Test Setup
19
SLVUAO6 – June 2016
Copyright © 2016, Texas Instruments Incorporated
bq501210 bqTESLA™ Wireless Power TX EVM
6.2.2.6
Thermal Protection, NTC
Thermal protection is provided by an NTC resistor connected to JP3. At approximately 1 V on the sense
side (T_SENSE), the thermal fault is set, and the unit is shut down. The status LED, D8, illuminates. The
system tries to restart in 5 minutes.
6.2.2.7
Foreign Object Detection (FOD) and Parasitic Metal Object Detection (PMOD)
The bq501210 EVM supports multiple levels of protection against heating metal objects placed in the
magnetic field during power transfer. PMOD is used for older WPC v1.0 version receivers and FOD is
used for WPC v1.1 and WPC v1.2.
The first level is a short FOD ping that detects most objects before any power transfer is initiated, analysis
of the impulse response is used. Resistors R47 and R48 are used to tune this function, see the data sheet
for additional information.
The second level is power loss accounting, a comparison between power transmitted to the receiver (RX)
with the power the RX reported receiving. The difference is lost power possibly going into a foreign object.
The transmitter determines the power sent to the RX by measuring input power and calculating internal
losses. The RX measures the power it received and also calculates losses. The RX sends this information
to the TX in a digital word, message packet. Unaccounted for power loss is presumed to be a foreign
object on the charging pad. Should this lost power exceed the threshold set by R8, a FOD fault is set and
power transfer is stopped.
Three key measurements for the TX FOD calculation:
•
Input Power
– Product of input voltage and current. Input voltage is measured at V_SENSE though
R10 and R11. Input current is measured using sense resistor R28 and current sense amp U7
(bq500100). Since these measurements are used to calculate the power lost through a foreign object,
both measurements must be accurate.
•
Power Loss in Transmitter
– This is an internal calculation based on the operating point of the
transmitter. The calculation is adjusted using FOD_CAL resistor, R49. This calculation changes with
external component changes in the power path such as MOSFETs, resonate capacitors, and the TX
coil. Recalculation of R49 and R8 is required if changes are made.
•
Receiver Reported Power
– The receiver calculates and reports power it receives in the message
packet
Received Power Packet
.
The FOD threshold on the EVM is set to 600 mW, R8 is set to 100 k
Ω
. Increasing R8 increases the
threshold and reduces the sensitivity to foreign objects.
This loss threshold is determined after making a measurement of transmitter performance using a receiver
calibrated for FOD similar to a unit manufactured by Avid
®
Technology. Contact Texas Instruments for the
FOD calibration procedure for bq501210.
The PMOD threshold on the EVM is set to 600 mW, R16 is 100 k
Ω
. Removing R16 disables this function.
6.2.2.8
HVDCP: High Voltage Dedicated Charger Port
The HVDCP Circuitry is shown in
. The purpose of this circuitry is to give a visual indication of the
negotiation result when using a USB power source that has HVDCP capability. The circuit has been
disabled by default on this EVM to allow more precise measurement of efficiency. To enable the circuit,
simply insert 0-
Ω
resistors for R60 and R61 along with an 8.66-k
Ω
resistor for R50.
To enable the USB port, jumper JP4 must be set to select USB. When an HVDCP supply is plugged into
J5, 5 V is passed to the system. The negotiation for higher voltage is done through the resistors on the
D+/D- pins.
Once the circuit is enabled, two LEDs will indicate the voltage level of the system. When D9 is on, it
indicates the system is about 9 V. D12 indicates the USB is > 12 V.