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I was looking at a TC4467's gate driver datasheet and there is one thing there that I haven't seen before when reading about H-Bridges. On the schematic of example H-Bridge motor control there is a diode in parallel to resistor on 2 of the gates:

Quad Driver for H-Bridge motor control

I've read that it's good to put a Schottky diode in parallel with resistor to make turn-onss slower and turn-offs faster, but then the diode's anode is facing gate, not cathode as in the schematic above. What is the purpose of the diode here? Also, why are these diodes+resistors missing from the remaining 2 gates?

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  • \$\begingroup\$ The resistors are missing too so that might be a clue. \$\endgroup\$
    – Andy aka
    Commented Mar 21, 2015 at 11:39
  • \$\begingroup\$ @Andyaka I updated a question, what I meant is why remaining transistors don't need anything more between a gate and a driver. \$\endgroup\$ Commented Mar 21, 2015 at 11:46

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The upper MOSFETS are P-channel, and the lower ones are N channel.

MOSFETs need the gate voltage to be roughly equal to the source voltage to turn them off. For the N-channel lower MOSFETs that's ground. For the P-channel upper MOSFETs that's "+5 to +15V".

P-channel MOSFETs are notoriously slow to turn off (slower than N-channel). Adding the diode allows the voltage at the gate to rise to the "+5 to +15V" level much faster at the cost of increased current.

The resistors slow down the turn-on of the P-channel MOSFETs giving the N-channel a chance to turn off first.

It's all about reducing the chances of shoot-through and engineering a dead zone in the switching - make sure the P-channel turns off before the N-channel switches on, and vice versa.

If you were to put resistors on the N-channel MOSFET gates as well it would negate much of that effect by slowing down the switching of the N-channel MOSFETs.

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The resistor / diode combination in conjunction with the gate capacitance of the upper FETs provide a slow ON / fast OFF switching action for the FETs. This reduces or eliminates the chance that both the upper and lower FETs are on at the same time.

Because the upper FETs are P-channel, the diodes are reversed compared to how they would be if they were on the lower FETs. That is: a HI level on the gates of the upper FETs turns the FETs OFF, a low level turns them ON.

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