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The Wireless Power Consortium’s (WPC) Qi wireless charging standard has entered a rapid developmental phase from 2022 onwards. Qi Version 2.0 was officially released in 2023, featuring an improved legacy Baseline Power Profile (BPP), Extended Power Profile (EPP) and the new Magnetic Power Profile (MPP) that enables 15W charging speeds through optimal device alignment. WPC has also been expanding wireless charging technology into industries such as smart kitchen appliances (using the Ki wireless power standard), electric vehicles (EV's), and manufacturing robots.
In this article, we’ll be zooming in on MPP Power Loss Accounting (MPLA), LQK measurement, foreign object detection, and other critical Qi2 compliance processes.
It is important for manufacturers to understand the power profiles supported by the Qi and Qi2 standards. All transmitter (Tx) and receiver (Rx) designs bearing the Qi2 logo must support both the MPP and BPP power profiles for backwards compatibility. While Rx authentication support is optional, Tx authentication support is mandatory.
Legacy power profiles such as the BPP and EPP are only supported by the Qi standard. Devices under Qi can support both the BPP and EPP power profiles of Qi versions 1.3.3 and 2.0.
| MPP | EPP | BPP | |
| Qi2 MPP Tx | 15W | n/a | 5W |
| Qi2 MPP Rx | Up to 15W | n/a | 5W |
| Qi EPP Tx | n/a | Up to 15W | 5W |
| Qi EPP Rx | n/a | Up to 15W | 5W |
Differences between Qi and Qi2 power profile support
Common objects such as coins, credit cards, or keys falling between an active power transmitter (PTx) and an active power receiver (PRx) may result in overheating and pose a safety risk to users. The MPP Power Loss Accounting (MPLA) feature mitigates this by enabling foreign object detection. Whenever a foreign object is detected between the transmitter (PTx) and the receiver (PRx), the transmitter renegotiates to a lower power level to minimize heat buildup in the interfering object. This process, known as 'best effort charging', prevents risks associated with overheating while still allowing continuous battery charging.
MPLA Formula
MPLA helps the transmitters estimate the power loss due to the presence of foreign objects in the power transmission path. Power loss can be accounted for with the equation below:
The transmitter measures how much power it is transmitting (Pin) and subtracts all the known losses in the transmission path before finally subtracting the power received by the receiver. Any difference between the known power losses and actual loss may be attributed to foreign object presence within the power transmission path.
Power loss from metal components within transmitters (PTX_FM) and receivers (PRX_FM) is also accounted for using friendly metal loss, which is calculated as PFM_LOSS.
PFM_LOSS = PTX_FM + PRX_FM
Friendly metal loss formula
Friendly metal loss is calculated based on a system model that accounts for DC bias magnetizing current required to maintain power transfer between coils. To accommodate the variance between transmitters and receivers in real world ecosystems, scaling factors are also considered during friendly metal loss calculation.
Ecosystem scaling & correction factors
In comparison, PTx_circuit_loss and PRx_circuit loss are highly dependent on manufacturing designs, with parameters determined by their respective manufacturers' power loss estimations. Calculating coil loss is slightly more complex compared to other parameters, as it depends on coil parameters and mated mutual inductance characteristics.
In addition, these parameters are tightly controlled by the specification which ensures that PTx and PRx coils’ abide by the system model as closely as possible. Mated parameters are measured in compliance testing using mated LQK measurements of the Tx and Rx coils with reference to the Golden Tx (GTPT) and Golden Rx (GTPR) coils respectively. PTx and PRx exchange sensitive MPLA parameter information using the MPLA protocol.
The MPLA protocol data exchanged between PTx and PRx during Configuration and Negotiation phases is used by PTx in power transfer phase to estimate the impact of foreign object presence.
The PTx and PRx exchange the MPLA parameters during the negotiation phase.
Assuming there is no foreign object present, the PRx will use the parameters received from the PTx and calculates the power received at the interface. The calculated power derived from the MPLA is then sent to the PTx during the power transfer phase.
After all known losses from the transmitters' input power (PIN) are subtracted as per the MPLA equation, any remaining difference between the estimated and actual power can be attributed to the presence of friendly metal within the power transmission path. This may also be used for in-power foreign object detection (FOD).
To visualize this, users may filter out protocol packets of interest and select only [Power Loss Accounting] and the [ACK packet] in GRL’s Wireless Power Tx test solution as shown below:
Foreign objects are detected using two primary approaches: Pre-Power and In-Power FOD.
During the Pre-Power phase, the transmitter calculates the open air quality factor (Q). While the transmitter may attribute significant open air Q change to foreign object presence, note that the PRx can also trigger false detection when placed on the PTx. To avoid this, PTx will attribute the Q deflection to a foreign object only if no ASK response is received. Once confirmed, the PTx will advertise safe operating power levels to preemptively mitigate risks.
In-Power FOD occurs when the Tx receives the estimated received power (Pest_rx) and rectified power from the Rx. From there, the Tx estimates the potential foreign object power (Pfo). Should the Pfo value exceed the Pfo threshold, the transmitter will throttle power transmission and renegotiate to a safer operating power.
Coil and friendly metal losses are highly dependent on mated coupling properties (LQK), which in turn refer to the key electrical observables of mated inductive coils. The breakdown of the concept is as follows:
In addition, a MPP Magnetic Boundary (MPPMB) is also defined to ensure interoperability between certified devices.
Graph illustrating relationship between LQK and MPP Magnetic boundary
When it comes to Qi2 wireless charging, MPP Magnetic Boundary (MPPMB) is pivotal for device interoperability. This boundary comprises of multiple levels: a tiny range for GTPT and GTPR pairs used in compliance testing and provided by approved tool vendors such as GRL, and a broader boundary for Tx Coil vs GTPR and Rx Coil vs GTPT pairs (Device vs Golden Pair Boundary). Devices within this boundary not only meet standards but also offer superior FO prediction and field interpretability. Compliance ensures seamless operation and reliable user experience across a diverse range of Qi2 devices.
There is minimal margin for error between the GTPT and GTPR pairs, which are used for LQK Measurement during compliance testing at WPC Authorized Test Labs. The next boundary exists for the Tx Coil vs GTPR and Rx Coil vs GTPT pairs. (Device vs Golden Pair Boundary). Should a device and a golden pair meet these limits, their Tx and Rx pairings can also be expected to fall within these boundaries. Device matting parameters that fall within the boundary can also be expected to have better FO prediction and good interoperability.
Ensure that you have at least two coils soldered with additional cable and SMA connectors to establish connection with the LCR Test jig for LQK measurement. Keep one additional cable & SMA connector for parasitic measurement of the additional soldered cable. Note that the cable's parasitic values should be removed when calculating the LQK boundary. Finally, ensure that your Tx or Rx coil is well packaged so that they can be fixed onto the test jig.
GRL positioning tool
Caution: This position is different from mating with magnetic attachment. The magnetic attachment position is also known as “Magnetic Center”.
Note: Ensure that your coil designs are optimal as they can significantly alter test results.
Watch the full webinar or download the presentation slides for the full list of LQK measurement. equipment.
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GRL-C3-MP-TPR & GRL-C3-TPT Qi2 Wireless Charging Transmitter & Receiver Testers
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Executive Vice President of Protocol and Power Solutions
Raja spent over 23 years in technology development at GRL, Tektronix, Intel and Prodigy Technovations. Raja is involved in developing new technologies and enabling companies to adopt in their product design, has deep experience in post-silicon validation methodologies and developing electrical and protocol compliance testing solutions, and is an expert in multiple technologies including USB, and Qi.