BQ51050B: efficiency

Jinwoo Lee

Prodigy 50 points

Part Number: BQ51050B

Hello.

We are working on new product using the BQ51050B for the wireless charging receiver.

We designed 4.2V Li-ion battery, and we charged 1.4A.

We confirmed that the board works well.

But, I confirmed that the RX coil temperature was too hot.(measured temperature was over 80 degrees Celsius)

I think the cause is bad efficiency.

The output voltage of AC adapter is 9V and current is 1.3A (9V x 1.3 A = 11.7W)

RX board is charging at 1.4A with 4.2V. (4.2V x 1.4A = 5.88W)

Approximate efficiency is 50%. Is it ok?

How can we become more efficient?

7 months ago

0 Bill Johns 7 months ago

TI__Guru 69641 points

Hello

What can you tell me about the RX coil you are using?

Also a schematic for review?

0 Jinwoo Lee 7 months ago in reply to Bill Johns

Prodigy 50 points

Hello.

I already reviewed a schematic and give RX coil information.

Refer to below link.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1330383/bq51050b-charging-current/5073888#5073888

0 Bill Johns 7 months ago in reply to Jinwoo Lee

TI__Guru 69641 points

What are the dimensions of the RX and TX coils?

What is distance between them?

0 Jinwoo Lee 7 months ago in reply to Bill Johns

Prodigy 50 points

Hello.

RX Coil :38mm x 21mm x 0.508mm

TX Coil : 58.8mm x 26.8mm x 3.4mm

Distance : 4mm

0 Bill Johns 7 months ago in reply to Jinwoo Lee

TI__Guru 69641 points

Thanks for additional information -- the coil dimensions and spacing look OK.

As a test Reducing spacing distance should improve efficiency.

Coil L is 7.2uH and Cs 247uF ( C552 / C551 / C553 ) this puts the tuning at about 120kHz, should be 100kHz would need Cs of about 350nF

This may help and using low ESR capacitors would help.

Adding magnetic shielding behind the coil, between coil and PCB / Battery may help. This will prevent AC field from entering the stray metal and heating them.

0 Jinwoo Lee 7 months ago in reply to Bill Johns

Prodigy 50 points

Thanks for your response.

I will try what you suggested.

I have one more question.

Typically, if a battery is charged as 1.4 A

How efficiency is it?

0 Bill Johns 7 months ago in reply to Jinwoo Lee

TI__Guru 69641 points

Typical efficiency at 1.4A can be in the 50% range. DCR loss in the coil, Cs caps, and IC will increase with current.

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BQ51050B: efficiency