IEEE Power Electronics Magazine - March 2023 - 25

FIG 7 Gen-2 BiDFET device: (a) blocking characteristics; (b) output characteristics.
Table 2. Comparing Gen-1 and Gen-2 BiDFETs.
-
Parameter,
Units
Chip Area, cm2
RDS,ON, mΩ
gM, S
CISS, pF
COSS, pF
CRSS, pF
EON, µJ
EOFF, µJ
ETOTAL, µJ
Gen 1
(2 Chips)
2.28
25
15
15100
1050
70
1350
460
1810
Gen 2
(1 Chip)
1.14
27
15
11730
600
70
1120
250
1370
Improvement
2x
-
-
1.3x
1.75x
-
1.2x
1.8x
1.3x
Advanced Packaging Approach
The target for this research was a low cost, reliable converter
operating in a harsh environment with preferred convective
cooling. Since the BiDFET has very low loss, an
advanced alternative packaging approach was adopted. An
investigation was undertaken to compare the use of ultrathin
epoxy resin composite dielectric (ERCD) as a replacement
to traditional DBC that uses plate ceramic. The ERCD
material recently introduced by RISHO KOGYO Co. Ltd. is
characterized as having 10 W/mK, 40 kV/mm B.V., modulus
of 53 Gpa, operation at ≤300 °C, and thickness of 120 µm.
The material is available as a metalized film, laminate or
clad on thick copper to create an insulated metal substrate.
Use of an organic approach allows for a metalized substrate,
or complete module, to be processed by high-end
PCB companies at substantially reduced cost and turnaround
time.
To compare ceramic to ERCD, a two-sided module
structure, Figure 8, was designed and simulated in ANSYS
as reported in [6]. Two SiC die were mounted and encapsulated
to allow heat flow from both surfaces. Spacers were
assumed to cover 60% of the die area. For a two-sided structure,
four possible combinations of topside and bottom-side
dielectrics of ceramic and ERCD were considered, with further
consideration of two types of ceramic, Al2O3 and AlN.
The ceramics were 380 µm thick clad with 127 µm of Cu,
and ERCD was 120 µm thick with 100 µm Cu. The thermal
conductivities were 24, 170, and 10 W/mK for Al2O3, AlN and
ERCD respectively. Comparative results for thermal resistance
are shown in Figure 9 and shows that the ERCD Rjc,eq
is 10% better than Al2O3. Though ERCD has lower conductivity,
the thinness allows the conductance to be higher.
Since cost is of concern and the power dissipation capability
with ERCD is well within the project requirements, the
more costly AlN and ceramic processing can be avoided.
Since cost is of paramount concern, a 19 mm × 32 mm
exemplar substrate was provided to three vendors for pricing.
The ERCD approach was one third or less than Al2O3
DBC, Figure 10. The ERCD is rated for continuous operation
up to 300 °C as noted in the paragraph above. Over temperature
of the BiDFET would not noticeably affect the cost
comparison, and the reliability issues would be dominated
by chip attachment in both DBC and ERCD.
The Gen-I BiDFET unencapsulated single-sided module
mounts the die on an ERCD insulated metal substrate (eIMS)
as shown in Figure 11(a) [6]. The ANSYS thermal analysis,
March 2023 z IEEE POWER ELECTRONICS MAGAZINE 25
FIG 8 Two-sided power package to compare ceramic and ERCD
dielectric isolated substrates.
IEEE Power Electronics Magazine - March 2023

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