Designing wireless charging systems

Robust circuit protection is the front-line defense against overloads, ESD, and other transients says Todd Phillips

Wireless chargers offer users great convenience for re-charging their battery-operated devices. The ease with which the charger can quickly put a portable device in a charging state, is driving growth in the mobile and wearables markets. In addition to consumer devices such as smartphones and smartwatches, medical products such as blood glucose monitors and industrial bar code scanners are transitioning to wireless charging. Furthermore, in-cabin charging for consumer devices is required in the latest new cars and trucks. As a result, the range of products that can employ wireless charging continues to expand (Fig. 1). The wireless market continues to grow at an explosive rate with a compound annual growth (CAGR) rate of 29%.

The need to directly connect to the portable device is no longer necessary due to wireless charging, which saves the user the inconvenience of looking for the proper charging cable with the appropriate connector to connect to the device.

However, electronic designers must face several challenges when developing wireless charging devices. For example, space limitations for circuitry and cost considerations challenge designers to develop their devices while using the fewest components possible without compromising quality. The next-generation mobile/wearables designs must also protect their products from failures related to overloads, ESD, and other transients. Additionally, electronics designers need to optimize their circuits for power efficiency. Perhaps the most important, products must comply with international standards for safety, surge and transient protection, and the USB protocol and Qi charge transfer power standards.

Elements of a wireless charging system

Wireless charging systems consist of three elements:

power adapter,

charging cable

wireless charging pad.  

The power adapter converts AC voltage from the power line into a DC voltage. The charging cable sends power from the adapter to the charging pad, which transmits the power wirelessly to the mobile device-to-be-charged. It may seem ironic to call this configuration "wireless" since the charging cable provides a wired connection between the adapter and charging pad. This set-up's convenience and "wireless" nature derive their name from the fact that last connection is no longer required; the connection between the power adapter and the device-to-be-charged. Fig. 2 provides an example wireless charging pad used to charge a consumer mobile phone, including the recommended circuit protection and sensing components for each section of the wireless charging system.

Power adapter and charging cable protection

A block diagram for the power adapter and charging cable is shown in Fig. 3. The Power Adapter must contend with the overload and transient conditions presented by the AC mains, the AC power line. Designers should protect the Power Adapter from overloads at the input stage. Common overloads include; switching surges, induced lightning voltage surges, electrostatic discharges, and overload fault currents.

Low-power power adapters use a variety of overvoltage protection solutions. Power adapters with 15 watts or greater output often use MOVs for overvoltage protection. Regardless of power adapter output power, fuses are the primary choice for overcurrent protection. Designers have a wide range of options for the form factor of a fuse. They can select cartridge fuses, thru-hole fuses, or surface mount fuses. Surface-mount fuses typically consume the least amount of circuit board real estate. The designer must ensure that the fuse has sufficient voltage and current interrupting ratings regardless of which form factor is selected. Designers should consider a time-lag fuse to avoid nuisance interruption from overvoltage events. Each fuse type has a different response characteristic to overloads. In addition, if designers are attempting to maximize energy efficiency, they should evaluate the watts-loss rating of the fuse.

High-frequency converter and clamp circuit protection and efficiency

Designers can maximize charger efficiency by selecting MOSFETs with low on-state resistance, low gate charge, and high dv/dt rating to reduce switching loss and obtain faster switch transition times. MOSFETS with low on-state resistance and high dv/dt parameters allow a higher frequency operation, enabling a switch-mode supply circuit topology to be more efficient. Designers should use MOSFETS with internal soft-recovery diodes, reducing turn-off transients and reducing electromagnetic emissions (EMI).

After the step-down transformer lowers voltage, Schottky diodes rectify the signal back to DC. Consider using Schottky diodes with low forward voltage drop and which can operate at high frequencies for this design section.

Some transients are known to find their way to the output rectifier, and some may be large enough to damage the power-semiconductor devices. Designers can consider transient voltage suppression (TVS) diodes to protect the circuit from those external voltage transients. TVS diodes can respond to a transient extremely quickly in under 1 pico-second. They also have low clamping voltages to protect sensitive electronic circuits. As shown in Fig. 4, designers can select either uni-directional or bi-directional configurations of a TVS diode.

USB Type-C port and charging cable overtemperature protection

The USB Type-C protocol allows up to 100 W charging to enable devices to be re-charged quickly. This substantial increase in available power over prior USB standards is enabled through USB Type-C connectors. These connectors have a 0.5mm pitch, five times less than USB Type-A connectors.

With more power in a much smaller space, there is an increased risk that dust and dirt can easily short pins on the connector and create an overtemperature condition. Electronics engineers should consider using a digital temperature indicator, like the PolySwitch setP, to detect an overtemperature condition. The setP temperature indicator is used in the Configuration Channel (CC) line of the Type-C connector to detect the over-temperature event and help protect the circuit.

The setP devices rapidly increase their resistance when their temperature reaches around 100°C. Fig. 5 shows the characteristic resistance versus temperature curve for two setP temperature indicators.

The setP temperature indicator is compliant with the USB Type-C standard for monitoring the temperature of USB Type-C connectors. Details on the circuit configuration for this protection scheme are in the USB Type-C cable and connector specification.

Wireless charging pad protection

The Wireless Charging Pad power input is either a proprietary DC input or a USB port. See Fig. 6. Designers should protect the DC input circuit from both overloads and transients. Regardless of the power input in use, protection is still necessary for the design.  

Designers should consider fast-acting fuses for the DC input circuit for overload protection. Small, surface-mount fuses with the proper DC voltage rating are ideally suited for this purpose. Surface mount TVS diodes are available for transient protection, which can provide up to ± 30 kV of ESD protection and 1500 W of peak transient power absorption. Low clamping voltages, typical for most TVS diodes, help avoid stressing downstream circuit components in the event of a transient strike. With a TVS diode and a fast-acting fuse, designers can have their wireless charging pad fully protected from overloads and transients.

When using a USB port for the power input of the wireless charging pad, provide thermal sensing and transient voltage protection via a setP temperature indicator and a TVS diode array.

Complying with applicable international standards

Designers need to be aware of the standards with which their wireless charger must comply. The standards define minimum safety requirements and provide testing instructions on evaluating various electrical hazards such as ESD, electrical fast-transients, and surge withstand requirements. Wireless chargers using USB communication must ensure interoperability according to the Universal Serial Bus (USB) standard. Designers should also be familiar with the Qi wireless charging protocol for transferring charge to a product’s battery. Table 1 lists the standards designers may consider applying to their designs. Failure to adhere to standard requirements can result in expensive re-design work and delays in product introduction and revenue generation.

Value of making safety a top priority

Proper protection of circuits helps ensure a positive end-user experience. The choice of appropriate control components can maximize product efficiency and reduce total power consumption. Protection and control components, such as the components recommended in this article also helps designers comply with the relevant standards.

Take advantage of the knowledge of the manufacturer’s experts on component selection. Involving a manufacturer’s application engineers early in the design cycle can save a designer substantial time and reduce design revisions.  

Equipped with this information and support from the manufacturer, designers can develop wireless chargers that are dependable and safe for their users.