Advanced Rugged LDMOS Transistors

Although GaN transistors have been popular in recent years for broadband and high-frequency applications, LDMOS is still the leading technology for high-power amplifier designs from HF to UHF. The sweet spot for rugged LDMOS is below 450MHz, and with the correct thermal design, rugged devices will withstand very high levels of VSWR up to 65:1. Initially designed for broadcast and industrial applications in the mid-nineties, we have seen power levels and breakdown voltages improve with successive generations.

Design Limitations

As generations of LDMOS advance (Ampleon is now working on Generation 12), I am often asked about even higher-power rugged transistors. However, there is a limit to power that can be produced at a given drain voltage. A designer can add LDMOS’ fingers’ to the layout (Figure 1), but there is a trade-off. More fingers increase the die size, and physically, there is a limitation in the package size. Reducing the distance between the fingers is possible, but it will increase parasitic capacitance.  

  Another option is to add more power to the LDMOS fingers, but the output impedance of the transistor lowers as you increase the power. This makes the transistor more challenging to use, harder to match with a high-Q, and makes broad-bandwidth designs harder.  

  Looking at our suppliers’ products, I believe the limit of around 1.5KW has been reached for 50 Volt devices. To increase power levels further, designers have opted to increase the drain voltage. Output Impedance RL = Drain voltage V2 / Output Power P, so by increasing the voltage, designers can increase the output power while keeping the output impedance at reasonable levels. There are now a few 65V transistors available from both Ampleon and NXP. Qorvo has also opted for 65V in GaN for some of their high-power parts targeting L-band Radar and avionics applications. 65V is not for everyone, though. Power supplies can be expensive compared to 48-50V versions, and some safety regulations also come into effect above 50V. Higher powers also provide more headaches in getting excess heat out of the PA. Other components, such as output-matching capacitors, must withstand higher voltages and dissipate higher temperatures. I have seen capacitors float off PCBs when the solder has melted, and PCB tracks sometimes need wires soldering to the surface to carry sufficient drain current. It isn’t always possible to make a track wide to handle high currents because it also lowers the impedance, affecting the RF performance of that track, and we are often limited in space on the PCB.

New Advanced Generation of Rugged LDMOS

Ampleon is in the process of releasing a new generation of rugged transistors. These are called Advanced Rugged Technology (ART) transistors. So far, there are 2KW, 1.6KW, 700W, 150W and 35W devices. We have also just started sampling a 450W device that will be available for general release in 2023. ART2K0FE is the highest power transistor at 2KW with a 65V supply and many customers consider water-cooled heat sinks at these power levels. The next transistor in the series, though, is more interesting. The ART1K6FH (Figure 3), offers a natural progression (and upgrade) from older devices like the BLF188XR and NXP’s MRFE6VP1K25.  

  Upon first glance of the datasheet, it may seem like the ART1K6FH has a little more power, with a 1.6KW headline , but take a closer look and you will see this device is qualified at up to 55V to provide this power level. At 50V, it is designed to be a replacement for the 50V BLF188XR with 1.4KW output power. In fact, customers are telling us that it “pretty much drops into the same circuit”. 65:1 VSWR is the highest mismatch that can be accurately measured during the production test of these devices, but the breakdown voltage is a handy parameter to consider ruggedness. I am reassured to see it is even higher than its predecessors. If we are short on power, as already mentioned, the ART1K6FH is qualified to 55V, and most 50V power supplies can be adjusted upwards by a few volts at least, so it is possible to achieve a slightly higher power output than the older devices without changing the PA power supply.

LDMOS in High-Power Amplifiers

The Gemini 2-1.2K from LinearAmp/The DX Shop uses the ART1K6 for high power in the 144MHz VHF band (Figure 4). This is an excellent example of a product that can use the ART1K6FH in place of the BLF188XR and the product page provides an interesting power graph comparing the two rugged devices.  

  Other transistors in the ART range fill many gaps in power levels. Ruggedness is essential for industrial applications such as CO2 lasers or Plasma generators, but I’m sure I’m not alone in saying that ruggedness is always welcome – even in communications amplifiers at lower powers. At the least, ruggedness provides peace of mind if there is an error on the output of a PA. Have more questions on integrating advanced LDMOS transistors into your design? Our experts at RFMW are available to assist. For more information on developing with next-generation LDMOS technology, contact RFMW today! About Tim Daniels

Tim Daniels is a Supplier Business Manager at RFMW specializing in transistor and MMIC technologies. With over 20 years of RF industry experience, Tim is a chartered engineer that holds a master’s degree in electronics innovation and a bachelor’s degree in electronics with communications. His expertise enables him to provide comprehensive design recommendations and technical support for RFMW’s customers. Tim enjoys working with engineers from around the globe and is eager to find innovative solutions to new challenges.