Freescale drives plastic-packaged LDMOS RF power transistors to 300-W CW

December 08, 2014 // By Freescale Semiconductor
RF power amplifiers designed for wireless base stations face huge challenges to accommodate the demands of high efficiency and linearity and lower power consumption. However, there are other applications that add another layer of complexity unparalleled in the wireless world. These applications, including FM and digital TV broadcast, aerospace and defense, land mobile radio, CO2 lasers, and industrial heating and sealing systems require that RF power transistors be stressed in astonishing ways. Fortunately, manufacturers like Freescale have applied technologies, architectures, and materials to the challenge to make them if not “bulletproof” then extraordinarily robust.

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.

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