How many MOSFETs can we safely parallel in condition of very high currents? I had problems with a motor application at 48V 1600A
At 1600A, I expect that you are approaching this problem from the wrong choice of switching components. TO-220 N-FETs soldered to copper boards seems insufficient for this application and the large number of devices means that probability of component failure is high and can be cascading.

For motor drive applications, module-packaged FETs may be more appropriate, even if substantially more costly per-unit.


Microsemi APTM20AM04FG
Digikey's stocked single N-FETs capable of high current


These modules would allow you to reduce the total number of switching devices in your design and allow you to couple them with bus bar rather than an assortment of bare copper-clad FR4.

Even switching to a different leaded/SMD FET package might be more appropriate and enable fewer components:


H2PAK-2 180A STH310N10F7-6
D2PAK7 240A IRFS3107TRL7PP
TO-247-3 209A IRFP2907PBF
TO-247-3 400A IXFH400N075T2
24-BESOP 500A MMIX1F520N075T2 (special heatsink required)


Remember: your time is worth something. Rebuilding the system each time you have catastrophic failure costs you and sets you back from completing and verifying the system. Better FETs may be expensive, but not blowing tens of them up for the Nth time will save you components and time.

For the diagnosis of your presented design:

On your driver board, it looks like you have too little bootstrap hold-up capacitance. 3x100nF almost certainly needs to be supplemented by additional 1s to 10s uF to ensure that the gate driver supply remains stable.

In your testing, have you verified that the channel-to-channel gate drive delay/timing variation is acceptable, even within your generous 2us of dead time? Module-to-module shoot through is also possible, particularly if a gate driver fails, leaving a FET turned on. Additionally, checking the case temperature during operation with a thermocouple or IR camera would allow you to verify that the parts are or are not overheating.

Your mention of 'enhancing' the lead of the transistor seems like it won't help too much, given the 246A silicon / 196A package rated limits of the IRFS7730. This is also additional work required to assemble the system, increasing the labor costs and potential unreliability.

Additionally, your rising and falling images indicate severe problems with bypass capacitance. You are dropping your bus voltage by ~50%! You MUST have sufficient bypass capacitance in both total value (100+ uF, likely) and in ripple current rating (>100Arms steady state, more during startup) to successfully implement your system. The supply "browning out" extremely hard may be part of the reason for your complete system failures. These capacitors will be expensive. Parts along the lines of these film capacitors may be appropriate, depending on your construction method and requirements.

Additional link: Infineon's app note on Current Ratings of Power Semiconductors and Thermal Design