Solving thermal issues in RF design

    Preventing circuit heat accumulation requires a certain amount of 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 device base).

    Understanding how heat is generated in various RF/microwave components of the system also helps with heat analysis. For example, the heating of a power amplifier is not solely due to its operation at the high-power stage. Factors such as amplifier efficiency, impedance matching (VSWR) at the amplifier output, and the thermal path originating from the amplifier output can all affect the generation of amplifier heat. Although a power amplifier with a 50% efficiency may seem impressive, it also wastes 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 losses of passive components such as filters and power dividers, as well as impedance mismatches (high VSWR) at the connections of components, coaxial cables, and other interconnecting devices, can also lead to heat dissipation barriers. Efficient thermal management requires an understanding of the heat flow process from the source (such as an amplifier) through all connecting cables and other components to the end point of heat dissipation.

    At the circuit level, thermal management is also a problem for the amplifier itself, as heat flows outward from the active components of the amplifier - some heat passes through the circuit board material, some enters surrounding components, and some flows into the air around the upper and lower parts of the circuit board. In an ideal scenario, a path can be provided to correctly dissipate heat from active devices, as the accumulation of heat around these devices can also shorten their lifespan. In addition, these heat may have harmful effects on certain devices, such as the continuous rise in temperature in silicon bipolar transistors, commonly known as "thermal runaway.".

    In the case of improper heat dissipation, some devices are more susceptible to damage than others. For example, the thermal conductivity of GaAs semiconductor substrates is only about one-third of that of silicon devices. At high temperatures, GaAs transistors may also be affected by memory effects (meaning that even if the temperature has dropped, the device may still operate in a specific gain state at high temperatures), leading to a deterioration in the 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, conversely speaking, it is the thermal conductivity of the material, which is an indicator to measure the thermal conductivity of the material. The data manual for thermal materials (such as thermal conductive adhesive and circuit board materials) generally lists this parameter, and 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 (which is a function of the power used), usually measured in ℃/W. When establishing thermal models for devices, circuit boards, and systems, all thermal effects must be considered, including not only the self heating effect of the device, but also its impact on surrounding devices. Due to the existence of these interactions, thermal modeling is generally completed by constructing a thermal matrix with all heating devices.

    On the circuit, even passive circuit components like capacitors may have an effect on heat dissipation. The application note "ESR Losses in Ceramic Capacitors" by American Technical Ceramics discusses how much power different types of capacitors can safely dissipate, based on their equivalent series resistance (ESR) rating. This note also provides a detailed explanation of how capacitors with high ESR values can leak electrical energy from batteries in portable devices, leading to a shortened battery life. Another useful reference is Hittite Microwave‘s application note "Thermal Management for Surface Mount Components", which explains how to incorporate surface mount components into circuit level thermal models.

    Of course, in order for the system to consider all thermal planning, the correct thermal design should start with the PCB level and the selection of PCB laminates that are most suitable for the power and thermal levels in a specific circuit design. When selecting circuit board laminate materials, it is not only important to simply choose materials with the highest thermal conductivity, but also to consider their electrical and mechanical stability at different temperatures.

    For example, a laminated board can be described by its coefficient of thermal expansion (CTE) in all three directions (length, width, and thickness) and the thermal coefficient of its dielectric constant. The first parameter represents the degree to which the material expands or contracts with temperature changes, while the second parameter indicates the variation of dielectric constant with temperature. The first parameter has a significant impact on reliability, while the second parameter may cause a deviation in the dielectric constant at different temperatures, ultimately leading to a change in impedance in microstrip circuits (for example, this change may alter the center frequency of the bandpass filter).

    Due to the high reliability and stable electrical performance required by many systems, including commercial communication and tactical military systems, circuit board material suppliers have been very concerned about thermal management issues in recent years. The materials developed can not only handle higher power levels in circuits such as power amplifiers, but also do not undergo electrical performance changes at high temperatures. For example, Rogers Corporation recently released the RT/duroid 6035HTC circuit material, which is a ceramic filled PTFE composite material with a thermal conductivity of up to 1.44 W/m/K, several times higher than the standard FR-4 circuit board material (see figure). This material integrates stable mechanical and electrical properties as well as thermal conductivity, making it an ideal material for high-frequency power amplifiers.