RC snubbers are commonly placed across secondary and bias diodes. The primary function of the snubber circuit is to limit the peak voltage at the instant when the diode turns off and to reduce the ringing.
The snubber capacitance should normally be chosen, so that it is much larger than the diode capacitance at the expected peak inverse voltage. The figure below shows how the ringing is decreased with the increased snubber capacitance. Note, however, that larger snubber capacitor is also associated with higher snubber losses. PI Expert selects a capacitor value to balance the losses and to provide significant damping at turn-off.
Typically, higher voltage rating diodes, which are operated at higher reverse voltages, have lower junction capacitance and need a smaller snubbing capacitor. Thus, at higher output voltages, the snubber capacitor is a lower value, which also ensures lower power loss.
The value of the resistor does not determine the total power loss. However, since the resistor will dissipate the energy accumulated in the capacitor, it has to be sized for its power rating accordingly. The value of the resistor can be selected so that the snubber corner frequency is below the ringing frequency.
When using bias you can optionally add an RC snubber comprising of a resistor and a capacitor. In the Feedback form select Snubber RC for Bias check box to turn on snubber.
In the Component box click the black down arrow control on the right of the calculator icon 
and select Change.
For resistor the Snubber Resistor dialog box opens. In this dialog click the calculator icon to open the Change Resistor dialog box and edit the Resistance, Tolerance and Rated Power values.
For capacitor you can select a new one from the active Component Set (see also Component Library and Component Sets Overview).