Noisy when BQ24650 is designed for 50W

TIN TIN NAING TIN TIN NAING

Other Parts Discussed in Thread: BQ24650

Hello,

I was designing a Solar MPPT charger for 12V SLA battery using BQ24650. I was testing using electronic load since I don't have compatible Battery.

When I was doing on 20W, thw noise sound is low. But, when I designed for 50W to charge a 12V SLA battery with max charging current 4A, the noise sound is very loud when the battery voltage is less than 9V, meanings that while charging at precharge current regulaion phase.

It would be very appreciated if someone can share me experience on BQ24650.

over 14 years ago

0 Jeff F over 14 years ago

TI__Guru**** 166996 points

Noise could be caused by various components, especially ceramic capacitors, vibrating due to control loop instability. We have seen electronic loads cause the charger to become unstable. Do you have a different electronic load? Have you tried it with a battery? Did you modify TI's bq24650 EVM or produce your own board? Can you provide an oscillscope of the voltage across the output capacitor when you hear the noise?

0 TIN TIN NAING TIN TIN NAING over 14 years ago in reply to Jeff F

Prodigy 40 points

Hi Jeff,

Thank you for your prompt reply.

(1) No, I didn't use EVM. I produced a new 2-layer PCB. But I strictly followed the PCB layout instructions as mentioned in application notes.

(2) First, I produced a 20W board to charge a 12V/5AH SLA battery. When I was testing with battery no noise is there. Only when I tested with electroinc load I can hear the noise. The scope image for output cap is attached.

(2) Then I used the same PCB foot print for 50W board. I set MPP voltage as 27V and tried to test with electronic load. Setting of electronic load is 18V, since I am trying to charging to 18V battery. Once the charger is charging in Fast Charge Current Regulation Phase, noise is not there. But the PCB heated. Once I set the load voltage to 16V I can hear a loud noise.

(3)I guess there is PCB layout issue and Inductor rating. But I would like to confirm.

Thanks and Best Regards,

0 Jeff F over 14 years ago in reply to TIN TIN NAING TIN TIN NAING

TI__Guru**** 166996 points

Is the ripple voltage actually close to 2Vpkpk?

0 TIN TIN NAING TIN TIN NAING over 14 years ago in reply to Jeff F

Prodigy 40 points

More than that. It is always around 4 to 6V pkpk.

0 Jeff F over 14 years ago in reply to TIN TIN NAING TIN TIN NAING

TI__Guru**** 166996 points

That is a bit to high. Are you certain that the inductor you choose is appropriate for your application? The datasheet equations on page 21 can help size the inductor and output capacitor for your application.

0 TIN TIN NAING TIN TIN NAING over 14 years ago in reply to Jeff F

Prodigy 40 points

I just followed the suggestions in SLVA437. I want to develope a board to charge a 18V SLA battery using 27V panel (since I cannot use supply beyond 28V). Here are the modifications I made.

(1) I changed V_{MPP_SET} programming resistors to be compitable to 27 V input.

(2) Changed the V_FB programming resistors to be compitable to 20.25V Buck voltage for 18V battery and followed the modifications in SLVA437 so that to match the battery's thermal characteristics.

(3) Since input voltage is 27V I changed the MOSFET to Si7960 (V_DS rating 60V).

(4) Since the charging current is programmed as 2A, I used the same output capacitor 11uF and output inductor 10uH.

Now I assumed that Eelctronic load may the reason for the noise. Because, even with EVK I can hear a low noise. Issue is the PCB heated. Due to my requirements, please advise me how should I modify to charge a 18V battery.

Here I attached the scope images for Vout ripple when I hear the noise, PH node voltage wave form while charging current is about 1 9A and MOSFET gate voltage waveforms while charging with the same current.