Ok to use MOSFET body diode as flyback protection?


by TheAnalogKid
Tags: body, diode, flyback, mosfet, protection
TheAnalogKid
TheAnalogKid is offline
#1
Oct20-10, 09:42 PM
P: 10
Hello,

I am designing a motor controller, and I was curious about the MOSFETs I'm using that have an internal body diode zener for Vds protection that is common in a lot of datasheets.

I am switching the coils of the inductor with these MOSFETs, and I would like to know if its unwise to assume to use the internal zener diode of the MOSFET as my flyback diode when I am switching the inductor.

The part I'm using is IRFP3206 and I see the max pulsed current is 840 Amps for the FET's diode, and I can do the calculations to approximate the flyback current, so I think it is ok to use.

But, I just am cautious to use the internal diode for a specific function, when it seems like it is put in more to protect the FET junctions as a general purpose protection. Is this done in practice?
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gnurf
gnurf is offline
#2
Oct21-10, 09:24 AM
P: 330
I'm not sure if it's normal practice or not (I'd guess not), but with a forward voltage of 1.3V you're wasting power compared to an external Schottky for instance. Also, the reverse recovery time of the internal diode is typically not as fast as it will be with an external diode. Hard to tell if these are issues with the limited information you provided. Good luck.
TheAnalogKid
TheAnalogKid is offline
#3
Oct21-10, 04:41 PM
P: 10
Quote Quote by gnurf View Post
I'm not sure if it's normal practice or not (I'd guess not), but with a forward voltage of 1.3V you're wasting power compared to an external Schottky for instance. Also, the reverse recovery time of the internal diode is typically not as fast as it will be with an external diode. Hard to tell if these are issues with the limited information you provided. Good luck.
Thank you very much for the response. I didn't take these factors into consideration, and they seem to be good arguments for using an external diode.

vk6kro
vk6kro is offline
#4
Oct21-10, 05:57 PM
Sci Advisor
P: 4,003

Ok to use MOSFET body diode as flyback protection?


When you switch an inductive load with a FET or transistor, the main spike is positive going, so a diode across the FET isn't going to help unless it is a Zener diode.

The diode needs to be a fast one and across the load with the anode connected to the drain of the FET.

The diode in Mosfets is usually shown as a Zener, but this would break down at some fairly high voltage (maybe 50 volts or so), so the power dissipated would be quite high and it would add to the heating of the FET.


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