Flyback power supply output voltage

[argintviu]

Hi all. I am working on a small step-up converter for a battery operated project and I discovered a little problem (LTSpice file and picture below).

The circuit works as follows: The primary's swing is limited by the 30V Zener diode (which also protects the transistor). After passing through the 1:10 transformer, the energy is stored in C4. The expected voltage on the VOUT node would be 300V minus one diode drop, but it only gets to about 270V. Probing the VPULSE node shows that the output pulses dip below 0 quite a bit. Why is this occuring? Is this negative component normal for the flyback topology or am I doing something wrong?

[xavier60]

The negative half cycle at the secondary is due to VCC applied across the primary while Q1 is on and multiplied by the turns ratio.
Although the Zenner is clamping the flyback pulse at the Collector of Q1 to 30V, the primary sees something less because of the presence of the VCC subtracting from it.

[argintviu]

Thanks, that's very helpful!