Today's users require the convenience wireless chargers offer for easily and quickly recharging their battery-operated portable devices. The ease with which wireless chargers can put devices in a charging state is mainly responsible for the mobile and wearables market growth. Other consumer devices, including smartphones, smartwatches, and healthcare products like fitness monitors, are transitioning to wireless charging. Furthermore, the latest cars and trucks require in-cabin charging for these consumer devices. As a result, the variety of products incorporating wireless charging continues to expand (Figure 1), resulting in a compound annual growth rate (CAGR) of 29%, according to IHS Markit.
Due to wireless charging, the need for directly connecting to the portable device is no longer required, which saves end-users the frequent inconvenience of searching for the proper charging cable with the correct connector.
However, design engineers must address numerous challenges when developing their wireless charging products. For one, space limitations due to size and cost considerations challenge today's designers to create their devices while using as few components as possible without compromising quality and reliability. Next-generation mobile/wearables must also have robust protection from failures resulting from ESD, overloads, and other transients. Electronics designers also need to optimize their circuits for power efficiency. Perhaps most important, products must comply with several international safety, surge, and transient protection standards, Qi charge transfer power standards, and USB protocol standards.
This article helps today's electronics engineers understand the methods for circuit protection, efficient control and safety sensing of wireless chargers. These recommendations also assist electronics designers in complying with the appropriate safety standards for their new products.
Wireless charging system overview
Wireless charging systems have three elements: a power adapter, a charging cable and a wireless charging pad. AC voltage from the power line is converted into DC voltage by the power adapter. The charging cable provides the adapter power to the charging pad, which wirelessly transmits the power to the mobile device being charged. It seems ironic to call this configuration "wireless" because the charging cable is a wired connection between the charging pad and power adapter. However, this set-up's convenience and "wireless" nature achieve their name because the last connection is not required (i.e., the connection between the power adapter and the device to be charged). Figure 2 illustrates a wireless charging pad used to charge a smartphone, including recommended circuit protection and sensing components for the wireless charging system.
Protecting the power adapter and charging cable
Figure 3 illustrates a power adapter and charging cable. Designers must protect power adapters against overload and transient conditions presented by the AC mains and the AC power line. Overloads frequently occur at the input stage, including induced lightning voltage surges, switching surges, overload fault currents and electrostatic discharges (ESD).
Low-power power adapters require several overvoltage protection solutions. Adapters with 15 watts or greater output typically use metal oxide varistors (MOVs). Fuses are the preference for overcurrent protection, regardless of the adapter's output power and designers have many form factor options, including cartridge fuses, thru-hole fuses or surface-mountable fuses. The surface-mount fuses consume the least amount of printed circuit board (PCB) space. Regardless of the form factor chosen, be sure the fuse has a sufficient voltage and current interrupting rating. To avoid nuisance interruption from overvoltage events, consider using a time-lag fuse. Every fuse option has a different response characteristic to overloads. If maximizing energy efficiency is a design requirement, be sure to evaluate the watts-loss rating of the fuse being selected.
Protecting high-frequency converter and clamp circuits while improving efficiency
To maximize charger efficiency, select MOSFETs with low gate charge, low on-state resistance and high dv/dt rating, reducing switching loss and providing faster switch transition times. MOSFETs with low on-state resistance and high dv/dt allow higher frequency operation, enabling an even more efficient switch-mode supply circuit topology. Use MOSFETs with internal soft-recovery diodes; this reduces both turn-off transients and electromagnetic emissions (EMI).
After the step-down transformer lowers the voltage, use Schottky diodes to rectify the signal back to DC. For this design section, consider Schottky diodes with low forward voltage drop, which can operate at high frequencies.
Some transients can find their way to output rectifiers, and some may be large enough to damage power semiconductors. To protect the circuit from external voltage transients, consider using transient voltage suppression (TVS) diodes, which can respond extremely quickly (under 1 pico-second) to a transient event. TVS diodes also have low clamping voltages that help protect sensitive electronic circuits and are available in uni-directional or bi-directional configurations (Figure 4).
Overtemperature protection for USB Type-C Port and charging cable
The USB Type-C protocol allows up to 100 W charging, enabling devices to be recharged quickly. The USB Type-C connector substantially increases available power over prior USB standards. These connectors have a 0.5 mm pitch, five times less than USB Type-A connectors.
Because there is more power in a significantly smaller space, there is an increased risk that dust and dirt can short the connector's pins, creating an overtemperature condition. Consider using a digital temperature indicator, like the PolySwitch setP, to detect any overtemperature conditions. Used in the Type-C connector's Configuration Channel (CC) line, the setP temperature indicator will catch the over-temperature event and help protect the circuit.
When the temperature reaches around 100° C, the setP devices rapidly increase their resistance. Figure 5 illustrates the characteristic resistance versus temperature curve for two setP temperature indicators.
The setP temperature indicator complies 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.
Protecting the wireless charging pad
The wireless charging pad power input is either a USB Type-C port or a proprietary DC input (Figure 6). Be sure to protect the DC input circuit from both overloads and transients. Protection is still necessary for the design, regardless of the power input in use.
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.
Comply with applicable international standards
Design engineers must be aware of the standards with which their wireless chargers 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 provides the standards that 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.
Safety: A top priority for mobile and wearable devices
By incorporating proper circuit protection as a forethought, not an afterthought, in their electronics design, design engineers help ensure a positive end-user experience. Selecting appropriate control components maximizes the product’s efficiency while reducing overall power consumption. The protection and control components recommended in this article also help designers comply with relevant safety standards.
Why not take advantage of the manufacturer’s experts’ knowledge 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.
Wireless Power Market Tracker. Q4 2018. HIS Markit.
Universal Serial Bus Type-C Cable and Connector Specification. Revision 2.0. August 2019. USB Implementers Forum (USB-IF), Inc.
Additional design guide references courtesy of Littelfuse Inc.