BQ24045: bq24045 + bq51013B + QI antenna

Hi Joaquim
If the heating is occurring after battery charge is complete with coil current lower it may be related to RX coil tuning.
Couple of thing to check:
1.) What is the value of Cs capacitors, between coil and AC1?
2.) Check the voltage at RECT pin during 10 - 12mA condition.
3.) Also check the TX operating freq during the above condition.

Hi Bill,

Thanks for your fast help. Here my answers.

1.) What is the value of Cs capacitors, between coil and AC1?

Please, refer to the attached image for values. All are NP0 and 25Vdc. The coil is 10uH from Würth, but mesuring it above TX, etc. the real value is about 12.5uH. Cs is 103nF and Cp is 2.43nF, so the tank resonator frequency is close to 140KHz.

2.) Check the voltage at RECT pin during 10 - 12mA condition.

7.5Vdc

3.) Also check the TX operating freq during the above condition.

Around 128KHz.

Hi Joaquim
The higher RECT voltage at light load may be causing the TX to generate a larger magnetic field and this is causing the heating.
BQ51013B will change RECT voltage with load, Dynamic Rectifier Control see section 7.3.2 in data sheet. It will improve transient response.

A test that you could do is to separate the RX coil from case, leads between coil and rest of circuit / case. You would want a spacer that will keep coil to coil distance the same. Lead length can be increased without a large impact on performance. Distance only need to be long enough to get the case outside of magnetic field.
This will give you a measure of the heat generated in the coil vs AC field entering case/battery/PCB.

Hi Bill,
I am trying disable the BQ51013B when the battery is fully charged, keeping the RX above the TX. In normal conditions, AD, EN2 and EN1 pins are pulled to "0". AD-EN pin is floating. I never use the wired power. According to the table 3 in section 7.3.6, I pull EN1 to "1" to deactivate the QI so as to reduce the temperature, but QI is still activated... Vout is 5V and Vrect is around 7V. The EN1 voltage level is 3.3V (minimum of 1.3V according to the datasheet). Can the OTG mode explain this? It is estrange because I don't have back-to-back PMOS and my battery is giving 4.2V.
Must I to use EN2 instead of EN1 to deactivate QI RX?

Thanks.

Joaquim Martinez.

Hi Bill,

Thank you very much for your help.

Yesterday I fixed the problem reported in my last post. It was a HW issue. Once solved, I could deactivate the BQ51013B. As you know, I am figthing to reduce the temperature once the battery is fully charged which now is around 41-42ºC for an ambient temperature of ~24-25ºC. Deactivating the QI, I can reduce it, of coarse, but the problem for me is that my device has two modes of performance; A mode is when the device is out of the charger, B mode is when it is charging or it is above TX. A and B modes are different, for instance, A mode allows to work some communication but in B mode, this communication is prohibited. Until now I am using PG (Power Good) pin of the BQ24045 in order to know if my device is above the TX (no matter if it is charging or not) or conversely it is removed from TX, so A or B modes are activaded/deactivated according to PG signal.

In general if I deactivate the BQ51013B, I lose the PG acknowledge. However, if I pull the EN1 = 1 and EN2 = 0, through a 3 sec polling timer (activating and deactivating EN1 pin) I get to know if my device is above the TX or not. I have 3 sec delay but can be enough, because the change from B to A mode has to be quite fast. Here the temperature is about 34ºC. The problem here is I am multiplexing every 3 sec the QI process, and it would seem no "nice" solution. 

Today I performed the change in my HW according to your suggestion, and the performance is different. Once the battery is full charged, I set EN1 = EN2 = 1, BQ51013B send "charge complete" to TX and QI charge is deactivated. After this, I activate again both EN1 = EN2 = 0, in order to do the PG signal polling, but TX takes 60 seconds to activate, then the PG signal is activated, etc. This time is not good for the app, because it will take 60 second to change from B to A mode, and many things can happen in these 60 seconds...

So, do you think switching ON/OFF QI charge is a good practique? Is there other ways to reduce the temperature of the RX coil without deactivating QI charge? or using other pins of BQ51013B / BQ24045 maybe? and the most important, how can I detect if the device is still above TX if I set EN1=EN2=1?

And finally, how works the QI charge in the smartphones? It is disconnected when battery is full charged? The temperature in my smartphone (S7 edege) is very low when battery is full charge and it seems QI charge is not stopped...

Many thanks.

Hi Joaquim
Can we discuss this offline, I have sent you a friend request?
Accept the request and then you can send me a message with attachments, privately.

Hi Bill,

Perfect. Thank you.