Table of Contents

Foreign Object Detection in the Qi standard

Classification of FOD methods

In the Qi standard, the transmitter emits power to the receiver via an high-frequency alternating magnetic field. Qi technology is defined as a low-power transfer standard which implies that it poses no health risk to living objects (LOs). In the Qi standard, no living object detection (LOD) is performed as there is no space between the charging pads where LOs (e.g. animals) can enter and interfere with the WPT. However, this does not mean that this magnetic field between the transmitter and receiver is weak. Metal objects (MOs), such as keys or coins, can induce eddy currents within themselves when present in the magnetic field. This can cause overheating of the MO and bring forth high-risk situations like fire hazard. (Zhang et al., 2019, p. 1,2).

In terms of detection principles, FOD technologies can be categorized into four types, as illustrated in Figure 1. The first type involves detecting electrical system parameters. It relies on monitoring changes in characteristics such as power loss, efficiency and the quality factor of the system. This method is considered the most cost-effective, as it utilizes existing hardware already integrated into the system design. It is also particularly suitable for metal object detection (MOD) in low-power systems, where the presence of a MO induces significant variations in electrical parameters. In contrast, the impact of a MO in high-power applications on the system parameters is comparatively small. The second category, non-electrical system parameter detection, involves the integration of additional sensors positioned near the charging area to measure variables such as temperature or pressure. Since this method does not rely solely on power transfer behaviour, it is applicable to both low- and high-power scenarios and is suitable for detecting both MOD and LOD (Tian et al., 2022, pp. 2-4,12-13; Zhang et al., 2019, pp. 2–5, 8–9).

While wave-based and field-based detection methods are theoretically applicable to low-power systems, they are not employed within the Qi standard. This is primarily due to their reliance on additional equipment such as optical sensors, infrared cameras or ultrasonic transceivers to detect FOD or potential fire hazards. This significantly increases system complexity and cost. The average Qi device has a compact, low-cost and mass-producible design (Technologies, 2023, p. 29; Zhang et al., 2019, pp. 8–9).

Figure 1 FOD methods (Zhang et al., 2019, p. 3)

FOD technology applied in Qi standard

Analysis of a Qi-authorized 15 W transmitter: the Infineon WLC1515, reveals that it uses three distinct FOD methods based on system parameter detection. These include Q-factor measurement, power loss monitoring and over-temperature protection (Technologies, 2023, p. 29).

1. Q-factor detection

The Q-factor (quality factor) of a coil is a measure of its efficiency in storing energy relative to the energy it loses. It is defined by the formula in Equation 1.

Q=2πfL/R Equation 1

Where

When a metal object (MO) enters the coil’s magnetic field, it disturbs the field and induces eddy currents in the object. These eddy currents cause the coil’s inductance 𝐿 to decrease and its resistance 𝑅 to increase due to additional energy losses. As a result, the Q-factor drops, which can be used as an indication of a foreign object in wireless power transfer systems. As examined by Tian et al., when metal objects enter the charging zone, the quality factor of the Rx coil exhibits a significant downward trend, making it a suitable index for MOD within WPT systems such as the Qi standard (Tian et al., 2022, p. 3; Wireless Power Consortium (WPC), n.d.).

Practically, the quality factor of the receiver coil will be measured in a pre-power transfer phase. Next, Figure 2 shows how Q-factor FOD is created (Infineon, 2023, p. 5; Technologies, 2023, p. 25).

Figure 2 Q-factor FOD (Infineon, 2023, p. 5)

2. Power loss detection

Power loss-based FOD is a real-time method used during active power transfer. This approach involves periodically measuring the transmitter output power and comparing it to the power values reported by the receiver via the Received Power Packet (RPP). The MO is detected when the measured system power loss exceeds the system power loss threshold value, as shown in Figure 3. By forcing the threshold breach to last for three consecutive readings, it prevents the system from responding to single outliers before cutting power. This increases detection reliability and user experience (Infineon, 2023, p. 9). As Tian et al. note, power loss-based FOD methods are easily affected by coil misalignment, making it difficult to distinguish between actual foreign object-induced losses and normal fluctuations in power transfer. The recent updates in magnetic alignment via the magnetic power profile (MPP) mitigates this issue by ensuring consistent coil positioning (Tian et al., 2022, p. 12)

Figure 3 power loss FOD (Infineon, 2023, p. 9)

3. Over-temperature detection

The last system parameter detection method is a non-electrical one which implies that an additional temperature sensor will monitor the temperature of the transmitter coil. This is typically a negative temperature coefficient (NTC) thermistor.

According to the WLC1515 datasheet, this thermistor enables the system to monitor the coil’s temperature in real time and disconnect power transmission if the temperature exceeds a configurable safety threshold (Zhang et al., 2019, p. 3).

References