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.
多 How much power can it handle? This is an unavoidable question for most devices in the transmitter, and it usually asks passive devices 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 lateral diffusion 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 question.
The transmitter requires power within the limits. Generally, these limits are set by government agencies, such as communications standards established by the US Federal Communications Commission (FCC). But in "unregulated" systems, such as radar and electronic warfare (EW) platforms, the restrictions come 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, part of the electrical energy will be converted into thermal energy. Circuits that handle large enough currents will generate heat-especially in areas with high resistance, such as discrete resistors. The basic idea of setting a power limit for a circuit or system is to use a low operating temperature to prevent any temperature rise that may damage the devices or materials in the circuit or system, such as the dielectric materials used in printed circuit boards. Interruptions in the flow of current / heat 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 the coefficient of thermal expansion (CTE) between different materials, can also cause reliability issues in high-frequency circuits and systems.
The heat always flows from the higher temperature area to the lower temperature area. This principle can be used to transfer the 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 a material that can clear or dissipate heat, such as a metal ground plane or a heat sink. Regardless, the thermal management of any circuit or system can only be best achieved by considering it from the beginning of the design cycle.
Generally, thermal conductivity is used to compare the performance of materials used to manage the heat of RF / microwave circuits. This index is measured by the power (W / mK) applied per degree of material per meter (in Kelvin). Perhaps one of the most important factors for any high-frequency circuit is a PCB stack-up, which generally has a lower thermal conductivity. For example, FR4 laminated materials often used in low-cost high-frequency circuits have a typical thermal conductivity of only 0.25 W / mK.
Conversely, copper (deposited on FR4 as a ground plane or circuit trace) has a thermal conductivity of 355W / mK. Copper has a large heat flow capacity, while FR4 has a thermal conductivity that is almost negligible. To prevent hot spots 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. The thinner PCB material allows shorter paths to the ground plane because it is possible to connect from the circuit traces to the ground plane using plated-through holes (PTH).
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 50Ω, so materials with higher dielectric constant values allow circuit designers to reduce the size of their RF / microwave circuits. That is, 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, the temperature rise of circuit materials with higher thermal conductivity is lower than that of lower thermal conductivity materials. Unfortunately, FR4 is no different from many other PCB materials with low thermal conductivity. However, the circuit's heat-treating and power-handling capabilities can be improved by specifying the use of PCB materials that have at least a higher thermal conductivity than FR4.
