Method for detecting a parasitic metal object on a charging surface, and associated charging device

A method for detecting a foreign metal object, by way of a charging device, comprising a microcontroller and a transmitting circuit, said transmitting circuit comprising at least one transmitting antenna, which are suitable for charging a portable item of user equipment placed on a charging surface at an operating frequency, the method comprising:

• a1) generating a predetermined number of voltage pulses at the terminals of the transmitting antenna at a parasitic resonant frequency, said parasitic resonant frequency being different and distinct from the operating frequency,
• a2) transmitting the generated predetermined number of voltage pulses at the terminals of the transmitting antenna, at the parasitic resonant frequency,
• a3) generating plural frequencies at the transmitting circuit, wherein a first oscillating frequency of the plural frequencies is determined by comparing a voltage at the terminals of the transmitting antenna with two threshold voltages for a predetermined period, and a second oscillating frequency of the plural frequencies is generated by a receiver in response to an electromagnetic field transmitted by the transmitting antenna;
• b) analyzing the plural frequencies of the transmitting circuit that are oscillating simultaneously such that:
• b1) when the first oscillating frequency is higher than the parasitic resonant frequency, a foreign metal object is detected on the charging surface,
• b2) when the first oscillating frequency is lower than the parasitic resonant frequency, the portable item of user equipment to be charged is detected on the charging surface, and
• b3) when the second oscillating frequency oscillates at a second resonant frequency and when the first oscillating frequency is higher than the parasitic resonant frequency, then the simultaneous presence of the portable item of user equipment to be charged and of a foreign metal object are detected on the charging surface, wherein the parasitic resonant frequency is between 200 kHz and 1 MHz, the second resonant frequency is different and distinct from the parasitic resonant frequency, and a value of the second resonant frequency is between 800 KHz and 1200 kHz.