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BQ51013B: Qi Ls and Ls' measurements - include battery or not?

tlongeri
Prodigy 70 points
Part Number: BQ51013B
Other Parts Discussed in Thread: BQ51013

I am a bit confused on whether to include the battery on the RX side when measuring Ls and Ls'. It seems most logical to me that, since it is essentially just one additional component on the RX side, it should be included in both measurements.

The WPC specification (version 1.2.3) says this (emphasis mine):

In these equations, Ls’ is the self inductance of the Secondary Coil when placed on the Interface Surface of a Power Transmitter and - if necessary - aligned to the Primary Cell; and Ls is the self inductance of the Secondary Coil without magnetically active material that is not part of the Power Receiver design close to the Secondary Coil (e.g. away from the Interface Surface of a Power Transmitter).

Based on this definition, I would have thought that the battery should be included in both the Ls and Ls' measurements. However, it seems like most of TI's content include it only in Ls' measurements:

I am looking for some confirmation and clarification.

  • 0
    TI__Mastermind 19195 points

    Hi,

    Ls is the free space measurement of the inductance, do not include the battery in this. 

    Ls' measurement should be taken as close the final RX solution as possible, this includes the battery and any other metal/shielding

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 70 points

    Hi,

    I'm sorry, but I am unconvinced about Ls being the datasheet value (which is what I think you are saying - and also what the FAQ clearly says).

    For one thing, if the Ls measurement were to include shielding and battery, that would make sense to me, as I would think of Ls as the inductance when away from the power transmitter and of Ls' as the inductance when on the power transmitter. I admittedly don't know/remember much about wireless power transmission, but I would expect any resonant frequencies would be determined by the joint effect of all components and not independently by the inductor.

    To add to that, the wording on the paragraph I cited in the original post suggests to me that they should be considered together. I also checked the reference designs part of the WPC specification, and the wording also seems to suggest the same (the following is from example 3):

    At the secondary resonance frequency fs = 100 kHz, the assembly of Secondary Coil and Shielding has inductance values Ls = 11.5±1 μH and Ls′ = 15.7±1 μH.

    Since the example includes some coil parameters (but not the inductance itself), I also went ahead and used this calculator to estimate the inductance of the coil, which comes out to around 6-7 uH (for example 3) (I tried matching the outer diameter), quite lower than the value of 11.5 for Ls that they use in the example. I'm not sure, but might that be explained by the ferrite shield increasing the inductance?