IEEE Power Electronics Magazine - December 2019 - 26

pad or a liquid gap filler is recommended, as this limits
mechanical stress and allows for a wider height tolerance
between components.
Both top-side and bottom-side thermal paths should
be optimized. First, the board-to-ambient thermal resistance can be reduced through the addition of thermal vias
(similar to the bottom-cooled design) but with the thermal
vias connecting to internal and external copper layers to
spread the heat laterally. Second, the TIM resistance can be
reduced in any of three ways:
1) by reducing the thickness of the device-to-heatsink
interface
2) by selecting a highly thermally conductive TIM

Rθ MB

Rθ LB TL

Rθ MA

Rθ B2

TS

TA
PL

PQ2

TJ2

TA

The half-bridge circuit is a common building block in many
power electronics circuits, and it will be used here as an
example for thermally modeling a GaN-based power stage.
A half-bridge circuit consists of two transistors, Q1 and
Q2, as well as the supplementary components, for
instance, gate drivers, regulators, and passive components
[21]. The thermal behavior of a half-bridge power stage
(e.g., a buck converter) using chip-scale GaN transistors
can be modeled with the lumped-parameter equivalent circuit shown in Figure 6. This model includes parameters
that represent most of the significant heat flux paths in the
converter, as demonstrated in Figure 7. The three primary
heat sources in this example system are power losses from
Q1, Q2, and the filter inductor [21]. The lumped-parameter
model neglects some of the more complex thermal interactions, such as treating the PCB temperature and heatsink
temperature each as a single temperature node. In reality,
a temperature gradient exists across each of these elements. Despite these simplifications, an equivalent circuit

Rθ CS1

Rθ SA

Rθ LA

TB

PM

Rθ JC
PQ1

Rθ JB

Thermal Model of a Power Stage With
Discrete GaN Transistors

TC1

TJ1
Rθ B1
Rθ BA

3) by placing the TIM on all sides of the devices and not just
the top. This reduces the thermal resistance of R iJC and
R iCS, as the device perimeter side walls increase the
overall device surface area by a significant margin.
Liquid gap filler is a convenient option for achieving sixsided cooling in this manner.
Further improvements are possible by utilizing dualsided heatsinking, forced-air cooling, liquid cooling, and
through the use of exotic PCB materials, such as direct
bonded copper [18], or insulated metal substrate [20].

Rθ JB

Rθ JC

TC2
Rθ CS2

Rθ BS
FIG 6 The thermal equivalent circuit for a half-bridge power
stage utilizing top-side cooling, with power losses dissipated by
two discrete GaN transistors, a filter inductor, and the surrounding system. (Source: EPC; used with permission.)

TA

Rθ SA
TS

PM

Shim

Rθ CS1
Si

Rθ CS2

TC1

Aluminum
Rθ JC
Nitride
Active GaN Device Region
Solder
Bars

Rθ JB
TB (VIN)

Heatsink

TB1, opp

TB(sw)

TC2

Aluminum
Nitride
Active GaN Device Region
Rθ JC

PL

Solder
Bars

Rθ BS
Copper
Traces

PCB

Si

Shim

Heatsink

PCB

TB(gnd)
TB2, opp

Rθ BA
FIG 7 A cross-sectional diagram of a half-bridge circuit consisting of two chip-scale transistors and a heatsink, highlighting the
physical heat flux paths from the two transistors corresponding to the equivalent circuit in Figure 6, as well as the heat contributions from the filter inductor and other system losses. (Source: EPC; used with permission.)

26

IEEE POWER ELECTRONICS MAGAZINE

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IEEE Power Electronics Magazine - December 2019

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