Can AC voltage surges propgate through AC SMPS power bricks? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 19, 2025, 02:15:54 am
content: Suppose some sort of transient occurs on a 120Vac line feeding an AC SMPS power brick outputting 24V. The transient does not damage the power brick.Under certain circumstances can a portion of the transient appear in the output of the SMPS power brick?If so, what makes are the mechanisms by which a transient might "get into the output" of the SMPS?

author: Someone
author_rank: Super Contributor
timestamp: April 19, 2025, 02:26:49 am
content: Yes, that is what EMC immunity test(s/ing) is for.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 19, 2025, 02:45:24 am
content: Transients passing from input to output are invariably coupled via capacitance, stray or otherwise.The remedy is also to use capacitance to shunt the transients and direct them away.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 19, 2025, 09:30:17 am
content: Quote from: incf on April 19, 2025, 02:15:54 amAny supply (linear also) transfers part of transient at input to output. Feedback loop reaction isn't enough fast to compensate any input voltage changes in time of 0ns.

author: tooki
author_rank: Super Contributor
timestamp: April 19, 2025, 10:37:54 am
content: Quote from: incf on April 19, 2025, 02:15:54 amWhat you’re asking about, in essence, is what’s called the PSRR: power supply rejection ratio. Good voltage regulator datasheets have graphs showing PSRR plotted against frequency.

author: MathWizard
author_rank: Super Contributor
timestamp: April 21, 2025, 06:10:08 am
content: The SMPS is no doubt going to have a high-frequency step-down transformer/coupled-inductors. So if the in-coming voltage gets real big, and no MOV acts quickly or controller IC, yeah I wonder how much would just make it through to the output.Yeah I've never really tested what happens in a simulator, with input voltage spikes, with the PSU's whose schematic's I have. I must try that.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 23, 2025, 02:44:04 am
content: Quote from: incf on April 19, 2025, 02:15:54 amSure, any pulse will capacitively couple from primary to secondary.Differential pulses (typically from other loads ON/OFF) will be attenuated by any filters.Common mode pulses (typically from lightning strikes) will have less attenuation and the few-pF from primary to secondary determine the pulse energy.Years ago, you could buy 3 pin power adaptors with a faraday shield connected to GND, that diverted much of any pulse to GND.Addit: I find this on ahttps://www.monolithicpower.com/video, gives some ball park values for 'good' and not-so-good isolation cap values :Quote

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 02:51:18 am
content: "A few pF" makes me think that a (non-destructive) lightening induced common mode pulse would be no worse than an ESD zap?And differential mode transients are almost totally negligible as well?

author: IconicPCB
author_rank: Super Contributor
timestamp: April 23, 2025, 08:37:22 am
content: Hey PCB Wizz,Can You expand on the adapter idea? Who made them? Who sold them?

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 23, 2025, 11:44:00 am
content: "Some sort of transient" is too vague to give an answer.Here's a few types of "transients" which come up in EMC testing:1. Voltage dips/interrupts (IEC 61000-4-11)2. Immunity to Surges (IEC 61000-4-5)3. Electrical Fast Transient/Burst Immunity (IEC 61000-4-4)4. Conducted immunity (IEC 61000-4-6)In practice, the device manufacturer gets to decide the acceptable behavior is during these tests. So if a 1kV transient causes the supply to hiccup, or its output voltage to jump temporarily up by 25%, and the manufacturer states that this is expected, then it passes the test.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 23, 2025, 11:57:57 am
content: Everythingwill cause interference, and everything, including the best power supplies and filters in the world, will pass interference, and everything will be susceptible to interference, it is only about the question "how much".This is all tightly regulated, the very general principle is that every device is allowed to interfere only to level x, and every device must accept interference up to level y without malfunctioning, such that x < y, with some quite significant margin.What x and y are exactly and how they are measured depends on the class of product, country, etc.