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half bridge motor control operation

John Saunders
Prodigy 140 points

Do the same trade-offs of synchronous vs non-synchronous half-bridge topologies as used for buck switching regulators apply to half-bridge motor control?  For example, if I have a brushed DC motor controlled by a MOSFET driver implementing a PWM control, the driver turns on the high-side MOSFET for part of the PWM period.  During the remaining part of the period it seems like you could just let the motor freewheel through a diode to GND or use a low-side MOSFET to get the equivalent of "synchronous" operation as is done in switching regulators.  Are the considerations the same for these 2 circuits (regulator and motor control)?  Are there any other considerations for motor control (besides making sure that both the upper and lower MOSFETs are not turned on at the same time)?

Then, the same question for a BLDC motor - what are the advantages of turning on the lower FET for the "low" part of the PWM signal when driving one phase of the motor vs. just letting the motor phase freewheel through the lower MOSFET body diode?  

Thank you.

  • 0
    TI__Intellectual 2061 points
    Hello John, I think I generally understand your concern. However, this is a quite a large topic unless we narrow down the dicussion topics. Therefore, Could you please help send the application and power stage schematic or simplified block diagram, the more detail the better for us to help. Then, we will discuss what will be the trade-offs over:

    1. Whether you need to have the moter operating in regenerative mode by feeding back energy to the DC source?
    2. DC link voltage, less than 48V or over than 400V?
    3. Bootstrap Capacitor energy refreshing?
    4. etc.

    Wei
  • 0 in reply to Wei Zhang
    Prodigy 140 points

    Hello Wei,

    Here is more info:

    1) we do not need to operate in regenerative mode.

    2) motor is powered by a 20V nominal Li-Ion battery.  Motor current is 25A continuous max current (current limited by the circuit)

    3) Bootstrap will be handled by driver / large bootstrap cap / logic of the microcontroller drive signals to driver IC

    4) Here is a rough schematic.  I am leaving out details where they are not needed so as to simplify the discussion.  Specific driver IC & MOSFETs are not yet selected. (if you can't see the schematic, then I am unable to add it)

  • 0 in reply to John Saunders
    TI__Intellectual 2061 points
    Hello John,

    Thanks very much for your reply. For some reason, I cannot see the schematic.

    Would you please send a email to w-zhang@ti.com.

    Regards,

    Wei