Quote:
Originally Posted by princeton
I apologize for my ignorance. I tried reading quite a bit of this thread. I still can't figure out why the diodes are necessary between the drain and V+ ??
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edown has it down pretty good. hehe. here's more info:
Flyback diode - Wikipedia, the free encyclopedia
Should try it: get a motor (or maybe a relay solenoid, something that's inductive) and a battery. Maybe an automotive relay and your car battery. Connect and disconnect the load to the battery and you'll see a spark when you remove the load. That spark is caused by a voltage that is high enough to break down the air and conduct electricity. the high voltage is induced by the inductance of the load and the changing current when the battery is disconnected.
I think you also asked about placing the diodes near the motor vs in the controller. while the motor may have the bulk of the inductance, the cables themselves that go to the motor from the controller have a finite inductance too (often called parasitic inductance). You can probably search the 'net to find equations for inductance of a wire. If the diodes are placed at the motor, the current in the cable will still experience a sudden change in current that is unclamped by the diodes. Coupled with the inductance of the wire, a large voltage spike will be produced and possibly take out the mosfets.
it's vitally important that the mosfets, diodes, and input caps are close together. draw your circuit and highlight the wires (or pcb traces) that are conducting current when the mosfet is on. draw it again, but this time highlight the wires that are carrying current when the mosfet is off. compare the two. there should be wires that carry current in one but not the other. the length of these wires should be minimized.