PSU startup spikes: where they come from? - Page 1
Forum section: electronics / projects

author: exe
author_rank: Unknown
timestamp: September 29, 2017, 01:43:35 pm
content: Hi guys,I just checked my precious DIY PSU () for startup overshooting and look what I found:  . There is some sort of EMI or something of relatively high voltage. Could someone explain the problem and provide a hint for a solution?The PSU itself is nothing special, just a transformer, bridge rectifier and two LT3080 to regulate voltage and current (and mains filter). There is a hardware button on front panel to switch it on and off, the button connects transformer to mains filter via 10cm leads (). The "funny" thing is I see this glitch on the scope even if it is not connected to the PSU. Might it be an "inductive kick"? I detect spikes both when powering it on and off.Another question: how dangerous is this for, e.g. stm32 microcontroller? Spikes:https://youtu.be/UKf34Uigzsg(shows two different starts, graticule is barely visible, sorry) PSU design discussion:https://www.eevblog.com/forum/beginners/please-critique-this-linear-psu/ Inside PSU:https://photos.app.goo.gl/6wqYwOVUuEEtBqI73

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2017, 10:31:55 pm
content: Let me guess, the sign and amplitude of the spike vary from shot to shot, also?At the instant the switch contacts close -- wait, let me take a step back for a moment.TLDR: switches are actually extremely complicated, physically speaking.Explanation:The switch contacts only close, as such, if the switch voltage is zero. In that case, when the points of the contacts, closest together, are within fractional nanometers (almost touching), electric current is able to tunnel over the gap. (Incidentally, the mean free path at STP is larger than this, so the gap effectively looks like vacuum. No need to consider air pressure or gas ionization.) Just a little ways further and the contacts are touching, proper (overlap of electron gasses in the metals), and the connection has low resistance.When the voltage is higher (above about 20V), a spark will jump, at a wider distance, instead of tunneling being the first moment of conduction. In that case, the spark begins as electron field emission from the surface with greater curvature (pointy!) and least work function (= ease of emitting electrons). The electrons strike gas molecules, ionizing them, freeing more electrons; the avalanche of electrons strikes the other electrode, kicking out atoms (and causing surface erosion). All the ions (mostly positive ions, i.e., lacking electrons) eventually float back to the negative electrode, where they push out still more ions and electrons, and so on; the process accelerates until the voltage drops to a limiting value (around 20V, corresponding to some ionization level of air and the electrodes). This all happens in less than a nanosecond.In either case, when the switch closes, the voltage drop at the switch becomes very small, very quickly. This launches an electromagnetic wave away from the switch, down each connecting wire. If the switch is in the middle of a cable (like a coaxial relay in a transmission line), then the waves propagate cleanly, and the amplitude is half the voltage that was across the switch, just before it closed. (Half goes one way, half goes the other.)What happens after that, depends on what's connected to the wires.In a typical mains circuit, there is wiring going back to the pole transformer, and wiring going forward into the circuit being switched. Usually, the circuit has some combination of capacitors and inductors. A switching supply may have shunt capacitors facing the switch, which will reflect the wave (just like a shiny mirror). You then get two waves going down the mains wiring, of the same sign (because a short circuit reflects the opposite polarity), separated by the delay between switch and short. This is absorbed by dielectric loss (mains wiring is usually PVC, pretty crummy stuff at radio frequencies), dispersed by poor wiring (the frequency response is not clean like an RF transmission line), and eventually absorbed in other loads, and radiated as EMI/RFI.Alternately, if the load is a transformer (or an SMPS with an inductor facing the switch), the wave piles up inside the transformer windings (it won't be transformed very well, unless a transmission line transformer is used, which is very unlikely; conventional designs have poor RF performance), and are eventually reflected (mostly in phase). "Eventually" meaning, the wave will be dispersed in the process (different delay and attenuation versus frequency), so it won't be much of a sharp pulse anymore.But that's a very ideal situation. In more conventional wiring, the impedance will be all over the place, and the waves won't stay in just the switch wiring, but will couple into nearby wires, willy-nilly. The effect is twofold:1. It's nearly impossible to model exactly what's going to happen, over the full bandwidth (i.e., including the sub-nanosecond time scale).2. We don't really /need/ to, as most of the energy will be in lower frequencies, where we can model things much easier as lumped inductance and capacitance.So, finally, TLDR over, what you're seeing is most likely the LC resonance of the switch, nearby wiring, and some loop area picking up induction from that event.To dampen it, you need to make the switch soft. An L||R in series has the effect of limiting current at high frequencies, and dampening resonance. (Conversely, an R+C in parallel with the switch, dampens the voltage spike when it opens under load.)To reduce induction, keep the mains wiring separate from the signal wiring. Keep all wiring close to the chassis (and needless to say, use a grounded metal chassis, if you have the option).If nothing else, tell the oscilloscope not to trigger on fast events: there's usually a trigger option for this (HF reject, noise reject, or simply reducing the vertical bandwidth).Tim

author: nugglix
author_rank: Regular Contributor
timestamp: September 30, 2017, 07:34:59 am
content: Dear Sir T3sl4co1l!Another amazing explanation.You have a very clear writing and can explain (complex) things in a way the non-initiated can grasp it.Consider it a gift.Given the number of "articles" you already have produced:did you ever think about putting them together and publish them?Would be a great resource for beginners/hobbyists like me.Thanks againGuido

author: exe
author_rank: Unknown
timestamp: September 30, 2017, 12:03:42 pm
content: Quote from: T3sl4co1l on September 29, 2017, 10:31:55 pmYes, this is the case. Thank you for the explanation.

author: strawberry
author_rank: Super Contributor
timestamp: September 30, 2017, 04:05:00 pm
content: It even increase useful life of power switch as well, due to damped arc between plates and contacts wont tremble as much due to air and metal detonation

author: ocset
author_rank: Super Contributor
timestamp: September 30, 2017, 09:42:41 pm
content: If you want you can put in a mains zero cross detector and switch the relay on at the zero cross, this will prolong the switch life and make switch n less noisy.But yes spikes like you show , even of the scope probe is not connected, will show up on the scope....as discussed its induced in to the probe.I worked in a radar power supply place once...and whenever a nearby contactor operated (switched) on the mains, our radar power supply which was plugged in on the other side of the lab woudl go into fault mode. (it wasnt connected to that contactor as such), but was just on the same mains circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2017, 11:33:43 pm
content: Quote from: nugglix on September 30, 2017, 07:34:59 amYou're welcome.I have some other articles here,https://www.seventransistorlabs.com/Articles/index.htmland calculators and some discussion here,https://www.seventransistorlabs.com/Calc/index.htmlI have a number of articles in the pipeline too, which I'll post eventually.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: September 30, 2017, 11:58:17 pm
content: Quote from: treez on September 30, 2017, 09:42:41 pmA proviso, switching at zero crossing will tend to maximize the inrush current to the transformer in a linear supply. Non-intuitively, to minimize the inrush current to a transformer you want to switch at the voltage waveform's peak (which obviously isn't good for any mechanical switches).

author: exe
author_rank: Unknown
timestamp: October 01, 2017, 09:14:58 am
content: I think I'll add a soft-start circuit (resistor bypassed with a time-delay relay). Should also reduce these spikes. What do you think?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 01, 2017, 11:24:13 am
content: A soft start can reduce the load the switch and inrush current from the transformer. It may be needed with a toroidal transformer larger than about 100 VA - mainly for the inrush current.To reduce the spike, the first steps would be keeping the primary and secondary side well isolated, keep current looks small and add a capacitance (a few nF) to ground (PE) from the supplies output. Much of the spike seen on the scope might be due to coupling after the regulator and via ground - so the load will likely not see much of this.