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LM5117: Help me tune this snubber

Part Number: LM5117
Other Parts Discussed in Thread: CSD18543Q3A, CSD19533Q5A,

Hello everyone,

I have a 100W 48V-to-14V Buck converter based on LM5117. I am using CSD19533Q5A for the synchronous FET and CSD18543Q3A for the control FET. I have also used Infineon BSZ099N06LS5ATMA1 for the control FET due to stock. Here's a snapshot of the schematic:

- There is more capacitance at the input and the output (outside of the area shown)

- The inductance is 33uH

The components shown in the schematic above are also in the layout here:

This is what I am struggling with:

1. Started with no snubber (R2, C6 left open) and saw the expected ringing

2. Used a 1500pF capacitor and a 5.6 Ohm resistor for the snubber. The C_oss of the sync FET is in the 350pF to 500pF range. I figured I would put some damping and tune it later. However,

3. The 1500pF capacitor didn't change the frequency of the oscillation at all -- actually reduced it just a bit! Sure, the amplitude reduced a bit and the ringing died sooner. But there was no significant change in the frequency.

4. The peak of the ringing/amplitude of the oscillations went up to a hundred volts at full load which meant that this snubber won't do

5. Tried increasing the capacitance (1800p, 2200p) but I saw NO CHANGE in frequency or amplitude

6. Ultimately I halved the resistance by soldering another one on top and got an acceptable margin. Some waveforms below:

I am convinced that a better snubber should be possible -- but how to get to it? How do I go about modeling this circuit with all the parasitics in a way that explains the observations?

Thanks!

S