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Selecting surface-mount MLCC for high-frequency apps

18 May 2015  | Brian Ward

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Lower ESR is a significant advantage in RF applications where power dissipation is a concern. To demonstrate the benefits of the lower ESR, continuous wave (CW) RF power testing is conducted. Two 180 pF, 1111 body size MLCCs are placed in parallel and subjected to an input power of 120 W at 360MHz. Thermal imaging (figure 4) confirms that the part made with 100 % palladium electrodes is hotter than the part made with the silver-palladium alloy electrodes. Measurements of the surface temperature over time show that upon reaching steady state, the part with the 100 % palladium electrodes is approximately 5°C hotter than the part made with silver-palladium alloy electrodes. A typical example is presented in figure 5.

Figure 4: Thermal image under load.

Figure 5: Temperature rise under load.

Other factors
While ESR and power handling are key performance characteristics for the RF capacitor, what happens to the frequency characteristics when the electrode material is different? Basically, not much.

Figures 6 and 7 show how the self-resonant frequency and parallel resonant frequency of 0402 and 0603 body sizes vary as a function of the capacitance value of the part. As can be noted, these is very little difference. Similarly the insertion loss – a measure of how much signal is lost during transmission – varies little between the parts manufactured with pure palladium electrodes and those manufactured with silver-palladium alloy electrodes. This is seen in figures 8 and 9.

Figure 6: SRF and PRF of 0402 body size as a function of capacitance.

Figure 7: SRF and PRF of 0603 body size as a function of capacitance.

Figure 8: Insertion loss of 0402 1 pF capacitor.

Figure 9: Insertion loss of 0402 10 pF capacitor.

Several factors come into play when choosing a capacitor for high-frequency applications. Power handling is key, and products made with less resistive electrode systems (an alloy of silver and palladium) prove to have an advantage over products made with the more costly pure palladium electrodes. This electrode material does not seem to affect other characteristics important in RF design, such as SRF, PRF, and insertion loss.

About the author
Brian Ward is with Vishay.

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