Power Electronics

Wireless Power Receiver IC Complements Existing Transmitter

A wireless Power Receiver, the bq51013, complements the bq500110 Power Transmitter introduced about six months ago. Both TI ICs comply with Wireless Power Consortium (WPC) Qi Standard Version 1.0.2 (April 2011) for wireless power transfer based on near field magnetic induction between planar coils.

Find a downloadable version of this story in pdf format at the end of the story.

Wireless power transfer employs a Power Transmitter with a primary coil that creates a magnetic field on a charging pad. When placed on the charging pad, a secondary coil that has an associated wireless Power Receiver converts the induced magnetic field into a dc output voltage. The bq51013 is a wireless Power Receiver IC with full-bridge synchronous rectification, voltage conditioning and wireless power control ( Fig. 1). The bq51013:
  • Complies with the WPC Qi Standard, producing up to 5W (5V @1A)

  • Enables powering or charging from TI's bq500110 or any available Qi-compliant transmitter.

  • Has 93% peak rectification efficiency that reduces thermal rise inside the system while allowing charge rates comparable to an AC adapter.

  • Has built in protection against voltage, current and temperature fault conditions, ensuring safe and reliable system operation.

  • Integrates voltage conditioning and full wireless power control

  • Is housed in a 1.9-mm × 3-mm WCSP package

The bq51013 allows designers to integrate wireless power technology into their existing and new applications with minimal impact on solution size. It is intended for portable consumer devices such as smart phones, gaming systems, digital cameras, along with medical and industrial equipment.

The Power Receiver works with the Power Transmitter in a wireless power transfer system. The bq51013 Power Receiver controls the power transferred by sending feedback (error signal) communication to the Power Transmitter's primary coil (e.g. to increase or decrease power). The Power Receiver communicates with the Power Transmitter by changing the load seen by the transmitter. This load variation results in a change in the transmitter coil current, which is measured and interpreted by the Power Transmitter's processor. Communication involves digital packets transferred from the Power Receiver to the Power Transmitter. Differential bi-phase encoding is used for the packets. The bit rate is 2-kbps. The WPC Standard defines various types of communication packets, including identification and authentication packets, error packets, control packets, end power packets, and power usage packets.

The Power Transmitter's coil stays powered off most of the time. Occasionally, it wakes up to see if a receiver is present by transmitting a “ping” to the Power Receiver. When a Receiver authenticates itself to the transmitter, the transmitter remains powered on. The receiver maintains full control over the power transfer using communication packets.

WIRELESS APPLICATION

Fig. 2 shows a system that uses the bq51013 as a 5V power supply while power multiplexing the wired (adapter) port. When placed on the charging pad, the Power Receiver coil couples inductively to the magnetic flux generated by the coil in the charging pad, inducing a voltage in the receiver coil. An internal synchronous rectifier feeds this voltage to the RECT pin which has filter capacitor C3.

The bq51013 identifies and authenticates itself to the Power Transmitter's primary coil by switching COMM1 and COMM2 in and out. If the authentication is successful, the Power Transmitter remains powered on. The bq51013 measures the voltage at the RECT pin, calculates the difference between the actual voltage and the desired voltage, VRECT-REG, (~7V for the bq51013 at no load) and sends back error packets to the primary coil in the Power Transmitter. This process goes on until the input voltage settles at VIN-REG. During a load transient, the dynamic rectifier algorithm enhances the power supply's transient response.

A voltage control loop maintains the output voltage at VOUT-REG (~5V for the bq51013) to power the system load (charge a battery). The bq51013 meanwhile continues to monitor the input voltage, and maintains sending error packets to the primary every 250ms. If a large transient occurs, the feedback to the primary speeds up to every 32ms in order to converge on an operating point in less time.

If the input voltage suddenly increases (e.g. a change in position of the equipment on the charging pad), the voltage-control loop inside the bq51013 becomes active, and prevents the output from going beyond VOUT-REG. The receiver then starts sending back error packets to the transmitter every 30ms until the input voltage comes back to the VRECT-REG target, and then maintains the error communication every 250ms.

If the input voltage increases beyond VOVP (overvoltage protection setting), the IC tells the primary coil to bring the voltage back to VRECT-REG. In addition, a proprietary voltage protection circuit is activated by means of CCLAMP1 and CCLAMP2 that protect the IC from voltages beyond the ICís maximum rating (e.g.20V).

Fig. 2 is an example application that shows the bq51013 used as a wireless power receiver that can multiplex between wired or wireless power for charging the selected battery. In the default operating mode pins EN1 and EN2 are low, which activates the adapter enable functionality. In this mode, if an adapter is not present the AD pin will be low, and /AD-EN pin will be pulled to the higher of the OUT and AD pins so that the PMOSFET between OUT and AD will be turned off. If an adapter is plugged in and the voltage at the AD pin goes above 3.6V, wireless charging is disabled and the /AD-EN pin goes to approximately 4 V below the AD pin to connect AD to the secondary charger. The difference between AD and /AD-EN is regulated to a maximum of 7V to ensure the V GS of the external PMOSFET (Q1) is protected.

The bq51013 includes a ratiometric external temperature sense function. The temperature sense function has two ratiometric thresholds that represent a hot and cold condition. An external temperature sensor is recommended to provide safe operating conditions for the Power Receiver.

An integrated, self-driven synchronous rectifier in the bq51013 enables high-efficiency AC to DC power conversion. This rectifier consists of an all NMOS H-Bridge driver where the backgates of the diodes are configured to be the rectifier when the synchronous rectifier is disabled (Fig. 3). During the initial startup of the wireless system, the synchronous rectifier is not enabled. At this operating point the DC rectifier voltage is provided by the diode rectifier. Once V RECT is greater than UVLO (undervoltage lockout), it enables the half synchronous mode until the load current surpasses 250mA. Above 250mA the synchronous rectifier stays enabled until the load current drops back below 250mA where half synchronous mode will be enabled instead.

POWER TRANSMITTER

The bq500110 (Fig. 1) provides intelligent control of wireless power transfer and facilitates communication in the single channel WPC compliant contactless charging base station. The bq500110 comes in a 48-pin, 7mm × 7mm QFN package and operates over temperature range from -40°C to 110°C.

Functions performed by the bq500110:

  • Periodically pings the surrounding environment searching for available devices to be powered.

  • Minimizes its idle power

  • Monitors all communication from the mobile device being wirelessly powered.

  • Adjusts power applied to the transmitter coil per information received from the powered device.

  • Manages fault conditions associated with power transfer and controls status signal (LEDs) to indicate operating modes.

The bq500110 can provide thermal protection to the transmitter. An external NTC resistor can be placed in the most thermally challenged area. The designer has full flexibility in choosing the NTC resistor, the value of the resistor, and to set the desired temperature when system shuts down. The system will attempt to restore normal operation after approximately five minutes being in the suspended mode due to tripping the over-temperature threshold.

The bq500110 can activate two types of buzzers to indicate power transfer begin. It can output a high logic signal for 0.5s which is suitable to activate DC type buzzers with built in tone generation. It can also output a 0.2s, 4000Hz square wave signal suitable for inexpensive AC type ceramic buzzers.

An inner PID (proportional-integral-derivative) loop feeds the variable frequency driver, which produces a 50% duty cycle with variable frequency. The inner PID loop calculates the necessary frequency, which is then generated by the variable frequency driver. The variable frequency is then fed into a MOSFET power train that excites the serial resonance transmitter coil.

The bq500110 can operate with several types of MOSFET gate drivers to accommodate various power train topologies. The most typical solution uses a synchronous buck driver like the TPS28225 that drives n-channel upper and lower power MOSFETs with a safe dead-time.

The bq500110 has an integrated power-on reset (POR) circuit that monitors the supply voltage. At power-up, the POR circuit detects the 3.3V rise (V33D). When V33D is greater than VRESET, the device initiates an internal startup sequence.

WPC WIRELESS POWER STANDARD

THE WIRELESS POWER CONSORTIUM (WPC) refers to a Base Station as a provider of wireless power and a Mobile Device as a consumer of that wireless power. The Base Station usually has a charging pad and the Mobile Device is placed on the pad to it can charge a battery. The Base Station contains a Power Transmitter with a primary coil and the Mobile Device contains a Power Receiver with a secondary coil. The primary and secondary coils form the two halves of a coreless resonant transformer that transfers power from the Base Station to the Mobile Device. The WPC Standard:

  • Enables wireless transfer of about 5 W, using an appropriate secondary coil with a typical outer dimension of about 40 mm.

  • This resonant power transfer system operates between 110 and 205 kHz.

  • There are two possible methods for placing the Mobile Device on the surface of the Base Station:

  • Guided Positioning helps a user properly place the Mobile Device on the surface of a Base Station that provides power through a single or a few fixed locations of that surface.

  • Free Positioning enables arbitrary placement of the Mobile Device on the surface of a Base Station that can provide power through any location on that surface.

  • A simple communications protocol enables the Power Receiver in the Mobile Device to control the transfer of power.

  • Exhibits very low standby power (implementation dependent).

Typically, power transfer from a Power Transmitter to a Power Receiver consists of four phases:

  • In the selection phase, the Power Transmitter monitors the interface surface for the placement and removal of objects. Initially, if it does not have sufficient information for this, the Power Transmitter repeatedly pings the Power Receiver. If the Power Transmitter does not select a Power Receiver for power transfer and is not actively providing power to a Power Receiver for an extended amount of time, the Power Transmitter goes to a standby mode.

  • In the ping phase, the Power Transmitter executes a digital ping, and listens for a response. If the Power Transmitter discovers a Power Receiver, the Power Transmitter may extend the Digital Ping, i.e. maintain the Power Signal at the level of the digital ping. This causes the system to proceed to the identification & configuration phase. If the Power Transmitter does not extend the digital ping, the system reverts to the selection phase.

  • In the identification & configuration phase, the Power Transmitter identifies the selected Power Receiver, and obtains configuration information such as the maximum amount of power that the Power Receiver intends to provide at its output. The Power Transmitter uses this information to create a Power Transfer Contract that contains limits for parameters that characterize the power transfer. At any time before proceeding to the power transfer phase, the Power Transmitter may decide to terminate the extended digital ping, which reverts the system to the selection phase.

  • In the power transfer phase, the Power Transmitter continues to provide power to the Power Receiver, adjusting its primary coil current in response to control data that it receives from the Power Receiver. Throughout this phase, the Power Transmitter monitors the parameters that are contained in the Power Transfer Contract. A violation of any of the stated limits on any of those parameters causes the Power Transmitter to abort the power transfer — returning the system to the selection phase.

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