Good evening Mincing .

I seen your question about snubbers and can copy-paste you a relative simple answer .
But i have to add that this is a dissapative snubber (i'm not always happy with that ,but it solves the ringing)
The information that follows is from the website of ridley engineering (they give a clear calculation method)

If you have any ringing waveforms in your power circuit, these waveforms must be damped or they can lead to device failure, excessive EMI, or instability. In many cases, you can damp a ringing waveform with a series RC network across the offending device.

Note: if the ringing frequency is not almost two orders of magnitude higher than the switching frequency, you may be in trouble. It will be hard to damp the ringing without excessive dissipation, and alternate circuit solutions must be found.

First, you need to measure the natural frequency of the ringing waveform.
To design the snubber for the power FET of a flyback circuit, for example, first run the circuit at low power with an oscilloscope probe on the drain waveform with no snubber.

Make sure you are using a low capacitance scope probe, otherwise the waveforms will be modified by the connection of the probe. If you suspect that the probe capacitance is too high, just set the scope to a higher sensitivity, and without making electrical contact to the FET drain, just bring probe close to the device. You will see the high-frequency ringing waveform due to the radiated noise.

Observe the ringing waveform at turn-off on the drain. Use a reasonably high input voltage (without destroying the FET, or course) since the resonant frequency of the ringing will be voltage dependent. Record the resonant frequency.

The ringing is caused by an equivalent RLC network. For a low-loss circuit, it will be quite undamped, and the oscillations will continue for many cycles. Step 1 is to add a damping R across the device. First , you must know one of the resonant elements, L or C. On the primary switch, the leakage inductance is the dominant L, and should be well known. For a secondary ringing, the diode capacitance will be a known quantity.

Calculate the characteristic impedance, of the resonant circuit.

If you know L, Z = 2 x 3.14 x f x L

If you know C, Z = 1/(2 x 3.14 x f x C)

Try an initial value of snubber resistor of R = Z. This usually suffices to control the ringing.

Using just a resistor across the power device will control the ringing, but the dissipation would be very large. A series capacitor is used to reduce the power dissipation in the damping resistor.

Calculate the C needed in series with R according to: C = 1/(3.14 x f x R)

Increasing C beyond this value will increase dissipation, but will not improve the damping. In some cases you will be able to decrease it by 30% or so, but any less than this and the snubber will be less effective.

Size the resistor according to the dissipation it will see: P = C(VxV)Fs

where V is the voltage on the device when it is off. Depending on the circuit operation, the actual dissipation may be closer to half this value in some cases, and the design will be conservative. Use thermal data from your circuit to determine if the resistor size can be reduced.

Build the snubber (keep leads short) and test the circuit. You should be close to the final solution on this first attempt.

If the problem remains ,i would advice you the book switchmode powersupply's by Pressman .
They have a chapter on all sort of snubbers (active passive etc. etc.)

Good luck Walkura