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ABLIC’s Wireless Power IC to Propel Rise of Compact Devices
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ireless power supply technology has started to become a standard feature in mobile phones and smartphones and has been recognized already as one of the common means of charging. A standard that supports up to 15W is the emerging basic feature targeting smartphones.

Qi has been one of the emerging international wireless power supply standards established by the Wireless Power Consortium (WPC).

The development of sensors for internet of things (IoT) applications, which incorporate a sensor and a wireless communication module, has been becoming active not only for wearable devices but also for various household devices. How to supply to these devices is an important factor that affects the value of products.

In particular, wireless power supply allows products to have waterproof characteristics, which is one of major advantages. From this kind of circumstance, there has been an increasing need for wireless power supply technology to have ultrasmall and low-power characteristics.

Structure of the system
Fig. 1: Structure of the system
ABLIC Inc., a manufacturer specializing in analog semiconductors, develops, manufactures and sells various analog semiconductor products, including charging and discharging protection ICs for lithium ion batteries. This article introduces a transmission IC and a reception IC for ultrasmall wireless power supply offered by ABLIC.

Structure of Wireless Power IC
As shown in Table 1, the wireless power supply IC consists of S-8471 power receiver control IC and S-8474 power transmitter control IC. It is ABLIC’s original simple analog wireless power supply system using electromagnetic induction. Table 2 shows system requirements and Fig. 1 shows the circuit structure.

Table 1: Specifications of S-8471 and S-8474
Specifications of S-8471 and S-8474
Table 2: System requirements of S-8471 and S-8474
System requirements of S-8471 and S-8474
Main Features

Simple LC resonance circuit
Features of LC (inductor/capacitor) resonance circuit blocks at the transmission side and the reception side are shown in Fig. 2. At the power transmission side coil, S-8474 controls resonance waveform similar to semi-sinusoidal half-wave in accordance with the On/Off of FET1. At the power reception side, LC resonance occurs only when FET2 is On, and energy is received efficiently. S-8471 would turn FET2 On and Off to control resonance and non-resonance states in accordance with the load at the reception side and controls power needed while driving the load.

LC resonance circuit blocks
Fig. 2: LC resonance circuit blocks
As a result, constant voltage of 5.0V is always obtained from the output of S-8471.

Operation modes
When there is no power reception device in a wireless power supply system, it is necessary to perform intermittent operation to save energy at the power transmission side. As shown in the waveforms in Figure 3, S-8474 operates in two operation modes: the continuous operation mode and the intermittent operation mode.

The switching between the continuous mode and the intermittent mode is judged during the reception detection period of 3 to 5msec from the start of output. Based on the judgment result, it is decided whether to operate (continuous operation) or halt (intermittent operation) for the remaining 25msec. The judgement of switching between the continuous operation and intermittent operation is repeatedly performed in a cycle of 30msec.

Operation modes
Fig. 3: Operation modes
Detection of power reception at the transmission side
The reception of power is detected by the comparison of peak voltage of the LX terminal voltage waveform of S-8474. In order to control the transmission of power so that the power reception side outputs constant voltage, the LX terminal voltage of the waveform of the power transmission side coil varies in the switching operation of FET2 (On→Off). The transmission side detects the reception of power by judging from the difference in the peak of the waveform. Upon detection of the reception of power, S-8474 shifts to the continuous mode.

Temperature protection
Both the S-8471 power reception control IC and the S-8474 power transmission control IC could halt operation at the time of high temperatures due to heating of the coils through the connection of a negative temperature coefficient (NTC) thermistor. This is effective to secure the safety of devices.

Example of incorporation in a smartwatch
Fig. 4: Example of incorporation in a smartwatch
Simultaneous reception of power from multiple power reception coils
As the system does not use an authentication method using communication for the power reception detection method, multiple power reception coils can share alternating current (AC) magnetic field created by a single transmitter coil, and simultaneously receive power.

Applications, Sample Modules
The ultrasmall wireless power IC is used in small and thin electronic devices that incorporate a small rechargeable battery and require waterproof and dustproof properties.

 Electronic pen and an example of simultaneous charging of two pens
Fig. 5: Electronic pen and an example of simultaneous charging of two pens
IoT sensor and an example of simultaneous charging of five sensors
Fig. 6: IoT sensor and an example of simultaneous charging of five sensors
 Electronic pen and an example of simultaneous charging of two pens
Fig. 5: Electronic pen and an example of simultaneous charging of two pens
IoT sensor and an example of simultaneous charging of five sensors
Fig. 6: IoT sensor and an example of simultaneous charging of five sensors
A sample of smartwatch is shown in Figure 4, which uses planar coils, while the likes of electronic pens and hearing aids in Figure 5 shows an example of simultaneous charging of two pens that employ inductor coils.

Figure 6, an example of IoT sensor and smart card, shows an example of simultaneous charging of five sensors that employ solenoid coils.

Note:
More information can be seen on ABLIC’s home page https://www.ablic.com/en/semicon/products/rtc/wireless-power-ic/intro/

About This Article:
The authors, Teruo Ebihara and Toru Machida, are from the Document & Technical Support Unit of ABLIC Inc.