In mobile phones and other small portable applications, the wireless power system is constantly recognized. The existing standard is limited to 5W power transmission, but the growing power demand for smartphones, tablets, and portable industries and medical applications has put forward higher requirements for power supply capacity. As output power increases, efficiency and thermal performance must be considered in the system design. This article reviews the implementation of the 10W wireless power system that can be produced and provides system design guidelines related to system performance optimization. An example of a transceiver (TX) and receiver (RX) coil that has been successfully tested in 10W applications has been given. The wireless power has already appeared many years ago, and there are also many forms, but it has become more common due to the emergence of industry standards. Smartphones and small tablets are the main product categories currently using wireless power. However, this technology has also begun to wearable devices and medical and industrial applications. When the wireless power supply is used in conjunction with wireless communication technology, the design of the fully closed device can be made possible. This makes the wireless power supply into all portable systems that need to be run in outdoor or humid environments.
Existing industrial standards have only limited power output capabilities, often in the range of 5W. The development of higher power standards is in progress, as of December 2014, has not been fully determined. Therefore, those that require higher power levels to charge for larger capacity batteries require custom or proprietary design. Although system designers may use standard components "from scratch", this method is difficult to achieve this goal of the end product to quickly launch the market. The complementary transmitter and receiver chipset on the market now enables immediate design of the 10W wireless power system for portable applications, including one and two battery section battery pack architectures.
Wireless power system architecture
The simplicity map of a tight coupled smart radio power system is shown in Figure 1. If viewed from the perspective of the schematic, it looks like a transformer coupled isolation power conversion circuit. Here, however, the primary coils and secondary coils are completely separated, rather than winding around the same magnetic core. Electrical energy is transmitted from the transmitter (primary, or tx) to the receiver (secondary, or) end, and the receiver circuit transmits the feedback to the magnetic coupling device in the form of a digital pulse.
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Figure 1: Typical wireless power system architecture diagram
It has to extend the power performance to 10W and have to take additional consideration. First, the silicon power element must be designed to handle the desired peak and the sustained power level. At the transmitter side, the power FET element is outside the transmit controller, so it can be upgraded to process peak current as needed. In the receiver end, the small size of the solution is important, and the integrated FET device is used to provide single-chip instruments. In order to provide high efficiency and improve thermal performance, the FET in the RX controller has lower RDS (ON) compared to previous 5W receivers. Magnetic components, that is, TX and RX coils must also have rating of higher peak currents required to process 10W power transmission. Since the magnetic field strength of the 10W system is higher, relative to the 5W system, the shielding range of the receiver is needed. This provides better shielding for metal components in the system, limiting the "approximate, contact metal" loss of the receiver end, and improving system efficiency as much as possible.
We note that the RX controller provides feedback to the TX controller, and requires TX to change its output power based on different load conditions, and coil alignment / coupling efficiency. A common method for changing output power is to excite the coil with a constant amplitude / variable frequency AC signal. Another alternative is to use a variable amplitude / fixed frequency excitation. Variable frequency control eliminates resonance tuning of the TX / RX resonant circuit for the TX / RX resonant circuit. When the TX operating frequency is close to the resonance point, possible power is transmitted from TX to RX. In order to reduce power delivered to the RX end, the TX controller increases its frequency, which is much higher than the resonance peak. The TX frequency tends to increase when RX requires less power or the like. However, this method makes the power transmission / control process to a large extent on the coil adjustment. When used at higher power levels, a variable frequency architecture also proposes some problems in electromagnetic interference (EMI) control.
The 10W transmitter system operates at a fixed frequency, but uses a tunable pre-regulator to change the DC voltage rail for the coil excitation. A full bridge circuit is used to generate an AC excitation current for TX coils. The basic block diagram of a fixed frequency (10W) wireless power transmitter system is shown in Figure 2. When RX requires more output power, the voltage of the DC voltage rail is supplied to the Tx coil power stage increases. The DC voltage decreases with the decrease in the RX load. Be
Figure 2. 10W wireless power transmitter with a wireless digital control
10W system adjustable output voltage and thermal performance
The 5W wireless power supply system typically generates a fixed 5V output voltage at the receiver side. This is enough to charge a single-section lithium-ion battery in the range of 1A in the range of 1A, which is essentially, which is similar to the USB type power supply that is visible everywhere. However, as the battery capacity in the portable device is increased, a higher current is required to maintain a fast charging time.
The BQ51025 10W wireless receiver output voltage can be adjusted with an external feedback resistor within a range of 5V to 10V. This enables charging of the tandem cell for one or two sections, and when combined with a wide input voltage range switch mode NVDC type charger, it is possible to maintain the high efficiency of the single cell charging [7]. In the case of a wireless RX output, the NVDC charger architecture enables efficient charging of the low voltage battery while reducing the input current required for higher voltage power supplies. Figure 3 shows the thermal response (Fig. 3A, B, B, B, and C) at 5V, 7V, and 10V outputs while providing a 10W power supply to the load. Obviously, the heat generated in the 10V output should be used in the case where the high-frequency switch mode charger can be used for battery charging. Be
Figure 3. Wireless receiver measurement under 10W load conditions.
The serial resonant capacitor (C1 in Fig. 4) on the receiver circuit is also the same key to optimizing thermal performance. In actual operation, multiple capacitors are connected together to provide the required total capacitance value. Be
Figure 4. Wireless power receiver and key resonant capacitor
The thermal performance difference between using C0g (larger package, low-serial equivalent resistance (ESR) and X7R (smaller package, higher ESR) is very considerable (Fig. 5). Be
Figure 5. Effect of capacitors on thermal performance
Smaller, high ESR capacitors will become places where the RX printed circuit board (PCB) is temperature. The PCB temperature caused by these capacitors will hinder the heat generated by the distribution of integrated circuit (IC) itself, which means that the overall temperature of the IC and PCB will increase. In addition, due to the smaller resonant capacitor, the total efficiency fell from 80% to 74%.
Figure 6 shows the overall system efficiency of 10W wireless power transmission that uses a wireless power transmitter (BQ500215) with a wireless power receiver (BQ51025), evaluation board (EVM), and appropriate components.
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Figure 6. End to end-to-end efficiency of 10W power supply system at 5V, 7V and 10V output settings
Coil Selection Guide
BQ500215 Transmitter Evaluation Module uses a wireless power distribution (WPC) type 29, 10μH, 30M? Coil, its rated current is 9A. In addition to the 10W receiver, this coil ensures compatibility with the previous 5W WPC type receiver.
At the receiver side, the coil parameters should be optimized to match the target output voltage of the application. In the case where the 5V output is required, the nominal inductance value of the RX coil should be in the range of 10 μH; for a higher output voltage of 7V or 10V, the Rx coil should be in the range of 15 μH.
Although the ideal state is limited to a DC resistance (DCR) of the coil, in the case of a higher output voltage, it is allowed to slightly add DCR to deal with lower current. Figure 7 shows two typical RX end coils. Rear shielding materials are required when all RX and TX coils are assembled.
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Figure 7. Typical RX coil technical specifications for 5V, 7V and 10V output requirements
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Figure 8. Reduce battery charging time with 10W wireless power system
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There are still many other details that need to be considered when designing a complete 10W power system. References provide a full guide and design calculation result of a system using TI 10W wireless power solutions.
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Source: Wiku Electronic Market Network
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