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Topic: (answered) Is a mosfet body diode a fly-back? (Read 597 times) previous topic - next topic

justPassingThru

Dec 15, 2017, 05:33 am Last Edit: Dec 15, 2017, 05:49 am by justPassingThru
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?)

Wawa

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..

justPassingThru

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?


larryd

#3
Dec 15, 2017, 05:45 am Last Edit: Dec 15, 2017, 05:47 am by larryd
It results from the manufacturing process.

Serves no real purpose.

.
No technical PMs.
The last thing you did is where you should start looking.

larryd

#4
Dec 15, 2017, 05:51 am Last Edit: Dec 15, 2017, 05:53 am by larryd
It does have a start up function when powering from USB, see schematic.
USBVCC
Click on the image for full resolution.
No technical PMs.
The last thing you did is where you should start looking.

Coding Badly

Serves no real purpose.
I believe it allows the Philips bi-directional level shifter to work correctly...
https://cdn-shop.adafruit.com/datasheets/an97055.pdf

But that is really the clever folks at Philips (Herman Schutte) taking advantage of the fact that the diode exists.


MarkT

#6
Dec 16, 2017, 05:03 pm Last Edit: Dec 16, 2017, 05:05 pm by MarkT
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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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