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I'm playing with a full-bridge driver:

full-bridge driver

Usually, you want to put an inverted PWM signal on IN1 and IN2 and take care of dead-time. While writing the code for MCU I tried a simpler pattern:

  • put a PWM signal on the high-side MOSFET
  • keep on the low-side MOSFET

The motor moves smoothly and nicely, but I wonder which are the disadvantages of such a "lazy" control approach. Is there a name for this pattern?

UPDATE

When I say I keep the low-side MOSFET on I talk about the opposite MOSFET to the one that has PWM input.

Example:

CCW

  • IN1 -> PWM
  • IN2 -> keep low-side on

CW

  • IN1 -> keep low-side on
  • IN2 -> PWM
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    \$\begingroup\$ EE top tip, assign unique designators to every component and refer to them at all times by their designator. That way you avoid “low-side MOSFET on I talk about the opposite MOSFET to the one that has PWM input”. Also, white background for screenshots please. \$\endgroup\$
    – winny
    Apr 9, 2021 at 14:51

1 Answer 1

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Edited after clarifications.

This mode of operation is not so unusual for DC motors. Effectively, it is a buck converter, with the fixed side of the load attached to V-. Thus it shares some pecularities with the buck converter, such as discontinuous current mode operation (DCM).

The difference is if you want to reverse direction.

In your operation mode, you either accelerate when the high side is on, or you do low side braking.

If you alternate both half bridges, you either accelerate forward, or accelerate backwards.

When current is low, your operation will result in DCM and a reduced braking torque. For the regular H-bridge operation, the torque will be constant as current goes through zero, avoiding DCM.

However: Dead-time is an issue even in your operation mode ! Actually acting with both H-bridges, does not complicate the dead-timing. Dead-Time needs to be obeyed on a single leg basis.

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  • \$\begingroup\$ Sorry, perhaps I explained it wrong. I leave only one low-side MOSFETs on. As you can see I don't have the control of all four MOSFETs. Example, left high-side PWM, right low-side on. And viceversa. \$\endgroup\$
    – Mark
    Apr 9, 2021 at 12:30
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    \$\begingroup\$ @Mark Ok this is important information, which you should update in the opening post. \$\endgroup\$
    – tobalt
    Apr 9, 2021 at 12:30
  • \$\begingroup\$ Done. Question updated. \$\endgroup\$
    – Mark
    Apr 9, 2021 at 12:32
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    \$\begingroup\$ @Mark I have updated my response to reflect your added info. \$\endgroup\$
    – tobalt
    Apr 9, 2021 at 12:36
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    \$\begingroup\$ The point where your control enters DCM could be very close to zero and most likely not be an issue. The dead time begins when one FET in a leg turns off and it ends when the other FET in the same leg turns on. It is most likely managed by your driver ICs. Dead time doesn't have to be obeyed between different legs. \$\endgroup\$
    – tobalt
    Apr 9, 2021 at 14:59

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