FOD function of BQ51221

Hi Jason,

FOD stands for Foreign Object Detection and works by performing a calculation between the amount of transmitted power (from the TX) and the received power (at the RX). The TX calculates how much power it sends and the RX tells the TX how much power it received.

A foreign object is an unwanted metal object (such as a coin) that can absorb a portion of the electromagnetic field and can heat up, causing an efficiency loss or a possible safety hazard.

The calculated power loss is assumed to be losses due to a foreign object. If the calculation reaches a certain threshold, the transmitted power is decreased and/or shuts down.

Theoretically, without foreign objects: TX_power = RX_power.

With foreign objects: TX_power = RX_power + ForeignObject_power.

The threshold value is dependent on the type of coil used, amount of “friendly metal” near the coil, and the tuning capacitors. FOD calibration is done by tweaking the R_FOD and R_OS resistors thus changing the voltage (V_FOD) on the FOD pin. The resistor values are determined through experimental testing based on individual systems and therefor does not have a “one size fits all” calculation. The Foreign Object Detection (FOD) Calibration/Tuning Tool software is located under the Tools & Software tab on the bq51221’s web page. (http://www.ti.com/product/BQ51221/toolssoftware)

The 0xE8 (not 0xE1) register contains the received power status. If the RX (phone) is outside of the available range of the TX, then the RX will not be able to communicate to the TX to tell it how much power it is receiving and therefore the TX will not transmit any power. In cases where the RX is not perfectly aligned (tightly coupled) with the TX but is still in the available range, the TX will increase the voltage to its coil, increasing the field strength to meet the RX’s requirements.

Regards,
Dylan

Hi Dylan:

Thank you so much for your detail and clear explanation.
Now customer want to achieve a function that if their phone is not perfectly aligned (tightly coupled) with the TX , the phone could tell the user to put into the right position to save the Tx power .

1)  Thus, considering this function , the reported power in the 0xE8 register is the real-time and actual power amount 51221 got , am I right ?
2)   Can customer set a threshold of this register value in the host , the reading value is below the threshold and trigger the event to tell customer ? If this is the case, what's the threshold i can set ?

3)  0xEF Bit6 "Align status" could resolve?

Hi Jason,

If you set the “ALIGN Mailer” bit (register 0xE0 B3) to ‘1’ you can monitor the V_RECT status (register 0xE3) in open loop configuration in real-time. The value of V_RECT will be proportional to the alignment (coupling) of the coils.

Take the V_RECT hexadecimal value (from register 0xE3), convert it to decimal, then multiply by 46mV (as specified in the datasheet, Table 10). From that, you can do some comparison math and set thresholds based on your design.

Regards,
Dylan

Thanks, Dylan

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