I built my own step-up transformer to be driven using a ZVS using a core from a flyback transformer in order to charge some 450 V capacitors. I wanted to see how well my transformer operated with a load on the secondary side and that's when I discovered an issue. With a 6.8 \$\Omega\$ resistor across the secondary I measured this on the oscilloscope.
It looked fine to me so I would expect around 2.4 V peaks on the primary since my turn ratio is about 10. But I measured this for the voltage on the primary.
So my step-up transformer is now a step-down transformer?? I initially thought the secondary was saturating due to the relatively large current causing the secondary inductance to drop below the primary inductance thus making it a step-down transformer. So I tested a 1.2 \$k\Omega\$ across the secondary to reduce the current but it was still acting as a step down transformer this time even more (4 V peak on the secondary side). I really don't know why this is happening.
Schematic:
Update:
I tested larger values for the secondary load as suggested by Antonio51. I started with 1M\$\Omega\$ just to be safe and it all worked fine, nominal secondary voltage off 440V peak. I then went down to 100k\$\Omega\$; also worked fine. 6.6k\$\Omega\$ also worked so I decided to test my luck and tried 180\$\Omega\$. This caused the secondary voltage to drop to around 330V peaks while the primary voltage did not drop in accordance. So I think it's safe to say it works for loads above 1k\$\Omega\$ but why it doesn't for smaller loads is beyond me.
Also, just to relieve any confusion, that resistor I tested in my orginal post which I thought was 1.2k\$\Omega\$ was actually 1.2\$\Omega\$.