Electrical – Flyback converter for a step-up application (4VDC to 48VDC)

I'd go for a full H-bridge driving a transformer.

With a 24V supply you can drive nearly 48 Vp-p onto the primary and this reduces the secondary winding turns to a ratio that produces 600 Vp-p - then I'd use a fast bridge rectifier and smoothing to give a 300 V dc output.

Step up ratio would probably be about 14:1 and if you wanted to control the amplitude I'd consider feeding the H bridge from a decent efficiency Buck convertor. Maybe make the turns ratio 15:1 so that a 20V output from the buck would do the business - leave a little overhead for loads etc..

My main consideration is avoiding the secondary winding having too much self-capacitance and acting like a parallel tuned circuit. It should be quite a way off with this type of topology.

It's not going to be a small lump of ferrite and I'd use the best I can get my hands on such as 3F4 material from Ferroxcube - you should be able to get 300 watts from an E68 planar transformer. I'm getting 200 watts from a slightly smaller E58 - using two half cores rather than a half core and a plate.

Mine's running at about 600 kHz so I can use PCB tracks for the coils - a sandwich of PCBs does the trick for the primary (4 turns in total) - mine is 1:1 so it's the same PCBs on the output - yours will need thinking about as to whether you can get 60 turns at 1A from PCB. I think you should just about squeeze them in.

Your not paying attention to the 'dead-time' in the transformer. As the pulse width increases the OFF time decreases, thus more output voltage and current. What you need to do is adjust R2 or R3 so that a usable maximum brightness is achieved-but no more than that. Your paying a penalty as it is by using pull-up resistors which slow down the rise time at the gate of the mosfet, causing it to be less efficient as a switch.

The transformer core MUST have a minimum OFF time so the magnetic field can collapse before the next drive pulse. Maximum safe ON time is likely about 95%, but your current design may push it close to 99% or more. Chances are the transformer may be getting warm as well, yet it can run cool while the mosfet gets very hot.

With no load the transformer can have large eddy currents, so overheating of the mosfet is no surprize. One thing that works for me is to find out the voltage across the output capacitor when LED's are at maximum brightness, then insert a string of 24 to 50 volt 5W zener diodes across the capacitor so that if no load is present, the zener diodes act as a dummy load and clamp the voltage to just 5 or 10 volts higher than if the LED's were hooked up. This way the circuit always has a load, either the LED's or the diode string.