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Thread: High side driver

  1. #1
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    Default High side driver

    Anybody game for trying to build a high side LD Driver to End the Curse of the Hot Can?

    I needed a current sensor at work. This works. It could use some tweeking but my favorite ZDS1009 was no longer available. This is a ZDS1009 clone

    Only issue I can think of is needing a few diodes in series with LD1 to get Vled above the two CE Drops of the transistor.

    See attached.

    Steve
    Attached Thumbnails Attached Thumbnails SENSE-I.png  

    Last edited by mixedgas; 11-11-2009 at 07:43.
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    Very interesting idea! People have been trying to cope with the hot can in various ways, but a driver like this would be the best by far..

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    Thanks for that.
    Any obvious reasons to go with this instead of a sense resistor and a diff amp on the high side?

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    Quote Originally Posted by FourDee View Post
    Thanks for that.
    Any obvious reasons to go with this instead of a sense resistor and a diff amp on the high side?
    no, not really, other then cheap and perhaps more available parts.

    Steve
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    My (ten minute) shot at a driver....

    The stuff up to T2 forms a simple current sink at the collector of T2 (approx 10mA/volt at the mod input).

    The output stage can be seen as a simple high side current source (Q1), with Vbe compensated by Q2 (which should be in thermal contact), and the current set at 1A/V by the voltage dropped across R1 and indirectly R2 (which means that current in R2 is converted to diode current at a ratio of 10mA = 1A.

    This current is the one controlled by the current sink around T2.

    Q3, C4, R9-11 form a soft start in that Q3 is initially biased hard on, holding Vbe of Q1 at zero and the diode current cut off. R11 limits the current required to do this if the opamp does something unfriendly during startup, it may not be required (depends on supply voltage), as C4 charges, the base current in Q3 decays and the output curent regualtion commences.

    Comments?

    Regards, Dan.
    Attached Thumbnails Attached Thumbnails driver.png  


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    For a 10 minute shot that's pretty good
    I'll have a closer look tomorrow, I need sleep right now

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    Here is another one:
    http://www.photonlexicon.com/forums/...ead.php?t=7802
    works well up to 400mA and higher currents are on the way.

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    Very nice, Dmills. You will want input over voltage protection and perhaps some loop compensation but on paper I dig the output setup. How precisely do you think Q2 will be able to compensate for thermal gain changes in Q1?

  9. #9
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    Q1 is an emitter follower so gain is not an issue (but both Q1 and Q2 should probably be darlingtons thinking about it), so the only effect that needs compensating is Vbe in the transistors.

    If you assume the base current is small (hence the darlingtons) then Ic = Ie and Ie is set by (Vb -Vbe)/R1 for Q1, so by offsetting the top of R2 dy a diode drop (Q2 is wired as a diode) we will approximately cancel the Vbe drop over a reasonable range (It is approximate as Ic in Q2 is 1/100 of Ic in Q1, but Vbe is dependent on the log of emitter current, so we should get away with it).

    Note that the current feedback is local to Q1 (And is provided by the influence of R1 * Ie on Ve) and there is no overall feedback and thus I suspect no stability problem as long as the opamp is unity gain stable.

    I might post a better variation tomorrow.

    Regards, Dan.

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    Very nice for 10min! What do you think the max current supplied to the LD would be?

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