Inductor selection for supercapacitor charger

Discussion in 'General Electronics Chat' started by BLUESHIFT, Sep 3, 2013.

  1. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
    4
    Hey team -

    I am working on this circuit for a supercapacitor charger based around the TI BQ24640 supercapacitor charger IC. [For reference, supercapacitor bank will be the power supply for an audio amplifier in an open-source, supercapacitor-powered boombox: www.blueshiftPDX.com ]

    http://focus.ti.com/docs/prod/folders/print/bq24640.html

    The BQ24640 is a PWM controller for a pair of MOSFETs - it sends constant current to the supercap stack. Duty cycle & voltage increase linearly as the caps charge. The PWM square-wave (600kHz) is LC filtered, TI recommends a resonant frequency of 12-17kHz.


    My question is about this filter: they recommend different components based on the charge rate. Each set of components has a similar resonant frequency, but more inductance for higher charge currents.

    I am trying to implement two different charge rates: fast charge (10A) from wall adapter and slow charge (500mA) from USB, solar, cigarette lighter, etc. Charge current is set elsewhere with voltage divider, so that's easy to switch.


    Question(s):

    Is the only reason they recommend a small inductor at small charge current space/money savings?

    Or is there some (stability?) issue I'm not seeing from running small current through the LC filter recommended for 10A?

    i.e. can I set this up for 10A and also run it at .5A?


    [​IMG]


    I think this is a non-issue but I'm guessing, about to build this and I'd love to have it kind of work on the first try...thanks for reading!

    Sam Beck
    BlueshiftPDX
     
  2. BLUESHIFT

    Thread Starter New Member

    Aug 23, 2013
    24
    4
    For the record - it is just a cost issue.

    The part that I missed is that peak current is partially a function of the inductance. That's why it makes sense to design a ripple at 20-40% of peak current - super low inductance means super high ripple current => high peak current. But big inductors are sort of expensive...kind of.

    Boring thread, wooooooo!
     
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