IEEE Power Electronics Magazine - March 2017 - 40

published power densities for several converter designs as
a function of their HMFOM values, estimated from the indicated material type [1], [3], [10], [11], [19]-[23]. These examples are restricted to air-cooled converters, at or above 110 V
ac or 380 V dc, that were physically built, wherein the objective was to maximize power density. For converters using
switches and diodes of different types, the root-product of
the HMFOM values is used. The data show a clear trend,
and extrapolating out to HMFOM values for AlN predicts
a power density of over 550 W/in 3 . This observed relationship is certainly not definitive, as power density depends

C
L2

L1

R

DUT

(a)

(b)

Magnitude (Ω)

Short-Circuit Zin Measurements
2

10

100
10-2

Package Is <1 Ω
at 35 MHz
107

108

109

107

108

109

Angle (°)

100
50
0

Frequency (Hz)
(c)

FIG 5 (a) The equivalent circuit used to fit parasitics, (b) the
package itself mounted for the network analyzer, and (c) the fit
to a package containing a short circuit [24].

100 °C

10
200 °C

8
Current (µA)

75 °C
40 °C

6
4
150 °C

2
0
0

2

4

6

8

10

12

Rethinking Inductor Design
While soft ferrites are ubiquitous in power electronics, the
energy density of these magnetic materials diminishes substantially at higher frequencies. This is driven by the need to
manage core losses within the material by limiting the peak
field strength. The two principal loss mechanisms are hysteresis and eddy current generation, each of which becomes
more troublesome at higher frequencies. To complement the
high-frequency operation of power converters, research and

14

z	March 2017

To realize the benefits of WBG and UWBG materials in
power electronic converters, the device packaging must tolerate the same stresses that the devices will see, without
interfering with performance. This means that packaging
must be developed that adds minimal impedance to the circuit while maintaining a high hold-off voltage. The packaging must also maintain insulation integrity at high temperatures. A prototype three-dimensional (3-D)-printed package
was presented in [24]. Therein, the prototype was designed
with a target of a 10-kV hold-off voltage. Additionally, the
package was designed with a 10-ns rise time in mind, and
therefore must have negligible impedance up to approximately 34 MHz. The pins of the prototype have a 1.4-cm
spacing and are separated from one side to the other with
0.254-cm-thick Ultem. Ultem is a high-temperature-resistant
resin produced by Sabic that allows for a 3-D-printed package that is capable of a high breakdown voltage even under
high-temperature operation. An added barrier between the
pins increases the pin-to-pin tracking length to 2.67 cm.
Therein, the pin-to-pin package impedance was characterized by a network analyzer over a wide range of frequencies
for a package containing a short circuit, an open circuit, and
a circuit with a 50- X load. The measured impedances were
then fit to an equivalent circuit model. The resulting equivalent circuit is shown in Figure 5, with R equal to 1 mX, L 1
equal to 1 nH, L 2 equal to 3.5 nH, and a C of 0.6 pF.
Additionally, the voltage hold-off of the package was
tested (using a 20-kV IV tracer) while in a high-temperature oven. Voltage sweeps of the part were conducted
with the oven at a variety of temperatures. The prototype
package was capable of holding off over 10 kV of voltage,
even at temperatures of 150 °C, but started to fail due to
increased leakage current as the temperature increased
to 200 °C (Figure 6). Alternative packaging materials and
methods of employing 3-D printing are being examined to
improve the high-temperature performance of this type
of packaging.

Second
Sweep

FIG 6 The package (inset) in the high-temperature oven at
breakdown, and the results of temperature-dependent voltage
hold-off testing [24].

IEEE PowEr ElEctronIcs MagazInE

Packaging: Reducing Parasitics and Handling Heat

100 °C

Voltage (kV)

40

on many variables, including topology, power quality, the
BoS, and reliability needs. Predicting the power density of
AlGaN-, AlN-, and other UWBG-based converters is highly
speculative, but the trend is illustrative of the potential that
UWBG technology presents. One thing is clear, however:
as UWBG devices push the limits of performance, so, too,
must the BoS.



Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2017

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