PCB News - Ultra-high power GaN devices

The operating voltage of most GaN devices currently on the market is 28 or 50V. 28V operating voltage devices are more common, but there are also some manufacturers that can provide 50V operating voltage devices for higher power circuits. At present, the 50V working voltage is the limit that most GaN devices can reach under the condition of ensuring long-term and reliable working performance. However, a few companies have been developing GaN devices with higher operating voltages for higher power application scenarios, and are seeking better heat dissipation solutions in these high power application scenarios. The author contacted several companies and got some examples of their working with devices with a working voltage greater than 65V, and received information from Integra Technologies and Qorvo. This article summarizes these contents and outlines what the author sees in the market Some of the cooling solutions.
Develop high-voltage GaN to replace vacuum electronics
Many aerospace and defense radars, satellite communications, and industrial, PCB scientific and medical(ISM) systems require more reliable and rugged devices, with RF output power levels of several kilowatts. These systems have historically relied on vacuum electronics (VED), such as traveling wave tubes (TWT), to generate kilowatts of power. In order to solve the increasing complexity and cost of VED-based systems, the utilization rate of semiconductor-based solid-state power amplifiers (SSPA) has surpassed some low-frequency and low-power devices. At first, the semiconductor used was silicon LDMOS. Later, GaAs was also used. For the manufacture of solid-state power amplifiers, most of them now use GaN. However, the problems of the high-power market are still mainly solved by VED.
In radar applications, LDMOS technology has only made little progress in terms of high RF power due to its low frequency limitation. Although GaAs technology can work above 100GHz, its low thermal conductivity and operating voltage limit its output power level. In order to realize high-power devices, GaAs amplifiers need to connect multiple devices in parallel, so the cost of using multiple devices is reduced efficiency and increased cost. Today's 50V GaN/SiC technology can provide hundreds of watts of output power at high frequencies, and can provide the robustness and reliability required by radar systems, but the challenges do not stop there.

Since 2014, Integra Technologies has been conducting research and development in the high-voltage (HV) GaN/SiC field to further expand the technology to achieve the multi-kilowatt power level required for next-generation radar systems. As system designers need to reduce the total life cycle operating costs while increasing the complexity of radars, it is more urgent than ever to promote solid-state solutions using commercial manufacturing platforms. Integra's HV GaN/SiC has proven that the efficiency can exceed 80% for a 100V continuous wave with a power density of 10W/mm and a 150V pulse with a power density of 20W/mm.

High-voltage GaN technology
Transistor-level operation at higher voltages opens up new degrees of freedom for the design of high-power RF amplifiers. This technology can make a better trade-off between higher power density and higher impedance. This flexibility allows single-ended transistors up to 10kW to be matched to a 50Ω load, and then through proper harmonic tuning optimization, 80% efficiency can be achieved at UHF frequencies. Integra has successfully demonstrated this performance on higher frequency bands such as L-band and X-band.

One of the challenges for devices operating at high power densities of 10 to 20 W/mm is to conduct heat away from the active area of the semiconductor device. Integra solves this heat dissipation problem by combining Integra's thermal patents and HV GaN/SiC epitaxial materials, device design and packaging.
Advantages of high-voltage GaN
For high-power systems in the 100kW range, system designers can only use VED technology or 50V GaN/SiC SSPA. For solid-state designs, a large number of power devices are required to achieve the required target power of several kilowatts. Integra's HV GaN/SiC can achieve higher power. At the same time, the number of RF power transistors, system complexity and total cost can be significantly reduced.

For example, a 200kW system built with 50V, 1kW transistors will require more than 200 transistors to reach the target power, but this will cause complex power combinations and related efficiency losses. With 10kW HV GaN/SiC transistors, the same 200kW system only needs about 20 transistors. Significantly reduce the number of transistors and the complex power combination brought by these devices, while ensuring higher efficiency. This allows radar system engineers to design more competitive and lower-cost radars, which can also reduce operating costs during their lifetime.

HV GaN/SiC technology can utilize mass production grade SiC substrates instead of more expensive and limited supply of more unique substrate materials, such as diamond. The HV GaN process is built on mainstream commercial materials and manufacturing platforms to reduce costs.

Integra's HV GaN/SiC provides a solid-state alternative to VED, and its technology leverages mainstream commercial supply chains. By using Integra's patented thermal enhancement technology, the platform solves the heat dissipation problem caused by high power density operation, thereby developing a more reliable and powerful technology that can meet the needs of next-generation radars.
160W GaN PA overcomes the heat dissipation problem of SMT packaging
Innovations in GaN technology allow devices to operate at higher powers, voltages and frequencies-all of which are key elements of advanced radar and other broadband communications in the L-band. GaN has a higher power density than LDMOS or GaAs. However, as RF power levels increase, thermal performance must be optimized to keep the junction temperature of the semiconductor low enough to minimize power consumption and ensure a long transistor life. When transistors are implemented using surface mount technology (SMT), the PCB needs to be carefully designed to optimize heat dissipation performance.

A reference example of a power amplifier (PA) used to solve this high voltage and heat dissipation problem is designed with Qorvo QPD1013, which is a high-power, wide-bandwidth high electron mobility transistor (HEMT). The device adopts the industry standard 7.2mm*6.6mm surface mount, dual flat no-lead (DFN) package. Compared with the traditional cermet package, it can realize simpler PCB assembly.

QPD1013 uses Qorvo's 0.5μm GaN/SiC technology and can work at 65V. The PA provides higher efficiency and wider bandwidth, suitable for many application scenarios from DC to 2.7GHz, including military radar, land mobile or military radio communications. The working frequency band of the example PA covers 1.2 to 1.8 GHz, can provide 160W of radio frequency output power, and the efficiency is about 55%, as shown in Figure 5. Although the efficiency of the PA is impressive, the power dissipation still exceeds 100W, highlighting the need for effective heat dissipation solutions.

In order to optimize the heat dissipation performance, the reference design PA utilizes "copper coin" technology. Copper coins are solid copper sheets or strips embedded in the PCB during the manufacturing process to allow efficient heat transfer from the transistor to the PCB carrier. Although the technology of filling vias with copper is very common and the most economical, copper coin technology can provide better heat transfer performance.

As shown in Figure 6, copper coins have a slight impact on the RF performance of the amplifier, which must be considered in the design. Although the copper coin improves the thermal resistance, care must be taken to ensure that the surface of the PCB is flat and that there is good contact between the copper coin and the DFN ground pad. Any air gaps or solder voids will weaken the inherent advantages of the copper coin method.