Each device 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 in these devices helps to handle higher power levels when designing circuits and systems.
How much power it can handle is an inevitable question for most devices in a transmitter, and is often asked for passive components such as filters, couplers, and antennas. But with the power levels of microwave vacuum tubes (such as traveling wave tubes (TWT)) and core active devices such as silicon laterally diffused metal oxide semiconductor (LDMOS) transistors and gallium nitride (GaN) field effect transistors (FETs) Increasingly, when installed in well-designed amplifier circuits, they will also be limited by the power handling capabilities of devices such as connectors and even printed circuit board (PCB) materials. Understanding the limitations of the different components that make up a high-power device or system can help answer this long-standing problem.
The transmitter requires power to be within limits. In general, these limits are governed by government agencies, such as those established by the Federal Communications Commission (FCC). But in "unregulated" systems, such as radar and electronic warfare (EW) platforms, the limitations are mainly from the electronics in the system. Each device 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 in these devices helps to handle higher power levels when designing circuits and systems.
When current flows through the circuit, some of the electrical energy is converted into thermal energy. Circuits that handle large enough currents will heat up—especially where the resistance is high, such as discrete resistors. The basic idea of ​​setting the power limit for a circuit or system is to use a low operating temperature to prevent any temperature rise that could damage the device or material in the circuit or system, such as dielectric materials used in printed circuit boards. Interruptions in current/heat flow through the circuit (such as loose or soldered connectors) can also cause thermal discontinuities or hot spots, which can cause damage or reliability issues. Temperature effects, including differences in thermal expansion coefficients (CTE) between different materials, can also cause reliability problems in high frequency circuits and systems.
Heat is always flowing from a higher temperature zone to a lower temperature zone. This principle can be used to transfer heat generated by high power circuits away from heat sources such as transistors or TWT. Of course, the heat dissipation path from the heat source should include a destination consisting of materials that can dredge or dissipate heat, such as a metal ground plane or heat sink. In any case, the thermal management of any circuit or system can only be optimally implemented at the outset of the design cycle.
Thermal conductivity is generally used to compare the properties of materials used to manage the heat of the RF/microwave circuit. This indicator is measured in terms of power (W/mK) applied per meter of material per meter (in Kelvin). Perhaps the most important factor for these materials for any high frequency circuit is the PCB stack, which typically has a lower thermal conductivity. For example, FR4 laminates, which are often used in low-cost high-frequency circuits, have a typical thermal conductivity of only 0.25 W/mK.
In contrast, copper (deposited on FR4 as a ground plane or circuit trace) has a thermal conductivity of 355 W/mK. Copper has a large heat flow capacity, while FR4 has an almost negligible thermal conductivity. To prevent hot spots from occurring on the copper transmission line, a high thermal conductivity path must be provided for the area 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 vias (PTH) can be used to connect from circuit traces to ground planes.
Of course, the power handling capability of a PCB is a function of many factors, including conductor width, ground plane spacing, and material dissipation factor (loss). In addition, the dielectric constant of the material will determine the circuit size at a given ideal characteristic impedance, such as 50 ohms, so materials with higher dielectric constant values ​​allow circuit designers to reduce the size of their RF/microwave circuits. That is to say, these shorter metal traces mean that PCB dielectric materials with higher thermal conductivity are needed to achieve proper thermal management.
At a given application power level, the temperature rise of a circuit material with a higher thermal conductivity is lower than that of a lower thermal conductivity material. Unfortunately, FR4 is no different from many other PCB materials with low thermal conductivity. However, the heat treatment capability and power handling capability of the circuit can be improved by specifying a PCB material that has a higher thermal conductivity than at least FR4.
For example, although the thermal conductivity level of copper has not yet reached, Rogers' several PCB materials can provide much higher thermal conductivity than FR4. The thermal conductivity of the RO4350B material is 0.62W/mK, while the company's RO4360 laminate thermal conductivity can reach 0.80W/mK. Although there is no significant improvement, it does have two to three times the heat/power capability improvement compared to the FR4 stack, enabling efficient dissipation of heat generated by RF/microwave circuits. These two materials are particularly well suited for amplifier applications with built-in heat sources (transistors), all of which have low coefficient of thermal expansion (CTE) values, thus minimizing dimensional changes with temperature.
Many commercial computer-aided engineering (CAE) software design packages are capable of modeling the heat flow through an RF/microwave circuit, including the thermal conductivity of the PCB, at a given application power level and given circuit parameter settings. These software design packages include a number of separate programs, such as SonnetSoftware's Electromagnetic Simulation (EM) tool, Fluent's IcePak software, ANSYS' TASPCB software, and Flomerics' Flotherm software. They also include a number of design software tool suites such as Agilent's Advanced Design System (ADS), ComputerSimulaTIonTechnology (CST)'s CSTMicrowaveStudio, and AWR's MicrowaveOffice.
These software tools can even be used to study the effects of different working environments on the power handling capabilities of RF/microwave circuits, such as arcs that may occur at high enough power levels at low atmospheric pressure or high altitudes. These programs also enhance the power handling of discrete RF/microwave devices by modeling the field distribution of energy as it flows through devices such as couplers or filters.
Of course, PCB material is not the only factor that affects heat flow in RF/microwave circuits or systems. Cables and connectors are also well known for limiting power/heat in high frequency systems. In a coaxial assembly, the connector can typically handle more heat/power than the cable it is connected to, while different connectors have different power ratings. For example, the N-type connector has a slightly higher power rating than an SMA connector with a smaller size (and higher frequency range). The average power and peak power of the cable and connector are rated and the peak power is equal to V2/Z, where Z is the characteristic impedance and V is the peak voltage. A simple estimate of 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. Some companies, such as TImesMicrowaveSystems, offer free downloadable computing programs that can be used to predict the power handling capabilities of their own different types of coaxial cable.
It is worth noting that this is an extremely simplistic treatment of complex topics. It also does not cover topics such as material breakdown voltage, PCB dissipation factor (loss factor) affecting the power handling capability of the circuit, the effect on the thermal expansion coefficient (CTE) of the PCB material, and the difference in heating between the continuous wave and the pulsed energy source.
Within devices, circuits, and systems, there are many complex phenomena that can affect power handling capabilities, including those with "open" and "off" states, which may have different RF/microwave power capabilities. In addition to software programs, tools for thermal analysis can also provide thermal imaging capabilities based on infrared (IR) technology that can be used to safely study heat buildup in devices, circuits, and systems.
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