Will this modified current sink work?

Discussion in 'General Electronics Chat' started by powersys2005, Jan 3, 2010.

  1. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    I found a voltage-controlled current sink as following:
    http://www.edn.com/article/CA6541375.html

    For the sake of convenience, the circuit schematic in the article is copied and pasted here as Figure 1 below.

    I'm not sure whether I could replace the "power supply under test" with the "armature winding circuit" as shown in the modified figure. Do you think the EMF generated by the DC generator will cause any damage to the circuit? Assume the output power of the DC generator is well below the rating of the circuit in Figure 1 (original).

    I plan to use an isolated DC power supply to source the +5V (for programmable input circuit) and +/-15V (for the OPA277). Do you think it is safe to connect the COM of the isolated power supply to the GND of the circuit?

    Thanks.

    [​IMG]

    [​IMG]
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I think it should work.

    As far as transients from the generator are concerned you could place a largish (?uF) capacitor value of suitable voltage rating across the +/- terminals to "quench" any back emf arising when the control is switched off. You might consider some precautionary but non-essential protection against inadvertent DC source reversal.

    You'll have to link the isolated power supply COM to the circuit GND. Again there should be no issues in doing this.
     
  3. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    I received the following reply from IRF. Do you think the following (underlined part) will happen to the circuit in Figure 1 above if Q1 is replaced by IR HEXFET MOSFET?

    "We don't recommend Mosfets used in linear application. IR HEXFET® MOSFETs are designed to be used as Switches. This does not preclude their use in linear applications but as the device technology advances to provide ever better performance as switches with lower Rds(0n) and gate charge figures, this makes the devices ever less suitable for linear applications. A lower Rds(on) equates to an output characteristic with a steep slope, which makes it difficult to maintain a suitable gate bias to keep the drain current operating point stable. As the drain current is deliberately varied in response to an applied input voltage, this changes the dissipated power within the part, changing the junction temperature and thus the value of Rds(on), i.e the slope of the output characteristic."
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's probably sensible practice to follow the manufacturer's advice. The point concerning the stability issue is important - it would be disappointing if the circuit didn't work as you hoped.

    You might consider modelling the circuit and seeing if there are potential instability issues.

    You could substitute a bipolar power transistor of suitable rating - however one needs to carefully consider the SOA issues for linear bipolar operation. Depending on the current and voltage it might require a parallel combination of transistors which is an added and non-trivial complication.

    I'm surprised EDN didn't pick up on the design issue for the original circuit but they presumably don't build the circuits to check them - the editors would vet the various reader contributions and only publish details only if they were sufficiently confident the design will work.
     
  5. lmartinez

    Active Member

    Mar 8, 2009
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    With out any thought into this, efficiency is being neglected in the above given circuit.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think the circuit would be more stable if C3 were omitted. The point of R3 and C4 is to isolate the op amp output from the MOSFET gate capacitance. C3 defeats this purpose.
    I also think IRF doesn't understand the way the MOSFET is used in this application. It's a source follower in a feedback loop. Their comments make no sense in this regard.
     
  7. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    Thanks for all great replies. At this stage, efficiency is not an issue yet. Therefore, please do not worry as I'm planning to place the Q1 on a huge heatsink and probably be blown by a cooling fan.

    Let's say now I wish to replace Q1 with another MOSFET with higher power handling capability (e.g. IRFP264). I find out the total gate charge Qg of higher power MOSFET is always higher. By way of example, the Qg of IRF530 is 21nC, whilst the Qg of IRFP264 is 230nC. Do you think the OPA277 in the circuit is enough to drive IRFP264? In fact, is there any formula to determine whether the output of an opamp is enough to supply the total gate charge of a MOSFET?

    By the way, kindly note that the IA in the modified circuit will all in one direction (by control the motor to drive the generator in one direction).
     
    Last edited: Jan 5, 2010
  8. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    By the way, do you think the modified circuit can provide a current regulation at +/-0.1A?

    Which part of the circuit that I need to pay attention in order to achieve +/-0.1A?

    Thanks.
     
  9. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    In fact I wish to have two different programmable current sink, i.e. one with higher power handling (0~5A) and another one with lower power handling (0~200mA). I wonder, if a proper MOSFET is used to replace Q1, the modified circuit can be used for 0~200mA current sink. Kindly advise.
     
  10. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    Vishay IRL530 is denoted as LOGIC MOSFET.
    Do you think this type of MOSFET is suitable for the application in the modified circuit?
    By the way, what is the difference between ordinary MOSFET and LOGIC MOSFET? I can't spot the difference.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    A logic-level MOSFET has a lower threshold than a standard MOSFET.

    I don't know why you are trying to use a linear regulator for this.

    Noise on the supply should not be a concern. That can be eliminated with capacitors.

    Efficiency is the big concern. Wasting power has been out of vogue for some time now.

    If you are adamant on wasting power as heat in your MOSFETs, good luck with your design.
     
  12. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    Hi SgtWookie, thanks for your reply. I need the circuit to perform a test in a specific application. I'm aware that the circuit is not efficient. Therefore, I plan to place the MOSFET on a huge heatsink blown by a cooling fan.
     
  13. 2Tell

    New Member

    Nov 29, 2009
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    <By the way, what is the difference between ordinary MOSFET and LOGIC MOSFET? I can't spot the difference.>
    LOGIC is TTL level 0.3V off _ 5v on Nmos
    ordinary is 4Vonoff 10-12Vfull on Nmos

    I think ?
     
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