I'm working on a Nixie tube clock and I'm having issues with the power supply. It's a flyback converter based on the LT3757 and the Coilcraft DA2032-AL flyback transformer. I designed to work with an input of 19 V and an output of 230 V at 38 mA of maximum load. The problem is that at its initial intended switching frequency of 100 kHz, it outputs 230 V under no laod and as soon as the tubes turn on, the voltage drops to about 140 V and the chip seems to enter some sort of protection mode, outputting a very low duty cycle gate drive signal. This happens at a load current of less than 10 mA.
I tried to change anything that could cause this: loop stability compensation capacitor, softstart capacitor, decreased current sense resistor, changed diode, tested with and without snubber, nothing helps.
I increased the frequency to 275 kHz and this caused the transformer to heat up to 80 degrees Celsius and the diode to fail after about 2 minutes. So I found a sweet spot at 185 kHz that provides enough power without excessive heat, but I'm still not happy with the final result because I did not find the root cause.
I simulated the circuit in LTSpice at 100 kHz and it can provide up to 70 mA, so there is a huge discrepancy between simulation results and practical results.
I think the limitation is due to the transformer somehow, but I'm not sure. So my question is: what could cause the circuit to fail to provide enough power at 100 kHz?
Here are some scope captures at 100 kHz under load, with snubber populated, during the described behavior:
CH1 - Q2 Gate
CH2 - Q2 Source
CH3 - Q2 Drain