Detailed explanation of component limit power loss and dispersion

Every component has a maximum power limit, whether it is an active device (such as an amplifier) ​​or a passive device (such as a cable or filter). Understanding how power flows through these components can help you handle higher power levels when designing circuits and systems.

How much power can it handle? This is an inevitable question asked of most components in a transmitter, and it is usually asked of passive components such as filters, couplers and antennas. But as the power levels of microwave vacuum tubes such as traveling wave tubes (TWTs) and core active devices such as silicon laterally diffused metal oxide semiconductor (LDMOS) transistors and gallium nitride (GaN) field effect transistors (FETs) increase, they will also be limited by the power handling capabilities of components such as connectors and even printed circuit board (PCB) materials when installed in carefully designed amplifier circuits. Understanding the limitations of the different parts that make up a high-power component or system can help answer this age-old question.

Transmitters are required to stay within power limits. Typically, these limits are set by government agencies, such as the communications standards set by the Federal Communications Commission (FCC) in the United States. But in "unregulated" systems, such as radar and electronic warfare (EW) platforms, the limits are primarily imposed by the electronic components in the system. Each component has a maximum power limit, whether it is an active device, such as an amplifier, or a passive device, such as a cable or filter. Understanding how power flows through these components can help you design circuits and systems to handle higher power levels.

When current flows through a circuit, some of the electrical energy is converted into heat. A circuit that handles enough current will heat up—especially where resistance is high, such as discrete resistors. The basic idea behind setting a power limit on a circuit or system is to take advantage of low operating temperatures to prevent any temperature rise that could damage components or materials in the circuit or system, such as dielectric materials used in printed circuit boards. Disruptions in the flow of current/heat through the circuit (such as loose or poorly soldered connectors) can also cause thermal discontinuities or hot spots that can cause damage or reliability issues. Temperature effects, including differences in the coefficient of thermal expansion (CTE) between different materials, can also cause reliability issues in high-frequency circuits and systems.

Heat always flows from areas of higher temperature to areas of lower temperature, and this principle can be used to transfer heat generated by high-power circuits away from the heat source, such as a transistor or TWT. Of course, the heat dissipation path from the heat source should include a destination composed of materials that can channel or dissipate the heat, such as a metal ground plane or a heat sink. Regardless, thermal management of any circuit or system can only be best achieved if it is considered at the beginning of the design cycle.

Thermal conductivity is commonly used to compare the performance of materials used to manage heat in RF/microwave circuits. This metric is measured in W/mK per meter of material per degree (Kelvin). Perhaps the most important factor for these materials for any high-frequency circuit is the PCB stackup, which generally has low thermal conductivity. For example, FR4 stackup materials, which are often used in low-cost high-frequency circuits, have a typical thermal conductivity of only 0.25W/mK.

In contrast, copper (deposited on FR4, either as a ground plane or circuit traces) has a thermal conductivity of 355W/mK. Copper has a large capacity for heat flow, while FR4 has almost negligible thermal conductivity. To prevent hot spots from developing on copper transmission lines, a high thermal conductivity path must be provided from the transmission line to the ground plane, heat sink, or some other high thermal conductivity area. Thinner PCB materials allow for a shorter path to the ground plane because plated through holes (PTHs) can be used to connect from the circuit traces to the ground plane.

Of course, the power handling capability of a PCB is a function of many factors, including conductor width, ground plane spacing, and the material's dissipation factor (losses). In addition, the material's dielectric constant will determine the size of the circuit given an ideal characteristic impedance, such as 50Ω, so materials with higher dielectric constant values ​​allow circuit designers to reduce the size of their RF/microwave circuits. That said, these shorter metal traces mean that PCB dielectric materials with higher thermal conductivity are needed to achieve proper thermal management.

At a given applied power level, circuit materials with higher thermal conductivity will have a lower temperature rise than materials with lower thermal conductivity. Unfortunately, FR4 is no different than many other PCB materials that have low thermal conductivity. However, the heat handling and power handling capabilities of a circuit can be improved by specifying a PCB material with at least a higher thermal conductivity than FR4.

For example, although not yet at the level of copper thermal conductivity, several Rogers PCB materials can provide much higher thermal conductivity than FR4. The thermal conductivity of RO4350B material is 0.62W/mK, while the company's RO4360 laminate can reach 0.80W/mK. Although not a dramatic improvement, it does provide a two to three times improvement in thermal/power capabilities compared to FR4 laminates, which can effectively dissipate the heat generated by RF/microwave circuits. Both materials are particularly suitable for amplifier applications with built-in heat sources (transistors), and they both have low coefficient of thermal expansion (CTE) values, thereby minimizing dimensional changes with temperature.

Many commercial computer-aided engineering (CAE) software packages are capable of modeling the flow of heat through RF/microwave circuits, including the thermal conductivity of the PCB, at a given applied power level and given circuit parameter settings. These packages include many individual programs, such as Sonnet Software's electromagnetic simulation (EM) tools, Fluent's IcePak software, ANSYS's TAS PCB software, and Flomerics' Flotherm software. They also include many design software tool suites, such as Agilent Technologies' Advanced Design System (ADS), Computer Simulation Technology's (CST) CST Microwave Studio, and AWR's Microwave Office.

These software tools can even be used to study the effects of different operating environments on the power handling capabilities of RF/microwave circuits, such as the arcing that can occur at sufficiently high power levels in the low atmospheric pressure or high altitude environments of an aircraft. These programs can also improve the power handling capabilities of discrete RF/microwave components by modeling the field distribution when energy flows through a component such as a coupler or filter.

Of course, PCB materials are not the only factor that affects the flow of heat in an RF/microwave circuit or system. Cables and connectors are also well known for their power/heat limitations in high-frequency systems. In coaxial assemblies, the connector can usually handle more heat/power than the cable it connects, and different connectors have different power ratings. For example, the power rating of an N-type connector is slightly higher than that of an SMA connector with a smaller size (and higher frequency range). Cables and connectors are rated for both average power and peak power, with peak power equal to V2/Z, where Z is the characteristic impedance and V is the peak voltage. A simple way to estimate the average power rating is to multiply the peak power rating of the cable assembly by the duty cycle.

Many cable suppliers, such as Astrolab, have developed specialized calculation programs to calculate the power handling capabilities of their coaxial cable assemblies, while some companies, such as Times Microwave Systems, offer free downloadable calculation programs that can be used to predict the power handling capabilities of their own coaxial cable types.

It is important to note that this is an extremely simplistic treatment of a complex subject. It does not touch on topics such as material breakdown voltage, how the PCB dissipation factor (loss factor) affects the power handling capabilities of the circuit, the impact on the coefficient of thermal expansion (CTE) properties of the PCB material, and the difference in heating effects between continuous wave and pulsed energy sources.

There are many complex phenomena within components, circuits, and systems that can affect power handling capabilities, including components that may have different RF/microwave power capabilities, such as switches with "on" and "off" states. In addition to software programs, tools available for thermal analysis can also provide thermal imaging capabilities based on infrared (IR) technology, which can be used to safely study heat buildup in components, circuits, and systems.