IEEE Power Electronics Magazine - September 2016 - 33

As
Inductor

PCB

(a)

(b)

FIG 3 The PSI2 packaging: (a) the power module and (b) the
structure.

(b)

FIG 4 (a) The inductor structure and (b) the corresponding
photograph.

Inductor Design
The inductor requirements are determined from the circuit
design and the target size for the module. Because the
inductor acts as the package, there must be enough room in
the cavity to accommodate the regulator chip, input and
output capacitors, and other small components (Figure 4).
The inductor winding itself occupies the rest of the volume.
The specifications are:
■■an inductance of 1 nH
■■a switching frequency of 800 kHz

Inductance (µH)

the top of the regulator to the ceiling of the cavity to trans■■a direct current (dc) of 6 A
fer heat from the regulator to the core. Benefiting from this
■■the lowest possible DCR (target < 15 mΩ)
packaging technology, the PSI2 power module has three
■■a width of 9 mm, a height of 2.4 mm, and a length of
main advantages over modules based on conventional plas15 mm (including the cavity).
tic packaging technology.
Because the inductor forms the overall package, it
1) Because there is no need to leave room for plastic mateuses the full width of the module itself, and the effective
rial, the inductor can be the full width of the package,
size of the functional portion is 9 × 9 × 2.4 mm. The outwhich means that both the winding and the magnetic core
side diameter of the coil must be significantly smaller
can be larger. The PSI2 module has a
than 9 mm to allow sufficient
magnetic material for mechanical
lower DCR and a higher inducstrength and ease of manufacture.
tance value because the volume of
Recent experimental
The variables in the design include
the inductor is increased. Losses
the number of turns, the cross secare decreased, and the efficiency
work has focused on
tion of the rectangular wire, the
is improved.
improving the packagbend radius of the coil, and the per2) In any point-of-load (POL) power
meability of the core material. A
module, the main heat sources are
ing of high-density
suitable metal-powder-composite
the semiconductor devices and the
power modules with a
core material was chosen first,
inductor. In a plastic-packaged
particular emphasis
so the permeability was known.
power module, the inductor is
Because the magnetic core has a
enclosed by the plastic material
on the inductor.
customized shape, and there are no
(typically, with a minimum thickexisting models for calculating the
ness of 1 mm), which increases the
inductance value, finite-element
thermal resistance to the ambient
analysis was used to simulate the inductance value and
air. By using the magnetic material as the package, the PSI2
the DCR.
module avoids this issue and allows the inductor to dissiFrom an initial rough calculation, there were several
pate heat directly to the ambient air. Some of the heat from
combinations of the wire width and bend radius that
the semiconductors is also conducted into the magnetic
could meet the inductance requirement with a reasonable
material, allowing a lower operating temperature and
number of turns. A simulation was carried out for each
improving the performance of the whole power module.
of these combinations to determine the option with the
3) Because there is no need for plastic molding, a major step
in the production process can be eliminated, reducing
module cost.
1.2
1.0
0.8
0.6
0.4
0.2
0.0

Simulation
Measurement
0

10
Current (A)

FIG 5 The inductance value versus the current.

September 2016

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2016