LDMOS FETs Deliver 25 And 100 W CW from 1.8 to 2000 MHz
By David Lester, Freescale Semiconductor
Most RF power transistors whether silicon, GaAs, or GaN, are typically tailored to deliver their best performance over a relatively narrow band of frequencies, such as one or two wireless bands. Achieving optimum performance over a much broader frequency span presents the normal challenges such as maintaining linearity, RF output power, and efficiency, along with others that may are important in some applications but not others.
Freescale Semiconductor has achieved the ability to satisfy the RF power generation needs of applications operating between 1.8 and 2000 MHz with two new LDMOS 50-VDC RF power transistors that deliver 25 and 100 W CW. Their high performance over this bandwidth along with extreme ruggedness make them useful as driver amplifiers and often final stage amplifiers in applications ranging from communications to avionics and radar, low-power television, “white space” transmitters, various ISM systems, and test and measurement.
The 25 W MRFE6VS25N and 100 W MRFE6VP100H combine the attributes of different LDMOS devices that Freescale has optimized for specific applications: linearity for base station transceiver devices, ruggedness for devices that can potentially experience extreme mismatch conditions (such as CO¬2 laser drivers, MRI, etc.), and advanced thermal management and packaging for all device types. The result is a family of packaged LDMOS RF power transistors that cover a frequency range that begins at the low end of HF and ends at PCS microwave wireless bands.
The MRFE6VVS25N has gain of more than 26 dB from 1.8 to 30 MHz and more than 25 dB at 512 MHz and its efficiency ranges from 50 to 73% depending on frequency. The MRFE6VP100H has gain of 26 dB at 512 MHz, more than 19 dB from 30 to 512 MHz, and efficiency ranging from 40 to 71%. The devices will operate without failure or degradation into an impedance mismatch greater than 65:1 at any frequency in their range and at any phase angle when driven with twice their rated RF input power. In addition to providing a highly robust RF power amplifier, the ability minimizes the need to employ complex external amplifier protection circuits that reduce power when extreme impedance mismatches are encountered. It’s important to note however, that some simple protection circuits are still required for components such as filters, transmit/receive switches, and other components that surround the transistors, but they can be comparatively simple and inexpensive. Detailed specifications for both devices are shown in Table 1.
Getting the Heat Out
Thermal management (and in particular thermal resistance) is one of the key factors in the design of RF amplifiers, especially with high-power-density technologies that produce large amounts of RF power from very small die. As the MRFE6VP100H is rated at 100 W CW over its bandwidth and under mismatch conditions may dissipate significantly more than its rated power, so thermal management was a major consideration in its design (as well as that of the MRFE6VS25N). Over its more than 15 years of LDMOS process and package development, Freescale has made significant strides in producing devices and their accompanying packages that have the lowest possible thermal resistance.
This parameter (measured in degrees C/W) when applied to semiconductors is the measure of effectiveness with which heat from a die is transferred to the exterior of its package. The greater the resistance, the more difficult it becomes to remove heat from the die and the higher the temperature rise of die. Consequently, reducing thermal resistance is arguably the most vexing challenges in achieving practical high-power RF amplifiers.
Through a combination of advanced packaging techniques and the inherent properties of LDMOS, the new devices have thermal resistance that is typically three times lower than a similarly-rated GaN device. This means they can extract and transfer heat from the device die to the outside of the package three times more effectively than GaN devices. The junction-to-case thermal resistance of the MRFE6VS25N is 1.2 oC/W and is 0.38 oC/W for the MRFE6VP100H. GaN RF power transistors have thermal resistance of ranging from about 1.4 oC/W, for a 90 W rated device to 4oC/W for a 28 W rated device.
As a result, new LDMOS devices can deliver higher RF power from die while maintaining low junction temperatures. Coupled with a high maximum operating temperature rating of 225 oC, this ensures long-term, reliable operation, and also reduces the cooling overhead such as heat sinking required to dissipate the heat. The comparison between LDMOS and GaN is appropriate in a frequency setting from HF through the low microwave region because GaN is the only technology that can produce the RF output power of LDMOS throughout this frequency range.
Covering the Wavefront
Freescale’s goal was to cover as many applications as possible within the devices’ 1.8 to 2000 MHz bandwidth. A good application example, although not entirely “mainstream”, is an amateur transceiver that covers 1.8 MHz (160 m) through 450 MHz (70 cm) continuously or in bands (no small achievement for a defense system let alone a product available to consumers). Multiple devices are currently employed to form the driver and final-stage amplifiers of these transceivers. Depending on the desired RF output, either the MRFE6VS25N or MRFE6VP100H (with an inexpensive driver) could serve as the only required RF power devices in such a transceiver. In addition, the transceiver would require only minimal VSWR protection as the devices inherently provide the highest levels of ruggedness available in any technology, and would represent the first “ultra-rugged” devices employed in amateur equipment at these frequencies.
In addition to this example, other typical applications include commercial HF transceivers, drivers or final-stage amplifiers for all types of radios from public safety at 150 to 512 MHz, 700 MHz, or 800 MHz, to military battlefield systems operating from 30 to 512 MHz (where they uniquely require only a simple single-stage matching circuit), low-power television amplifiers, emerging “white space” systems, VHF and UHF television transmitters, and drivers for airborne radar systems from 900 to 1400 MHz (L-band).
At their highest rated operating frequency, the devices are considered by Freescale as narrowband, as their usable bandwidth in “only” about 300 MHz. Operation at low frequencies required stability enhancements within the device structure to ensure gain does not rise to unmanageable levels that could otherwise cause oscillation appearing as modulation on the signal.
In some of the aforementioned applications, the conservative rating of the two devices makes them even more appealing as they do not need to be “driven hard” into compression to achieve the RF output required by the system. In addition, unlike GaAs pHEMT devices (an alternative at some frequencies) that require a negative bias supply voltage, the LDMOS devices require only positive supply voltages. Many of the applications likely to be served the MRFE6VS25N and MRFE6VP100H are in applications with long production and service lives and customers expect that a device will be available for a decade or more, which is just the opposite of the volatile wireless industry. As a result, Freescale has placed the devices in its “longevity” program, which guarantees that they will be available for at least 10 years.
The MRFE6VS25N is housed in Freescale’s TO-270-2 over-molded plastic package and the MRFE6VP100H transistor is available in Freescale’s NI-780-4 or NI-780S-4 air cavity packages. Both devices are now in production and are supported by reference designs and other tools.
About the author
David Lester has been with Freescale (formerly Motorola) since 1999 and is a technical marketer for the RF division with responsibility for industrial, scientific and medical applications. He has 25 years of experience in the set-top box industry and 9 years of experience in digital video and conditional access systems.
1. The MRFE6VS25N and the MRFE6VP100H deliver 25 and 10 W CW respectively from 1.8 to 2000 MHz.
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