LTspice sim of LTC3756 and LTC3766 wont work?

With your circuit, I get an time step to small error referencing something internal to U1. As we don't know what it looks like inside, it's difficult to diagnose.

The built in example circuit for the LT3765 (which includes the LT3766) works. What did you do to break it?

I understand that you're changing the example circuit to work at different voltages. I suggest starting with the working reference circuit and gradually make changes to it, running the sim every few steps. That way you'll know what change breaks it.

Hi Thanks, it now works to an extent, as attached (LTspice sim and jpeg scm)

The attached shows a 400w 360vin , 100khz, two transistor fwd with active clamp.
The LTSPICE Sim of this chipset runs so slowly it is of very limited practical use.
The dissipation in the clamp cap snubber is obviously too much, and was calculated with the damping factor of the datasheet of ltc3765.
The LTspice sim cct doesnt work at all unless you short out the ISMAG resistor.

The circuit works very badly.
There are enormous current spikes in the synch fets.
The Secondary power synch_fet turns on before the primary power fet....this is never wanted........you want the primary one to turn on first, then the sec diode to conduct, then the sec synch fet should short out its diode.

Anyway, I am noticing that at certain times of the simulation, the “synchronous” FET is staying on for longer than a switching period…this surely should never be allowed?

Also, there are huge switching current spikes in the synchronous FETs…and this cannot be resolved no matter how the delay resistors are adjusted.
Also, the sec side forward synch fet comes on before the primary FET…and this should surely never happen?.....surely the sec side forward synch fet should only cone on after its diode has started conducting?

Do you have any way of righting this sim?.....even any way of speeding it up?…it is ridiculously slow