Flyback inverter has primary switch connected in series with the transformer winding. A snubber circuit is required to damp the high voltage spike that occur as a result of resonance that occur because of the transformer leakage inductance and the mosfet capacitance. Without a snubber circuit the mosfet on primary side can burn out.

An RCD snubber is commonly used in flyback configurations and consist of a diode, capacitor and resistor. In this configuration ,the diode must be able to operate at the same frequency as the switching frequency. It is because the leakage inductance will create resonance during the turn off time.Therefore, if the diode in RCD snubber is not able to switch at switching frequency it will not work and hence cannot protect the mosfet. C should be greater than or equal to the thrice of the C_oss of the mosfet. While R = V/I where V is the max voltage the switch can handle.
Figure 1 below shows the effect of snubber on the switching pattern of a mosfet. It is pertinent to mention that the below mentioned waveform are recorded under different input conditions.

Without RCD snubber (Click to zoom)

With snubber (Click to zoom)