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I have a SMPS which I would like to current limit. The current limiter should have a negligible voltage drop when not limiting (say, <0.5%). I thought of inserting a MOSFET and a hall-effect current sensor in series, and switch the MOSFET on and off up to 20kHz to limit the current to a certain value when it is reaching that value. The output would be smoothed out by a capacitors.

Is it going to work? The SMPS is switching at about 50kHz, providing an adjustable voltage between 1.2V and 12V, and the current limit should be settable between 10mA and 5A.

What transistor should I choose for the job (what considerations etc.)?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Have you found a hall sensor that will work over a 500:1 range? 10 milliamps to 5 amps may not be feasible. A current-sense resistor will have the same problem. Multiple hall sensors may work. \$\endgroup\$ – Mark Apr 18 '16 at 20:58
  • \$\begingroup\$ I used to be a test engineer for hall current sensors. good luck getting one much lower than 5A. And even if you do, the output will be pretty noisy. \$\endgroup\$ – Brendan Simpson Apr 19 '16 at 0:19
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Sure, this seems plausible. This idea isn't really much different than using a low value resistor in series with the output to convert a current into a voltage for the purposes of making a current source. The current sensor is merely doing the resistor's job, with significantly less voltage drop and heat dissipation.

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  • \$\begingroup\$ Glad to hear it. Sorry I had forgotten to add the main question, which is about selecting the MOSFET. \$\endgroup\$ – user42875 Apr 18 '16 at 21:00
  • \$\begingroup\$ Make sure your FET can handle the surge currents and also that it switches fast enough for you. It should have a low rds-on so that heat dissipation is low. Read some application notes from switching controller vendors on FET choosing. \$\endgroup\$ – Brendan Simpson Apr 19 '16 at 0:21

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