Not really.
The first adds resistance to the diode, which significantly reduces efficiency, and only damps the ringing from the diode capacitance, not from any other stray capacitance.
You need the second circuit, with the resistance in series with the capacitor, to absorb the tank energy.
The added capacitance swamps the stray capacitance so that most of the energy is absorbed in the resistor.

Thank you, crutschow

 

As I said, the DC voltage is your output voltage feeding back through the inductor when it stops conducting in the forward direction.
Once current stops flowing in the forward direction through the inductor there's nothing to stop it from flowing in the reverse direction to charge the stray capacitance across the diode.
No need to add a transistor leakage current to account for that.

If you add a snubber capacitor, it's also usual to add a small resistor in series with the diode to absorb the ringing energy from the LC tank (L from the inductor and C from stray capacitance), otherwise you will just change the ringing frequency but not get rid of the ringing.
Values of a few tens of ohms up to about a 100 ohms are typical. You can experimentally determine the best value.

Can you explain why we can kill those ringing after a snubber circuit is added in detail? Is it because inductor and capacitor phase are inverse? What happen to inductor and capacitor?

Thanks

 

Are you familiar with LC resonance?
The LC forms a resonant circuit that rings at its resonant frequency due to the abrupt stop of the inductor current by the diode.
The resistor in series with the tank absorbs the resonant energy and thus damps the oscillation.
With proper selection of the resistor value to give a critically damped circuit, the energy is dissipated within one cycle of the resonant frequency.

 

Are you familiar with LC resonance?
The LC forms a resonant circuit that rings at its resonant frequency due to the abrupt stop of the inductor current by the diode.
The resistor in series with the tank absorbs the resonant energy and thus damps the oscillation.
With proper selection of the resistor value to give a critically damped circuit, the energy is dissipated within one cycle of the resonant frequency.

Thank you. You are a very nice mentor. I got it.

 

Are you familiar with LC resonance?
The LC forms a resonant circuit that rings at its resonant frequency due to the abrupt stop of the inductor current by the diode.
The resistor in series with the tank absorbs the resonant energy and thus damps the oscillation.
With proper selection of the resistor value to give a critically damped circuit, the energy is dissipated within one cycle of the resonant frequency.

Mr. Crutschow,
There is other application for the same circuit above. The difference is that R3= 3.0K and R4 = 3.0k, R2 = 4.9k. But this switch node is very smooth and output(3.3V) are very smaller around 70mV (spec 120mV). Moreover, the C6 value is different too. What happen to those application? (R36 = 0). You can see the previous captures to compare.




the output captures as follows;