I've designed and built a circuit based on the LT3798. It's very similar to one from the datasheet. I need to input 90-240VAC and output about 4A @ 24VDC (about 100W). I've got it to work fairly stably but it only puts out up to 40W before the output voltage drops. I'm using an almost identical circuit to the one from the datasheet but it's been adjusted for an output of 4A instead of 2. I'm also using a custom transformer (4:1:1). There are two main problems:
1. The output voltage drops once the load gets to about 30W (~1.2A). It's stable up to that point.
2. The snubber circuit gets very hot regardless of whether I use the one from the datasheet or from the reference design.
Here is an image of the schematic. Ignore the dots on the transformer, I just used a generic symbol from my library. I'm doing my rectification on another board, and that part works fine but right now I'm running this from a 120VDC power supply. I've also added about 2mF of output capacitance. In the interests of not making this post too long, I'm going to stop here so if there's something you'd like to know, just ask.
Your converter is probably current limiting.
The final snubber components you will need depend on the leakage inductance of the transformer. If it's >2%, yes, it will get very hot, and it can cause many other problems for you at higher power.
I'm including a picture of a 100 watts application I did a while back. A transformer with a similar ratio as yours was used. Note the highlighted parts.
Your output diode and main FET will need heat-sinks. And, your diode needs to be rated for >200V.
Sorry for the slow reply, I've been working on this and other projects in the background. Thanks for your advice. I've made some changes and I managed to briefly get to about 96W so I there's definite improvement.
Quick question: Why does the output diode need to be rated for >200V? The output voltage gets nowhere near that. Even if regulation fails, with the transformer windings I have, the output voltage would be about 60V when run from 240VAC.
During normal steady state behavior, with 240VAC at the input, the diode sees Vout+(Vin/n). since your turns ratio (n) is 4:1, that corresponds to 24+(340/4)=109. Assuming your transformer is perfect, no leakage inductance, you need to account for transient events, such as some startup overshoot.
If you have any switching spikes, and I am sure you do, these spikes will add to the diode stress, hence the suggestion for the higher voltage diode.