why did ST add a 100pf 1000V 1210 parallel capacitor with MOSFET in LLC resonant topology in AN4599?

[Aghar.1]

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[Peter BENSCH]

Welcome, @Aghar.1​, to the community!

this is extensively discussed in AN2644, appendix C: Power MOSFET effective Coss and half-bridge midpoint's transition times.

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

[Aghar.1]

Thanks for your good answer @ peter Bensch , actually we have used L6699D with Infineon Coolmos IPAN50R500CE, and it worked properly.

But when we changed this Mosfet with IPD60R600P7S it couldn't start under full load, I think it is a problem from startup. finally, we solved it by adding a 100pf capacitor in parallel with the lower Mosfet. Would you please help me whats the root of this problem? Or do you have a better solution? Please do not hesitate to contact me if any further information is needed.

I have attached the Datasheets of these two Mosfet.

Alireza