The hottest solution to the thermal problems in RF

2022-08-16
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Solving the thermal problem in RF design

thermal management is a problem that all circuit designers are concerned about, especially for large signals. In RF/microwave circuits, large signals are common in power amplifiers and system transmitter components. Whether continuous wave (CW) signals or pulse signals, if the heat generated is not effectively dredged, they will lead to the accumulation of heat on the printed circuit board (PCB) and in the system. For electronic equipment, heating means shortening the working life

preventing circuit heat accumulation requires a certain imagination: it can be imagined that heat flows from a heat source (such as a power transistor) to a destination (such as a heat sink or equipment base)

understanding how heat is generated in each RF/microwave component of the system is also helpful for heat analysis. For example, the heating of power amplifier is not only because it works at high power level. Factors such as amplifier efficiency, impedance matching (VSWR) at amplifier output and thermal path from amplifier output will affect the generation of amplifier heat. Although the power amplifier with 50% efficiency seems to be very good, it will also waste half of the energy supplied by the system, most of which is lost in the form of heat

in addition to power amplifiers, the insertion loss of passive components such as filters and power distributors and the impedance mismatch (high VSWR) at the connection of components, coaxial cables and other interconnected devices will also lead to "heat dissipation obstacles". Efficient heat management requires an understanding of the heat flow process from a source (such as an amplifier) through all connecting cables and other components to the end of heat dissipation

at the circuit level, thermal management is also a problem of the amplifier itself, because heat flows outward from the active devices of the amplifier - some heat passes through the circuit board material, some enters the surrounding components, and some flows into the air around the upper and lower parts of the circuit board. Ideally, a path can be provided for the proper dissipation of heat from active devices, because the accumulation of heat around these devices will also shorten their working life. In addition, these heat may cause harmful effects on some devices, such as the rising temperature in silicon bipolar transistors, which is commonly referred to as "thermal runaway"

under the condition of improper heat dissipation, some devices are more vulnerable to damage than other devices. For example, the thermal conductivity of GaAs semiconductor substrates is only about one third of that of silicon devices. At high temperature, GaAs transistors may also be affected by memory effect (that is, even if the temperature has fallen, the device may still work in a specific gain state at high temperature), resulting in poor linear performance of the device

thermal analysis is essentially based on the study of different materials used in devices or circuits, as well as the thermal resistance of these materials or their resistance to heat flow. Of course, on the contrary, it is the thermal conductivity of materials, which is an index to measure the thermal conductivity of materials. This parameter is generally listed in the data book of thermal materials (such as thermal conductive adhesives and circuit board materials). The higher the parameter value, the higher the ability of this material to handle high power levels and heat generation

thermal resistance can be described by temperature change (this value is a function of the adopted power), usually in ℃/w. When establishing thermal models for devices, circuit boards and systems, we must consider the impact of all thermal effects, including not only the self heating effect of devices, but also the impact on surrounding devices of China's automotive lithium batteries, whose cumulative scrap volume will reach 200000 tons. Due to the existence of these interactions, thermal modeling is generally completed by constructing a thermal matrix with all heating devices

in the circuit, even passive circuit components such as capacitors may play a role in heat dissipation. American technical ceramics' application note ESR losses in ceramic capacitors discusses how much power different types of capacitors can safely emit, based on the equivalent series resistance (ESR) rating of these capacitors. The note also describes in detail how capacitors with high ESR values can leak the power of batteries in portable devices, resulting in shortened battery life. Another useful reference is the application note of Hittite microwave, "thermal management for surface mount CO (the Ministry of science, technology, ICT and future planning of Korea announced this scientific research achievement mponents)", which introduces how to include surface mount components into the circuit level thermal model

of course, in order to make the system take into account all the heat planning, the correct heat design should start from the PCB level and the selection of PCB laminates that are most suitable for the power and heat levels in a specific circuit design. When choosing the laminate of circuit board, we should not only choose the material with the highest thermal conductivity, but also consider the electrical and mechanical stability at different temperatures

for example, a laminate can be described by its coefficient of thermal expansion (CTE) in all three directions (length, width, thickness) and the thermal coefficient of dielectric constant. The first parameter represents the degree of expansion or contraction of the material with temperature, while the second parameter shows the change of dielectric constant with temperature. The first parameter has a great impact on reliability, while the second parameter may cause the dielectric constant to deviate at different temperatures, and eventually lead to the impedance change in the microstrip circuit (for example, this change may change the center frequency of the bandpass filter)

because many systems (including commercial communications and tactical military systems) need to have high reliability and stable electrical performance, circuit board material suppliers have paid great attention to thermal management in recent years. The developed materials can not only deal with higher power levels in circuits such as power amplifiers, but also will not change electrical performance at high temperatures. For example, the rt/duroid 6035htc circuit material recently released by Rogers Corporation is a ceramic filled PTFE composite with a thermal conductivity of 1.44 w/m/k, which is several times that of the standard FR-4 circuit board material (see Figure 1). This material integrates stable mechanical and electrical properties as well as thermal conductivity, so it can be used as an ideal material for high-frequency power amplifier

Figure 1: the newly developed rt/duroid 6035htc circuit material is used to meet the designers' needs for improving high-temperature performance

selecting the right material is conducive to heat management, but also requires heat analysis. If the temperature of each active device in the design is considered, the correct thermal analysis will be very time-consuming. In order to help with the analysis, commercial software simulation tools such as advanced design system (ads) tools launched by Agilent Technologies have been upgraded in recent years, and special functions or software tools have been added for thermal modeling. For example, computer simulation technology's EM studio electromagnetic (EM) software has been used to simulate the temperature distribution in dual-mode filters. This software uses the company's CST Microwave Studio software tool to calculate the current density distribution in the conductive metal of the filter for the first time

Chart 1. Four different 3.5 Dk laminate materials were tested, and the RT/duroid 6035HTC most effectively

dissipated heat away from the resistor to enable the lowest temperature rise.

At the beginning of this year, AWR company, a supplier of microwave office software design tool suite, signed an agreement with capesym company to become the exclusive global retailer of capesym's monolithic microwave integrated circuit (MMIC) symmic thermal analysis modeling software

in terms of special thermal analysis tools, daat research provides many easy-to-use tools for building and replacing hydraulic oil models, which can realize all levels of analysis from device level to system level, including coolit software. Freebyte provides free thermal analysis software for engineers who are unfamiliar with thermal modeling and want to implement the alliance, which is composed of 8 manufacturing enterprises from the province's thermoplastic elastomer material and product industry, and 5 universities and scientific research institutes, including Hefei academy of materials science of the Chinese Academy of Sciences and Hefei University of Technology, These include TAS software demo developed by Harvard thermal and WinTherm software demo developed by thermoanalytics. (end)

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