Magnetics Business & Technology - May/June 2023 - 34

TECH TIPS
Figure 5, Power Transformer Artwork, 1:1.67 Winding Ratio
Bifilar and Sector windings can be implemented with two copper
layers while the overlapped windings require 4. For the 7.5kV
transformer, an over-lapping winding was selected, primarily for
the properties of high voltage isolation and low leakage inductance.
Besides inductance, transformers have the secondary parameters
of winding resistance (RDC), leakage inductance (LL) and
inter-winding capacitance (Cww). RDC and LL are loss mechanisms
and should be kept low to maximize efficiency. Cww
provides a conduit for switching noise to cross from the primary
to secondary winding and also should be kept low. In Figure 3,
the windings are drawn with consistent width conductors. The
designer has the option to also flare and shape the windings to
manage these secondary properties.
Figure 5 shows the artwork for the 27uH transformer. The
secondary windings have 15 turns and are implemented on the
inner layers; Layer 1 and Layer 2. The conductor widths are
0.4 mm wide and separated by 0.13 mm. Between the layers,
the windings are connected with 0.25 mm plated-through- hole
(PTH) vias. The primary winding has 9 turns and is implemented
on the outer layers; Layer 3 and Layer 4. The vertical via barrels
between layer 3 and layer 4 are placed 0.4 mm away from the
copper conductors on the inner layers, a distance which provides
≥ 18kV of isolation.
For the overlapped windings, we rely on the pre-preg laminate
layers to provide the isolation between the primary and secondary
windings. For this design, an automotive grade pre-preg was
selected. This material is a lead-free and has a voltage breakdown
rating ≥ 45kV/mm. The laminate sheets come in different
thicknesses. To provide ≥7.5kV of isolation, 3 sheets of pre-preg
with a 1080 fiberglass weave is used. This stacks up to a thickness
of 0.183 mm, which corresponds to an 8.2kV breakdown
voltage. On the top and bottom surfaces, a polyimide cover-lay
was applied to cover the conductors. The polyimide film and
bond-ply adhesive film are each 0.25 mm thick. Polyimide has a
breakdown voltage of ≥137kV/mm and the 0.25 mm film thickness
provides >3.5kV. Since the transformer is being over-molded
in the SIP package, the polyimide cover-lay does not neces34
Magnetics Business & Technology * May/June 2023
sarily need to meet the full 7.5kV AC breakdown requirement.
Figure 7 shows finished transformers and summarize the specifications
and test results for 5 parts. On average, the primary
inductances came in much higher than the 27uH spec. Winding
resistance on the primary side was measured at 100mΩ, which is
low for a transformer of this size and good for low loss and high
efficiency operation.
Since the clearance distance was below IEC 62368-1 specifications,
Hipot testing was done while the transformers were
submerged in a bath of insulating oil. A lot sample of 10 devices
was tested and all passed 7.5kV AC and 10KV DC for 60
seconds without voltage breakdown between the primary and
secondary windings. As mentioned earlier, once assembled onto
the SIP substrate, the assembly is over-molded with epoxy. The
final assembly also passes Hipot at 7.5kV AC and 10kV DC.
Of course an alternate construction is to combine the embedded
transformer with PCB substrate. Figure 8 shows an assembly
where the embedded transformer becomes the substrate upon
which a digital isolator in a DIP package is mounted on the top
surface along with biasing resistors and filter capacitors. Combining
the transformer with the PCB substrate brings additional cost
savings and can reduce the SIP module footprint.
Conclusion
A design example has been provided for integrating a low profile
power transformer into a SIP package. The transformer was
implemented using embedded magnetic construction and fabricated
on a standard PCB line. The transformer was implemented
in a 4 layer design, where the breakdown voltage properties of
the laminates are critical for maintaining voltage isolation. FR-4
Laminate and coverlay materials were selected to successfully
achieve a Hipot breakdown voltage exceeding 7.5kV AC. Given
that the line widths and separation between the transformer
windings are defined by photolithography the electrical performance
is highly consistent between individual devices and
fabrication lots. With such consistency, it is practical to do Hipot
testing on a lot sample basis and not necessary to test each
device in production.
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