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Components for Wireless Power Transfer Charger Design

Created: October 9, 2020
Updated: October 10, 2024

Don’t want your customers to add another cord to their collection? Add wireless power transfer to your design.

The other day, I received a wireless charging pad in the mail from my old university. I’m not sure why they decided to send this out, but it’s pretty convenient for keeping my phone charged while I listen to podcasts during work. It outputs just enough power to keep my phone charge constant while I have audio playing and while I occasionally check Facebook. The physics that govern these systems are easy to understand, and with the number of components available on the market, they are easy to build as well.

Wireless power transfer is about more than just convenience for keeping your phone charged during work. Environments packed with wireless IoT products will need some way to extend lifetimes as long as possible without manually changing batteries. Wireless power transfer is one way to accomplish this goal without sending a technician around to change batteries. If you’re interested in wireless power transfer, here’s what you need to know and some options for components you’ll find on the market.

Types of Wireless Power Transfer

Wireless power transfer occurs in two possible modes: inductive coupling and resonant inductive charging. Both methods are near-field methods, i.e., the device being charged needs to be quite close to the charger. Most wireless charging systems specify a range of less than 50 mm, and placing the receiving device closer to the charger provides faster charging.

The primary difference between the two is in terms of tuning. For an inductive coupling charger, the transmitting and receiving devices both use a large coil with inductance in the μH range. The transmitting and receiving coils are typically arranged so that the receiving device steps up or down the voltage/current to be in the appropriate charging range for the battery. The design goal is to set the received voltage/current such that charging time is minimized while preventing overcharging, which decreases battery life.

In a resonant inductive charger, a capacitor is used with the coil to create a series LC resonator. The resonant frequency of the LC circuit can be tuned to match frequency of the received signal, which maximizes current in the receiver. This can be done with a varactor diode, a small microcontroller, and a small current-sense amplifier with a feedback loop. This is then used to adjust the capacitance from the varactor to sit within a certain range.

There are two sets of standards on wireless power transfer products as specified by the Wireless Power Consortium (the “Qi” standard) and the Power Matter Alliance. The Qi standard can be made compatible with USB-PD for devices that would normally charge over a Type-C cable. The table below summarizes the device standards specified by both organizations.

For wireless power transfer without charging, the same concepts apply: power is received inductively, and it is sent downstream to power the device without charging. Simply use a standard regulator without battery management features and you’ll be powering a device from a distance.

Components for Wireless Power Transfer

The components shown below can be used for either inductive coupling or magnetic resonant inductive wireless power transfer modes. The components you need fall into 3 areas:

  • Power regulation and battery management: This includes FETs for on/off switching at the Tx end, a standard power regulator at the Rx end, or a regulator with battery management at the Rx end.

  • Transmission and reception: Power needs to be efficiently transmitted and received with high-inductance coils; you’ll then need to add a capacitor or varactor diode for tuning the resonant frequency to the Tx frequency.

  • Rectification: DC power is needed to charge batteries, so a small rectifier and power capacitor will be needed to convert the received signal to DC.

  • Control and tuning: You might want to switch the unit on or off, as well as control any ICs over standard interfaces.

Würth Elektronik, 760308103204

The 760308103204 Rx charging coil from Würth Elektronik is designed for a range of applications in larger devices. This coil provides flat inductance out to high current (10 A) and high switching frequency (~2 MHz), as shown in the graphs below. Würth Elektronik offers similar components in wireless arrays both for Rx and Tx sides of a wireless power transfer system. In addition, Würth Elektronik offers coils that combine wireless power transfer and NFC reception into a single package.

Inductance vs. frequency and current in the 760308103204 from Würth Elektronik. From the 760308103204 datasheet.

Analog Devices, LTC4124

If you’re looking for a compact solution for charging low-capacity batteries in small wearables or other low-power devices, the LTC4124 from Analog Devices is a good choice. This small SMD component provides pin-selectable voltage and current output (up to 100 mA and 4.35 V maximum, respectively). For temperature-qualified charging, this component includes an NTC resistor input, which eliminates the need to implement a control feature with an MCU.

Wireless charging application circuit with the LTC4124 wireless power transfer controller. from the LTC4124 datasheet.

Infineon, BSC065N06LS5ATMA1

The BSC065N06LS5ATMA1 N-channel MOSFET from Infineon is part of the OptiMOS line of MOSFETs. This component is rated for 60 V output, low on-state resistance (6.5 mOhm), and 64 A drain current. The logic-level drive means this MOSFET has low gate threshold voltage, allowing this component to be driven with a 5 V output from a microcontroller. This makes the component a central part of an inductive coupling or resonant inductive wireless power transfer circuit. The 8-pin SMD package also allows this component to easily fit into a small PCB.

Package and pin diagram for the BSC065N06LS5ATMA1 MOSFET. From the BSC065N06LS5ATMA1 datasheet.

Other Components for Wireless Power Transfer

A wireless power transfer system will need other components at the Tx and Rx ends to help maximize power transfer to/from coils, condition the DC output, and provide tuning for resonant power transfer circuits.

Whether you want to add wireless power transfer capabilities to a new mobile device or you’re designing a power transmitter, you can find the components you need with the advanced search and filtration features in Octopart. Octopart gives you a complete solution for sourcing and supply chain management for nearly any new electronic system. Take a look at our integrated circuits page to start searching for the components you need.

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