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I have noticed that a MOSFET symbol has a little diode in it (or at least what looks like a diode). Does this mean I do not have to worry about using a diode in a circuit that runs a motor using one? I would have used a diode in order to prevent reverse voltage from turning the motor by hand.

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  • \$\begingroup\$ "I would have used a diode in order to prevent reverse voltage from turning the motor by hand." That's not what a flyback diode is for. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 16 '15 at 23:48
  • \$\begingroup\$ It is called a body diode and it can be used as a flyback diode. \$\endgroup\$ – Passerby Mar 16 '15 at 23:49
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    \$\begingroup\$ For single MOSFET circuits driving an inductive load, the body-diode will NOT work as a replacement for a flyback diode as it conducts in the wrong direction. Consider for example an N-channel MOSFET as a low-side driver for a solenoid. When the MOSFET turns off, the voltage spike at the drain will be positive. The body diode will NOT conduct as it is reverse biased. The exception is avalanche breakdown and some MOSFETs are "Repetitive Avalanche Rated" (often drawn as a zener diode). \$\endgroup\$ – Tut Mar 17 '15 at 11:17
  • \$\begingroup\$ @electricviolin If you add a Schottkey diode per Nick's suggestion, put it across the load, NOT in parallel with the MOSFET body diode (unless your circuit is an H-bridge and you are adding diodes across all 4 MOSFETs). See my comment above. \$\endgroup\$ – Tut Mar 18 '15 at 10:15
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Yes, power MOSFETs have a parasitic diode called Body Diode.

MOSFET with body diode

As a result of this diode, a single MOSFET can work only as a unidirectional switch. A single MOSFET can't switch-off the opposite direction, because the diode conducts independent of the gate.

The body diode is usually fairly slow to turn on. I advise against using it as a the only flyback diode. Add an external Schottky diode for flyback protection.

(There are power MOSFETs with a built-in Schottky diode. They have 2 diodes: the native body diode, and an additional Schottky. But such MOSFETs are relatively uncommon.)

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  • \$\begingroup\$ Okay, thanks for the help! Would I have to use a Schottky if I am not worried about voltage drop? I am running it at 12V, and I don't mind if a loose 1V or 1.5V. Or did you use the Schottky for the fast switching rate? \$\endgroup\$ – electricviolin Mar 18 '15 at 0:39
  • \$\begingroup\$ @electricviolin Schottky for the fast switching speed. \$\endgroup\$ – Nick Alexeev Mar 18 '15 at 2:45

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