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So for some MOSFETs, specially with Power MOSFETs, the body diode is a flyback diode that (as far as aI can tell) can be conveniently added to the MOSFET's bulk as there already PN junctions (correct me if I'm wrong).

I've looked at the datasheet of a Power P-MOSFET and it has parameters like "Internal Source Inductance" and "Internal Drain Inductance". So, is the build up of reverse voltage on the source and drain sides, when said MOSFET is suddenly turned off, a behavior intrinsic to (at least some) MOSFETs? Or are these from the rest of the circuit, say inductive loads?

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  • \$\begingroup\$ I've done a bit more skimming and found that body diodes are ALWAYS there in any MOSFET. Ok, ignore first paragraph... Probably should've done more reading before posting this... \$\endgroup\$ – Dehbop Jul 2 '15 at 8:21
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    \$\begingroup\$ If your question has changed, you can use the 'edit' link to amend it. \$\endgroup\$ – Nick Johnson Jul 2 '15 at 8:28
  • \$\begingroup\$ There is always some inductance in the system, the question though is how much the device is designed for. \$\endgroup\$ – PlasmaHH Jul 2 '15 at 8:44
  • \$\begingroup\$ @Nick: It provides more content... Like sawdust. \$\endgroup\$ – Dehbop Jul 2 '15 at 8:59
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The body diode is part of the MOSFET structure and is always there. Sometimes a Schottky diode is added (co-packaged) because the body diode is slow recovery. Since the Schottky diode has a lower Vf it conducts most of the current.

It should be noted that the diode only comes into play in a configuration such as a half-bridge or H-bridge where the diode conducts when a different switch turns off. The internal diode is in the wrong place for a single switch controlling an inductive load- when the switch turns off the voltage across the switch rises rather than reversing.

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  • \$\begingroup\$ So what exactly are "Internal Source Inductance" and "Internal Drain Inductance"? I can imagine capacitances, but inductance? Maybe this is how current pinches off and that can be quantified by growing resistance and time delay to turn off (L = R*t)?? \$\endgroup\$ – Dehbop Jul 2 '15 at 10:38
  • \$\begingroup\$ Just what it says- a very small inductance in series with the drain and source leads. Even a short straight wire (like the bonding wires) has a bit of inductance. Usually the drain inductance is swamped by (adds to and is much less than) external inductance, but the source inductance can be a problem if you are driving the MOSFET hard- di/dt affects the gate-source voltage. Modules have higher source inductance, but it's still nH. \$\endgroup\$ – Spehro Pefhany Jul 2 '15 at 10:53
  • \$\begingroup\$ High frequencies... Never considered that... That's why it was included. But I've never seen BJTs that includes them... Might have to check JFETs too. \$\endgroup\$ – Dehbop Jul 2 '15 at 11:02
  • \$\begingroup\$ Definitely important in RF parts, but usually presented differently. \$\endgroup\$ – Spehro Pefhany Jul 2 '15 at 11:25

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