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For brushless DC motor, we use 6 step commutation. For voltage control, in each phase, PWM is used to drive the Bridge MOSFETs. And for the sensorless algorithm, Back EMF of non-energised phase is measured.

I have found 2 basic PWM strategy, Can you explain which one better and why?

enter image description here

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    \$\begingroup\$ If you're turning the high side FETs fully on (no PWM), beware of the bootstrap circuit time constants., or they will slowly start turning off. The low side FETs don't have his problem. \$\endgroup\$ – Brian Drummond Jan 15 '18 at 14:47
  • \$\begingroup\$ My Gigh Side 3 MOSFETs are equipped with 3 numbers of Charge pump capacitors. So what is the difference between these two PWM strategies? \$\endgroup\$ – litun bls Jan 16 '18 at 14:36
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In your schematic, there is no theoretical advantage to either approach. @Brian Drummond makes good points about it being easier to switch the low side, because the gates of these transistors are always at referenced to ground level, while the high side gates voltage must follow the motor phase voltage.

Many controllers with a high side charge pump only charge the capacitor when the gate voltage is low, so the capacitor's voltage can droop at low speeds and provide insufficient voltage to drive the high side gate. Keeping the high side switch "on" (using PWM strategy 2) for long periods at low motor speed will then cause the switch to drop out.

So, as in many cases, you make a design decision. You can commutate the low side if you know your speed range and use a big enough charge pump capacitor and charging diode, and you can let the freewheeling body diode to handle the high side complimentary PWM commutation - simple circuitry. If you commutate on the high side (or high side and complementary low side - if you don't want to rely on the body diode) you have to get more complicated, but at low speeds, since you will presumably be commutating anyway, your charge pump will always be happy.

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