Plastic-Packaged LDMOS RF Power Transistors Deliver Up to 300 W CW
By David Lester, Freescale Semiconductor
RF systems such as FM and digital TV broadcast, aerospace and defense, land mobile radio, CO2 lasers, and industrial heating and sealing systems stress RF power transistors and amplifiers to a far greater degree than most other applications. RF input power often exceeds device ratings, DC voltages vary, and impedance mismatches can rise to levels that essentially resemble a direct short. Nevertheless, the end product must continue to operate to specification over long periods without failure.
To accommodate such hostile operating environments, power amplifiers have traditionally employed RF power transistors housed in air-cavity ceramic packages, especially at higher RF output levels. However, Freescale Semiconductor has changed this paradigm in recent years with LDMOS FET RF power transistors that deliver high RF power levels when housed in overmolded plastic packages. The company has recently increased these power levels to 150 W CW and 300 W CW, which represent the highest RF power outputs available from any plastic-packaged RF power device.
Like their ceramic-packaged counterparts, these devices will perform without failure or performance degradation when driven by twice their rated RF input power into an impedance mismatch (VSWR) greater than 65:1 at all phase angles. The new MRFE6VP5150 and MRFE6VP5300, available in both straight-lead and gull-wing packages, deliver 150 W and 300 W CW over a frequency range of 1.8 to 600 MHz. Detailed specifications are provided in Table 1.
Freescale rates its RF power products conservatively and at the highest frequency in the range, which ensures that they will deliver their rated output levels with considerable margin. For example, Figure 2a shows the RF power output of the MRFE6VP5150 operating at 230 MHz with an input signal having a pulse width of 100 µs and a 20% duty cycle. P1dB RF output power ranges from slightly over 3 W with a 13 dBm input signal to over 158 W with 30 dBm of drive. In Figure 2b, MRFE6VP5300 operating at the same frequency and with an input signal having similar characteristics delivers about 10 W with 17 dBm of drive and greater than 398 W with just over 31 dBm of drive.
The Future Is Plastic
Freescale pioneered the use of overmolded plastic packages in RF power applications in 1997 after first successfully introducing the technology for automotive and industrial applications in the 1980s. The company has subsequently dramatically increased the use of this packaging technique in a wide array of small-signal and LDMOS RF power transistors, and most recently in LDMOS devices that deliver even greater amounts of RF power.
Achieving higher power levels such as those represented by the new MRFE6VP5150 and MRFE6VP5300 presents a major challenge in packaging design as the devices must simultaneously accommodate 225° C die operating temperatures and greater heat dissipation without compromising electrical performance, ruggedness, or reliability. One of the key metrics in this area is thermal resistance (measured in °C/W), which represents how effective the die/package combination is in transferring heat into a heat sink or cold plate. The higher the thermal resistance the more difficult it becomes to remove heat from the die so it must be as low as possible. Freescale has made consistent advances over the years in reducing thermal resistance, and the new devices not only match the thermal performance of Freescale’s ceramic-packaged LDMOS FETs delivering the same RF output powers but actually improve upon it.
Replacing traditional ceramic air cavity with overmolded plastic packages has significant advantages, the most obvious being lower cost. However, unlike bolt-down ceramic-packaged devices, they are also compatible with automated pick-and-place manufacturing, which makes them suited for moderate-to high-volume production applications. This has significant benefits for OEMs. For example, ceramic air cavity packages are usually hand soldered down to the circuit board, which is not only time-consuming but much less precise than what automated equipment can achieve. Typical accuracy with hand soldering is 0.12 mm while automated equipment can achieve precision of about 0.025 mm, which can reduce or even eliminate the need for tuning that is required to compensate for the imprecision of hand soldering.
Why Such Hostility?
Compared to applications that require extremely rugged RF power transistors, systems such as wireless base stations, smartphones, and other wireless-enabled devices are far less demanding. Their operating environments are far more stable and not subject to wild variations in DC operating voltages, RF input power, and often (but not always) impedance mismatches. In contrast, CO2 lasers, plasma exciters, and even FM and over-the-air TV transmitters not only require high RF power output but present some or all of the aforementioned anomalies, often on a frequent basis. Lasers and plasma generators must ramp up quickly and place severe stress on RF power transistors and amplifiers, which can easily exceed the specifications on their data sheets.
Even with the help of power monitoring circuits in the radio that help maintain RF output power with reductions in supply voltage, they do so by increasing drive to the final amplifier, producing an overdrive condition that many devices have difficulty accommodating. However, Freescale’s high-ruggedness LDMOS devices can simultaneously shrug off variations in DC power, twice their rated RF input level, and mismatches that occur resulting from poor connections, user abuse, and cable discontinuities.
The MRFE6VP5300 is in full production and the MRFE6VP5150 is expected to be available in production quantities in the third quarter of this year. Freescale provides a wide range of support tools including broadband fixtures, models, and reference designs. More information can be obtained by visiting our website.
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