The function of the Clamp circuit is to protect the primary switch from excessive Drain- Source Voltage that is imposed during the off time of the switch. The two most common clamp circuits are RC (resistor and capacitor) with blocking diode and Clamping Zener with blocking diode. An RC snubber with blocking diode is the lower cost option, but this should be used with caution. Make sure that drain voltages are clamped to a safe level under worst-case conditions.
For low voltage forward converter applications, a common clamp circuit (see figure below) is a single Zener diode placed directly across the primary Switch, together with a small capacitor across the primary winding.