Magnetics Business & Technology - May/June 2023 - 33

TECH TIPS
Figure 4 Examples of Bifilar, Sector and Overlapped Windings
Embedded Magnetics Construction
To begin, a cavity is routed into a FR-4 substrate, to receive
the magnetic core. A ferrite toroid is dropped into the cavity and
encapsulated with a low shrink epoxy. Some DC/DC converter
architectures, like the popular Flyback converter, require that the
core first be gapped. Once the cores are in place and encapsulated,
the panel is planarized to remove any excess epoxy and
prepare the panel for laminating printed circuit layers. At this
point the panel follows standard PCB processing steps. Copper
foil is laminated to the substrate with thin sheets of pre-preg,
which is a woven fiberglass impregnated with partially cured
epoxy. Under the pressure and temperature of a laminating
press, the pre-preg epoxy flows and bonds the copper foil to the
substrate. Winding layers are applied using standard drilling,
plating, imaging and etching processes. PCBs can be processed
in large panel formats. For the small devices that might reside
in a SIP package, a standard 610 x 457mm panel may contain
over 1000 devices. So every time a technician handles the panel
and puts it in plating or etching tank, they are handling a batch of
1000 devices. These batch processes provides much efficiency,
compared to the traditional method of winding wires on individual
cores, one at a time. Figure 2 (opposite page) shows a cut-way
view of an embedded magnetic device.
Design Example
As an example, we can look at a 3W power transformer that
Shennan Circuits developed for a digital isolator SIP. As depicted
in Figure 1, the transformer is part of a DC/DC converter
that provides supply voltages to the high voltage side of the
isolation barrier. In this particular application, the transformer is
required to have a 5:6 winding ratio and isolation barrier that can
exceed 7.5kV AC. Additionally, the transformer is packaged in a
SIP module that is over-molded with epoxy and requires that the
transformer have a low profile of ≤ 2.2mm. The over-molded epoxy
essentially covers the I/O pads on the transformer so that the
clearance/creepage requirements apply to the exposed metal on
the I/O of the SIP package and not the transformer package.
Figure 3 depicts the transformer implemented in a SIP package.
The transformer and bias components are first surface mounted
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to the substrate, prior to die attaching and wire bonding the digital
isolator ICs. Transfer molding is then used to cover the whole
assembly with epoxy.
Figure 3, Digital isolator SIP assembly
PCB laminates are quite good insulators. General purpose FR-4
has a breakdown voltage of 20kV/mm while grades designed for
high voltage applications have breakdown voltages exceeding
45kV/mm.
For this design, a Manganese Zinc (MnZn) ferrite material was
selected with a relative permeability of 3000. A T6.5x1.5x1.0 size
core was selected, which has an inductance factor (AL) of 0.33
uH/T2. The power converter circuit requires that the transformer
provide an inductance of > 27uH and a 1:1.67 winding ratio to
provide a voltage step-up between the primary and secondary
windings. Considering the AL factor, it was calculated that
there should be 9 on the primary side and 15 on the secondary
side. There are a few basic configurations that can be used for
implementing the printed windings. As shown in Figure 4 (top of
page), they can be labeled Bifilar, Sector or Overlapping. Each
has their own benefits and drawbacks.
Continued on page 34...
May/June 2023 * Magnetics Business & Technology 33
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