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A further complication arises at higher power levels—typically around 100 W device output power. At this level, ROUT and COUT for +28-VDC devices result in low output impedances that approach 2 Ω or less, making it difficult to achieve impedance matching for 50-Ω systems. A typical +28-VDC LDMOS device at 100 W adds an output impedance prematch to arm an amplifier designer with more reasonable terminal impedances. The added complexity increases the package size, however, nearly doubling the printed-circuit-board (PCB) area needed for a +28-VDC device for the same power level.

To simplify this problem for the sake of comparing +28- and +48-VDC devices—looking only at the resistance and capacitance characteristics of the amplifying die—it is possible to show that the available bandwidth for a given mismatch also follows the voltage ratio. A +48-VDC device will provide about 70% increased bandwidth potential compared to an equivalent powered +28-VDC device.

If the system requires only moderate bandwidth, there are still good reasons for using a +48-VDC solution. The lower Q or transformation ratio allows for more simplistic matching topologies since the load impedances require less transformation. These simplistic topologies also tend to be more resistant to manufacturing tolerances (i.e., PCB and matching component variations due to assembly).

High-Voltage GaN-on-Si Devices Deliver High Power, Table 2

For a realistic comparison, Table 2 shows fundamental properties for two similar 100-W GaN-on-Si devices, the +28-VDC model NPT1010 and +48-VDC model NPT2010 devices. Both devices use a 100-W die with no output impedance prematching and the same substrate thickness, die attach technique, and metal air cavity package. Among these measures, two differences stand out. The optimum load impedance of the +48-VDC device is significantly higher with the ratio of the real part at 3:1, consistent with theory.

The second obvious difference is thermal resistance (RθJC). This is the exception in the comparison of devices, whereby the smaller +48-VDC die can push performance in the wrong direction on account of more power being produced in a smaller area. Both devices are designed to operate with adequate margin to achieve rated output power levels below maximum rated junction temperatures for a given ambient condition. A significant thermal limitation for these high-power devices is the AC360 air-cavity flange package. The limitations of the package serve as a reminder of the need for more enhanced thermal packaging techniques to further enhance +48-VDC device operation.

In addition to enabling broader bandwidths, higher output impedances also enable alternative matching topologies, some of which were previously impractical. One of these, for example, is a broadband amplifier targeting 90 W output power from 100 MHz to 1 GHz. A ferrite-based impedance transformer is the best choice for a decade-plus bandwidth in the VHF/UHF bands; these topologies are ideally suited for N2:1 impedance ratios such as 1:1, 4:1, and 9:1.

From Eq. 1, an amplifier based on +28-VDC devices would require a load resistance of about 4.5 Ω, and 9:1 would be the ideal ratio for a transformation to 50 Ω. But a 9:1 impedance transformer is difficult to realize. The higher permeability materials needed to enable the low-frequency transformation become lossy at high frequencies, preventing full band coverage.1 Realistically, a high-power ferrite transformer with 9:1 transformation ratio will suffer from high loss and struggle to work above a few hundred MHz.

High-Voltage GaN-on-Si Devices Deliver High Power, Fig. 2

With a +48-VDC drain supply, the load resistance is much higher, nearly 12.5 Ω, and a lower-ratio 4:1 transformer is better suited to make the impedance transformation to 50 Ω. This lower transformer ratio is ideal for frequency coverage from 100 MHz to 1 GHz. With a nominal 100-W device, the simple and low-cost design shown in Fig. 2 delivers more than 80 W output power over the entire frequency band, achieving 50 to 70% efficiency with a single device. Figure 3 shows the efficiency performance for this design.

High-Voltage GaN-on-Si Devices Deliver High Power, Fig. 3

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