This guy says that the IRL540's body diode protects the mosfet from inductive flyback.
Is that correct?
(Contrast that circuit to this one, which shows the diode across the inductor, but no body diode.)
(Supplementary question: do all mosfets have body diodes, even though the symbol might not show one?)
justPassingThru:
This guy says that the IRL540's body diode protects the mosfet from inductive flyback.Is that correct?
No.
Inductive kickback increases voltage on the drain, so the body diode is in the wrong place to stop it.
Nick Gammon's site is correct.
Leo..
Wawa:
No.
Inductive kickback increases voltage on the drain, so the body diode is in the wrong place to stop it.Nick Gammon's site is correct.
Leo..
That's what I thought, thanks.
So what's the body diode for?- just against "normal" every day mistakes of reverse polarity?
It results from the manufacturing process.
Serves no real purpose.
.
It does have a start up function when powering from USB, see schematic.
USBVCC
Click on the image for full resolution.
larryd:
Serves no real purpose.
I believe it allows the Philips bi-directional level shifter to work correctly...
But that is really the clever folks at Philips (Herman Schutte) taking advantage of the fact that the diode exists.
justPassingThru:
That's what I thought, thanks.So what's the body diode for?- just against "normal" every day mistakes of reverse polarity?
Its an unavoidable consequence of a vertical current flow FET - the substrate is the drain, so the
source forms a diode to the drain.
With IGBTs, which are like MOSFETs but have an extra layer for the collector, there is no
body diode as the drain is between source and collector(subtrate).
(some devices incorporate one anyway as they are very useful in power bridge circuits driving inductive loads).
With an H-bridge each MOSFET's diode acts as a free-wheel for the other MOSFET in that leg of
the bridge. This is where the misunderstanding comes from.
