Application Note

Noise Suppression Methods in Qi Standard Wireless Chargers

Ever since Qi standard^*1smartphones came onto the market in 2011, the number of products suitable for wireless power supply has been increasing. Wireless power supply has the advantages of being easily rechargeable and being suitable for watertight/dust-proof designs. On the other hand, it has been associated with concerns over possible noise problems, such as the generation of switching noise during the conversion of an electric current from DC to AC, or the leakage of magnetic flux from a gap between the power-transmitting and power-receiving coils.

In this article, we present the strategies against noise in Qi standard wireless chargers (hereinafter referred to as chargers).

What Is Wireless Power Supply?

Wireless power supply is, as its name suggests, a technology for the wireless transmission of electric power. For instance, the electromagnetic induction method employed in the Qi standard, unlike the conventional charging methods that plug in a power supply cable to a smartphone, uses the principle of electromagnetic inductance to charge a device. That is, magnetic flux, generated by the flow of an alternating current in the coil of the power transmitter (charger), couples with the coil of the power receiver (smartphone), thereby inducing an electric current in the receiver (Fig. 1).

Hence, chargers additionally require an inverter to convert a direct current to an alternating current and a coil to generate the magnetic flux. Smartphones need a coil to receive the magnetic flux and a rectifier circuit to convert an alternating current to a direct current.

Fig. 1 Wireless Power Supply Using the Electromagnetic Induction Method

Noise Problems in Wireless Power Supply

Our research into noise generated by wireless power supply has revealed that wireless power supply is associated with the increase of radiated emissions and the suppression of receiver sensitivity in wireless communication.

(1) Increase of radiated emissions

Radiated emission is the magnitude of electromagnetic waves radiated from electronic devices. Because large radiated emissions may possibly give rise to malfunction at other electronic devices, the EMC regulation of each country sets the limit for radiated emissions. If your products exceed the limit set out by a country, you are not allowed to distribute the products in that country.

If no noise suppression measures are taken for chargers, large radiated emissions may occasionally be observed in the frequency band between 30MHz and 100MHz. Figure 2 presents the result of the measurement of radiated emissions between 30MHz and 1000MHz as indicated in a blue line. The observed radiated emission of this charger exceeded the limit of CISPR22 classB^*2 near 60MHz, which means that the product is not marketable as it is, and therefore the noise suppression measures will be necessary.

Fig. 2 Radiated Emission (Vertical Polarization)

(2) Suppression of receiver sensitivity in wireless communication

Receiver sensitivity is the lowest power level of receiving signal that wireless communication devices, such as smartphones, can detect. If receiver sensitivity is suppressed (i.e., deteriorated), the devices can no longer detect weak signals and communication is no longer possible.

Various frequency bands have been used for wireless communication in Japan: for example, multimedia broadcast (200MHz), TV broadcast (400 to 700MHz), voice/data transmission (700 to 900MHz, 1800 to 2100MHz), and so forth. If noise with the same frequency as these afore-mentioned frequency couples with the antenna of smartphones, weak signals become hard to be distinguished from these sorts of background noise, leading to the suppression of receiver sensitivity.

Fig. 3 Receiver Sensitivity (900’sMHz)

If no noise suppression measures are taken in the chargers, the receiver sensitivity may possibly be suppressed in the multimedia broadcast, TV broadcast, or voice/data transmission in the frequency band between 700MHz and 900MHz. Figure 3 illustrates an example of the measured receiver sensitivity of voice/data transmission at the frequency band of 900’sMHz shown in a blue line. In this charger, the receiver sensitivity suppression of as large as maximum 14dB was observed over the entire frequency range from 925MHz to 960MHz. Hence, communication or receiving calls may possibly be hindered during the recharging, and therefore appropriate noise suppression measures will be necessary.

The Mechanism of Noise Interference

Our research on the source and the propagation routes of noise in wireless power supply has revealed that the noise generated from chargers is more dominant than the noise generated from smartphones. We inferred the mechanism of noise interference in wireless power supply as presented in Fig. 4.

The source of noise is the inverter of a charger. Although the switching frequency of the inverter that converts a direct current to an alternating current is around 100kHz, its higher order harmonic components exist up to several hundred MHz, giving rise to noise problems, such as the suppression of receiver sensitivity in wireless communication or the increase of radiated emission.

Fig. 4 Noise Interference Mechanism of Wireless Power Supply

The noise observed as the radiated emission is emitted primarily from the power supply cable of the charger. The noise affecting the receiver sensitivity of wireless communication is emitted predominantly from the coil of the charger, but some of it may come from the circuit board of the charger. The noise emitted from these sources couples with the antennas of smartphones, thereby suppressing the receiver sensitivity. There are two types of noise: a normal mode noise^*3 and a common mode noise.^*4 Therefore, you will need strategies against both normal mode and common mode noise.

suppression measures will be necessary.

The Noise Suppression Method

Figure 5 shows the noise suppression method in the charger. The method employing a common mode choke coil (CMCC^*5) or capacitor is effective.

As the strategy against the radiated emission, a CMCC DLW5BTM142SQ2 is installed at the base of a power supply cable. This noise suppression strategy reduced the radiated emission by 21dB at most as indicated by a red line in Fig. 2, suppressing the noise below the limit of CISPR22 classB.

Fig. 5 Noise Suppression in Chargers

As an effective strategy against the receiver sensitivity suppression in wireless communication, a CMCC DLW5BTM251SQ2 and a low ESL (Equivalent Series Inductance) type capacitor LLL185R71H222MA01 as an across-the-line capacitor (hereinafter referred to as X-capacitor) may be installed immediately behind the inverter. If the inverter and the CMCC are kept apart, noise may be emitted from the circuit board. If the constraint of the charger layout forces the inverter and the CMCC apart, the additional installation of LLL185R71H222MA01 as a line-bypass-capacitor (hereinafter referred to as Y-capacitor) immediately behind the inverter solves the problem. These noise suppression strategies improved the receiver sensitivity in voice/data transmission by 13dB at most as indicated in a red line of Fig. 3, achieving the receiver sensitivity comparable to the sensitivity of the devices that do not use wireless power supply.

The Characteristics of the Noise Suppression Components Used in Wireless Chargers

Although DLW5BTM142SQ2, used to suppress the radiated emission, has a compact size with a low profile of 5 × 5mm (t=0.235mm/max), it has the common mode impedance of 530Ω and 1650Ω at 30MHz and 100MHz respectively as indicated in Fig. 6, showing an excellent capability of noise suppression in the frequency band where countermeasures against the radiated emission are necessary.

Similarly, DLW5BTM251SQ2, which we recommend as a countermeasure against the receiver sensitivity suppression in wireless communication, has a compact size with a low profile of 5 × 5mm (t=0.235mm/max). However, it has a high rated current of 5A and the common mode impedance of 400Ω and 200Ω at 200MHz and 1000MHz respectively as indicated in Fig. 6, showing an excellent capability of noise suppression in all the frequency bands where countermeasures against the receiver sensitivity suppression are necessary.

LLL185R71H222MA01, used as an X- and a Y-capacitor, is a low ESL capacitor, called an LW reverse-type capacitor. Although it has a compact size with a low profile of 0.8 × 1.6mm (t=0.5mm), it maintains low impedance characteristics up to high frequency, with the impedance of 0.30Ω and 0.55Ω at 200MHz and 1000MHz respectively as indicated in Fig. 7. It shows an excellent capability of noise suppression in all the frequency bands where countermeasures against the receiver sensitivity suppression are necessary.

As described above, using the noise suppression components suitable to the types and frequencies of the noise that you want to suppress, you will be able to solve the noise problems in wireless power supply.

Fig. 6 Characteristics of DLW5BT Series

Fig. 7 Characteristics of LLL185R71H222MA01

Conclusion

The market for wireless power supply is expected to grow from 2014 onward as well. Presently, its use is mainly for lower power consuming devices, such as smartphones; from now, its application to high power consuming devices is expected to grow to include tablet and laptop computers, and in the future even electric vehicles. Meanwhile, as its application to the high power consuming devices progresses, concerns about noise are expected to grow. Hence, we believe that the strategies against noise will be more important than ever.

Murata has established noise suppression technologies not only for wireless power supply, but also for various kinds of devices. We will continue to strive for the development of noise suppression technologies, so that everyone will think of Murata first whenever the noise suppression is necessary.

Glossary

*1 Qi standard:

A standard for the wireless power supply using the electromagnetic induction method established by the Wireless Power Consortium. The products compliant with the standard are sold worldwide.

*2 CISPR22 classB:

An international standard underlying the EMC regulations of information technology equipment in each country. Class B is the limit for devices used in residential environments.

*3 Normal mode noise:

Noise generated between signal lines or power supply lines. Each noise current flows in the opposite direction. Noise suppression countermeasures, such as across-the-line capacitors (X-capacitors), are required.

*4 Common mode noise:

Noise generated between signal or power supply lines and ground. Between the signal lines or power supply lines, noise currents flow in the same direction. Noise suppression countermeasures, such as common mode choke coils (CMCCs) or line-bypass capacitors (Y-capacitors), are required.

*5 CMCC:

A coil that transmits normal mode signals, blocking only common mode noise.

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