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Mosfet and Flyback

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Makaram

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Hey,
Just noticed on the schematic for my chosen fet that it has what appears to be a flyback diode.
I'll be using these mosfet in an H bridge switching 12V motors for shot bursts (automotive door locks).

Do I need external flybacks to protect these fets or is that unnecessary?

DATASHEET
https://www.vishay.com/docs/62667/si7615adn.pdf
 
Hey,
Just noticed on the schematic for my chosen fet that it has what appears to be a flyback diode.
I'll be using these mosfet in an H bridge switching 12V motors for shot bursts (automotive door locks).

Do I need external flybacks to protect these fets or is that unnecessary?

Judging from the specs (-80A limit), no. Unless you expect greater currents to be present in your design.
 
That diode shown on the part's schematic symbol is actually called the intrinsic diode. It is fairly slow. If you need "flyback:" protection, you might be advised to add an external, faster diode.

John

EDIT: Please ignore the above and see added comments in my post #7.
 
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John,

Just as an aside, how are you defining "slow"?

All the intinsic diode timing aspects for this device are listed as no more than 50ns.

Surely this is fast enough for the OP's stated purpose.

CBB
 
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@cowboybob

You are right. The comment I posted was based on a note I had kept of comments by an experienced member on another forum. That note has now been deleted.

While researching this, an almost identical question, but involving N-channel mosfets, was posted on physicsforums : https://www.physicsforums.com/showthread.php?t=440122
Go to the last post.

It appears that an external diode may be used for protection, not because the intrinsic/body diode is too slow, but because its zener voltage is too high.

Also found this interesting application not on the same subject: https://www.electro-tech-online.com/custompdfs/2013/01/transient_voltage_protection_of_mosfets.pdf

In the case here, Vds is -20V and the voltage being used is 12V. The body diode breakdown voltage is not given, so one might consider using a zener diode for protection.

John
 
From my understanding, the diode is there because of the way the FET is produced and can not be done away with and I imagine it is shown in the diagram to let us know it is there and that it is not provided for any particular purpose, I even believe it is referred to as a parasitic diode. It's been a while since I read up on FETs, so if I'm wrong, additional comments would be appreciated.
 
The question of whether a mosfet can be made without the diode was asked of International Rectifier last year on its Q&A forum, and the answer was that with current technology, such a device cannot be made.

Here is a little more about that diode:

Source= https://www.electro-tech-online.com/custompdfs/2013/01/IXAN0061.pdf

Body Diode’s Reverse Recovery Characteristics:
The body diode exhibits very slow reverse recovery with large reverse recovery current as shown in Figure 8.


Source= https://www.electro-tech-online.com/custompdfs/2013/01/Power_MOSFET_Basics.pdf

During diode reverse recovery, its reverse current also goes to the lower FET in Figure 11, along with the load current; in addition, the reverse recovery di/dt can cause large voltage overshoots (Ldi/dt) due to circuit stray inductance. These voltage overshoots are minimized if the di/dt during the second phase of the trr (after crossing IRM) is kept low. Such a diode is said to have soft recovery. Lower QRR leads to lower switching loss. This is often the largest single component of switching loss in a switching converter. AOS SDMOS and SRFET have been designed with advanced technology specifically to improve body diode reverse recovery performance with low Qrr and good softness coefficient compare to regular MOSFETs, which can greatly reduce the voltage overshoot, and improve the overall efficiency.

So, that diode is slow in recovery, but it has been made better.

John
 
You might want to post your proposed circuit, but I think you will find the inductive kick to be of the wrong polarity to turn on the body diode in which case you would be relying on the avalanche breakdown of the fet to stop the voltage rise from the inductor. This will work in some cases if the resulting pulse is within the avalanche rating of the FET. But if you think about it the breakdown voltage is about 30% higher than the Vds rating (usually) and the inductor current passes thru the fet so the power is very high. A reverse diode is much safer and easier to calculate.:D
 
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