Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 29 subscribers
  • Views 357 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

3-Phase Isolator MOSFET Circuits

Michael Raad
Prodigy 10 points

I am referencing the document Designing High-Side and 3-Phase Isolator MOSFET Circuits in Motor Applications by Garrett Walker. What is the specific purpose of the phase isolator MOSFETs? Are they to disable phases, for example, in the case of a short elsewhere to allow the motor to operate with reduced functionality? Also, is it possible to implement a control strategy to use the isolator MOSFETs to "disconnect" the inactive phase at any given time to reduce or remove the back emf on that line and what would the benefit be? 

  • 0
    TI__Intellectual 2595 points

    Hi Michael,

    Usually, the purpose of the phase isolator MOSFET is to cut off the motor from a damaged power stage. For some systems, the priority may be to disconnect all three phases from the motor to avoid potential rotor lock or unintended motion. In others, as you suggested, it may be desired for a "limp home" functionality with the remaining two phases.

    Even if not dealing with a damaged power stage, disconnecting the phases can block the flow of current to the motor when it is being turned externally and operating as a generator, preventing overvoltage on the motor power supply.

    However, in my experience, I haven't seen phase isolator MOSFETs used as dynamic switches to disconnect/reconnect motor phases to avoid back EMF as part of normal operation. It's usually used as a largely static on/off switch; the MOSFET is on during normal operation, and switched off in a fault or shutdown condition. To dynamically switch the phase isolators as part of the commutation would require fairly precise control of the switching/timing of the MOSFETs - I don't know that you could reliably achieve this with discrete transistors.

    Thanks,
    Garrett

  • 0 in reply to Garrett Walker
    Prodigy 10 points

    Thank you for the quick reply Garrett,

    Out of curiosity, lets say you could achieve precise control of the MOSFETs to disconnect phases, would that completely remove back emf? And would this be purely beneficial to the operation or does removal of back emf have any negative effects on the system?

  • 0 in reply to Michael Raad
    TI__Intellectual 2595 points

    Hi Michael,

    The back EMF is always present, it's just easiest to see in the inactive phase because there's no switching to interfere with it. If using a sinusoidal-wound motor, the BEMF will be a continuous sinusoidal signal, and we see the zero crossing occur while the phase is in the "inactive state". The positive peak occurs when the phase is in the "active high-side" state, and the negative peak occurs when in the "active low-side" state, so I'm not sure what benefits disconnecting during the inactive state would have.

    On the flip side, many sensorless commutation implementations monitor the back EMF on the inactive phase for position detection, so disconnecting the phase would not work well with these control methods.

    Thanks,

    Garrett