ZVS with energy recuperation. This specific design with a single ended driver is usually difficult to sustain, as normally the drain voltage spikes to hundreds of volts and requires either a passive snubber, or a very hugh voltage mosfet, which usually come with their own set of limitations. Due to the very low coupling coefficient betwen primary/secondary, the majority of the cycle energy typical stays undelivered to the secondary (stored as leakage). The traditionally nasty spikes are the result of the MOSFET turning off, and the stored leakage magnetic field collapses due to the abrupt current flow interruption. The energy recuperation snubber essentially takes the remaining energy at turn off, and feeds it back to the primary (similar to traditional freewheeling diode clamps). The trick here is the addition of an extra inductor (22uH). Instead of fully dumping back the excess back into the primary, it is partially recycled through the power supply and stored within the inductor's magnetic field. This way, the averaged power consumption reduces, while the output maintains a continuous boost to continue increasing the amplitude. Essentially the recuperation network takes excess voltage, converts it to current, and buffers it, rather than dissipating, like an RCD snubber usually does. The self-oscillating nature of the circuit allows for ZVS drive of the MOSFET, where there's almost no overlap between voltage/current switching, thus reducing mosfet loss. Nonetheless, there's a small (30ns) window, where the rising/falling edges overlap due to turn on delays, RC delays and so on.
