Next, we will explain the simple equivalent circuit (noise suppression circuit) of the wireless receiver module.
Ferrite beads are installed at the base of the receiver coil (2 locations) as shown in the figure.
As a result, noise that is emitted from the charger and flows to the receiver coil is prevented from propagating within the receiver module.
Ferrite beads are installed in the power line and ground line connected to the smartphone.
As a result, noise that is generated from the receiver IC is prevented from flowing to the smartphone.
Use of the BLM15PD800SN1 ferrite beads featuring a compact size and support for large currents is recommended.
The following noise suppression measures enable improved reception sensitivity for voice communication.
If the flux leakage from the charger is large, noise suppression measures for the receiver side only may not improve the reception sensitivity.
Noise suppression measures must also be fully implemented at the charger.
Also, if the flux leakage from the charger is large, the noise that is emitted from the transmitter coil will flow directly to the antenna, and so measures that use ferrite beads will not be effective.
In this case, the electromagnetic wave absorbing sheet can be enlarged beyond the receiving coil (as if affixing to the entire case) for preventing the noise from flowing to the antenna and improving the reception sensitivity.
We studied whether the noise suppression measures in the above figure have an adverse effect on charger operation.
This is an issue because, if the impedance characteristics formed by the circuit (receiver coil + series capacitor, receiver coil + series capacitor + parallel capacitor) are changed when ferrite beads are installed, the power transmission and reception also change.
In the Qi standard,
it is specified that
Resonance frequency for the receiver coil + series capacitor = 100 kHz+5%-10%
Resonance frequency for the receiver coil + series capacitor + parallel capacitor = 1000 kHz±10%
Then, we calculated how the impedance characteristics of the circuit (receiver coil + series capacitor in this example) change when ferrite beads were installed. This was calculated assuming that the inductance value and capacitance value were two different types.
Installing the ferrite beads has no effect on the impedance characteristics of the circuit.
As a result, installing the ferrite beads had no effect on the impedance characteristics of the circuit. And so, we think that, even if ferrite beads are installed, their effect on charger operation is small.
Compared to the inductance value (about 10 to 20 uH) of the receiver coil, the L value of the BLM15PD800SN1 is a small value of only 210 nH (actual value measured by LCR meter), and so the resonance frequency can be said to remain unchanged.