Trying to Reduce Transient Ripple for a Buck SMPS - Page 1
Forum section: electronics / projects

author: kml183
author_rank: Contributor
timestamp: July 13, 2023, 05:27:15 pm
content: Hi there! I am trying to reduce the ripple generated by my SMPS designed to buck 6.5v down to 5v. The IC I am using is the TPS565242. This is the inductor I am usinghttps://www.we-online.com/components/products/datasheet/74404043033A.pdf.This power supply is suppose to power both a atmega328 and a 1W rf module. Here is the power supply circuit I am testing.At steady state with only about a 50mA draw, I am seeing a lot of ringing within a generated PWM signal from the MCU I am powering it with the SMPS.When I transmit on the RF module there is a few micro second transient from 50mA to 400mA. I see the switching frequency increase and a lot more ripple in the output. The output delta looks to be 2.320v. In the screenshot I have the yellow signal be the trigger that the rf transmission is about to start and the purple signal is the output from the SMPS.How could I reduce the noise in both the normal (50mA) and transient (500mA) loads?I can't increase the capacitance as after about 220uF the output becomes unstable, which corresponds to the datasheet for the SMPS chip

author: ConKbot
author_rank: Super Contributor
timestamp: July 13, 2023, 05:47:54 pm
content: What exactly is C9 doing? A huge chunk of low ESR capacitance from SW to gnd, will make for a ton of ringing on edges, as you're seeing. If you're putting anything between SW and GND it could be a snubber.Unless I'm missing something, pull C9, then if ringing is still too much followhttps://e2e.ti.com/blogs_/b/powerhouse/posts/calculate-an-r-c-snubber-in-seven-stepsIf this is a one-off or low production, or prototype, shoehorn the snubber R and snubber C onto the pad of C9 in an "A" shape, and you should be doing better in terms of noise.

author: redkitedesign
author_rank: Regular Contributor
timestamp: July 13, 2023, 05:52:18 pm
content: What is your layout? (you can ignore this if removing C9 solves your issues).

author: kml183
author_rank: Contributor
timestamp: July 13, 2023, 07:13:42 pm
content: Quote from: redkitedesign on July 13, 2023, 05:52:18 pmRemoving C9 Worked! Ya I got this design from the TI power designer app. I wonder why it suggested I added that capacitance there. It did see weird that I would be adding a LC filter before the load with a square wave switching signalI breaded boarded the current design, so stray capacitance/inductance will be added here, but here is the layout I am going to use for the PCB.[header=1]Quote from: ConKbot on July 13, 2023, 05:47:54 pmThanks for the suggestion! I am going to add the circuit for an RC snubber then if I need, I will populate it later on.The transient is much better and looks like the datasheet. The MCU and the RF moduleseems(famous last words) to operate fine with that transient.

author: CosteC
author_rank: Regular Contributor
timestamp: July 14, 2023, 07:48:41 am
content: Quote from: kml183 on July 13, 2023, 07:13:42 pmCan you post screenshot of this "designer app"? I am curious why it suggested you to do such thing.With ripple measurements it is also important to probe circuit correctly. Long ground lead of oscillosocope may add ripple, depending on case.

author: redkitedesign
author_rank: Regular Contributor
timestamp: July 16, 2023, 08:45:36 am
content: Quote from: kml183 on July 13, 2023, 07:13:42 pmTurn C5 180 degrees, so VDC_FM has an easier connectionObviously, remove C9Put some ceramic capacitance on the spot of C9, so there is some low ESR capacitance next to the L.

author: Smokey
author_rank: Super Contributor
timestamp: July 16, 2023, 10:11:29 am
content: https://www.ti.com/tool/WEBENCH-CIRCUIT-DESIGNER

author: Warhawk
author_rank: Frequent Contributor
timestamp: July 16, 2023, 08:37:33 pm
content: Quote from: Smokey on July 16, 2023, 10:11:29 amCould you please make a print-screen of the suggested circuit? C9 does not make sense at the location. Sometimes, there's a snubber circuit from the switchnode to ground. This, however, requires acapacitorresistor.If webench suggest C9 it is likely a bug. If this is the case, I would like to get it fixed.

author: Smokey
author_rank: Super Contributor
timestamp: July 16, 2023, 09:03:39 pm
content: Sorry, webbench doesn't make it easy to actually link to a specific schematic, and the EEVBlog forum doesn't make it easy to paste screenshots into posts.Webbench does NOT recommend a "C9" cap from SW to ground. The Webbench schematic looks fine.