Gauge Power Limits On Passive Components

By avoiding high insertion loss and impedance junctions in passive components, high power levels can be transferred without unnecessary buildup of heat or creation of damaging hotspots.

Passive components are often required to handle large amounts of RF/microwave power. When subjected to high continuous-wave (CW) or peak power levels, the signal path or paths through a passive component can also be thought of as thermal paths, and any impediment to the conduction of heat can limit the power-handling capabilities of the component. Understanding how well different passive components were designed for thermal flow can provide some insight into their power ratings.

Two of the key passive-component specifications that will determine how well the component will handle high power levels are insertion loss and VSWR (or return loss). Signal energy can be lost as a result of dissipative losses in the materials of the component, including metal conductors and dielectric substrates. The lost energy is usually converted into heat, which must be dissipated. VSWR is a measure of signal reflections occurring at changes of impedance along the signal path, such as from a coaxial connector pin to a printed circuit board (PCB). Any such impedance junctions are also points at which heat can build up.

Even within the parts of a passive component are points where heat can build up, such as in a coaxial connector. For this reason, connector manufacturers evaluate their products in test fixtures with large signal levels, to determine the maximum safe CW and peak operating power levels. Studies by numerous connector companies, such as Amphenol RF, have clarified differences between average power and peak power to improve customers' understanding of their connectors' power ratings.

The firm also points out that the average power rating for a connector or cable/connector combination is inversely proportional to frequency (since resistive losses increase with increasing frequency), and connectors generally have higher power ratings than the cables to which they are attached. A connector's peak power capability is related to its peak voltage (V) rating, according to V2/Z, where Z is the characteristic impedance (usually 50 ) of the connector. Peak power is usually determined for a very short duty cycle, and is inversely proportional to VSWR, but not dependent on frequency. Both peak and average power-handling capabilities decrease with altitude.

To reduce insertion loss, some connectors incorporate air gaps, although these can appear as discontinuities in the thermal path. Standard SMA connectors are rated for about 100 W CW power. For most coaxial microwave components within the frequency range of the connector, the SMA connector will establish the power-handling limit of the component. Higher power levels are possible with ruggedized SMA connectors or larger connectors such as Type N connectors.

For example, Southwest Microwave manufactures a "Super SMA" connector usable through 27 GHz at power levels of 250 W CW and more, depending upon operating temperature. The firm offers an application note, "Power Rating for Coaxial Connectors," which addresses the power capabilities of mated pairs of coaxial connectors, and how large current flow through a small contact area between the two connectors can lead to heating.

SMA female connectors are used in the model 3164-90 miniature hybrid coupler from ARRA. By maintaining low insertion loss of 0.25 dB and low VSWR of 1.25:1 throughout its 1-to-2-GHz frequency range, it can handle CW power to 100 W CW. It is rated for 5 kW peak power, when tested with 5-microsecond pulses at a duty cycle of 0.05 percent.

It is also possible to manage 100-W CW power handling in a hybrid coupler without connectors, as Werlatone has demonstrated with its model QH7785 component. The hybrid coupler operates from 200 to 1000 MHz and achieves the high power-handling capability with the help of low 0.5-dB insertion loss and 1.30:1 maximum VSWR. In spite of its high power rating, the model QH7785 measures just 2.3 x 0.7 x 0.15 in. in a drop-in package.

How can designers maximize the power-handling capabilities of their circuits? At the circuit level, the choice of substrate or laminate material is critical to the ultimate power-handling capability of the design. The thickness of the dielectric material as well as the conductor metal influence the powerhandling capabilities of a circuit. The thermal conductivity of the substrate material should be as high as possible and dissipation factor (loss) as low as possible to ensure minimal heat buildup on the circuit board from high input power levels.

Designers can also mount high-power circuits on heat sinks to improve the flow of heat away from the circuit board. A heat sink formed of a metal with high thermal conductivity, such as aluminum or copper, can aid the transfer of thermal energy away from a circuit board and prevent hot spots on the circuit at thermal junctions, such as solder joints for mounted components. For reliability, the coefficient of thermal expansion for the heat-sink material should be as closely matched as possible to that of the circuit-board material, so that any expansion and contraction of the materials as a function of temperature is similar to avoid mechanical stresses. Often a layer of thermal grease is added between a heat sink and component to facilitate the flow of heat.

Designers also have a number of computer-aided-engineering (CAE) tools available to create thermal models of their designs to study the effects of different power levels on their circuits and assemblies. Thermal modeling tools such as CELSIUS from Integrated Engineering Software, RadTherm from ThermoAnalytics, Flo- THERM from Mentor Graphics, Sauna from Thermal Solutions, Icepak from ANSYS, and software modules from COMSOL and ITP Engines UK can help identify hot spots in a design before undergoing potentially hazardous testing at high power levels. A number of firms, including Motorola and Advanced Logistics Development, offer thermal testing and modeling services to evaluate components and circuits at different power levels.

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