Buck Converter Snubber Design - Page 1
Forum section: electronics / projects

author: jonathan_nadar
author_rank: Contributor
timestamp: November 10, 2021, 11:46:01 am
content: I am designing a snubber circuit for my buck converter. I am following the procedures mentioned in the attached application note. When I do the procedure mentioned on page 3, step 2 where it says to connect a capacitor, a 1nF, 100V 0603 ceramic cap in my case, the low side mosfet (SIR882BDP-T1-RE3) heated and eventually blew up. The test conditions were as follows: Vin=80V, Vout 12V, Iout=1.7A, fsw=180kHz.I am attaching an image of Vds waveform of low side FET without snubber, there is an overshoot of about 120V, which is higher than the rated value, but for some reason, it didn't blow up before.I'll be taking into account the DC bias of the capacitor during calculations and using a ground lead adaptor as mentioned in the note.Do let me know how I should proceed with the design.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 10, 2021, 02:42:19 pm
content: A buck converter doesn't need snubber, as the bemf is taken by the output capacitor and flows through the diode at the input of the coil, I've never seen any buck converter with that.Step up converters don't need it either because of the same reason.The problem comes when using transformers, there're two separate coils, so the primary bemf will kick back when opening the driver, there's no direct current path in the very moment it's switched off, until the secondary rectifier takes the stored power.Maybe the coil causes a small spike before the diode starts conducting, but they're nowhere as huge spikes as withflyback converters.That's also why you use very fast diodes in such converters.I'm not a expert here, so I could be wrong, coils are a bit abstract for me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2021, 07:54:13 pm
content: 0. Schematic and layout?1. Use 150V+ transistors. 100 is way too close; what's nominal maximum input voltage? Transients? Unless it's supplied by another regulated source, that thing is dead even without the overshoot.2. Just slow down the gate drive. It doesn't look like you need to push it all that fast.Tim

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 13, 2021, 01:21:54 am
content: It should be obvious that adding a snubber capacitor is also adding current through the switches that will at least partially charge/discharge it.You may have over stressed the MOS by loading it with the test capacitor.Switching at too high frequencies can make the compromise on snubbering even harder between killing the ringing and killing the switches.Maybe a light RC snubber and TVS if you don't want to change switching frequency?

author: jonathan_nadar
author_rank: Contributor
timestamp: November 13, 2021, 11:28:39 am
content: Quote from: T3sl4co1l on November 12, 2021, 07:54:13 pmI have attached the schematic and layout.The input comes from a 20s Li-ion battery pack, so that's 84V peak and 72V nominal. I have a TVS diode at the input to take care of the transients.I wanted to keep the converter compact and lightweight hence opted for a higher switching frequency. I have increased the high side gate resistance to 4.7 ohms to increase the turn on time.

author: jonathan_nadar
author_rank: Contributor
timestamp: November 13, 2021, 11:33:24 am
content: Quote from: shakalnokturn on November 13, 2021, 01:21:54 amI measured the waveforms with and without snubber and there doesn't seem to be any difference. I increased the input voltage to 8V and doubled the output power by adding another bulb and the converter continued to work without any issues. As suggested by DavidAlfa, I concluded that they might not be there present and are due to probe parasitics, which I have tried to minimize.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2021, 01:59:07 pm
content: I can't see half the routing or pads, can you turn on transparency and screenshot again?Suggest this setting (extract the attached zip then load it from Layer Settings):Also, what's the controller? Why is current sense at the end, not at the inductor?Tim

author: jonathan_nadar
author_rank: Contributor
timestamp: November 15, 2021, 05:24:04 am
content: Quote from: T3sl4co1l on November 13, 2021, 01:59:07 pmThe converter is controlled by an STM32 microcontroller. The current sense is used for charging a 12V battery, so it's after the inductor.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 11, 2025, 11:35:34 am
content: Quote from: Terry Bites on November 12, 2021, 05:00:14 pmJust google "snubber design converter"or just "snubber design"Here are a few results.I think the AD is the most easy to understand.Every man and his dog has a snubber design guide. AD, TI, Infineon, Rohm......

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 11, 2025, 01:49:35 pm
content: The approach in the document in the OP's post is fine, no need to bring in other app notes.If simply adding a 1nF cap across the FET results in its self-destructing, then that's a big red flag that the the normal design is extremely marginal. Agree that a Vdds rating of 100V isn't enough for a nominal input voltage of 80V.Quote from: DavidAlfa on November 10, 2021, 02:42:19 pmRC snubbers are commonly used on both buck and boost converters, though usually only at high voltage/power levels where the semiconductors are in large packages.

author: Cpuch22
author_rank: Contributor
timestamp: August 12, 2025, 03:16:16 am
content: These two videos might help:Part 1:Part 2:

author: trobbins
author_rank: Frequent Contributor
timestamp: August 12, 2025, 09:37:12 am
content: Post #1 plots indicate a 100MHz transient overshoot damped ring that you are hoping to suppress the peak of. I guess that 100MHz relates to an L1 loop through output side caps.Have you checked that overshoot characteristic operation for a few changes like slower gate drive, different L1, different FET, different D5, adding a parallel D5 to solder direct to drain and output cap land (ie. smaller loop) ?

author: electron_plumber
author_rank: Regular Contributor
timestamp: August 12, 2025, 08:25:44 pm
content: Quote from: jonathan_nadar on November 13, 2021, 11:28:39 am... and did that help?Quote from: jonathan_nadar on November 13, 2021, 11:33:24 amDo you have an active probe (low C) to verify? Another quick/easy verification of the ring frequency can be made by a spectrum analyzer and a loop of wire to pick up near field emissions like I did below. It's a 30 second test if you have the equipment. Left=damped, right=undamped: