Thread: Discussion H-bridge motor controller
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Old Apr 04, 2012, 05:07 PM
CombatWombat is offline
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United States, NJ, Edison
Joined Mar 2008
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Originally Posted by jeffs555 View Post
With those mosfets you should be ok with 3.8v Vgs drive.
I rehooked my motor, and you are correct. 3.8V must be enough, because the motor IS running.

I know it is a puzzle why you are not getting the full 5v gate drive. One end of the bootstrap cap is being pulled low by the low side mosfet and the other is pulled to supply voltage thru internal schottky diode, so I would expect the bootstrap caps are actually charging to about 4.5v(assuming 5v supply). Have you measured across them?
Well, they DO go to 4.5V with the high side mosfets off. I measure across with the probe and it's ground lead directly touching each end of the capacitor. I don't think my scope supports differential input probes.

I'm close to calling this "working well enough for a DIY thing slapped together in a couple days".

edit: I take back what I said about it working well enough. I think these mosfets might just be too damn big for these drivers. Here's a shot of Vgs in action with the duty cycle continuously varied back and forth.
H-Bridge Test Run.mp4 (0 min 36 sec)

The voltage is 3.8 at best. Then sags down at high duty cycle (kind of expected that, no big deal). Oddly enough, it *does* jump to 5V at the start of each new cycle. Weird.
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Last edited by CombatWombat; Apr 04, 2012 at 06:35 PM.
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