Saturday, May 25, 2019

Compact PA Solution Using Novel GaN Process and Specialized Software

Parent Category: 2018 HFE


The Design Challenge

Modern electronic warfare (EW) and battlefield awareness require reliable, high-output-power, high-efficiency, and large-bandwidth power amplifier (PA) solutions. With many applications requiring single PA compact solution covering a broader frequency range, GaN on SiC has become the technology of choice due to its high-power density, high-frequency capability and robust design ability.

Designers in this case were tasked with using a next-generation 28 V GaN process technology (G28V4, 0.25µm) to design a wideband general-purpose device capable of meeting the demands of the EW market for dismounted or man-portable uses that operate from battery power. Wolfspeed selected the 0.25 µm process for its benefits of 6 W/mm power density, >120 V breakdown, and 28 V operation to deliver the highest power gain possible from a single-stage RF device.

Design goals for the PA, shown in Figure 1, were to create a single-ended amplifier with the ability to deliver over 70 W (CW) from 0.5 - 3.0 GHz, while maintaining an efficiency high enough to operate at a case temperature of 65° C.

1807 HFE pa 01

Figure 1 • Compact PA solution designed using a next-generation 28 V GaN process technology.

Other critical parameters included gain flatness of +/- 1 dB for easy system integration and good input/output return losses. To achieve the large bandwidth, a packaged input matched approach was used to reduce the burden on the input matching network (IMN), while maintaining a small form factor. An input equalizer design was implemented on the printed circuit board (PCB) to shape the gain across the band to meet the required specification. A key requirement for the equalizer was to maintain the gain flatness in band and add stability margin while handling the maximum input power needed to drive the transistor into saturation without the need of non-surface-mount components. For this reason, careful simulations were needed to see the dissipation in each of the components on the PCB and make sure the power ratings were not exceeded during operation.

The Solution

The Wolfspeed design team chose the NI AWR Design Environment platform, specifically Microwave Office circuit design software, for the design of this device. The final product (CG2H30070) demonstrated 80 - 115 W of output power from 0.5 - 3.0 GHz with greater than 48 percent drain efficiency. The input equalizer enabled the gain flatness to be within +/- 1 dB of the nominal value. Because surface-mount components were required to reduce the complexity of the PCB, aluminum nitride (AlN) thick-film chip resistors were used on the equalizer network to handle the high-power requirement, as shown in Figure 2. Electromagnetic (EM) simulations, in conjunction with harmonic balance, were used to verify the power dissipation in each of the components and to confirm that the maximum rating was not exceeded. Using Microwave Office graphs, the dissipated power versus input drive was plotted to determine the maximum rated input power of the PA, so that a first-pass design was feasible on the bench.

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Figure 2 •Equalizer network used on IMN to achieve gain flatness across required bandwidth.

For the full design, a high degree of correlation was achieved between the simulations and measured results, as demonstrated in Figure 3. The predicted drain efficiency was typically within 2 percent of the measured data and the output power was within 0.5 dB across the frequency range.

1807 HFE pa 04

Figure 3 • Measured (dashed) versus simulated (solid) data showing a high degree of correlation.

Why NI AWR Design Environment

The Wolfspeed design team chose the NI AWR Design Environment platform for the design of the CG2H30070F 0.5-3.0 GHz GaN on SiC PA utilizing a packaged pre-match to reduce the required transformation ratio and on-board equalizer to achieve optimal gain flatness. The ability to quickly add a full Ansys HFSS package transistor model in Microwave Office software along with a full AXIEM EM simulation of the PCB enabled the team to analyze the circuit in detail and achieve industry- leading performance in a short period of time. The resulting measured-to-model correlation validated the approach and enabled quick release of a full device model.

Special thanks to Matthew Pizzella, RF Design Engineer and Brad Millon, Applications Engineering Manager, for their contributions to this story.

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