I'm using my PWM arduino to controle a mosfet to control an High power motor (30amp, can peak to 100amp I guess)
For the motor, I would need a flyback diode, but I really don't know how to choose a diode for that. I guess there will by a very little current, but for the voltage?
The mosfet is a IRF3709PBF
The motor is EV warrior
Thanks for your help
Bernard
Quite the opposite - the diode carries the current when the MOSFET switches off, since a rapid
change in current is what generates high voltages.
The diode needs to have a peak current rating as big as the load current, and voltage say twice the
supply. I'd recommend some dual schottkys in TO247 or TO220 package, and heatsink them if going
for 100A (50A per diode of the pair).
Something like: http://uk.farnell.com/multicomp/mbr30100ct/diode-schottky-30a-100v/dp/1625138
Thanks markT,
What the advantage of dual diode? is it only to distributed the load or is there an important reason? Can I use two diode in parallel?
Bernard
Hi
Would it be dangerous connecting a diode across a motor? What happens when switched off, if its a p.m. motor the diode will attempt to dump all the power while the motor behaves as a dynamo. Most of the motor drivers use an H bridge or similar.
john
Beurnii:
What the advantage of dual diode? is it only to distributed the load or is there an important reason? Can I use two diode in parallel?
It just happens to be a convenient choice because that is a common form factor, used as a full-wave rectifier for switchmode power supplies. Indeed, the two diodes in parallel can carry 1.7 or so times the current of each single one.
john_m0ers:
Would it be dangerous connecting a diode across a motor? What happens when switched off, if its a p.m. motor the diode will attempt to dump all the power while the motor behaves as a dynamo.
You have that thoroughly confused!
What you have described is the very reason a diode is actually not required across a motor, but there is nevertheless no a problem in placing a diode there.
There is the problem with an inductor, that when you disconnect the supply, the field will collapse and cause the voltage across it to reverse in order to maintain current flow. A "kickback" diode permits this current to dissipate (slowly).
A motor does not behave as an inductor - its kinetic inertia keeps it turning and the "back EMF" appears in the same polarity as the voltage that was exciting it. It has no need for a diode to conduct any current; the energy contained in its inertia will be dissipated by mechanical losses.
The only reason for placing a diode across a motor is to account for the possibility of parasitic inductances which may be present due to design limitations, and the inductance of field windings. As a PM motor has no such field windings, it should by rights, not cause any such "kickback"
john_m0ers:
Most of the motor drivers use an H bridge or similar.
Which is not relevant. While H-bridges should contain "catch" diodes, this is again only to account for subtle deviations in motor action from the "theoretical perfect" motor. (Incidentally, you cannot use a single H-bridge to reverse a motor with field windings!)
Beurnii:
Thanks markT,What the advantage of dual diode? is it only to distributed the load or is there an important reason? Can I use two diode in parallel?
Bernard
Cheapness - two diodes in one package is a high volume product for switchmode power supplies.
Doesn't the mosfet already have the diode in it ?
michinyon:
Doesn't the mosfet already have the diode in it ?
The MOSFET has a diode in it, which conducts if the MOSFET is reverse biased. That is not relevant to this immediate discussion which is a about a diode across the motor except that such a diode means that extra diodes are unnecessary if such MOSFETs are used in an H-bridge. 
Yes, if you have a half-H-bridge (two MOSFETs, one as high side switch, one as low-side switch)
then each MOSFETs body-diode acts to protect the other MOSFET against over-voltage. The diodes
are integral part of the structure of a high current vertical-current-flow device, they come for
free with power MOSFETs. However they are not schottky diodes which have lower losses,
sometimes people add schottky diodes across MOSFETs to reduce losses in high power bridges.
Should this diode do the trick? digikeyLink
page not found.
Hi
Would it be dangerous connecting a diode across a motor? What happens when switched off, if its a p.m. motor the diode will attempt to dump all the power while the motor behaves as a dynamo. Most of the motor drivers use an H bridge or similar.john
The diode will only conduct if the motor is forced to turn backwards, and during switching transients.
Normally it is reverse biased.
The MOSFET body diode will conduct (and charge up the power rail) if the motor is turned forwards
while the power is off.