc. in Cleveland, Ohio USA using
printed circuit designs developed by Prism Circuits in
Ontario, Canada (now UPC Canada Ltd). More information
about this high-power unit can be found in the
March 2000 issue of PCIM Magazine (name changed recently to Power Electronics Technology Magazine).
Planar Magnetics Players…..
Up until the early 1980's, planar magnetic design
techniques were mostly confined to in-house applications
in products for the military and aerospace industries.
One of the first persons to publish design information
on planar magnetics in that time period was
Alex Estrov, then a staff member of Theta-J
Corporation in Wakefield, MA USA. In 1986, he published
the details some of his work associated with
the low-profile magnetics design of a 1 MHz resonant
converter. Shortly thereafter, Estrov formed his
own company, Multisource Technology, which
offered a variety of off-the-shelf planar power transformers
and inductors. He is now an executive
member of the Payton Power Magnetics Ltd. group
based in Boca Roca, Florida USA. Figure 5 is a
photo of a collection of the many "off-the-shelf"
planar assemblies offered by Payton Ltd. today.
Payton's planar assemblies, as well as comparable
units available from Signal Transformer,
(another major US manufacturer of planar magnetics),
employ special patented isolation bobbins
to meet safety creepage and spacing requirements.
Figure 6 shows an "exploded" view of typical
planar transformer assembly with these special
bobbins identified. Also shown in this illustration
are two different planar winding types. For the primary
winding, two PCB-style windings are used
while, for the two high-current single-turn secondaries,
stamped metal windings (lead frames) are
indicated. In this latter case, the 1-turn windings
are physically supported and secured in the assembly
by the isolation shrouds. Good energy transfer
between primary and secondary is accomplished
in this construction by the interleaving scheme for
the windings in Figure 6.
Over the past ten years, many other manufacturers
have entered the planar magnetics arena. In
most cases, the planar designs offered employ lowprofile
E-E or E-I cores. However, with the recent
introduction by core manufacturers of new planar "circular"
cores of the RM, PQ and DS varieties, this situation
will change soon. The base reason for the extensive
use of E-E and E-I cores up to now has been lower
cost and availability than the "circular" shapes, plus
the E-E/E-I cores are open shapes, permitting easy
access to internal windings. Circular designs offer
lower overall PCB winding resistance than do rectangular
designs for a given winding area. Therefore, in
designs where winding losses must be minimized as
much as possible, planar circular core shapes with circular
PCB windings are preferred.
Rolling Your Own…..
Even though there is a wide variety of planar magnetics
to be purchased on the open market today, in
many instances they may not meet the needs of a particular
power conversion design. Such needs include
shape, size and, of course, height as well as special electrical requirements.
An example of the latter need are high-frequency transformers
with a multiplicity of primary and/or secondary windings. In most
cases, "off-the-shelf" planar (or surface-mountable) power inductors
can be found for the majority of applications. Unfortunately, these planar inductors are usually more expensive than
conventional inductors who have the same electrical
characteristics.
Today, there is more selection of low-profile cores
for planar magnetics than was the case five years ago.
Most major magnetic core manufacturers offer some
types of ferrite planar cores now. Most of them can
also cut and/or grind a standard core size down to the
dimensions needed at minimal expense. This is an
excellent prototyping alternative, allowing a designer
to optimize core dimensions during the design phase
without investing money in new tooling for pressing a
special core size at this point in the design cycle.
Summary….
Planar magnetic design and construction techniques
today offer viable cost-effective alternatives to
traditional magnetic manufacturing approaches. More
and more applications are being found now for
these unique components, and some say that
planar construction methods will replace conventional
bobbin-oriented magnetic components
in the years to come. |
