Where to Place Snubbers in a Buck Converter
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- Опубликовано: 16 июн 2024
- We're continuing our Buck Converter simulation tutorials by exploring ringing on the switching node. This problem happens generally during high voltage switching with MOSFETs and Tech Consultant Zach Peterson is here to tell you what to do about it.
0:00 Intro
0:54 Buck Converter Review
2:22 Parasitics and Ringing
6:11 Adding Snubbers
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zach is an amazing engineer and presenter
Thank you!
@@Zachariah-Peterson a Rockstar briefly :)
Okay, now Zach finally looks more nerdy 🤣🤣🤣
What about a snubber in parallel with the actual inductor?
My converter is subject to fast changes in load which are causing voltage spikes. Output is 9V, spikes are 21V.
Is there a way to determine an actual parasitic inductance based on a layout?
Yes, it can be determined in Altium Designer using the impedance calculator inside the stackup manager. If you look in the right side of the calculator panel, you will see values for inductance per inch and capacitance per inch when you calculate the impedance. You can set whatever trace width you want and it will provide an impedance value. Some online calculators will also return the capacitance or inductance values per unit length. The same idea applies in an online calculator, you can set whatever width you like and it will just tell you the inductance value.
It seems like the parasitic inductance for Q2 really belongs between its source and ground...
(1) Do you actually want a resistor for the RC snubber, or are you just using the parasitic R that comes with any track? (2) When you say the LC filter works only at the load, are you really just saying that the stuff between the switch and the L can still experience noise that might leak out and interfere with stuff you do care about (therefore minimize that area)?
Discrete resistance is definitely needed
(1) If you find that the package and lead inductance are unacceptably large, and you decide you need a snubber to affect the ringing, then yes you would use a resistor in that snubber. There are a few app notes on this topic in terms of the circuit design. It is true that the trace has some resistance, but to get to even a small resistance of a few OHms requires a really long, really thin trace. That loop inductance path for that snubber section would be big, so you would be adding in inductance with a long route.
(2) Yes exactly. The filter only affects whatever conducts out of the regulator, but the noise produced on that weakly damped transient response can radiate in the MHz range.
Thank you for video! It's very interesting!
I see that snubber should use in handmade dcdc. What is about IC? Do I need use snubber or not?
@Вова yes for an IC, snubbers are okay to use. Rohm has a good application note on this topic, in their application note they cite an original IEEE paper for the formulas used to calculate the value of R and C in a snubber. In general, you need a bit small resistance and capacitance if you determine that a snubber is needed. The title of the app note is "Snubber Circuit for Buck Converter IC" (sorry I can't post a link, if I do then youtube will delete this comment).
I would like to point out that a snubber is not always required. It can be useful if you have a lot of ringing on the switching node on the PWM signal edge transitions. Using a snubber and changing the layout together can help to greatly reduce the switching noise.
@@Zachariah-Peterson I think that order-of-magnitude estimate for what "small" means is some background I'm personally still missing, though I don't know how much of the rest of your audience is. Thank you.
For my point of view, the 22u snubber (bigger than output filter) is way too big. I'm typically reduce the ringing frequency by 3-5 times, then adjust snubber resistance to best snubbing. Best solution - to adjust switching timings to prevent ringing at all - but its complicated.
Yes it's large, typical will be much smaller but it does depend on the ringing frequency, generally higher ringing frequency requires a smaller cap. If you do a sweep of capacitor and resistor values with the Parameter Sweep tool in a SPICE simulator you can get to a good pairing of R and C values.