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High or low side current sensing

Discussion in 'Electronic Design' started by Raveninghorde, Feb 8, 2010.

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  1. I'm reviewing an existing charger design which I'm expanding to 10

    At the moment I use high side current sensing with an opamp and fet
    level shifting the signal so that it is ground referred.

    The disadvantage is that this circuit can not measure the current
    during a short and relies on the swticher's own current limit for

    I've used a current mirror circuit before to get around this but it's
    too many parts.

    If I low side sense then this problem disappears. But then I loose the
    simplicity of a common ground.

    I can't make my mind up. Time for a whisky.
  2. Joerg

    Joerg Guest

    You could consider current monitor chips that have a helper voltage and
    where common mode includes ground. But it's not as cheap. If each charge
    channel has its own switcher, can't you sense current there?
  3. Joerg

    Joerg Guest

    If the battery terminals short out you do get a total loss of voltage.
    But it's no big deal if there's a helper voltage that, for example,
    provides a negative supply if the opamp CM range doesn't include VSS.
  4. Jamie

    Jamie Guest

    Operate the sensing circuit from a separate supply that does not get
    effected from the main supply drop.

    In your circuit, I assume you're using the +/- inputs of the opamp to
    form a dif shift via a shunt R ? The op-amp and Fet you're using can be
    operating from a secondary supply that can also share the same common.

    of course, if you throttle back the drive using the current sense,
    you wouldn't get a total loss of voltage!.
  5. What are your desired current, voltage and measurement accuracy ?
  6. Thanks.

    That looks good and appears to get around the short circuit problem.

    It adds 3 resistors to my component count. Under half a cent extra
    component cost plus placement time.
  7. I'm happy with 3% accuracy on current. Who cares if a battery charges
    at 970mA rather than 1A.

    I'm charging lithium ion so voltage is more important. On a 4 cell
    pack I expect to be between 16.70V and 16.75V so that's +/- 0.15%
    after adjustment.
  8. In this application I am using National simple switchers so they don't
    help with the current measurement. They do provide an internal current
    limit which does provide short circuit protection.

    I don't like the short circuit current being higher than the charging
    current. In the other job I've got to tidy up for production I'm
    using a sepic which does need a proper current limit.

    I like to use common building blocks and MooseFET's circuit looks good
    at the moment.
  9. I have battery good detection in the micro. The charger has to be safe
    even if the micro dies.

    The customer checks short circuit by putting a switch across the
    output and a then a fuse in line to the battery. They then start
    charging and throw the switch.

    Similarly for reverse battery protection they fit a reversing switch
    into the wiring. Again they start charging and throw the switch.
  10. Joerg

    Joerg Guest

    On a SEPIC that's easy. You can sense the current at the base of the
    second coil and run that into an inverting opamp.

    So you owe him a beer then :)
  11. Your circuit is the same as what I use at the moment except + and -
    inputs of the op amp are swapped.

    The op amp does come from a higher supply, but as you relaized that
    doesn't help.

    I'm using a switching regulator so measuring in front doesn't work.
  12. Joerg

    Joerg Guest

    Raveninghorde wrote:


    What kind? Any chance to add a C and an L and make it a SEPIC? Then
    current sense would become easy.
  13. I'm using an LM2675 simple switcher in a buck configuration. Changing
    it to a sepic doesn't make sense as the extra cost would outweigh the
    cost of high side current sensing.
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