Electronic Design
Take Charge When Creating A Qi-Compliant Wireless-Power Accessory

Take Charge When Creating A Qi-Compliant Wireless-Power Accessory

Standardization among different transmitters and receivers receives high priority within the emerging wireless-power market. Previously, any company that sold a wireless-power receiver also had to sell a corresponding transmitter. This hinders the market adoption of wireless power, and eventually leads to a proliferation of different, incompatible wireless power technologies.

To address the problem, the Wireless Power Consortium (WPC) came up with Qi (pronounced “chee”)—the first global standard that enables interoperability between compliant transmitters and receivers at power levels up to 5 W.1 The Qi standard, introduced in July 2010, defines the operating frequency, operating voltages, and basic coil configurations for a wireless-power system. In addition, a communication protocol is defined whereby a receiver can communicate information to a transmitter, such as when the transmitter should terminate power (i.e., enter a power-saving mode when the phone is no longer charging), how much power the receiver requires, and whether output power should be increased or decreased.

The end result is that accessory products now can offer wireless power for a mobile device without the need for a basestation (wireless power transmitter). A common approach is to offer a sleeve, back door, battery pack, or holster that contains the receiver coil and electronics in the accessory product.

WPC (Qi) System Overview

In a WPC-based inductive wireless-power system, the transmitter consists of an ac-dc power converter, driver, transmit coil, voltage and current sensing, and controller (Fig. 1). The receiver features a receive coil, rectification, voltage conditioning (i.e., regulation), and controller. The load can be any battery-powered device, e.g., a cell-phone handset.

Power transfers from transmitter to receiver via a coupled magnetic field that’s created when an ac current flows through the transmitter coil. If a receiver coil is in close proximity (less than a 5-mm gap in X-Y or Z dimension), a significant portion of the transmitter field lines will be coupled to the receiver coil. These coupled field lines create an ac current in the secondary, which can be rectified to produce a dc voltage. This results in a power source for a cell phone or other portable device. Note that the wireless power link is essentially a loosely coupled, air-core transformer.

Accessory Architectures

The quickest path to Qi-compliant products is to provide accessory solutions with either a power supply or a direct battery-charge implementation while leveraging the standard for basestation sources (wireless transmitters). In this case, “accessory solution” refers to wireless-power functionality that’s added as an option for the mobile device.

The two most popular accessory implementations are a sleeve and a back cover. A sleeve refers to a plastic shell that contains the wireless power circuitry, clips on to the mobile device, and provides power to the mobile device via external contacts. A back cover is a replacement for the mobile device’s standard back cover, which includes the wireless-power circuitry. An alternative accessory solution features wireless-power circuitry in the mobile device’s battery pack and charges the battery directly.2

Power-supply accessory: A wireless-power receiver can mimic the operation of a power-supply adapter by providing a 5-V, 5-W power source to the mobile device (Fig. 2). In the simplest implementation, only two contacts—wireless power and ground—are required between the receiver and the mobile device.

Since most first-generation Qi products will still contain a wired connector, charging can be achieved from either a wired adapter or a wireless power source (Fig. 2, again). Both sources connect to a power multiplexer inside the mobile device. Typically, the adapter power is selected by default and wireless power becomes enabled in the absence of an adapter.

During wireless-power transfer, operation should be interrupted when an adapter is present or upon termination of battery charging. This can be achieved by sending a message to the transmitter to stop power transfer when the receiver observes a no-load condition. To simulate this condition, open the wireless-power receiver switch in the multiplexer. Additional contacts can help provide more information about the specifics of no-load condition.

Two-contact accessory: Though the two-contact solution is the least expensive interface between the wireless-power output and receiver, its functionality is limited. When there are only two contacts, only wireless power (i.e., 5-V output) and ground can be connected to the mobile device. Furthermore, the mobile device must detect independently when to switch between adapter power and wireless power.

This solution’s main drawback is the mobile device’s difficulty in signaling to the transmitter that charging has terminated. In a typical wireless-power system, a charge cycle may start when the user goes to bed at night, and the charging typically continues for around two hours. Once charging is complete, it’s desirable for the receiver to signal end power to the transmitter (as defined by the WPC protocol) so that the transmitter can go into a low-power standby mode. However, the receiver can only detect termination when the output current drops below a certain threshold. While this method enables the transmitter to enter standby mode, it’s limited due to the supply current being the sum of the system current plus the charging current.

Three-contact accessory: This solution improves on the two-contact solution by adding a control signal along with the wireless power and ground. The control signal can be an input to the wireless-power receiver that’s driven by the mobile device.

A typical application would involve the charger inside the mobile device detecting termination of charging and signaling this condition to the receiver. The receiver, in turn, can communicate end power to the transmitter, and the transmitter enters a low-power standby mode.

The mobile device is continuously powered by a battery; therefore, it can continue asserting termination to the wireless receiver for an indefinite period of time. As a result, total transmitter power consumption during the complete charge cycle will be very low. Moreover, the transmitter can use the end power information from the receiver to let the user know that charging has terminated (e.g., by signaling an LED).

This method also is more accurate in determining the termination condition. That’s because the mobile device can measure battery current directly, while the wireless-power back cover can only measure the sum of the system current and battery current.

Four-contact accessory: A four-contact solution provides even more options than the three-contact option. For instance, it provides two control signal inputs—one could signal termination to the transmitter while the other signals that a fault condition is present in the mobile device.

Figure 2 shows an alternative four-contact implementation. An external adapter can be input to the device receiver and an adapter-FET gate-drive signal can be output from the receiver and connected to the mobile device. In this way, the receiver is able to detect the presence of an adapter, turn off the wireless power transmitter, and then apply the adapter voltage directly to the receiver.

Mobile device power multiplexer: During the first wave of wireless-power accessory adoption in the marketplace, the wired adapter port will remain alongside the wireless power input. This requires a power multiplexer between the wired and wireless power supplies (Fig. 3).

Such an approach leverages the receiver accessory by sensing the adapter voltage (AD) and providing the gate drive (AD_EN) in the presence of adapter voltage. The FET must be wired in a “back-to-back” configuration to block both reverse and forward conduction when the switch is turned off. The wireless-power receiver then can disable power transfer once the adapter is present and keep the gate drive active via the adapter supply. This approach requires at least a four-pin interface between the accessory and mobile device (wireless power, AD, AD_EN, and GND).

The amount of pins required between the power-supply accessory and the mobile device can be reduced by leveraging an automated power multiplexer (Fig. 4). This approach doesn’t require the AD and AD_EN connections. Priority is given to the wired power path via the VSNS connection. If a voltage is detected at VSNS, the wired power path is active. Otherwise, the wireless power path is active.

For the receiver electronics to detect the presence of the adapter port to terminate wireless-power transfer, it must monitor the supply’s output current. Monitoring the output current makes it possible to detect a true light load (e.g., near zero output current) when the wireless-power path switch is turned off. Then the receiver can send a command to the transmitter to terminate power transfer.

Battery-pack accessory: Integrating the electronics and receiver coil into the mobile-device battery pack allows the end user to recharge the battery pack while either installed in the mobile device or by placing it directly on the transmitter pad (similar to a cradle charger user experience). However, the implementation of the handoff between the wired- and wireless-power charger becomes restricted. It would be optimal to provide a solution that doesn’t add to the battery pack’s pin count.

A temperature sense resistor (NTC) embedded in the battery pack ensures safe operating temperatures of the battery cell while charging (Fig. 5). However, in this unique application, the receiver electronics can use it to detect whether wired charging is active or disabled.

When the mobile-system battery charger is active, there will be some voltage across the NTC resistor. When inactive, the NTC resistor pulls down to the ground reference of the pack. Therefore, the receiver electronics in the pack can detect the presence of this voltage and immediately disable the wireless-power charger. This scenario only occurs when the receiver is placed on the pad while the wired adapter is connected. Though uncommon, delivering twice the charge current can be detrimental to the cell’s safety.

Such an approach gives the wired-power path priority because the receiver detects the NTC signal and takes proper action. To prioritize wireless power, the detection algorithm should be employed in the mobile system. This reverses the system’s detection routine, which monitors the NTC pin for the voltage that’s present when the wireless-power charger is active.

Figure 5 includes the voltage and current loops immediately after the rectification stage of the receiver electronics. As a result, the controller can implement the charging algorithm, which optimizes integration and efficiency by removing the “voltage conditioning” stage shown in power-supply accessories.

References:

  1. Wireless Power Consortium, http://www.wirelesspowerconsortium.com/
  2. For more about WPC implementations and information about design challenges, go to http://www.ti.com/wirelesspower
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