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I am in the middle of doing some research on gate driver ICs and I'm wondering why it is more common for people to use two half bridge driver as apposed to using two gate driver ICs.

Here is the more common approach:

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(credit https://320volt.com/en/h-bridge-dc-motor-driver-circuit-with-ir2101/)

and here is the approach I was intending to use:

enter image description here

Is there something fundamental that I am missing with my design that will cause me issues when I try to power my motor?

Luke Bradley
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  • Yes. Your approach CAN be adapted to work for restricted supply voltages and PMOS high side switches; but otherwise you need different voltages to correctly drive all the gates. –  Aug 18 '21 at 12:37

1 Answers1

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Is there something fundamental that I am missing with my design that will cause me issues when I try to power my motor?

Absolutely. Your second circuit that attempts to share common drive signals to (say) upper left and lower right MOSFETs cannot utilize bootstrapping and this means that the upper MOSFET will never turn on properly.

In your first diagram, both upper MOSFETs were driven by a bootstrapped driver and that allows their respective gate drive voltages to rise around 10 volts higher than the 24 volts feeding the H-bridge shown in that diagram. This makes a really big deal. Inside a bootstrap driver: -

enter image description here

Picture from here

Andy aka
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  • thankyou for the links! But why would the top left MOSFET not turn on properly? From what I can tell from my circuit diagram, when the top gate driver is outputting high, then the voltage on the gate of Q1 and Q4 will be Vcc with respect to the ground. Is it because the source pin of Q1 is not actually connected to the ground but is instead referencing the drain of Q4 through the load? – Luke Bradley Aug 18 '21 at 11:51
  • @LukeBradley exactly. The top MOSFETs are source followers hence, to turn on the MOSFET properly the source would have to be between 3 volts and 10 volts below the gate voltage AND, given that the gate voltage is at Vcc, the MOSFET's source has to be at least 3 volts lower hence, it cannot be fully turned on and conducting good from the Vcc rail. – Andy aka Aug 18 '21 at 11:57
  • ah I see, thank you for the detail! Out of curiosity, why do companies provide ICs specifically for low side gates if you will always need some form of bootstrapping in order to drive high side n-MOSFETs? do people use motors with only one grounded MOSFET? – Luke Bradley Aug 18 '21 at 12:46
  • @LukeBradley driving a gate of a MOSFET to avoid significant time in the partially conducting area (big power loss) requires an injection of several hundred mA to many amps to charge up the gate-source capacitance. This somewhat necessitates a bespoke driver. A single grounded MOSFET to activate or deactivate a motor is very common. – Andy aka Aug 18 '21 at 13:35