This is my first post in a long time as I've only just re-subscribed.

Hi Sean,

Does your application require the FETs to be switched so quickly?

My first thought (if possible) was to eliminate the cause of the problem 
by slowing the switching rather than filtering the effect?

Regards,

Nigel
 

Sean Breheny wrote:
>  
> Hi all,
>
> I have a circuit involving FETs switching 50 amps. The FETs are driven
> by a gate driver IC which has two supply voltages, one which is for
> the logic side and the other for the drive side.
>
> When a FET switches on, an inductive spike develops between the source
> lead and various parts of the ground plane, due to the really high
> di/dt (it switches on in about 150 nanoseconds). To minimize turn-on
> time, I locate the drive-side driver ground reference very close to
> the source lead. However, the reference for the logic side has to be
> with the logic circuits. The spec on the IC says that there can be no
> more than 5V between the two references, so this means that I must
> make sure that the inductive spike is no greater than 5V. I have had
> ICs die for apparently this reason (latch-up).
>
> To minimize the spike, I am using film caps close to each FET source
> to return the current to the positive supply rail (where it ultimately
> comes from). There are also bulk electrolytic caps to prevent the
> current spikes from going back to the battery. Here's my problem: in
> order to properly place and size the caps, I need to be able to
> estimate the inductance between two points on the ground plane.
> Simulation shows that this makes a big difference.
>
> I can find all kinds of formulas and calculators online for
> inductance, but I cannot find a copper sheet inductance formula.
>
> I even found a reference from Dr. Howard Johnson saying that one
> cannot calculate the inductance of a ground plane, only of a wire OVER
> a ground plane.
>
> I think he is right if it is a perfect ground plane, but his comment
> doesn't apply to a real, thin copper sheet.
>
> Certainly, my circuit is ALSO a closed loop (the current is not just
> in the plane but in wires and components, too), but I do not care
> about the TOTAL inductance, only that between two points on the plane.
>
> Any ideas?
>
> Thanks,
>
> Sean
>   

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