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Part Number: LM5117
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?
I don't directly support LM5117, but this is the procedure I use:
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In reply to JohnTucker:
In reply to Saurabh Tewari:
In the plots below, the capacitor is 1500pF. Reducing the resistor changes the peak of the spike, and the frequency. These are plots (c) and (d) in my original post. I do not understand why changing the resistance would change the frequency. In both cases, the ringing dies pretty quick though.
if you compare your wave forms to this example from the snubber document below:
You can see the there is persistent ringing on the example and your waveform ringing is damped out within one to one and a half cycles. I don't think you can improve much more beyond that.
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