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Original question:

DRV8316: current sampling issue

DRV8316: Will the sense FET sense the current flowing through the body diode of the low-side FET?

Xiang Maverick
Intellectual 750 points
Part Number: DRV8316

1)  I can understand the current can be sensed if it flow through the main path by the sense FET beside the low-side FET. Will the sense FET sense the current flowing through the body diode of the low-side FET?

2) Whe the high-side or low side FET is switched on，can the current flow from the source pin to drain pin? It can obviously flow the source pin to drain pin through the body diode (figure b). Can it flow through the main path (figure c)?  From the figure c extracted from the page 41 of the part datasheet, we know it can. But it is different from my common experience of a N-MOSFET.

  • +1
    TI__Genius 14935 points

    Hi Xiang,

    1) The sense FET is primarily designed to sense current through the main path, not the body diode of the low-side FET. The reason being is because this diode is typically freewheeling during certain phases of the commutation cycle and may not be part of the primary current flow that the sense FET monitors.

    2) When the high-side or low-side FET is switched on, current can flow from the source pin to the drain pin through both the body diode and the main path. The main path refers to the conducting channel formed when the FET is switched on, allowing current to flow in the conventional direction, contrary to the body diode which allows current to flow in the reverse direction when the FET is off. This behavior may seem different from common experiences with N-MOSFETs, but it aligns with the specifics of the circuit and design.

    Regards,

    Yara

  • 0 in reply to Yara Al Shami
    Intellectual 750 points

    "The sense FET is primarily designed to sense current through the main path" .

    Do you mean if the body diode has current flow, it can not be sensed by the sense FET?

    question 2 is well answered.

  • 0 in reply to Xiang Maverick
    TI__Genius 14935 points

    Hi Xiang,

    When I mention that the sense FET is primarily designed to sense current through the main path, it implies that its main focus is on monitoring the current flow through the conducting channel formed by the FET when it is switched on.

    While the sense FET may detect some characteristics of the body diode, its primary function is to measure current in the main path. Current flowing through the body diode may not be as accurately sensed because the sense FET is optimized for monitoring the primary current flow during commutation.

    Regards,

    Yara

  • 0 in reply to Yara Al Shami
    Intellectual 750 points

    When the low-side FET is switched on and current flow from the source pin to the drain pin through the main path, can sense FET accurately sense the current?

    Does a PMOS or NMOS allow current flow from  both (D to S)  and (S to D), so long as the mosfet is swiched on? Does it obey the the circuit theory?

  • 0 in reply to Xiang Maverick
    TI__Genius 14935 points

    Hi Xiang,

    I'll provide an update to your answer soon.

    Regards,

    Yara

  • +1 in reply to Xiang Maverick
    TI__Genius 14935 points

    Hi Xiang,

    Xiang Maverick said:
    When the low-side FET is switched on and current flow from the source pin to the drain pin through the main path, can sense FET accurately sense the current?

    The sense FET can accurately measure current as it flows through the main path from the source to the drain pin. This allows precise monitoring of the current in the motor phase for effective control and commutation.

    Xiang Maverick said:
    Does a PMOS or NMOS allow current flow from  both (D to S)  and (S to D), so long as the mosfet is swiched on? Does it obey the the circuit theory?

    Yes, both PMOS and NMOS transistors allow current flow in both directions (from drain to source and from source to drain) when they are switched on. The direction of current flow depends on the applied voltage and the transistor's configuration.

    I will be closing this thread, if you have any other questions feel free to look through the E2E forum or post another question.

    Thank You,

    Yara