Half bridge motor driver; best motor placement?

Question

The two options:

I have been scrolling through forum after forum for a unilateral agreement on what the best motor placement would be within the half-bridge driver circuit: Putting the motor across the high side MOSFET or, across the low side MOSFET. As an example, I have included both options. (Please do not write an essay about my schematic design, these are just aids to describe the issue question!)

Motor across the high side FETS

Motor across the low side FETS

The average current draw for the proposed example would be around 24 A, hence the parallel arrangement for the FETs.

Also, as seen in the images, I have swapped the 'HO' and 'LO' outputs of the IR2111 driver in order to give the correct output if the 'PWM_In' input was to change. The data sheet diagram for the IR2111 can be found here: IR2111 Datasheet

The research I have conducted so far seems to suggest for applications that are not to drive motors, having a load across the low side FETs is most beneficial as seen in the second image. But for motor driving the first arrangement is more beneficial, at least from what I gather.

Please keep any response casual and respectful, I just want to hear some more experienced opinions on this!

Answer 1

If this is supposed to be a unidirectional motor driver using a half-bridge, then it's rather pointless because that would only allow for unidirectional motor drive. But in a unidirectional motor drive you don't need both high and low-side MOSFETs.

If this is your goal you only need either high-side MOSFETs or low-side MOSFETs, not both. But note that if you did choose high-side MOSFETs you cannot use a boostrap capacitor gate drive since those rely on a low-side MOSFET being present and switching frequently enough to refresh the bootstrap capacitor by allowing current to flow from 12V to ground through the capacitor. This also means 100% duty cycle is not allowed.

You would either go low-side MOSFETs only with low-side gate drive. Or if EMI is a concern and you need the motor to stay connected to ground you would use a continuous high-side gate drive (such as removing the bootstrap diode and connecting a isolated floating voltage supply where the bootstrap capacitor would normally go.

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If this is supposed to be an bidirectional motor driver using an H-bridge (which is different than a half-bridge).

1. Only looking at the right-hand side, that is not an H-bridge. The motor does not go in either of those places. Google schematics of an H-bridge and look at where the motor goes. You placed a permanent dead short where the motor is supposed to go.

2. You connected the low-side gate drive outputs to the high-side MOSFETs and you connected the high-side gate drive outputs to the low-side MOSFETs.

3. Looking at the way the left-hand side is wired into the right-hand side, Your drive signals are also wrong in more than just #2. I think the entire reason your circuit is mangled is because you looked at the gate driver schematics for a half-bridge and then tried merging it with an H-bridge. The 'H' in H-bridge does not stand for half-bridge. An H-bridge is four MOSFET arranged to shape like an H. So the left-half of an H-bridge is one half bridge, and the right-side of an H-bridge is another half-bridge.

   Your H-bridge needs two gate-driver ICs so that you have two high-side gate drives and two low-side gate drives, such that you have one for each of the four MOSFETs present.

Answer 2

Your best option is number 3, this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Choose your freewheeling diode to take at the very lease the full supply voltage and max inrush current, preferably more. Your PWM source may be too weak to drive a 24 A capable MOSFET, so you may need a buffer in between.

C1 is your local decoupling, unless you really know what you are doing.