Why Plug-and-Play, GaN in Plastic Power Modules will Revolutionize Next Generation Radar Systems
By Damian McCann, Director of Engineering, Long Beach Design Center, MACOM Technology Solutions
The Importance of Size, Weight and Power (SWaP) in Future RADAR Components
RADAR systems typically work by connecting an RF power transmit line-up to an antenna that emits a high-frequency, pulsed signal. To be resistant to jamming, the RADAR needs to be able to change signal frequency and pulse pattern at will. To scan an area of interest, the RADAR signal needs to point in different directions. In the past this was done mechanically, but today the signal is usually manipulated electronically by splitting the RF signal into multiple paths, which are then electronically delayed to different antenna elements, typically spaced in an array pattern. Using electronic steering methods, the beam, formed by the combination of signals from each antenna element, can then be swept as needed without any mechanical movement. This type of system is called an Active Electrically Scanned Array or AESA. The total power of the final focused beam can be any combination of antenna elements, of which there can be hundreds or thousands depending on the frequency, and typically will make up many KWs.
Today’s system designers are under continuous pressure to achieve aggressive size, weight and power (SWaP) profiles that can help ensure a system advantage. But with so many elements in the final array, achieving higher power and smaller, lighter components using conventional silicon and GaAs-based power transistors is an ever mounting challenge. For these devices, limitations in component power density, breakdown voltage and thermal reliability introduce increasingly problematic performance constraints, with significant implications for system reliability, ruggedness and functionality to meet new mission objectives. Never mind that many are packaged in traditional metal/ceramic packages, which are large and impractical.
The Benefits of 50V GaN in New RADAR Systems
Not everyone recognizes just how important high-voltage operation is to a RADAR system where the RF power is transmitted in a pulsed mode. To facilitate pulsed RF power, a burst of energy is required that is derived by rapidly discharging a previously charged capacitor. As the energy stored in a capacitor is equal to ½*CV2, a higher voltage will give a considerably greater pulse power. As the pulse duration increases, so does the need for energy storage, which, in turn, influences the size of the capacitor.
Figure 2 shows the energy storage derived from a 28V supply using a very typical 10000uF cap — it is roughly 4 Joules. Operating at a voltage of 50V, the number is increased approximately 3 times to 12.5 joules as shown in Figure 3.
Practically speaking, high-value, high-voltage caps are not small, nor are they inexpensive. Figure 4 is a picture of an eval board with an associated charge storage cap showing the physical size of these caps. Reducing overall size and weight is an important consideration.
Another key aspect is that the energy storage needs to increase as the pulse duration increases and this often can show up as a pulse droop. As pulse durations increase, so will the storage cap size, which makes higher operating voltage even more important in long-pulse duration applications. Pulse rise and fall times also benefit from the lower current draw.
Given the importance of high-voltage operation, it is no wonder that GaN is fast becoming the semiconductor technology of choice to achieve the required power performance for RADAR systems operating across a wide range of frequencies. GaN power devices have significantly higher gain, power density, breakdown voltage and thermal performance when compared to Si Bipolar and LDMOS.
Reliable 50V operation in GaN is made possible by having a typical breakdown voltage of 340V using 0.5um field plate GaN technology. This high breakdown voltage results in unprecedented levels of ruggedness and superior reliability under various load mismatch conditions.
The increased transistor power density is another key benefit of GaN, but again the power density is directly related to the voltage. In general terms, if you change from 28V to 50V you roughly double the power density of the transistor used. In terms of cost, the designer reaps the benefits of reduced board area and weight generating reduced system cost due to the doubled power density.
This increase in power density from 28V to 50V also results in a smaller device size for a given power, which results in lower inherent device capacitance. Lower capacitance leads to lower Q, and, as a rule, ½*Q = 2*BW. This is a key benefit in new, broader-band RADAR systems by allowing wider frequency sweeping that makes a system even more resistant to jamming.
Matching impedances are also made higher by an increased voltage – driven simply by the fact that the load impedance is defined as Load = Vcc2 / (2 * Pout) – thus making power matching smaller and more compact.
Surface Mount GaN in Plastic Packaging
While it’s clear that high-voltage GaN semiconductor technology sets a new standard in power, efficiency and bandwidth performance to enable new multi-function RADAR systems, meaningful forward progress on reducing size and weight hinges on our ability to develop and manufacture - smaller, wider bandwidth, lighter and functionally more flexible power transistors that promote multifunction integration. What’s needed in all of these cases is a new approach to power transistor design and packaging technology that provides greater overall power performance in a smaller form factor with the greatest possible ease of assembly. What’s needed is GaN in Plastic.
Scaling to high-pulse power levels of 100W, GaN in Plastic transistors defy the power, size and weight limitations of competing ceramic-packaged offerings to enable a new generation of high-performance, ultra-compact military and civilian RADAR systems. As a result, customers can use these products to provide new capabilities and take advantage of the total system cost reductions associated with reductions in size, weight, and cooling requirements.
With packaging as small as 3 x 6 mm dual-flat no leads (DFN) and standard small outline transistor (SOT-89) packages, GaN-in-Plastic transistors perform at 50V drain bias resulting in outstanding power density and performance, higher efficiency, and smaller impedance matching circuits due to improved device parasitics.
GaN in Plastic-based power transistors are also extremely lightweight compared to the existing ceramic-packaged offerings that are currently available. Measured in aggregate across the hundreds of power amplifiers within a typical modern multi-element radar system, this can reduce overall system weight considerably. The resulting weight reduction ensures greater ease of movement for mobile radar systems.
High performing and innovative GaN in space-saving Plastic enables RADAR system designers to take full advantage of GaN technology and achieve new levels of power density while reducing system size and weight significantly. Utilizing sophisticated packaging and thermal management techniques to maximize design efficiency and component reliability enables designers to overcome challenging development hurdles and pioneer a new generation of high-performance, rugged RADAR systems.
Ultra-Compact SMT RADAR Power Amplifier Module
The GaN in Plastic approach also allows for ultra-small, fully matched, integrated module solutions. The next evolutionary step is to develop high gain power modules based upon the GaN in Plastic power transistors for the L- and S-Band radar markets based around a common platform. These modules would need to be fully matched with two stages of high gain and realized using Surface Mount Technology (SMT) assembly on a very small RF board.
The module should use robust laminate technology with low gold plating, thermally enhanced via technology.
The small size of GaN in Plastic devices allows for full SMT assembly using standard reflow techniques, rather than Chip-on-board (COB). Combining “Known Good Die” (KGD) assembly with strict adherence to “best commercial IPC layout rule” practices will result in excellent assembly and RF yield. An SMT GaN power module is shown in Figure 6 demonstrating very compact, lumped element matching to achieve full 50 Ohm matching across the band. The module can be easily plugged into a radar system front-end.
Key features include:
• 100W pulsed power
• 50V operation
• >60 % power added efficiency
• Flat performance over large range of input power
• Surface mount assembly
• Common module outline – plug and play
The Creation of a Versatile Common RADAR Power Amplifier Platform
Both L-Band and S-band modules share what is referred to as a “Common Platform.” This means that each amplifier module looks and feels the same and shares the same Pin configuration, mechanical outline (Figure 7) and footprint (Figure 9).
The advantage of this commonality is that operations managers can take either matched module and use it seamlessly in all their RADAR platforms without needing additional optimization. This makes the modules resemble what is often referred to as a COTS (commercial-off-the-shelf) device.
As with any small power device, questions often arise with regard to the thermal behavior of a device once mounted to a package and then mounted to a customer board. Using our transient thermal imaging camera, we have measured the thermal behavior of the GaN die when mounted on a module in situ, Figure 10.
In addition to the thermal imaging results in Figure 10, which confirm the thermal stability of the module mounted to an evaluation module, Figure 11 also shows the pulse droop exhibited by the module under the extreme pulse condition of 3mS pulse duration and 10% duty factor. The pulse droop is measured from the 10% point of the pulse to the 90% point of the pulse.
Performance data for output power and Power Added Efficiency (PAE) (see Figure 12a and 12b), shows stable performance over a wide range of temperatures, making the module a very versatile solution under different environmental conditions.
Breaking the Boundaries of SWaP
The ability to offer a full SMT solution using GaN combines the best of advanced military power technologies and high-volume commercial manufacturing expertise. With this combination, it is possible to break through the current boundaries of SWaP and realize a new level of performance and capability in future RADAR systems.
By making use of existing SMT technology, capacity and best commercial practices to create highly manufacturable, cost-effective modules that can be used in a “True SMT” assembly, while also reducing the size, and weight of power solutions of power solutions, RADAR system designers will be able to achieve new levels of multi-function performance. These modules, combined with additional RF components to form complete T/R modules in AESA RADAR systems, will lead the way towards a truly modular RF solution for future AESA RADAR and multi-function systems.
MACOM Technology Solutions
this article to a friend!