My question is what is the best DC-DC converter style to use for this, we are currently looking at flyback converters but are having problems with modelling them using TINA.
It depends...............
If you are running fairly high power all the time, push pull dc/dc square wave inverters will have much smaller magnetics, and are the best choice for size and cost.
But at much reduced power levels they can be horribly inefficient. The problem is the magnetising current.
Even with zero output load current, it may take significant input current to swing the transformer back and forth over it's peak to peak voltage range. So a 1Kw inverter may draw 100 watts even with zero load. That may concern you, or it may not.
Now flybacks are very interesting, and especially discontinuous flybacks.
They can work over very large ranges of both input and output voltage, and they are extremely efficient at very low power levels. The only problem is the size of the magnetics required at high power levels.
But you can overcome that by using several smaller flyback operated in parallel.
Two flybacks can be run 180 degrees out of phase, to reduce the ripple current at both input and output. With two, you get twice the ripple frequency at half the amplitude, which is MUCH easier to filter with capacitors. You can run four 90 degrees out of phase, or any number.
It also offers some redundancy. Four 250 watt dc/dc flyback modules would produce 1Kw of dc with low ripple at both input and output. And at extremely low power you could completely turn off modules, thus increasing the low power efficiency even more.
Flybacks operate in current mode, and can be designed to be very reliable. Push pull square wave inverters can generate some horrific current spikes in the switching devices, especially if the transformer saturates. Flybacks are much more benign.