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High frequency triangle signal sensing

Nathan Ding
Genius 10085 points

Hi Team,

My customer needs to sense a high frequency triangle current, about 1~5MHz, max current is about ~15A, min current is close to 0A, common mode voltage is about ~10V.

Because current sensing amplifier doesn't have a very high bandwidth product so I turn to look for some discreate solution, by using shunt resistor to sense the current, and then using amplifier to  amplify the voltage drop of the shunt resistor. The shunt is about 1mohm, so the voltage drop (input voltage of amplifier is 0~15mV)

High DC CMRR and AC CMRR is needed because one of the node of the sense node is switching node.

Do you have any proposed device or solution? Thanks.

Regards,

Nathan

  • 0
    TI__Guru 56571 points

    Hi Nathan,

    Nathan Ding said:
    The shunt is about 1mohm, so the voltage drop (input voltage of amplifier is 0~15mV)

    A typical precision shut, say 1mΩ has parasitic inductance associated to it, which is typically <5nH (high quality ones). So the shunt is configured with  1mΩ + 2nH parasitic inductance in series (assume the parasitic is 2nH, which it is a very low inductance shunt on the market). At DC or low frequency, we can ignore the impedance contributed from the parasitic inductance. At 1 MHz or higher switching frequency, 2nH will contribute > 12.566 mΩ (X_L = sL = 2*pi*f*L), which it is in series with 1mΩ shunt. The higher the switching frequency, the worst shunt's sensing voltage errors will get from the current sensing measurement. 

    Enclosed in an application talked about the DC current sensing in AC switching power supply, which it may be useful to you.   

    https://www.ti.com/lit/an/sboa571/sboa571.pdf 

    So the above sensing approach is very difficult to compensate for it without getting a large current sensing errors. I do not believe that the application is required to sense the entire high current switching profile. If the current sensing application is able to sense the average switching current and feed back to the controller, this may meet the customer's requirements.

    Please let us know what is the customer's design requirements. The customer may have use other sensing method to capture the switching current information, such as use of current transformer, hall effect current sensor or other sensing techniques. 

    Best,

    Raymond 

  • 0 in reply to Raymond Zhang1
    TI__Guru 56571 points

    Hi Nathan, 

    The current sensing via shunt techniques at high frequency are very challenge, where the errors can be very large, even the user is trying to compensate for it (correction, calibration or using software to correct the parasitic inductances). Thus, I do not recommend the current sensing approach using shunt. 

    I am going to close this inquiry. If you have any additional questions, please let me know and I will be happy to assist you. 

    Best,

    Raymond