The significant advantages of gallium nitride (GaN) have mostly been discussed in relation to delivering high peak power in narrowband pulsed applications, primarily active electronically scanned array (AESA) radar. However, as GaN devices are inherently broadband, they are also appealing for use in electronic warfare (EW) or applications with multiple narrow frequency bands, such as military radios. To serve these, Freescale Semiconductor has introduced the MMRF5014H, a GaN on SiC HEMT power transistor that delivers at least 125 W, CW or pulsed, up to 2700 MHz and 100 W CW from 200 to 2500 MHz.
FEATURES AND BENEFITS
A device with broad bandwidth allows designers to use a single device, rather than several, to cover an entire range of operating frequencies. Most military radios and other systems operating in the frequency range of the MMRF5014H employ silicon power transistors. While these are rugged and achieve high levels of performance, they are usually designed and matched to operate over narrow frequency bands. Consequently, a system covering an octave bandwidth requires three or four transistors, one for each segment of the frequency range. This design approach increases system complexity and cost. The MMRF5014H can be matched over a much wider frequency range, requiring fewer amplifiers and transistors – in some cases only one. Also, manufacturers can design a range of products that cover different bands from HF through S-Band using just the MMRF5014H as the output transistor.
In many applications, power amplifiers are subjected to significant impedance mismatches that can destroy power transistors. While some “high-ruggedness” LDMOS transistors can survive extreme mismatches without performance degradation or failure, this has generally not been true for GaN HEMTs, whose rated maximum VSWR is generally 2:1, and usually no greater than 5:1. The MMRF5014H is an exception, designed to deliver full CW power into a 20:1 VSWR without degradation or failure, even at twice its rated input drive. With this level of ruggedness, the MMRF5014H is well suited for battlefield radios and industrial systems such as RF heating, where significant impedance mismatches are common.
The thermal design of the system is a challenge when using a GaN power transistor, since the high power density (W/mm of gate periphery) of GaN generates greater heat concentration than other device technologies. So the thermal resistance from device junction to package base is a key parameter. It should be as low as possible to allow the heat to efficiently spread away from the device and into the heat sink. At less than 1° C/W, the MMRF5014H has the lowest thermal resistance of any commercially available GaN HEMT. The device is housed in Freescale’s NI-360H ceramic air-cavity package.
RF PERFORMANCE
Detailed specifications for the MMRF5014H are shown in Table 1. In Freescale’s 200 to 2500 MHz applications circuit, the power transistor achieves greater than 40 percent drain efficiency over the full frequency range and 58 percent at narrower bandwidths. Gain is greater than 12 dB and increases to 16 dB over narrower bandwidths.
Like all Freescale RF power transistors, the MMRF5014H is conservatively rated, as demonstrated in Figure 1. In this pulsed radar application between 1300 and 1900 MHz, the gain, efficiency and RF output power are significantly higher than rated. This is also true over the full bandwidth of the transistor, as shown in Figure 2.
Samples and evaluation circuits for the MMRF5014H are available now. The device can be ordered on tape and reel.
Figure 1 Narrowband gain, efficiency and RF output power under pulsed operation.
Figure 2 Wideband gain and efficiency at 100 W CW output power.
Unlike the volatile wireless market, defense systems require a commitment to long-term availability, as systems remain operational for many years after being deployed. To satisfy this requirement, the MMRF5014H and all products designed for defense applications are part of Freescale’s product longevity program, which guarantees availability for 15 years.
Freescale Semiconductor
Tempe, Ariz.
www.freescale.com/RFmilitary