無線電源傳輸 Wireless Power Consortium (WPC) Communication

Universally Compatible Wireless Power Using the Qi Protocol

Wireless charging of portable electronic devices is here now. It will become ubiquitous when all such devices adhere to the same standard.

By Upal Sengupta and Bill Johns, Texas Instruments

The Wireless Power Consortium (WPC) has developed a standard for wireless power systems referred to as Qi (pronounced as 「chee」). This Qi-compliant wireless power system allows compatibility between devices from multiple manufacturers. It also allows handheld equipment from original equipment manufacturers (OEMs) to focus exclusively on the design of their equipment without having to design a customized wireless power pad, since multiple Qi-compliant power sources are already available.

The key to interoperability within the Qi standard is the communication protocol. This article explains the fundamentals of how the receiver (RX) device (handheld equipment) communicates with the transmitter (TX) device (charging pad providing energy). Communication packets sent by the receiver (and the corresponding response from the transmitter) are illustrated. Communication from the receiver to the transmitter allows the closed-loop control and regulation of the receiver circuit’s output voltage.

Introduction

Wireless power systems are emerging as a practical option for conveniently recharging mobile phones and other handheld devices. Implementing an industry standard interface allows a common charging pad (TX) to recharge multiple types of battery-operated devices (RX). The WPC developed the Qi standard for wireless power systems with up to 5W of output power. This allows complete interoperability between transmitters and receivers independent of device manufacturer.

This standard defines the means of implementing a number of functions that enhance the utility and efficiency of a wireless power system, for example:

• The charging pad does not consume significant amounts of standby power when idle (no device placed on the pad).

• The TX can detect the presence of an object placed on the pad, and further determine that it is a valid, Qi-compliant RX device.

• Once an object is placed on the charging pad, the transmitter can output a variable power level based on the TX’s requirements.

• The RX unit communicates its power needs back to the TX unit over the same magnetic coupling used for power transmission

Figure 1 shows a block diagram of the overall Qi-compliant wireless power system. 

References 1 and 3 provide a more complete description of the wireless power system.

Basics of Communication

The resonant converter in the TX circuit generates a quasi-sinusoidal AC signal in the range of approximately between100-200 KHz across the primary TX coil. This signal is magnetically coupled into the secondary RX coil, where additional circuitry rectifies and regulates it to provide DC output to the handheld device.

Communication from the RX back to the TX uses the same magnetic coupling path as the forward power transfer. A simple load modulation method is used to communicate status and commands back to the TX controller. When the receiver circuit is powered-up, it can apply a controlled pulsed load across the secondary coil (Figure 2). This results in an amplitude modulation of the primary coil voltage which is detected and demodulated by the TX controller.

The load modulation can use either a resistive or capacitive load element. The modulated load is internal to the RX side circuitry and independent of the actual system load (battery or portable device).

 

Figure 3 shows an example of the actual effect seen on the primary TX coil voltage as a result of the modulation pulses on the secondary RX side. These waveforms correspond to the ideal waveforms illustrated in Figure 2. The modulating signal on the RX side is measured at the point labeled 「COMM DRV」 in Figure 1.

Communication packets

The data in Figure 3 shows that load pulses on the RX side correspond to an amplitude modulation effect on the primary side (TX coil voltage and/or current). The communication from the RX side uses a 「differential bi-phase」 bit encoding scheme. Since there is no separate clock line or control signal path, a fixed clock frequency of approximately 2 Khz is used with a start bit before each 8-bit transmission, followed by parity and stop bits. Figure 4 illustrates the bit / byte encoding schemes as defined in the WPC Specification v1.0.2 (Reference 1).

A communications packet consists of four specific sections:

• Preamble: a fixed sequence of several 「1」 bits, which allows the TX circuit to detect the start of communications, synchronize to the bit stream, and detect the start bit of the header byte to follow.

• Header byte: defines what type of information is to be transmitted (「packet type」) and the length of the message to follow.

• Message field: typically this is one byte, but could be larger depending on the type of information required; the size of the message field is determined by the packet type.

• Checksum byte: used at the end of each packet to allow the TX side to verify that no errors occurred after each packet transmission.

A number of specific functions are defined by the types of packets that can be sent. As of now, not all possible options are implemented, but room is still available for expansion of functionality with future versions of the Qi standard. Reference 1 provides a complete description of existing packet types. The most common packets and their functions are:

• Signal strength: used to help align the RX unit on the charging pad. This allows the charging pad to provide a visible or audible indication to the user when the signal strength sent back from the RX unit is good enough to allow power transfer (for example, the RX coil is properly positioned with respect to the TX coil).

• Control error packet: this returns a signed integer value (–128 to +127) that indicates the degree of error between the value of the input voltage seen by the RX and its desired input voltage. The TX circuit adjusts its output using a proportional-integral-differential (PID) algorithm in response to the signed value of the control error packet. When a large error exists between the actual and desired value of the RX coil voltage, the RX controller sends the error packets at a faster rate of approximately 32 ms intervals. As the coil voltage gets closer to the desired setpoint, the error packets are sent at a reduced rate of approximately 250 ms intervals (Figure 5).

• End power transfer packet: request by the RX unit instructing the TX to terminate power output. Typically this is due to a fault condition or if the receiver device no longer requires power, for example, when the RX device's battery has been fully charged.

• Rectified power packet: this is an unsigned integer value that communicates the amount of power the RX sees at the output of the rectifier circuit. The TX uses this information to determine the overall coupling efficiency as well as to determine when the RX is at its maximum power limit. The TX terminates power transfer when no packets are received for a fixed interval (350 ms minimum, 1800 ms maximum), indicating that the RX device has been removed from the pad.

 

Figure 5. Control error packet transmission and VRECT voltage response to a negative load transient (500 mA to 0 mA).
 
Optimizing load transient response using RX-to-TX communication

The ability of the RX side circuitry to communicate back to the TX side circuitry allows the overall wireless power system to act as a true closed-loop regulated power system, since the equivalent function of an analog error signal feedback is accomplished by the RX controller's control error packets being sent back to the TX controller. From an overall system point of view, the implementation shown for the wireless power TX/RX combination can be treated as a switch-mode converter with a low-dropout (LDO) post-regulator.
  
The raw input voltage to the RX coil can be highly variable in an "open loop" configuration as it will fluctuate significantly with variable load. To maintain good regulation of the final DC output (for example, +5V), the feedback provided from the RX side adjusts the input to the linear regulator (VRECT) up or down, based on the load current conditions.

Figure 6. Control error packet transmission and VRECT voltage response to positive load transient (0 mA to 500 mA).

When the output is lightly loaded, the RX circuit sends control error packets back to the TX controller to increase the VRECT input voltage applied to the LDO stage up to approximately +7V. The regulated output is set to +5V. Since the load current is light, the 2V input-output differential across the LDO does not represent significant power loss.

The reason for setting the VRECT level higher at light loads is to anticipate the effect of a low-to-high current transient. When this transient occurs, the VRECT voltage initially sags until the RX controller can respond. The error packets sent by the RX controller request the TX controller to raise the output voltage. Leaving the VRECT level high at light loads provides enough headroom to prevent the +5V regulated output from collapsing until the digital communication (feedback) can be sent.

At higher load currents, the VRECT voltage is kept as low as possible to minimize power loss across the LDO (and maximize total system efficiency). For example, at the maximum load current of 1.0A, the VRECT signal is set to approximately 5.20V. For example, the dropout performance of the LDO regulator stage within the bq51013 (wireless receiver IC) allows it to maintain a regulated 5.0V at 1.0A load current.

Figures 5 and 6 illustrate the adjustment in the VRECT setpoint based on load current described earlier. Note that the COMM signal bursts shown correspond to complete packets rather than individual bits due to the time scales of the plot. When the VREC deviates significantly from the desired setpoint by a large error, the COMM packets are sent at a faster interval. As the VRECT approaches the desired setpoint, the COMM packet transmission interval is decreased.

Figure 7 shows the system output voltage response to a large load transient (corresponding to the maximum load transient case of 0A à 1A). The maximum load transient results in less than 100 mV droop at the output, such as an approximate two percent deviation from the regulated output voltage.

Measurement of RX and TX signals

Qi-standard wireless power devices allow a system designer to implement a Qi-compliant power system using an integrated solution, such as the bqTESLA™, and requires no programming implement the communications protocol. Evaluation modules (EVMs) are available for both TX controller and RX controller sections.

Figures 8 and 9 are partial schematics from the bqTESLATM evaluation modules which highlight the measurement points used to collect the data in Figures 3, 5, 6, and 7. In the case of the discrete receiver circuit (>SLVU420), the COMM DRV signal can be directly measured since the load modulation FETs are external to the RX controller IC. When using the integrated RX circuit (SLVU477) the load modulation FETs are integrated within the RX controller and their gate drive signals cannot be accessed. However, the communication pulses from the RX controller still can be detected by measuring the differential voltage across the load capacitor C13 as shown. The complete schematics of the bqTESLATM EVM kits are provided in Reference 4 and 5.

Summary

When using a fully integrated Qi-standard chipset solution, all of the communication from receiver to transmitter is handled automatically with no user programming required. However, a basic understanding of the communication protocol can help the system designer know how to test and verify that the system is operating properly.

At a fundamental level, the communication can be thought of as amplitude modulation (AM) with a modulation frequency of 2 KHz and carrier frequency ranging from 100 to 200 KHz. This simple, robust protocol defined by the WPC Qi standard allows communication to occur along the same inductively coupled path as the forward power transfer, and does not require a separate set of contacts or magnetics.

The wireless power receiver’s ability to communicate its power needs back to the transmitter (based on load conditions) allows the system to maintain a stable output voltage under constant or transient load conditions. The closed-loop nature of the overall system is achieved by using a Qi communication protocol.

Acknowledgements

The authors would like to thank Steve Terry, Tony Antonacci, and Michael Day for their technical and editorial contributions to this article.

References

• 「Qi low-power specification,」 Wireless Power Consortium website.
• 「An introduction to the Wireless Power Consortium standard and TI’s compliant solutions,」
  by Bill Johns, Texas Instruments, Analog Applications Journal, Texas Instruments, 1Q2011.
• 「bq25046EVM-687 Evaluation Module User’s Guide,」 SLVU420, Texas Instruments, June 2011.
• 「bq51013EVM-725 Evaluation Module User’s Guide,」 SLVU447, Texas Instruments, March 2011.
• 「bqTESLA Wireless Power Transmitter Manager EVM User’s Guide,」 SLVU467, Texas Instruments, June 2011.
• TI bqTESLA Home Page, Wireless Power Products, Links and FAQs.

Communication

Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to send power and how much.

In order to regulate, the receiver must communicate with the transmitter whether to increase or decrease frequency.

The receiver monitors the rectifier output and using Amplitude Modulation (AM), sends packets of information to the transmitter.

A packet is comprised of a preamble, a header, the actual message and a checksum, as defined by the WPC standard.

The receiver sends a packet by modulating an impedance network.

This AM signal reflects back as a change in the voltage amplitude on the transmitter coil.

The signal is demodulated and decoded by the transmitter-side electronics and the frequency of its coil-drive output is adjusted to close the regulation loop.

The bq500410A features internal digital demodulation circuitry.

The modulated impedance network on the receiver can either be resistive or capacitive.

Figure 1 shows the resistive modulation approach, where a resistor is periodically added to the load,

Figure 2 shows the resulting amplitude change in the transmitter voltage.

Figure 2 shows the capacitive modulation approach, where a capacitor is periodically added to the load and the resulting amplitude change in the transmitter voltage.

 

無線電源接收器RT1650之簡介

摘要

RT1650是全集成的無線電源接收器，可爲移動設備提供7.5W的電源供應。本文解釋了無線電源傳輸的基本原理，簡要介紹了各類無線電源傳輸的標準，重點介紹了名爲Qi的WPC 1.1低功率標準的實現方法，對RT1650的主要特性進行了講解。文章最後以Nokia DT601無線電源發射器和以RT1650爲核心的EVB結合在一塊兒所構成的無線電源傳輸系統爲例對實際應用中可能遇到的各類問題進行了講解，具備重要的參考價值。

1.無線電源傳輸之優勢

在兩個裝置之間，無實體纜線鏈接而能傳輸電源之方式有許多優勢：

• 在兩裝置之間徹底的電流隔離，可以使應用更加安全。
• 電源接收器可徹底封閉，使應用裝置較易達到徹底防水。
• 省去體積較大的鏈接器可以使總體應用的體積減小，對於如藍牙耳機、智能手錶和健康（醫療）等方面的穿戴式裝置而言，是很是重要的。
• 非接觸式的電源傳輸很是方便：無需插上或拔出鏈接器，只需將接收器放在發射器的表面上，便可開始電源傳輸。
• 無線電源傳輸多用於行動裝置電池的充電，所以也常被稱爲「無線充電」。

2.無線電源傳輸之原理

最多見之無線電能傳輸方法是透過兩個耦合的電感器之間的磁感應來實現的，交流變壓器即爲應用此原理之一例，線圈附近的磁場強度是隨距離呈指數關係降低的，換言之，若要高效率的電源傳輸，就必須使發射器線圈和接收器線圈之間的距離儘量地近，或是其距離要遠小於線圈的直徑，以達到高效率的電源傳輸。

另外一種容許發射器和接收器線圈之間的距離較大的方法稱爲諧振感應耦合法，。在這種系統中，發射器和接收器的內部都有諧振在相同頻率上的LC電路，電源就在這一諧振頻率上被傳輸。兩個線圈之間的諧振可增強相互之間的耦合，並改善電源傳輸的效率。此方式容許的發射器和接收器線圈之間的距離可較大，但和磁感應式相比，其最大可傳輸功率較低。

3.無線電源的標準

隨着須要按期充電的便攜設備（如手機，平板計算機和筆記本計算機）的用量的增長，無線電源聯盟（Wireless Power Consortium ，WPC）於2008年成立，其使命爲制定在電子裝置之間進行無線電源傳輸的標準。 2009年，WPC 推出了命名爲Qi 的低功率標準，使得任何符合 Qi 標準的裝置之間可以以磁感應方式傳輸5W如下的電源。

目前，針對無線電源傳輸的標準有三種：Qi，PMA和A4WP。Qi和PMA 都是應用磁感應原理，而A4WP則是採用諧振感應原理。表一顯示了這三種標準之間的差別。

Qi 和PMA標準的性能很是接近，允許電源傳輸在短距離（一般約爲5毫米）上對單一接收裝置進行，發射器和接收器之線圈必須對齊以實現高效率的電源傳輸，兩裝置之間的通信透過所傳輸的電源信號進行，避免了額外硬件的使用。Qi和PMA在通信協議上是有差別的。

Rezence（先前的稱呼是A4WP）使用磁諧振感應耦合技術， 容許在較大的距離下 (高達約50毫米) 傳輸電源，發射器和接收器之線圈不須要很好地對齊，其缺點是全系統效率較低，能傳輸的功率較低。接收器和發射器之間以藍牙做爲通信手段，這使多個設備之間的通信成爲可能，所以允許多個裝置從一個發射器接收電源。因爲須要額外的硬件實現藍牙鏈接，此解決方案的成本較高。

Qi WPC 1.1*低功率標準（5W）是目前被最普遍採用的手機無線供電方法。

*新的WPC 1.2標準已於2015年6月發佈，RT1650接收器能夠兼容WPC 1.1和WPC 1.2。

從接收器到發射器的ASK通訊

WPC 1.1標準中，接收器對發射器的通訊採用ASK（幅度鍵控）的反向散射方法進行：接收器經過對接收到的來自發射器的信號的幅度進行調製，該信號幅度的變化會被反射到發射器一側，再經解調、解碼之後供系統使用。

在咱們的案例中，接收器一側的ASK調製由開關控制的與接收到的交流信號並聯的電容來實現，這至關於給交流信號增長了一個額外的負載，它將致使交流信號電壓的降低（或者說是增長了交流信號的電流）。這種被改變了的交流信號幅度會被反射到發射器一側，發射器能夠檢測到交流信號的變化，這種變化可在信號電壓或電流的包絡上看出來，通過對它的解調製操做之後便可取得其原始信息。

經過圖13的波形能夠看出ASK調製的基本表現形式。因爲是串行信號，它包含了時鐘信息和數據，其第一部分是前置的用於時鐘同步的信號，緊接着是數據包的頭部、被編碼的信息，最後是校驗碼。信息的傳輸採用了差分雙相編碼方式，以下圖所示