Location of Freewheel Diode

[jbarrett]

free wheel diode

I am working on a project to control the speed of a DC motor with PWM. This is for an automotive application, and will be high-side drive. My dilemma is where to locate the freewheel diode in my design. The distance from the controller to the motor could be 10 feet or longer. From research I have found that the diode is generally placed more or less across the motor terminals, as in (A) of the attached schematic. For simplicity, would it be acceptable to place the diode at the mosfet, as in (B)? I feel that this would be easier on the mosfet, as the wire inductance could be large. The motor draws approximately 15 A at 12V, and will be wired with 10 gauge.

Thanks in advance for any help!

Jeremy

 

[VVV]

mosfet as freewheel diode

The diode should be placed as close to the MOSFET as possible. You need to protect the MOSFET from spikes, which would be produced by the wiring inductance, if the diode was placed far from the MOSFET.

The extra inductance that is added by the wire simply adds to the motor inductance, so from this point of view, it does not matter (it matters for EMI to keep the connections short). Although the wire inductance can be significant to cause large spikes to make the MOSFET fail, it will be negligible compared with that of the motor.
But even if it was not negligible compared to the inductance of the motor, it would not produce any negative results, to the contrary: the larger inductance would cause lower current ripple. In fact, sometimes an extra inductor is added in series with the motor, in order to reduce the current ripple, especially if the motor has a very low inductance.
This arrangement suggests right away that the diode should be placed close to the MOSFET.

 

[jbarrett]

free wheel diode

Thanks for the help! My thinking was along the same lines, but I wanted some other opinions to make sure I wasn't far off.

Jeremy