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circuit senses high-side current

Discussion in 'Electronic Design' started by raf, Feb 9, 2007.

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  1. raf

    raf Guest
    Dear all,
    Here i have a current sensor (high-side) to measure the current and to
    offset the common mode voltages. I would like to know how the circuit
    is working. If i give 300mV as input (assumed the shunt resistor with
    0.5m ohm and current 60A) then the output is measured at the
    transistor (IRF150) which is connected to the op amp. So how the
    circuit is removing the common mode voltage. for this the transistors
    (dual type) are connected which are doing the common mode voltage
    offset. The circuit i am giving is PSPICE model in which i do not have
    exact components so i took available from PSPICE.
    Any suggestions are appreciated
  2. raf

    raf Guest

  3. Was that circuit an ED Ideas for Design, or similar? Ref, please?

    The high-side circuit is a two-stage current mirror, fed from a low-
    side current source. Any voltage drop across the current-sensing
    shunt resistor unbalances the mirror by a current Ix = Is Rs/R1.

    The low-side circuit is a servo that senses the difference between
    the current source and the mirror's output, and adds a current Ix
    through the MOSFET to rebalance the mirror. This current is the
    aforementioned Ix = Is Rs/R1, and is sensed by an output resistor
    Rg, so the output voltage Vo = Is Rs Rg/R1 shows the current Is.

    The MOSFET (which could have been a BJT instead) is operated at
    very low currents, and should therefore be a small-die part (such
    as the suggested 2n7000, not a large-die part like an IRF150), to
    keep the capacitance down.

    Contrary to the drawing, the opamp does not need to be a rail-rail
    opamp. Its inputs should work to the Vee rail, e.g., like an LM324.
  4. raf

    raf Guest

    Hi Mr. Hill,
    thanks for u r reply. Yes, exactly it is from ED design ideas. In that
    the operation is not explained in detail. If u could expalin more in
    detail it would be appreciated.
  5. I'm sorry, RAF, I thought I had done so, e.g., Vo = Is Rs Rg/R1,
    etc. Could you tell us exactly what issue of ED's Design Ideas
    it came from? Thanks!
  6. raf

    raf Guest

    Hi ,
    I am searching for a current sensor for motor application (high side)
    which can withstand high common mode voltages( 100V). The Circuit in
    the ED's design ideas is meant for avoiding high common mode voltages
    which the current mirrors do. So i need detailed operation of each
    component so that i can proceed further.
    Thanks and Regards
  7. raf

    raf Guest

    In the ED'd design on March1,2001, EDN123 was published.
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