Tackling Thermal Issues In RF Design

Just as electrical energy must overcome resistance in a circuit, the flow of heat through a design relies on overcoming thermal obstacles before it can be safely dissipated.

Thermal management is a concern for all circuit designers, especially at large-signal levels. In RF/microwave circuits, large signals can be found in power amplifiers and components in the transmit side of systems. Whether these are continuous-wave (CW) signals or pulsed signals, they can cause heat buildup on a printed circuit board (PCB) and in a system if the resulting heat is not properly channeled. For electronic devices, heat is synonymous with short operating lifetimes.

Preventing heat buildup in a circuit requires imaginationthe visualization of heat flowing from a source (such as a power transistor) to a destination (such as a heat sink or an equipment chassis).

Understanding how heat is generated in various RF/microwave components in a system can also help in the thermal analysis. For example, a power amplifier doesn't simply generate heat because it is operating at high power levels; factors such as the efficiency of the amplifier, the impedance match (VSWR) at the output of the amplifier, and the thermal path from the output of the amplifier can all impact heat generated in the amplifier. Although a power amplifier with 50-percent efficiency may seem impressive, it is also burning one-half of the energy supplied to it, much of which is lost as heat.

Beyond the power amplifier, the insertion losses of passive components, such as filters and power dividers, can also contribute to "thermal roadblocks," along with impedance mismatches (high VSWRs) at the connections of components, coaxial cables, and other interconnecting components. Effective thermal management involves picturing the flow of heat from the source (such as an amplifier), through all the connected cables and other components, and to the eventual location for dissipation of the heat.

At the circuit level, thermal management is also an issue within the amplifier itself, since heat is flowing from the amplifier's active devices outwardsome through the circuit-board material, some into surrounding components, and some into the ambient air above and below the circuit board. Ideally, a path can be provided for proper thermal flow away from the active devices, since heat buildup around these devices will also lead to shorter operating lifetimes. In addition, it can lead to unwanted effects in some devices, such as the effect of ever-elevating gain with temperature in silicon bipolar transistors, known as "thermal runaway."

Some devices are more susceptible than others to damage from improper dissipation of heat. For example, GaAs semiconductor substrates have only about one-third the thermal conductivity of silicon devices. At high temperatures GaAs transistors can also suffer from memory effects, in which the device operates at a particular gain state from an elevated temperature even after the temperature has been reduced, causing degraded device linearity performance.

Thermal analysis is essentially based on a study of the different materials used in a device or circuit, and the thermal impedance of those materials, or its resistance to the flow of heat. The inverse, of course, is the thermal conductivity of the material, which is a measure of a material's capability to conduct heat. This parameter is typically listed on data sheets for thermal materials, such as thermal adhesives and circuit-board materials, with higher numbers representing greater capabilities at handling high levels of power and generated heat.

The thermal impedance or resistance is represented in the change in temperature as a function of applied power, or typically in C/W. In developing thermal models for devices, circuit board, and systems, all heating effects must be considerednot only the self-heating effects of a device but its effects on surrounding devices. Because of these interactions, thermal modeling is typically performed by building a thermal matrix with all contributing devices.

At the circuit level, even passive circuit elements such as capacitors can contribute to the dissipation of heat. A four-page application note from American Technical Ceramics, "ESR Losses in Ceramic Capacitors," reviews just how much power can be safely dissipated in different types of capacitors, based on their equivalent series resistance (ESR) rating. The note also details how capacitors with high ESR values can drain power from a battery in a portable device, resulting in shorter battery life. Another useful reference is a six-page application note from Hittite Microwave Corp., "Thermal Management for Surface Mount Components," which shows how to include components in surface-mount packages into a circuit-level thermal model.

Of course, for all of the thermal planning that can go into a system, proper thermal design starts at the PCB level and in the selection of a PCB laminate material that is best suited for the power and thermal levels of a particular circuit design. In choosing a circuit-board laminate, it is not simply a matter of picking one with the highest thermal conductivity. Electrical and mechanical stability with temperature are also concerns.

For example, a laminate is characterized by its coefficient of thermal expansion (CTE) in all three directions (length, width, and thickness) as well as its thermal coefficient of dielectric constant. The first parameter is a measure of how much a material expands or contracts with temperature, while the second parameter indicates how the dielectric constant changes with temperature. The first can greatly impact reliability, while the second can cause deviations in dielectric constant with temperature that result in impedance changes in microstrip circuitschanges that shift, for example, the center frequency of a bandpass filter.

Because of the need for high reliability and stable electrical performance in a wide range of systems, including commercial communications and tactical military systems, circuit-board-materials suppliers have paid a great deal of attention to thermal-management issues in recent years, developing materials capable of handling higher power levels in circuits like power amplifiers without variations in electrical performance at high temperatures. For example, the recently announced RT/duroid 6035HTC circuit material from Rogers Corporation is a ceramic-filled PTFE composite material that features high thermal conducitivity of 1.44 W/m/K, or several times that of standard FR-4 type circuit-board materials (see figure). Because it also combines stable mechanical and electrical properties with its ability to channel heat, it is ideal for use in high-frequency power amplifiers.

Selecting the right materials can help manage heat, but a thermal analysis may also be required. A proper thermal analysis can be time consuming, if the temperature of each active device in a design is calculated. To aid in the analysis, commercial software simulation tools, such as the Advanced Design System (ADS) tools from Agilent Technologies, have been enhanced in recent years with additional capabilities or added software tools for thermal modeling. The EM Studio electromagnetic (EM) software from Computer Simulation Technology, for example, has been used to simulate the temperature distribution in a dual-mode filter, using the firm's CST Microwave Studio software tools to first calculate the current density distribution within the filter's conductive metals.

Earlier this year, AWR Corp., suppliers of the Microwave Office suite of software design tools, announced an agreement with CapeSym to serve as the exclusive global reseller for that firm's SYMMIC thermal analysis modeling software for monolithic microwave integrated circuits (MMICs).

For dedicated thermal analysis, Daat Research Corp offers a number of easy-to-use modeling tools for all levels of analysis, from the device through the system level, including its Coolit software. And for those to whom thermal modeling may be new and wish to experiment, Freebyte offers free thermal analysis software, including a demonstration version of the TAS software created by Harvard Thermal, Inc. and a demonstration version of the WinTherm software created by ThermoAnalytics.

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