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BQ25756: Heating Problems with 48V input and 10S lithium-ion battery charging at 10A

Deniz Ilhan
Prodigy 120 points
Part Number: BQ25756

Tool/software:

We started designing our own charging solution relying on BQ25756, and starting from BQ25756EVM. Our goal is to be able to charge a 10S lithium-ion battery at 10A. We use a 48V AC/DC converter as the input to the system.

What we are realizing is that both the inductor and the FETs in buck configuration heat up more than expected, easily 40C temp rise even though we are below saturation current for the inductor and the FETs are not specced to heat up this much at the set current level. We made a couple changes compared to the eval board so we decided to go back and repeat the same test there. We realized the eval board similarly overheats at 10A.

Do you have any suggestions on how we can mitigate this?

- inductor choices

- FET choices and using more than one in parallel

- switching frequency changes

- modifications on the config? We tried to change FETs speed and dead times but couldn't get any improvements out

Adding the schematics and a snapshot of the board for reference:

/cfs-file/__key/communityserver-discussions-components-files/196/ChargeControl.pdf

/cfs-file/__key/communityserver-discussions-components-files/196/ChargeFETs.pdf

/cfs-file/__key/communityserver-discussions-components-files/196/PCBA_2D00_3D.pdf

  • 0
    TI__Mastermind 31425 points

    Hello Deniz,

    Thanks for working on this. If possible, can you send us the full layout of the PCB? Just an image will work fine.

    In the meantime, I have a few suggestions to reduce the heat:

    • Can you change the switching frequency to 200kHz? This will reduce the switching losses.

    • Can you increase the gate drive current to 7V or 10V? This will reduce the conduction losses of the charger.

    • Can you try setting EN_CONV_FAST_TRANSIENT=0?

    • Can you measure the waveforms on SW1 and SW2?

    • Does the system heat up at all variations of the input or output voltage? Or, does the system heat up in just a small range of the input or output voltage?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 120 points

    Hello,

    Thank you for responding. Is there a way for me to send the design layers just to you or will I need to share it publicly here?

    I'll order the appropriate resistor to switch down to 200KHz. Our own prototype was at 300kHz and the evalboard is at 250kHz.

    Would you like me to increase the gate driver voltage on the evalboard? It will be trickier to do this on the prototype since I will need to cut some traces but I can give it a try. As is, the internal supply is driving the gates.

    I will try setting EN_CONV_FAST_TRANSIENT to 0 tomorrow and report accordingly. I can also measure SW1 and SW2 while doing this.

    We observed the overheating at various battery voltage levels but did not carefully characterize the relationship. Would you expect it to be worse closer to 42V or closer to 25V?

  • 0 in reply to Deniz Ilhan
    TI__Mastermind 31425 points

    Hello Deniz.

    Deniz Ilhan said:
    Is there a way for me to send the design layers just to you or will I need to share it publicly here?

    Can you send me the design layers over E2E private message?

    Deniz Ilhan said:
    Would you like me to increase the gate driver voltage on the evalboard?

    This would be a good thing to test.

    Deniz Ilhan said:
    Would you expect it to be worse closer to 42V or closer to 25V?

    I would expect efficiency to worse when VAC and VBAT are separated by a large voltage or when VAC≈VBAT.

    Let me know what you find out.

    Best Regards,
    Ethan Galloway