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Gauging Ruggedness In RF Power Transistors

Dec. 16, 2011
Some of the latest high-power RF transistors claim notable ruggedness ratings, allowing them to operate into severe load mismatches at full power without damage or even performance degradation.
Model MRFE6VP61K25HR6 is an enhancement-mode LDMOS transistor rated for 1250 pulsed and cw output power at frequencies to 600 MHz, with the capability of operating into load mismatches equivalent to a VSWR of 65.0:1.

High-power RF transistors are being made more rugged than ever before. Devices designed for extreme durability can withstand severe mismatches, even at full output levels. Several manufacturers now offer high-power silicon laterally diffused metal-oxide-semiconductor (LDMOS) transistors promoted as capable of withstanding operating into loads equivalent to a 65.0:1 VSWR. But are these transistors truly bulletproof? And for what type of applications does such ruggedness pay off? This report will look at a sampling of the latest ruggedized high-power LDMOS transistors, their electrical characteristics, and a comparison of how they are tested to determine their durability levels.

Some transistors, such as silicon bipolars, have been known to continue functioning even when some of the semiconductor cells have been damaged due to short circuits or load mismatches. Consequently, defining a device as a "rugged transistor" can get into murky waters. Tests of ruggedness for silicon LDMOS transistors usually refer to a device's capability to withstand severe load mismatch conditions at high output-power levels without performance degradation or device failure. when a transistor operates into a mismatched load, much of its output power is reflected back into the device where it must be dissipated in the transistor. But in comparing different rugged transistors, it is important to examine the conditions under which different device manufacturers arrive at their ruggedness findings, since test conditions may vary widely from manufacturer to manufacturer.

Transistor ruggedness testing typically involves three electrical parameters that may or may not be varied during the test: the input power, the DC bias supply to the transistor under test, and the load impedance presented to the device under test. In some cases, transistor manufacturers may used fixed (nominal) values of input power and device bias, and vary the load mismatch impedance. While such testing indicates that the device can survive operation under those specific conditions, it does not provide a great deal of insight into what the device will do when faced with real-world conditions, where all three parameters might be changing simultaneously.

For some devices, ruggedness testing consists of establishing baseline performance levels under nominal operating conditions, operating the device under stress conditions (such as a severe load mismatch), and then returning the device to its baseline operating conditions to test for degraded performance levels. A drop in output power or DC parameters that are degraded by 20% or more during these second baseline tests typically indicates a failed device.

VSWR is typically used as a figure of merit to describe the degree of load mismatch. with a load mismatch equal to a VSWR of 5.0:1, for example, about one-half of the transistor's output power is reflected back to the device. For a load mismatch equal to a VSWR of 20.0:1, about 80% of the transistor's output power is reflected back to the device and must be dissipated as heat. These VSWR values are among the many different mismatch levels that are used to characterize an RF power transistor as a "rugged" device.

To understand what is required to qualify an RF power transistor as a rugged device, it may help to compare a few examples. Requirements for ruggedness are often defined by an application. For long-range radar systems, such as UHF weather radars, icing on antennas can give rise to severe load mismatch conditions represented by load VSWRs of 10.0:1 or higher. As an example, STMicroelectronics supplies several N-channel enhancement-mode lateral MoSFeTs rated for operation into load mismatches as severe as a 20.0:1 VSWR. Model STEVAL-TDR016V1 is designed for VHF marine-band radio from 155 to 165 MHz and delivers 30 W CW output power from a +20-VDC supply. It offers 14.7 dB power gain with at least 60% efficiency. The firm offers several higher-frequency "rugged" transistors that can handle load mismatches equal to a 20.0:1 VSWR or higher, but at considerably lower output-power levels.

For several devices now on the market, the 20.0:1 VSWR rating might even appear conservative. For example, NXP Semiconductors offers at least one high-power device rated to withstand load mismatches equivalent to VSWRs of 65.0:1 or higher, while Freescale Semiconductor is shipping a family of devices capable of surviving load mismatches equal to VSWRs of 65.0:1 or higher. Both types of devices are silicon LDMOS transistors.

NXP Semiconductor unveiled its first rugged transistor, model BLF578XR, earlier this year at the IEEE MTT-S International Microwave Symposium 2011 (IMS 2011) in Baltimore, MD. This ruggedized version of the company's popular model BLF578 transistor, the Class AB model BLF578XR, is suitable for broadcast and ISM-band applications and can be matched for use from HF to 500 MHz. It provides peak pulsed output power of 1400 W at 225 MHz when operating with a 100-s pulse width and 20% pulse duty cycle. The device also achieves 24-dB power gain and 71% typical drain efficiency under those conditions, and features integrated electrostatic-discharge (ESD) protection. In addition, it is rated to provide 1200 W output power under CW conditions at 108 MHz.

To qualify this device as a "rugged" transistor, it is operated at 225 MHz with a load VSWR equivalent to 65.0:1 through all phases, with a drain-source voltage of +50 VDC, quiescent drain current of 40 mA, and 1200-W pulsed output power into the load. Obviously, in addition to the capabilities of the semiconductor die, the package is instrumental in allowing this device to survive such severe mismatch conditions. It is supplied in a balanced, flange-mount ceramic package. The device exhibits thermal resistance from junction to case at +125C of typically 0.14 K/W and transient thermal impedance from junction to case at +125C of typically 0.04 K/W, helping prevent any buildup of heat from the reflected output power during severe mismatch conditions.

Currently boasting the largest portfolio of "rugged" silicon LDMOS RF power transistors, Freescale Semiconductor is unique in qualifying its devices with the same pulsed and CW output-power ratings; all are tested to survive a load mismatch equivalent to a VSWR of 65.0:1 or higher without damage or performance degradation. This group of devices ranges from the highest-power model MRFE6VP61K25HR6 enhancement-mode LDMOS transistor (see figure), rated for 1250 W pulsed peak output power (100- s pulses at 20% duty cycle at 230 MHz) as well as 1250 W CW output power at 230 MHz, to the 125-W output power model MRF6VP8600HR6 which is aimed at broadcast analog and digital television transmitters from 470 to 860 MHz. In between, model MRFE6VP5600HR6 is an LDMOS power transistor that can deliver 600 W pulsed or CW output power from 1.8 to 600 MHz and model MRFE6VP6300HR6 is an enhancement-mode LDMOS power transistor capable of 300 W from 1.8 to 600 MHz.

As with the other Freescale "rugged" devices, these transistors are qualified for ruggedness at 230 MHz at a load mismatch equivalent to a VSWR of 65.0:1 into all phase angles, under pulsed conditions with 10-s pulses at 20% duty cycle. They are tested at a supply of +50 VDC with 100 mA quiescent drain current. In addition, they are evaluated with input power levels equal to twice their rated maximum input power levels, which they survive without damage or performance degradation.

The high-power transistors, which can be used single-ended or in push-pull configurations, are actually characterized for use with supplies from 30 to 50 V to allow a great deal of design flexibility. Packaging is also critical to these devices, since under adverse operating conditions most of the output power will be reflected back into the active device and surrounding package.

While such screening to determine whether an RF power transistor is truly rugged may seem excessive, some applicationsincluding RF ablation, CO2 lasers, semiconductor processing equipment, plasma generators, and magnetic resonance imaging (MRI) systemsmay require such ruggedness. In addition, high-power transmit applications, especially where antennas must weather a variety of different environmental stresses (such as snow and ice), can experience severe load mismatches as a normal matter of course. Having a set of transistors designed to "take the abuse" can greatly extend the operating lifetime of a solid-state transmitter.

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