Wireless power transmission Wireless Power Consortium (WPC) communication

Universally Compatible Wireless Power Using the Qi Protocol

Wireless charging of portable electronic devices is here now. It would 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 have to design a customized wireless power pad, since multiple qi-compliant power sources is already Available.

The key to interoperability within the Qi, the communication protocol. This article explains the fundamentals of what the receiver (RX) device (handheld equipment) communicates with the transmit ter (TX) device (charging pad providing energy). Communication packets sent by the receiver (and the corresponding response from the transmitter) is 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 is emerging as a practical option for conveniently recharging mobile phones and other handheld dev Ices. Implementing an industry standard interface allows a common charging pad (TX) to recharge multiple types of Battery-operat Ed 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 wireles s 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's a valid, qi-compliant RX D Evice.

• Once An object was placed on the charging pad, the transmitter can output a variable power level based on the TX ' s Requirem Ents.

• 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 between100-2 XX KHz across the primary TX coil. This signal was magnetically coupled into the secondary RX coil, where additional circuitry rectifies and regulates it to P Rovide 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 was used to communicate status and commands back to the TX controller. When the receiver circuit was powered-up, it can apply a controlled pulsed load across the secondary coil (Figure2). This results in an amplitude modulation of the primary coil voltage which are 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 dev ICE).

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 was measured at the point labeled "COMM DRV" in Figure 1.

Communication packets

The data in Figure 3 shows this load pulses on the RX side correspond to a amplitude modulation effect on the PR Imary 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 are 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< /c2>).

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 transmitted ("Packet type") and the length of the message To follow.

• Message field: typically this was one byte, but could was 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 all packet to allow the TX side to verify that no errors occurred after each p Acket transmission.

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

• 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 fro M The RX unit is good enough to allow power transfer (for example, the RX coil are properly positioned with respect to the TX coil).

• Control error packet:  this returns a signed integer value (–128 to +127) that indicates th E 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 Val UE 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 p Ackets at a faster rate of approximately-ms intervals. As the coil voltage gets closer to the desired setpoint, the error packets is 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 was due to a fault condition or if the receiver device no longer requires power, for example, when the RX de Vice ' s battery has been fully charged.

• Rectified Power packet: A unsigned integer value that communicates the amount of power the RX sees at th E output of the rectifier circuit. The TX uses this information to determine the overall coupling efficiency as well as to determine when the RX was at its MA Ximum power limit. The TX terminates power transfer when no packets is received for a fixed interval (ms Minimum, 1800 MS Maxi Mum), indicating that the RX device had been removed from the pad.

Figure 5.  control Error packet transmission and Vrect voltage response to a negative load transient (at Ma to 0 MA). /strong>
 
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 a analog error signal feedback are Accomplished by the RX controller's control error packets being sent back to the TX controller. From a 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 is highly variable in an "open loop" configuration as it would fluctuate Significa ntly with variable load. To maintain good regulation of the final DC output (for example, + +), 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).

When the output was lightly loaded, the RX circuit sends control error packets back to the TX controller to increase the VR ECT input voltage applied to the ldos stage up to approximately +7v. The regulated output is set to + +. Since the load current was 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 are to anticipate the effect of a low-to-high current Transien T. 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 in light loads provides enough headroom to prevent the + V regulated output from collapsing u Ntil the digital communication (feedback) can be sent.

At higher load currents, the vrect voltage are kept as low as possible to minimize power loss across the LDO (and maximize Total system efficiency). For example in the maximum load current of 1.0A, the vrect signal are set to approximately 5.20V. For example, the dropout performance of the LDO regulator stage within the BQ51013 (Wireless receiver ICS) allows it to Mai Ntain 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 O f the plot. When the Vrec deviates significantly from the desired setpoint by a large error, the COMM packets is sent at a faster int Erval. 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 TR Ansient case of 0AÀ1A). The maximum load transient results in less than MV droop at the output, such as an approximate, percent deviation f Rom 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 a integrated solution, such as the bqtesla™, and requires no programming implement the communications protocol. Evaluation Modules (EVMs) is available for both TX Controller and RX controller sections.

figures 8 and 9 is partial schematics from the BQTESLATM evaluation modules which highlight the measurement poin TS 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 mo Dulation FETs is external to the RX controller IC. When using the integrated RX Circuit (SLVU477) The load modulation FETs is integrated within the RX controller and their Gate drive signals cannot is accessed. However, the communication pulses from the RX controller still can is detected by measuring the differential voltage Acros s the load capacitor C13 as shown. The complete schematics of the BQTESLATM EVM kits is provided in Reference 4 and 5.

Summary

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

At a fundamental level, the communication can is thought of as amplitude modulation (AM) with a modulation frequency of 2 KHz and carrier frequency ranging from. This simple, robust protocol defined by the WPC Qi standard allows communication to occur along the same inductively COUPL Ed 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 it power needs back to the transmitter (based on load conditions) AL Lows 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 contributio NS 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 transmitter, where the receiver tells the transmitter to send POW ER and how much.

In order to regulate, the receiver must communicate and 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 TRANSM Itter.

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 a impedance network.

This is 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 it coil-drive output is ad Justed 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 was 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 was periodically added to the load and the resulting a Mplitude change in the transmitter voltage.

Wireless Power receiver RT1650 Brief summary

The RT1650 is a fully integrated radio source receiver that provides 7.5W power supply to mobile devices. This paper explains the basic principle of radio source transmission, briefly introduces the standard of various radio source transmission, introduces the implementation method of WPC 1.1 low power standard named Qi, and explains the main characteristics of RT1650. At last, the wireless source transmission system composed of the Nokia DT601 Radio source transmitter and the eVB with RT1650 as the core is taken as an example to explain the various problems that may be encountered in practical application, which has important reference value.

1. Advantages of Radio source transmission

Between two devices, there are many advantages in the way that you can transfer power without physical cabling:

• Complete galvanic isolation between the two devices makes the application more secure.
• The power receiver can be completely closed, making the appliance easier to achieve complete waterproofing.
• The absence of larger connectors can reduce overall application volume and is important for wearable devices such as Bluetooth headsets, smart watches, and health (medical).
• Contactless power transmission is convenient: without plugging in or unplugging the connector, simply place the receiver on the surface of the transmitter to start the power transfer.
• Wireless power transmission is used to charge the battery of mobile devices, so it is often referred to as "wireless charging".

2. Principle of Radio source transmission

The most common radio energy transmission method is achieved through the magnetic induction between two coupled inductors, the AC transformer is one example of the application of this principle, the magnetic field strength near the coil is exponential relationship with the distance, in other words, to efficient power transmission, It is necessary to make the distance between the transmitter coil and the receiver coil as close as possible, or the distance is far less than the diameter of the coil, in order to achieve efficient power transmission.

Another method that allows a larger distance between the transmitter and the receiver coil is called the resonant-inductive coupling method. In this system, both the transmitter and the receiver have an LC circuit resonant at the same frequency, and the power supply is transmitted on this resonant frequency. The resonance between the two coils strengthens the coupling between each other and improves the efficiency of the power transmission. This method allows a larger distance between the transmitter and receiver coils, but has a lower maximum transmit power compared to the magnetic induction type.

3. Standard for radio sources

With the increase in the amount of portable devices that require regular recharging, such as mobile phones, tablets and laptops, the Radio Source Alliance (Wireless Power Consortium, WPC) was established in 2008 and its mission is to develop standards for the transmission of radio sources between electronic devices. In 2009, the WPC introduced the low power standard named QI, which enables the magnetic transmission of power supplies below 5 watts between any QI-compliant device.

Currently, there are three standards for the transmission of radio sources: QI,PMA and A4WP. Both QI and PMA apply the principle of magnetic induction, while A4WP uses the principle of resonant induction. Table I shows the differences between the three standards.

The performance of the Qi and PMA standards is very close, allowing the power transmission to be carried out on a single receiving device at short distances (usually about 5 mm), the coils of the transmitter and receiver must be aligned for efficient power transmission, and the communication between the two devices is carried out through the transmitted power signals, avoiding the use of additional hardware. There are differences in the communication protocols between QI and PMA.

Rezence (formerly known as A4WP) uses magnetic resonance inductive coupling technology to allow transmission of power at large distances (up to about 50 mm), the transmitter and receiver coils do not need to be well aligned, with the disadvantage that the system is less efficient and can transmit less power. Bluetooth is used as a means of communication between receivers and transmitters, which makes communication between multiple devices possible, allowing multiple devices to receive power from one transmitter. This solution is expensive due to the need for additional hardware to enable Bluetooth connectivity.

The Qi WPC 1.1* low Power Standard (5W) is currently the most widely used mobile wireless power supply method.

* The new WPC 1.2 standard was released in June 2015 and the RT1650 receiver can be compatible with WPC 1.1 and WPC 1.2.

Ask communication from receiver to transmitter

In the WPC 1.1 standard, the receiver communicates with the transmitter using the Ask (amplitude keying) Backscatter method: The receiver modulates the amplitude of the received signal from the transmitter, and the amplitude of the signal is reflected to the transmitter side, which is then demodulated and decoded for use by the system.

In our case, the ask modulation on the receiver side is implemented by switching the capacitance in parallel with the received AC signal, which is equivalent to adding an additional load to the AC signal, which will cause the AC signal voltage to drop (or increase the current of the AC signal). This changed AC signal amplitude will be reflected to the transmitter side, the transmitter can detect the change of AC signal, this change can be seen on the signal voltage or current envelope, after the demodulation of its operation can obtain its original information.

The basic representation of the ask modulation can be seen through the waveform in Figure 13. Because it is a serial signal, it contains the clock information and data, the first part of the pre-set for clock synchronization signal, followed by the packet header, the encoded information, and finally the check code. The information is transmitted using a differential two-phase encoding, as shown in

Wireless power transmission Wireless Power Consortium (WPC) communication