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BQ51050B: QI Charging IC heating up

GoodSirEric
Prodigy 60 points
Part Number: BQ51050B
Other Parts Discussed in Thread: BQ25100, BQ51003

Our battery has been exceeding CE certification temperatures during charging (45C), and we have determined the source to be the BQ51050BYFPR using an IR camera. 

Is there anything that can be improved such as passives around U3, or do we need to switch to a different QI charging IC? 

Thanks

  • 0 in reply to Nick Brylski
    Prodigy 60 points

    Hi Nick,

    Thank you for the quick response. The IR Camera photo here shows the coil and charging base off to the side of the PCB. The heat is from the BQ5150BYFPR and the coil and battery do not heat appreciably. 

    From the FAQ: "At high loads, the receiver IC and other ICs like linear battery charger will also generate heat. Adding a better PCB heat sink can also help here." 

    We are not at high load, we are somewhat below the minimum recommended charging amount at only ~70mA. 

    Will the tuning of the coil cause the BQ5150B IC to heat up? 

    Thanks,

    Eric

  • 0 in reply to GoodSirEric
    TI__Mastermind 19195 points

    Hi Eric,

    Tuning the coil is referring to selection of C1/C2.  I can see how that might be mistaken as actually altering the coil itself, will revise that.

    1. We recommend 3 capacitors in parallel for C1 for lower ESR and heating
    2. What coil are you using? Note Ls' value can change when in the housing of the device. 

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 60 points

    Hi Nick,

    I inherited this design, was hard to find info on the Coil. It used a custom designed coil. 

    Characteristics from 'proposed' Datasheet:

Ls [ uH] : 34.8 uH typ @100kHz
Rdc [ ohm] : 1.75 Ω Max.

Measured: Ls' = 43.2uH and Ls = 39.4uH. 

Calculated C10 and C12 ~ .03uF and C13 and C14 ~ 320pF
We are using .047uF and 470pF

    Fairly close but not exact. Would this be far off enough to cause substantial heating inside the BQ5150BYFPR  during a 70mA transfer? 

  • 0 in reply to GoodSirEric
    TI__Mastermind 19195 points

    Hi Eric,

    The slight error in value would probably not cause heating issue. This could be ESR of C10 / C12 adding to heating.

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 60 points

    Hi Nick,

    The heat generated inside the BQ51050B IC appears due to inefficiency at low (70mA) current. Am I reading these graphs correctly? (from Page 10 of datasheet)


    Is there a form-factor-replacement part or other part that you would recommend for charging a 120mAH battery using QI? 
    Hoping to not redesign the PCB if we don't need to, but we are aware we might not have a choice in order to keep the charging temperature under 45C. 

    Thanks,

    Eric

  • 0 in reply to GoodSirEric
    TI__Mastermind 19195 points

    Hi Eric,

    For integrated RX + charger the BQ51050b is currently the only option, another solution for improved temps would be BQ51003 + BQ25100. 

    Best,