Freescale drives plastic-packaged LDMOS RF power transistors to 300-W CW
A recent video produced by Freescale demonstrates an RF power amplifier in a fixture being subjected to an enormous impedance mismatch. When power is applied, a flame erupts on the board’s microstrip line and like a fast burning fuse sends a stream of fire down the line, through the device (which is only about 1 x 1.5 inches), and back to the source. The fact that the device continues to function exactly as it did before the test seems to defy logic. Nevertheless, devices like these are designed to just withstand this type of abuse along with double their specified RF input power, and wide-ranging DC voltages both lower than and in excess of the data sheet rating.
A tough neighborhood
Wireless applications have their own unique requirements but their operating environments are generally 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, the applications mentioned above not only require high RF output power but present some or all of the conditions encountered in wireless infrastructure. Lasers and plasma generators, for example, must ramp up quickly and in doing do place severe stress on RF power transistors and amplifiers that can easily exceed the specifications on their data sheets.
Even in mobile radio applications, in which power monitoring helps maintain RF output power with reductions in supply voltage, the radios do so by increasing drive to the final amplifier, potentially producing an overdrive condition that many devices have difficulty handling. Defense systems are routinely subject to abuse as they are often operated by soldiers who have no time to ensure everything is working “just right” nor the technical expertise required to understand how their radios work. They just need to work, all the time. Freescale’s high-ruggedness LDMOS devices can simultaneously shrug off these challenges.
Putting power in plastic
As important as device enhancements, advancements made by Freescale in overmolded plastic packaging are equally important and today serve even devices whose RF power levels are quite high. Freescale was the first semiconductor manufacturer to use overmolded plastic packages in RF power applications, introducing its first device in 1997 after first introducing plastic package technology to automotive and industrial applications in the 1980s.
Overmolded plastic packaging has significant advantages, the most obvious being lower manufacturing 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 factor alone is a significant benefit to OEMs, as it reduces the amount of labour required in assembly when compared to ceramic air cavity packages that are hand soldered to the circuit board. In addition to being time-consuming, manual placement is inherently less precise than automated equipment. That is, the placement accuracy that can be achieved with hand soldering is about 0.12 mm while automated equipment can reach 0.025 mm.
Such accuracy can also reduce or even eliminate the need for tuning required to compensate for the imprecision of hand soldering. Not surprisingly, overmolded plastic packages are replacing ceramic air-cavity packages at higher and higher RF power output levels. The highest yet offered is 300-W CW delivered by the latest Freescale MFRFE6VP300 described below. There are currently a wide array of plastic-packaged LDMOS RF power transistors in the Freescale portfolio, and higher power levels are on the company’s road map.
Figure 1: The MRFE6VP5300 delivers at least 300 W CW over a frequency range of 1.8 to 600 MHz.
Achieving power levels of 150-W CW delivered by the MRFE6VP5150 and 300-W CW by the MRFE6VP5300 requires packages to simultaneously accommodate 225°C die operating temperatures and high heat dissipation without compromising electrical performance, ruggedness, or reliability. Thermal resistance (measured in °C/W), is a measure of the effectiveness of a die/package combination in transferring heat into a heat sink or cold plate. As removing heat from the die becomes more difficult as thermal resistance increases, the latter must be reduced to the lowest possible level. Since the first overmolded plastic packages were introduced, Freescale has made consistent advances in this area, leading to the current ability to match and sometimes even exceed the thermal performance of ceramic-packaged LDMOS FETs while producing the same RF output power.
Reaching higher power levels
Freescale’s MRFE6VP5150 and MRFE6VP5300 are excellent examples of this capability. They are available in straight-lead and gull-wing packages, operate over a frequency range of 1.8 to 600 MHz, have high efficiency of at least 75%, and combine excellent overall performance with the level of ruggedness typically associated with ceramic packages. They will deliver their rated performance 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. A 65:1 VSWR is for practical purposes a direct short, as it represents a return loss value of zero.
Since its days as Motorola Semiconductor, the company has rated its products conservatively. They are tested at frequencies both below and above their specified operating ranges to ensure they will deliver their rated output levels with considerable margin. This is depicted in Figure 2a, which 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, the MRFE6VP5300 operating at the same frequency and with similar input signal characteristics delivers about 10 W with 17 dBm of drive and greater than 398 W with just over 31 dBm of drive.
Figure 2: The MRFE6VP5150 (a) (top) delivers up to 159 W (P1dB) at 230 MHz with 30 dBm of drive. The MRFE6VP5300 (b) (bottom) delivers up to 398 W (P1dB) with slightly less than 32 dBm of drive.
The MRFE6VP5300 and MRFE6VP5150 ARE in full production quantities. Freescale provides a wide range of support tools including broadband fixtures, models, and reference designs.