Electronics > Beginners
MOSFET intrinsic diode vs. Schottky (PWM motor circuit)
T3sl4co1l:
Note that you need the diode beside the transistor, and what the diode returns to (+V), needs a bypass to FET GND in the same location. The diode shouldn't go directly across the motor. The FET is causing the flyback, the diode needs to be at the FET.
Tim
MagicSmoker:
--- Quote from: T3sl4co1l on July 05, 2019, 06:46:11 pm ---Note that you need the diode beside the transistor, and what the diode returns to (+V), needs a bypass to FET GND in the same location. The diode shouldn't go directly across the motor. The FET is causing the flyback, the diode needs to be at the FET.
Tim
--- End quote ---
More specifically, any stray inductance between the freewheeling diode anode and the MOSFET drain causes a delay in the transfer of current from one to the other and also induces nasty little semiconductor-destroying spikes. Similarly, stray inductance between the input capacitor and the switch/FWD will also induce nasty little spikes and ringing every switch turn-off.
EDIT - this reply is intended for the OP's benefit; Tim likely doesn't need it :-DD
jmelson:
--- Quote from: blueskull on July 04, 2019, 07:05:41 pm ---0.56uF at 10kHz is insane.
Your snubber is WAY too large. You are forcibly charging a cap every time you turn on your FET.
--- End quote ---
Yes, but the whole idea of a snubber is to DISSIPATE energy, so you need a resistor in series with it.
A very low resistor of maybe 10 - 25 Ohms might be around right.
Jon
Zero999:
--- Quote from: jmelson on July 05, 2019, 07:12:49 pm ---
--- Quote from: blueskull on July 04, 2019, 07:05:41 pm ---0.56uF at 10kHz is insane.
Your snubber is WAY too large. You are forcibly charging a cap every time you turn on your FET.
--- End quote ---
Yes, but the whole idea of a snubber is to DISSIPATE energy, so you need a resistor in series with it.
A very low resistor of maybe 10 - 25 Ohms might be around right.
Jon
--- End quote ---
But a snubber is probably not needed here, certainly not such a large one anyway.
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