Designing circuits and systems for high-power applications requires a good deal of imagination. Knowing the limits of the different parts that make up a high-power component or system can aid the process enormously.
How much power can it handle? That is a question inevitably asked about most components meant for transmitters—and usually for passive components, such as filters, couplers, and antennas. But the increasing power levels of microwave vacuum tubes, like traveling-wave tubes (TWTs), and hearty active devices, such as silicon laterally diffused metal-oxide-semiconductor (LDMOS) transistors and gallium-nitride (GaN) field-effect transistors (FETs), when mounted in well-conceived amplifier circuits, will also be limited by the power-handling capabilities of such components as the connectors and even the printed-circuit-board (PCB) materials. Knowing the limits of the different parts that make up a high-power component or system can help to answer that enduring question.
Transmitters require power, within limits. Typically, those limits are set by governing agencies, such as the United States’ Federal Communications Commission (FCC) for communications standards. But in the case of “ungoverned” systems, such as radar and electronic-warfare (EW) platforms, the limits will primarily arise from the electronic components in the system. Every component has a maximum power limit, whether it is an active component, such as an amplifier, or a passive component, such as a cable or filter. Understanding how power flows through these components can help when designing circuits and systems to handle higher power levels.
When current flows through an electric circuit, part of the electrical energy is converted into heat energy. A circuit handling sufficient current will generate heat—particularly at areas where resistance is high, such as a discrete resistor. The basic idea behind setting power limits on a circuit or system is to prevent any rise in temperature that can cause damage to components or materials in the circuit or system with the low operating temperatures, such as the dielectric materials used in the printed-circuit boards (PCBs). Interruptions in the current/heat flow through the circuit, such as a loose or poorly soldered connector, can also result in thermal discontinuities or hot spots that can result in damage or reliability problems. Temperature effects, including differences in the coefficient of thermal expansion (CTE) among materials, can also cause reliability problems in high-frequency circuits and systems.
Heat will always flow from an area at a higher temperature to an area at a lower temperature, and this principle can be used to conduct heat produced in a high-power circuit away from a heat-generating source, such as a transistor or TWT. Of course, the thermal path from a heat source should include a destination composed of a material capable of channeling or dissipating the heat, such as the metal ground plane or a heat sink. To that end, thermal management in any circuit or system is best accomplished when it is included at the beginning of a design cycle.
Materials for managing heat in RF/microwave circuits are generally compared in terms of their thermal conductivity, which is measured in terms of applied power per degree of temperature (in Kelvin) per meter of material (W/mK). Perhaps the most important of these materials for any high-frequency circuit is the PCB laminate, which tends to have low thermal conductivity. FR4 laminates, for example, are often used in low-cost, high-frequency circuits, even though they exhibit a typical thermal conductivity of only 0.25 W/mK.
In contrast, copper (deposited on the FR4 as a ground plane or as circuit traces) has thermal conductivity of 355 W/mK. Copper has a large capacity for heat flow, while FR4 has almost negligible thermal conductivity. To prevent hot spots from building in the copper transmission lines, high-thermal-conductivity paths must be provides from the transmission lines to the ground plane, a heat sink, or some other area of high thermal conductivity. Thinner PCB materials allow shorter paths to the ground plane, using plated through holes (PTHs) when it is possible to connect from a circuit trace to the ground plane.
Of course, the power-handling capabilities of a PCB are a function of numerous factors, including the width of the conductors, the ground-plane spacing, and the dissipation factor (loss) of the material. In addition, the dielectric constant of the material will determine the dimensions of the circuitry for a given desired characteristic impedance, such as 50 Ω, so that materials with higher dielectric constant values will allow circuit designers to reduce the size of their RF/microwave circuits. That said, these smaller metal traces will imply the need for a PCB dielectric material with higher thermal conductivity for proper thermal management.
Circuit materials with higher values of thermal conductivity will exhibit lower rises in temperature above the ambient temperature for a given applied power level than materials with lower values of thermal conductivity. Unfortunately, FR4 is not unlike many other PCB materials in having a low value of thermal conductivity. However, the thermal- and power-handling capabilities of a circuit can be improved by specifying a PCB material with somewhat higher values of thermal conductivity, at least compared to FR4.
For example, although not in the range of copper, several PCB materials from Rogers Corp. (www.rogerscorp.com) offer considerably higher thermal conductivity than FR4. RO4350B material has a thermal conductivity of 0.62 W/mK, while the firm’s RO4360 laminate has a thermal conductivity of 0.80 W/mK. Although not significant increases, they do represent two and three times the thermal/power capacities, respectively, of FR4 laminates for effective dissipation of RF/microwave circuit-generated heat. These two materials, which are ideally suited for amplifier application with their built-in thermal sources (transistors), both exhibit low values of coefficient of thermal expansion (CTE), allowing for minimal dimensional changes with temperature.
Many commercial computer-aided-engineering (CAE) software design packages include capabilities for modeling the thermal flow through an RF/microwave circuit for a given applied power level and given set of circuit parameters, including PCB thermal conductivity. These packages include individual programs, such as the electromagnetic (EM) simulation tools from Sonnet Software (www.sonnetsoftware.com), IcePak Software by Fluent (www.fluent.com), the TAS PCB software by ANSYS (www.ansys.com), and Flotherm software by Flomerics (www.flomerics.com). They also encompass design suites of software tools, such as the Advanced Design System (ADS) from Agilent Technologies (www.agilent.com), CST Microwave Studio® from Computer Simulation Technology (CST; www.cst.com), and AWR Microwave Office™ from AWR Corp (www.awrcorp.com).
These software tools can even be used to study the impact of different operating environments on RF/microwave circuit power-handling capabilities, such as the arcing that can occur at sufficient power levels at low atmospheric pressure or at the high altitudes found in aircraft. Such programs can also improve the power-handling capabilities of discrete RF/microwave components by modeling the field distribution of the energy through a component, such as a coupler or a filter.
Of course, PCB materials are not the only culprits when it comes to managing the thermal flow through an RF/microwave circuit or system. Cables and connectors are well known for their power/heat limitations in high-frequency systems. In a coaxial assembly, a connector can usually handle more heat/power than the cable to which it is attached, and different connectors have different power ratings. For example, a Type-N connector has a somewhat higher power rating than an SMA connector with its smaller dimensions (and higher-frequency range). Cables and connectors are rated in terms of average and peak power capabilities, with peak power equal to V2/Z, where Z is the characteristic impedance and V is the peak voltage. A simple estimate for average power rating is to multiply the peak power rating of a cable assembly by the duty cycle.
Many cable suppliers such as Astrolab (www.astrolab.com) have developed proprietary computer programs to calculate the power-handling capabilities of their coaxial cable assemblies. Some, such as Times Microwave Systems (www.timesmicrowave.com), offer free downloadable calculator programs to predict the power-handling capability of different types of their own coaxial cables.
Note that this has been a greatly simplified treatment of a complex topic. It has not touched on such topics as breakdown voltages in materials, how a PCB’s dissipation factor (loss factor) can impact a circuit’s power-handling capabilities, the effects on performance of a PCB material’s coefficient of thermal expansion (CTE), or the differences in heating effects between CW and pulsed energy sources.
Within components, circuits, and systems, there are many complex phenomena that can affect power-handling capabilities, including components such as switches that have “open” and “closed” states that may have different RF/microwave power capacities. In addition to the software programs, tools available for thermal analysis offer impressive imaging capabilities based on infrared (IR) technology, and can be used to safely study thermal buildup in components, circuits, and systems. These thermal imagers are available from various sources, including FLIR Systems (www.flir.com) and Fluke (www.fluke.com).