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Design007-Feb2020

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74 DESIGN007 MAGAZINE I FEBRUARY 2020 Most commercial PCB-based applications that use high power are typically associated with cellular base station technology; howev- er, there are other applications. There are also several things to consider when working with high-power RF applications. This column will concentrate on PCB-based power amplifiers used in base station applications, but the ba- sic concepts discussed here will apply to other high-power applications. Most high-power RF applications will have issues with thermal management, and there are a few fundamental relationships to consid- er. One relationship is the loss-heat relation- ship. A circuit with higher loss will cause high- er heat to be generated when high RF pow- er is applied. Another issue is frequency heat, which causes more heat to be generated at higher frequencies. Additionally, any dielectric material that is subjected to an increase in heat will have a change in Dk (dielectric constant), and that is the thermal coefficient of dielectric constant (TCDk). There will also be a change in insertion loss with a change in temperature. These changes in Dk due to TCDk can impact the RF circuit performance and could be prob- lematic for the application. There are multiple materials and PCB prop- erties that should be addressed for the loss- heat relationship. Sometimes, when a design- er chooses a low-loss material for a PCB appli- cation, they will only consider the dissipation factor (Df, or loss tangent). The Df is related to the dielectric losses of the material; however, a circuit will have other losses. The overall loss of a circuit, as it relates to RF performance, is PCB Materials for High-power RF Applications Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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