• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 64 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25756: Gate driver circuit configuration issues

Xiong ZeHua
Prodigy 10 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25758A

Tool/software:

Hello, technical team. I am using this chip to implement a function that charges an 8.4V battery with Vin=25V. My gate drive peripheral circuit is exactly the same as the typical application in the data manual, which is a diode + MLCC boot circuit.

However, in my application scenario, I found through actual testing that the buck half-bridge switches smoothly; however, the boost half-bridge does not switch, or more precisely, the upper transistor is always on and the lower transistor is always off. The gate driver is powered by an external 12V supply, and when outputting 8.4V, the upper transistor's Vgs is only around 3.6V.

I looked back at the datasheet, and section 8.3.3.3 does indeed contain a statement supporting the above phenomenon.

However, the application information in section 9.2.1 of the datasheet provides the following curve:

If the upper transistor's activation is entirely based on bootstrapping, how does the upper transistor on the boost side remain constantly open in the typical application described in Section 9.2.1? I couldn't find any mention of an on-chip charge pump/isolated power supply in the datasheet.

I would like to ask how the boost upper transistor in 9.2.1 remains constantly open. If I need to keep it constantly open, what key points in the circuit should I pay attention to?

Additionally, if the upper transistor's activation is entirely dependent on bootstrapping, can I replace the bootstrapping with an isolated power supply to power the upper transistor's gate?

  • 0
    TI__Mastermind 31425 points

    Hello Xiong,

    Thanks for working on this.

    Xiong ZeHua said:
    However, in my application scenario, I found through actual testing that the buck half-bridge switches smoothly; however, the boost half-bridge does not switch, or more precisely, the upper transistor is always on and the lower transistor is always off.

    This is expected behavior.

    Xiong ZeHua said:
    If the upper transistor's activation is entirely based on bootstrapping, how does the upper transistor on the boost side remain constantly open in the typical application described in Section 9.2.1? I couldn't find any mention of an on-chip charge pump/isolated power supply in the datasheet.

    There's a charge pump in the IC to provide power to the high side FETs when those FETs aren't switching.

    Xiong ZeHua said:
    If I need to keep it constantly open, what key points in the circuit should I pay attention to?

    There's not a way to keep the high side FETs constantly ON or OFF. For the DC-DC converter ICs in the BQ2575X family or ICs, the BQ25758A can keep the high-side FETs ON with the bit EN_BYPASS=1.

    Xiong ZeHua said:
    Additionally, if the upper transistor's activation is entirely dependent on bootstrapping, can I replace the bootstrapping with an isolated power supply to power the upper transistor's gate?

    Just to clarify, you want to use an isolated power supply to hold the high side FET's gate high? I think this should be possible, but I haven't tested this in the lab.

    Best Regards,
    Ethan Galloway