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RF Power Capabilities of High-Frequency PCBs

I often hear this question: “How much RF power can be applied to a high-frequency PCB?”

My answer sometimes surprises engineers. I tell them that they can put as much RF power into the PCB as they want, with the assumption that the PCB does not exceed its maximum operating temperature (MOT). MOT refers to the maximum temperature to which a circuit can be exposed without degradation of critical properties. The actual RF power limit of a PCB is based on the MOT of the circuit, and that is dependent on the circuit material, the PCB construction and fabrication process.

The relative thermal index (RTI) is a rating given to UL-rated circuit materials for the maximum temperature to which the raw material can be exposed indefinitely without degradation in material properties. But when the raw material is made into a circuit, MOT is the rating that is most applicable to the power-handling capability of a circuit. The MOT is always less than a circuit material’s RTI. When reviewing the maximum RF power-handling capability of a PCB, MOT is used as the maximum temperature of which a circuit can be exposed over long periods of time.

For example, a circuit with a heat rise of +70°C above an ambient of +25°C must endure a temperature of +95°C indefinitely. The RF power which creates this heat rise is acceptable if the circuit has a MOT rating of +105°C. But if the circuit’s heat rise is greater than +80°C above ambient, the applied RF power level that created the heat rise would not be acceptable.

When considering circuit heating due to applied RF power, modeling the heat rise of high-frequency PCBs can be difficult. Many variables influence heat rise, and they must be taken into account. Insertion loss is the total RF loss of a high-frequency PCB and is equal to the summation of conductor loss, dielectric loss, radiation loss and leakage loss. Insertion loss is the cause of the heat generated when RF power is applied. A circuit with a high level of insertion loss will generate more heat than a circuit with lower insertion loss, when considering the same amount of applied RF power. Insertion loss can also be difficult to model, because there are sub-components that make up insertion loss. Typically, the major contributors to insertion loss are dielectric loss and conductor loss. 

To read this article from the April 2015 issue of The PCB Design Magazine, click here.