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BQ24610: High PH Node Ringing at Light Load and Risk to IC/MOSFETs

Craig Peckett
Prodigy 110 points
Part Number: BQ24610

Tool/software:

Hi team,

I'm using the BQ24610 in a 24V-powered backup charger design for a 12V lead-acid gel battery, based on the "Charging Without Termination for Backup Application" configuration from SLUA992.

The system operates well overall, but I’m seeing significant PH node ringing at light load, and I have concerns about stress on both the MOSFETs and the PH pin of the IC.

Mag Observations:

  • At ~500 mA load, PH node is clean and square

  • As the load drops (e.g. battery near full charge), ringing increases:

    • ~300 mA load: PH waveform begins to dip

    • Ringing reaches ~30–32V peak-to-peak

    • Frequency is ~2–3 MHz

    • Appears during early switching cycles and worsens with less load

I understand some ringing is expected at light load (likely due to DCM), but the amplitude seems excessive compared to datasheet examples (e.g., Figure 7), which show much more controlled transitions.

Warning️ Reliability Concern:

During earlier testing, I managed to blow the 40V MOSFETs (SQS484EN) on all 5 units under various conditions (e.g., probing, light load, startup).

Since upgrading to 60V MOSFETs, I have not been able to reproduce the failure, and the BQ24610 ICs appear to have survived — however:

The PH node ringing previously reached voltages high enough that it may have exceeded the 36V absolute max rating of the IC’s PH pin.

This still concerns me long-term, especially if the ringing also threatens the HIDRV gate driver.

Camera PH Node Scope Captures:

All captures taken with 0Ω gate and BTST resistors (i.e., no edge slowing or filtering).

    • ~500 mA load (clean switching)

    • ~400 mA load (start of ringing)

  • ~300 mA load (clear sinusoidal ringing)

Question Questions:

  1. Is this level of ringing (~30Vpkpk at light load) considered normal or excessive for BQ24610?

  2. Could this have been the likely cause of the 40V MOSFET failures?

  3. Given the PH pin's absolute max is 36V, could this ringing also be a risk to the IC’s internal gate driver?

  4. Are there any recommended techniques (e.g., snubber, layout tweaks) to further control this behavior under light load?

Thanks in advance — would appreciate any guidance on whether this behavior is within safe limits or something I should address further.

Best regards,
Craig