How to Resolve the resonation of Coss and Leakage inductor at No Current output in a Phase Shifted Full bridge Converter?

[Pulasthi_Perera]

I have been designing and testing a closed loop phase shifted full bridge.

When my Current Reference to the controller is 0A.bridge of the Phase shifted full bridge is switching at 50% with no overlapping. At this moment i observe high EMI induction in the Surroundings because in the Deadtime when one FET is open and other is also open but it Coss is charging to 400V.Now this Coss is resonates with the leakage inductance of the Transformer and produce EMI in the deadtime.

The Transformer primary waveform is attached and you can see the voltage spikes that are coupling to the secondary.
And my understanding of what is happening is drawn in the other attachment.

Can anybody did me on how to solve this issue? how can i Discharge the Coss of MOSFETS without delivering power to the secondary and prevent resonation.I see heavy EMI in this occation enough to make my monitor flicker.

 

[cupoftea]

5 ohms doesnt sound that high....you can do more i think....dont forget you have ZVS at turn on...use a diode and get fast_off____slow_on.
It sounds to me you switch fet to fast...(please tell Cgs?) (and Qgd and Qgs).

 

[Pulasthi_Perera]

FYI this is the waveform of the primary side of the transformer when 400V input 72 output and 8A output. At this time converter has a 91% efficiency. So i think my design can be validated for power delivery. Total design is 2.5kW but i only have loads at this moment to go up to 600W.Don't mind the voltage scale i forgot to change it to 500x

 

[mtwieg]

This is a good point, but means your series caps in your gdt (if you use GDT) will ring with the Lm of the GDT. ( due to the repeated sudden duty cycle change from 50% to nothing) This can cause spurious on-coming of the FETs.

Good point, depending on how the GDT is implemented burst mode may have issues.

Confessedly you may alternatively use bootstrap drive...but these are dodgy when the input bus is 400V+.
...
Bootstrap = deathtrap, as has been discussed much on this forum.

Wrong, but I'm not going to derail the thread about this.

 

[cupoftea]

Wrong, but I'm not going to derail the thread about this.

....thanks, correct..in the ideal world....in the real situance, we cannot get great layout, and the bootstrap's tricks start to play out on us.