EMI Suppression Filters (EMC and Noise Suppression)Noise Suppression Measures for Qi-Standard Wireless Power Supply Modules

Contents

In this study, we conducted noise assessments and examined noise suppression measures for both wireless power supply transmitter and receiver modules compliant with the Qi standard, and we established methods for implementing effective noise suppression measures.
Because measures are required by implementing remedies at both the receiver and transmitter sides, the measures implemented for each circuit type will be explained separately.

A study was conducted on whether the reception sensitivity for voice communication was reduced when supplying power wirelessly.
As a result, it was found that the reception sensitivity was reduced in the 800 MHz band when power was supplied wirelessly. Also, a reduction in reception sensitivity occurred uniformly in all frequencies.

As a result of our noise identification study, we proposed a mechanism by which noise is likely generated in the receiver module.

There are two main sources of noise.
The first source is the charger.
The second source is the receiver IC, which is located in the receiver module.

This noise is thought to be emitted directly from the receiver module board or to flow through the power line or ground line and be emitted from the smartphone board or wires, and this noise results in a reduction in reception sensitivity when it flows to the antenna.

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.

Note:
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.

We evaluated the reception sensitivity when power is supplied wirelessly while the noise suppression measures described in the previous section were implemented.

As a result, the reception sensitivity improved for all frequencies.
In this sample evaluation, the reception sensitivity improved by up to 5 dB, and the drop in reception sensitivity when power was supplied wirelessly was zero.

Noise generated by the charger flows from the transmitter coil to the receiver coil, and the insertion of a wireless circuit reduces the reception sensitivity of the wireless circuit.

As shown below, ferrite beads are used at the receiver circuit to implement noise suppression measures and enable improvement of the reception sensitivity of the wireless circuit.

Following the receiver module, next, we will examine the noise that must be suppressed in the transmitter module. In the transmitter module, the inverter used for supplying AC to the transmitter coil is a source of noise. Noise emitted from the power supply cable becomes an issue as externally-emitted noise, and noise that flows into the transmitter coil side is emitted externally and also reduces the reception sensitivity of the smartphone unit.
Consequently, we recommend noise suppression measures for preventing noise emissions and for preventing drops in reception sensitivity.

First, we implemented the measures for preventing emission noise.
This consists of noise that leaks to the power supply cable side from the inverter and noise that leaks to the transmitter side, and so noise suppression circuits are installed for each location. (Suppression measures (1) and (2))
Emission noise consists primarily of noise transmitted in common mode, and so a common mode choke coil is used. Also, because the transmitter coil side contains a large amount of high-frequency noise, a line bypass capacitor is also used.

Use of noise suppression measures at both the power supply cable side and transmitter coil side enables improvements of up to 14 dB in the emission noise.

A noise suppression component was connected to the transmitter coil side. This noise suppression measure enables improvements of up to 12 dB in the emission noise.

Use of noise suppression measures at both the power supply cable side and transmitter coil side enables improvements of up to 14 dB in the emission noise.

Next, we implemented measures for preventing drops in the reception sensitivity. The noise suppression measures were implemented in the same way as those at the transmitter coil side for preventing emission noise, and a common mode choke coil and line bypass capacitor were used. As a result, the reception sensitivity was improved by up to 12 dB.