Current measurement for a mosfet h-bridge

Discussion in 'General Electronics Chat' started by rpicatoste, Jul 11, 2013.

  1. rpicatoste

    Thread Starter New Member

    Jul 11, 2013
    Dear all,

    I have a mosfet h-bridge feeding the phase of an electric motor.

    For measuring the current we were using so far a LEM LTSR 15-NP but now the ADC will be in the high power side and therefore the isolation given by this component is not an advantage anymore.

    I am trying to get an accuracy of around 0.15 A for a ±15 A range. (0.5 % of full range)

    Is this kind of device still a good option or a sense resistor could give better results? (or other way)

    I think that probably a precision resistor (0.1% tolerance for example), plus the amplification and offset (to get a proportional shifted voltage between 0 and 5 V to be read by the ADC) could give better accuracy than the LTSR (0.7% accuracy plus 0.1% non-linearity), but probably the common mode voltage applied to the resistor could make a precise measurement difficult.

    The switching frequency is 50 kHz, and the voltage supply for the bridge 170 V.

    Thank you very much in advance for any advice!
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    I guess you have to ask yourself if the increased accuracy is worth the effort of making a new measurement system. Do you really need .1%? Is the high power side really that critical? If the method you used worked before, it might be the best choice. That said, either method would yield good results.
    rpicatoste likes this.
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    I suppose a resistor as sensing element has to be cheaper than that tranformer-based device. If it's worth doing the design work to save the money, it should be pretty straightforward.

    If it were my project, I'd have used Hall-effect sensors like this:

    But I do see that they only claim "typical output error of 1%", so maybe that wouldn't have been good enough.
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    While 1% error might sound like alot, remember that using a series resistor to sense current will add error to your system. For example, say you're measuring 10A on a 10V system, using a .01ohm resistor. For the same circuit without the current sense resistor, the current should be 10.1 amps. This is a 1% error, even if using a very precise resistor. Add to that errors in your amplifier, VOS, IOS, etc...