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[FAQ] How Do I Select Capacitor Values for a Wireless Power RX Coil?

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Replies: 1

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This FAQ is applicable for wireless power RX like bq51013B, bq51050B, bq51003, and similar.

Nick Brylski

Applications Engineer

Battery Chargers | Texas Instruments

  • The coil, C1, and C2 make up the dual resonant circuit. The capacitors values must be selected to work with the chosen coil. The equations below show how to calculate C1 and C2 for given coil parameters. Recommend having C1 as 3 capacitors in parallel and C2 as 2 capacitors in parallel. The parallel configuration reduces ESR resulting in higher efficiency and reduced heating.

    Ls’ is the WPC test fixture inductance and must be measured manually. Ls is the free-space inductance given in the coil datasheet. Since the environment of the coils will impact the inductance value, care must be taken to get the correct Ls’ value. Any metals, magnets or other materials within the device housing may impact the Ls' value.  The receiver datasheet (see BQ51003 d/s figure 34) recommends a method for measuring Ls' but also notes that the final value is impacted by its final industrial design. If housing of device changes, Ls’ must be measured again which would necessitate different C1/C2 values.

    Also note the quality factor requirement; this is part of the WPC specifications for RX coil:

    Nick Brylski

    Applications Engineer

    Battery Chargers | Texas Instruments

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