Page 44 - EETimes Europe June 2021

44 EE|Times EUROPE

MARKET & TECHNOLOGY TRENDS
Wolfspeed’s GaN on SiC Tackles Broadband

Design Challenges

By Scott Blum, RF Applications Engineer, Cree | Wolfspeed

ireless applications are the largest and fastest- band. Wolfspeed already had a solution using two 70-W CG2H30070
growing segment in the electronics industry. From radar devices for the output stages, and it was already known that 43 dBm,
to satellite communications, two-way radios to cellular or 20 W, of RF power was needed to feed into that stage to get the
Winfrastructure, the sub-6-GHz broadband is where most required 125 W.
of those applications reside, including certain implementations of 5G It is recommended to set up the specs for the stages looking back
networks. from the final output, and this, therefore, became the target output
Designing amplifiers for these applications is, however, a con- power specification for the middle stage that is the focus of this study.
siderable challenge. Unlike typical amplifiers that must offer Wolfspeed has a wide portfolio of GaN high-electron–mobility tran-
high-performance operation in a narrowly defined set of conditions, sistors, including 25-W devices. But with the necessary compromises
broadband amplifiers are required to operate over one octave or more. for broadband operation, the 35-W CG2H40035F was selected after
Many of the performance parameters vary with frequency, and ensuring evaluating load-pull data (Figure 1).
that the entire amplifier lineup delivers the required performance con- From the load-pull data, too, it was apparent that 2 W of input power
sistently across the entire band is a challenge. would be required, which became the spec for the pre-driver stage. That
Fortunately, wide-bandgap semiconductor devices like gallium requirement is met by another Wolfspeed device, the CMPA0530002S.
nitride help address this challenge and support continued growth in The main consideration for the driver stage was to get enough power
this market. For instance, Yole Devéloppement estimates that the GaN out of the device while operating it efficiently enough for reliable oper-
RF market will reach US$2 billion by 2025, growing at a CAGR of 12%. ation, and this set the limits for tradeoffs.
And in that market, GaN on silicon carbide offers the best perfor- CG2H40035F is rated for a maximum junction temperature (T j ) of
mance with clear advantages over silicon devices, including LDMOS, as 225˚C, a maximum case operating temperature (T c ) of 85˚C, and
well as devices fabricated with GaN on a Si substrate. junction-to-case thermal resistance (Rθ jc ) of 3.4˚C/W.
Wolfspeed, A Cree Company, uses GaN on SiC to provide its custom- Therefore, the power dissipated must be:
ers higher efficiency, wider bandwidth, higher power, and smaller size P DISS ≤ (225˚C – 85˚C) / 3.4˚C/W = 41.2 W
in order to cost-effectively meet requirements from multiple mar- The minimum efficiency is then derived from the maximum DC
kets, such as military and aerospace, communications, and industrial power, P DC , that will keep T j < 225˚C.
applications. The company also supports its customers by designing P DC + P RF_IN = P DISS + P RF_OUT
reference boards for these products. Or
The issues with broadband amplifier design are better-understood by P DC = 41.2 + 20 – 2 = 59.2 W
following Wolfspeed’s approach to solving the amplifier requirement The minimum efficiency is then:
for the 500-MHz to 3-GHz telecom band. EFF > P RF_OUT / P DC = 20 W / 59.2 W = 33.8%
Therefore, in load-pull analysis, the region of success is where the
MEETING REQUIREMENTS, SETTING SPECS device simultaneously has >10-dB gain with >43-dBm P RF_OUT and
A recent customer requirement was for the driver stage in a three-stage >34% drain efficiency, plus some margin for various losses, including
lineup to deliver a 125-W continuous-wave solution to cover the entire mismatch loss.

Figure 1: The pre-driver, driver, and power amplifier stages in the lineup

JUNE 2021 | www.eetimes.eu