IEEE Power Electronics Magazine - December 2019 - 28

Temperature Rise (°C)

against the shim, without any solid or liquid TIM between
the heatsink and shim. The screws were tightened to
attach the heatsink securely to the PCB, while the shim
absorbed the mechanical stress of mounting. This step
caused excess liquid gap filler to be extruded from
beneath the heatsink, and this pressure forced the gap
filler into air gaps across the uneven surface of the PCB.

100
90
80
70
60
50
40
30
20
10
0

Q1
Q2

IMAX = 19 A
With Heatsink
IMAX = 12 A
Without Heatsink

9
11
13
15
Output Current (A)

FIG 9 The transistor temperature rise for the example given in
Figure 8, before and after heatsink attachment, with a cutoff
of 100 °C for either transistor [21], [23]. (Source: EPC; used
with permission.)

5) Excess gap filler was cleaned off the PCB. After several
hours, the liquid gap filler cured to a solid state, and the
assembly was complete.
Figure 9 shows the junction temperature rise of each
EPC2045 GaN transistor in this example when operated
as a 700-kHz, 48- to 12-V synchronous buck converter. The
ambient temperature was maintained at 25 °C, with 800
linear feet per minute (LFM) forced air. Assuming a maximum allowable temperature rise of 100 °C above ambient
for each transistor, the addition of the heatsink increased
the converter's output current capability by 60%, from 12
to 19 A. Because of the close contact with the half-bridge
circuit, the thermal impact of the Vishay IHLP series output
filter inductor was also considered in this example [21].
As a second example, a 45 × 45 × 25-mm cross-cut
heatsink was attached to a larger power stage using two
EPC2206 GaN transistors and an off-board filter inductor.
The heatsink was attached using only the TG-X thermal pad
in this example, without liquid gap filler. Figure 10 shows this
power stage and the junction temperature rise of each transistor, operated as a 125-kHz, 48- to 12-V synchronous buck
converter. In this case, the maximum allowable temperature
rise of each transistor was limited to only 60 °C above ambient, with 800 LFM forced air. The addition of the heatsink
doubled the output current capability from 25 to 50 A [21].

Summary
This article reviewed the thermal management considerations for GaN transistors and GaN-based converter designs.
The thermal characteristics typically included in device data
sheets were explained, such as thermal resistances and transient thermal impedances. Thermal design schemes for bottom-side cooling and multisided cooling were discussed,
including selection of the heatsink and TIM, as well as key
design considerations for heatsink attachment.
Additionally, some thermal models for GaN transistorbased converters were defined, including an individual
power stage with and without a heatsink. The typical
options for temperature measurement were reviewed.
Finally, this article gave two application examples for thermal design with chip-scale GaN transistors, from a single
high-density buck converter to a much larger converter
with higher voltage, more transistors, and a larger heatsink.

(a)
Temperature Rise (°C)

60
IMAX = 25 A
Without Heatsink

50
40

30
Q2

About the Authors

IMAX = 50 A
With Heatsink

10
5

20 25 30 35
Output Current (A)
(b)

FIG 10 The heatsink attachment example using two EPC2206
GaN transistors, a nylon shim, TG-X thermal pad, and 45 × 45 ×
25-mm cross-cut heatsink. (a) The PCB used for thermal evaluation and (b) transistor temperature rise before and after heatsink attachment, with a cutoff of 60 °C for either transistor [21].
(Source: EPC; used with permission.)

IEEE POWER ELECTRONICS MAGAZINE

z December 2019

Edward A. Jones (jones.edward@gmail.com) received his
B.S.E.E. degree from Virginia Tech, Blacksburg, in 2007,
where he was awarded the prestigious Bradley Scholarship
and cofounded the Virginia Tech chapter of Engineers Without Borders. He received his Ph.D. degree from the University of Tennessee in 2017, where he was a Chancellor's Fellow, a Center for Ultra-Wide-Area Resilient Electric Energy
Transmission Networks Fellow, and a Bredesen Energy Sciences and Engineering Fellow. He is a senior GaN applications engineer with Infineon Technologies. His research
interests include wide-bandgap device characterization and

IEEE Power Electronics Magazine - December 2019

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