- Definition
- Foreign object detection (FOD) is the requirement that a wireless charging transmitter detect metal objects within its field and reduce or stop power transfer before they heat up.
This page defines the term. For what it means in practice, see Qi2 explained: what the magnetic profile changes in practice.
Why it exists
A wireless charging transmitter creates an alternating magnetic field. Anything conductive inside that field will absorb energy and heat up — a coin, a key, a paperclip, the metal backing of a phone case, a bank card with a metal layer in it.
The object does not need to be large. A small coin sitting under a phone on a pad can reach temperatures that damage the pad, the phone, the furniture beneath, or the user. FOD is the function that prevents this.
How it works
A transmitter continuously estimates how much power it is delivering and compares that with how much power the receiver reports receiving. The difference is the loss, and part of that loss is attributed to the coupling. When the difference exceeds what the coupling can explain, something else is absorbing energy, and the transmitter reduces or cuts power.
The second approach uses a quality factor measurement before charging begins, or a dedicated sensing array. Different implementations trade sensitivity for false positives: too sensitive and a phone case with a metal plate stops the charger working; too tolerant and a coin gets hot.