CR95HF and Wireless Charger on 13,56MHZ

[AShub.1]

Hello!

We have combined the CR95HF receiving antenna and wireless charging capability on 13.56Mhz. 

We monitor placed device by user on wireless charging and in this mode we disable attempts to read RFID.

When if wireless charge is enable, on ST_R0 is 10.4V.

This voltage on ST_R0 is present when reading RFID.

This value exceeds the recommended 7V of AN4327, accordingly over 18V maximum differential input voltage between pins RX1 and RX2.

This schematic solution have quality of reading RFID tags is normal.

My questions:

1. It's no damaged problem for CR95HF, if on ST_R0 have 10.4V when RFID card readed ?

2. It's no damaged problem for CR95HF, if on ST_R0 have 10.4V when located on wireless charging ?

In this state voltage on WRLS_CHRG pin up to 20V.

In addition to pin RX, voltage affects on pin TX.

3. In the case of where there is an attempt to cycling read RFID, user transferred device to the electromagnetic field of wireless charging. It is possible that the phases of the oscillations of the field CR95HF and the external field of wireless charging will coincide. In this case, there will be a sharp increase in the voltage amplitude at the input pins of the CR95HF.

It won't make the damage problem CR95HF ?

In this case will it be some effect its internal logical state register, which will ultimately affect its communication with the MCU ?

 

[AShub.1]

It will be possible to answer all questions if there is internal schematic CR95HF.

Can it be this obtained?

[Henry Crane · Accepted solution]

Hello,

to make it simple, ST_R0 pin is a rectified voltage of RX1-RX2 RF voltage. As a consequence DC voltage on ST_R0 is proportional to the amplitude of RX1-RX2 RF differential voltage (with a gap due to AC/DC rectification). ST_R0 pin and RX1/RX2 pin are electrically connected and connecting a resitor on ST_R0 is equivalent to connecting a resistor between RX1 and RX2 to damp the VRX1-RX2 voltage (method explained in AN4327).

Concerning RX pins, the critical parameter is the RX1- GND and RX2-GND voltage. CR95HF embeds internal protections that are activated as soon as RX1-GND and RX2-GND voltages reach 9V. Above 9V internal protections start to clamp the voltage on RX1 and RX2.

When CR95HF operates as reader/writer, if the signal on RX pins (carrier wave + tag response) exceeds 9V , tag response signal may be clipped by Rx protection circuit causing tag response decoding issues. This is the reason why it is recommended to tune the reader with a carrier wave below 9V. 9V on RX1 and RX2 pins with respect to GND is equivalent to 18V differential voltage between RX1 and RX2. This explain the recommendation in note 3 of table 32 for RF reader operations.

Based on the explanations above having 10.4V ST_R0 means more than 10.4V on RX1 and RX2 pins with respect to GND causing a strong performance decrease in tag response decoding and read range. It is strongly recommended to increase R1 and R3 resistor value to maintain RX-GND and RX2-GND below 9V during reader operations.

Absolute maximum ratings in table 28 of the datasheet states that a RX1 and RX2 voltage exceeding 14V may lead to permanent damage. During charging (CR95HF not in operating mode) 10.4V on ST_R0 may correspond to a RX1 and RX2 voltage close to 14V.

Same table states than maximum voltage on TX1 and TX2 is 7V.

based on this I can state

1) 10.4V on ST_R0 during CR95HF read/write operation won't probably not damage CR95HF assuming RX1 and RX2 voltage with respect to GND don't exceed 14V. However, reader read performance is certainly affected and it is strongly recommended to limit RX1-GND and RX2-GND voltage below 9V increasing R1 and R3 values.

2) based on answer 1), 10.4V on ST_R0 when located on charger will probably not damage the CR95HF, assuming voltage on TX pins never exceed 7V.

Limiting RX1 and RX2 voltage below 9V during Reader read/writer operation as recommended in answer 1) will automatically also limit RX1 and RX2 pin voltage below 9V in front of charger.

WRLS_CHRG pin up to 20V may also destroy C7 and C8 if their maximum voltage is below 20V.

3) it is strongly recommended to avoid CR95HF to communicate (that is generate voltage on Tx pins) while set on charge because as you anticipated, signal generated by CR95HF may combine with signal captured from charger and cause voltages on TX pins and RX pins to exceed the absolute maximum ratings. CR95HF may be permanently damaged by Electrical Over Stress and stop to operate.

Best regards,

Henry Crane

STMicroelectronics RFID/NFC support team.

[AShub.1]

Dear Henry, thanks for your reply