RCD Snubber design - Page 1
Forum section: electronics / projects

author: XaviPacheco
author_rank: Regular Contributor
timestamp: June 17, 2018, 10:01:01 pm
content: Hello,I want to design a RCD snubber network like the one attached. I have read some guidelines and books regarding to it. But they all use a VDS/IDS vs time waveform to determine the turn off time. That graph is not given in datasheets. What is given in datasheets are the turn-off delay and fall times for specifics values of ID, VDD, RG and VDS. So, does the mosfet have to get through empiricals tests before designing a snubber?This is the mosfet that I want to snubber:https://www.infineon.com/dgdl/Infineon-IRF250P224-DS-v01_00-EN.pdf?fileId=5546d4625a888733015a8bb640137c6e

author: CopperCone
author_rank: Super Contributor
timestamp: June 18, 2018, 02:07:29 am
content: has anyone seen an active measuring snubber ?

author: Neilm
author_rank: Super Contributor
timestamp: June 18, 2018, 07:01:23 pm
content: I try to simulate it before committing to the snubber. I made a high isolation DC-DC flyback supply a few years ago that had to fit in a really small space. I just measured the leakage inductance in the transformer then ran the simulation. Designed a lossless snubber and the simulation agreed with real life to within one volt (peak was 90V)

author: Siwastaja
author_rank: Super Contributor
timestamp: June 19, 2018, 07:24:01 pm
content: Your missing parameter is parasitic inductance, and since it comes fromyourPCB layout, it cannot be in the datasheet.My suggestion is to try your best in minimizing the loop area, and use proper low-ESL (small ceramic) DC link capacitorsveryclose. You need to think about this loop: input capacitor + -> high side switch -> low side switch -> input capacitor -, and mininize that loop.Modern SMD parts have made this much easier.Now, you can add footprints for an RC snubber across both FETs for prototyping, but chances are you won't need them.Run first at lower voltage and look at the scope trace. If your layout sucks, you may easily have 2x overshoot and ringing in the switch node voltage - this is why you run at lower voltage first so that you won't blow the FETs on excess Vds. OTOH, if the overshoot is below about 1.5x the DC link voltage, your FETs Vds rating is fine with it, and there is no requirement for EMC, then snubbers would only waste some energy.In modern motor controllers or switch mode designs, most of the time there are no snubbers. Do the layout right.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2018, 11:27:08 pm
content: Yeh... what's on the other side of that connection dot?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2018, 11:34:19 pm
content: Quote from: Siwastaja on June 19, 2018, 07:24:01 pmThis may be possible but it isn't a general solution.Example, a 5V 50A converter built with DFNs still has too much stray inductance for this to be a viable approach.For a general solution, the key is realizing, the switching loop is an impedance like any other: a transmission line that must be terminated. Maybe you aren't going for termination as such -- you generallywantreflections in a switching supply, as long as they're in-phase so the energy recirculates back into the supply -- but whatever the case, this observation provides much needed guidance on the situation.A snubber is nice, but also not always possible, what with the stray inductance of two components in series, even. (On that note, I wish they made SiC schottky in SMT packages... at SMT prices, mind.) The only thing you can do at that point is make the channel smaller (so that the load impedance becomes higher than the switching loop impedance, thus reducing voltage overshoot), and use multiple acting in parallel. Multiphase is also a win for ripple current. This does of course carry the unfortunate side effect of needing more drive and control circuitry.Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: June 19, 2018, 11:50:31 pm
content: The application is based on a PMDC motor controller like the one attached for treadmills. I'm trying to figure out if the snubber is strictly necessary in this application.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2018, 11:55:55 am
content: Do you have a layout to show?Tim