Inrush causes overvoltage in Mains SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 28, 2023, 06:58:55 pm
content: Hi,In the attached LTspice and Jpeg of an Offline SMPS input filter....if the R28 is any less than 2R2, then the voltage on the input electrolytic capacitors goes above 400V when inrush occurs. (they are only 400V rated)Do you agree, we have no option but to fit this 2R2, and use a 2W rated axial resistor.During the inrush, it dissipates 15kW for a 200us pulse.....but a general, wirewound 2W resistor will be able to handle this for many years, would you agree?

author: mzzj
author_rank: Super Contributor
timestamp: April 29, 2023, 11:22:00 am
content: Your component values are off by about a factor of 100.With spice simulations garbage in = garbage out.I'll leave it as an exercise for You to do the engineering and find out what's wrong with the sim.

author: Faringdon
author_rank: Super Contributor
timestamp: April 29, 2023, 03:43:04 pm
content: Thanks, but the component values are correct, its for a 17W LNK3209 Buck converter with 24Vout.(Attached jpeg and LTspice sim shows its good)The X2 capacitor needs to be that size due to some Triacs which are connected to the neutral.The diff mode inductors are too big for diff mode, but they are in line and neutral, so they give some common mode attenuation too......so it saves using a common mode choke.The caps have to be that value to stop the dc bus going below 70V.....they cant be any more in capacitance as we have no more room.

author: mzzj
author_rank: Super Contributor
timestamp: April 29, 2023, 05:26:37 pm
content: Think harder

author: Faringdon
author_rank: Super Contributor
timestamp: April 29, 2023, 07:31:33 pm
content: Thanks, there is not a single component there that is over voltaged....i agree that the input electroytics suffer approx twice their ripple current rating...but that is because the product only gets used for 20 mins per day, and the whole circuit has strong blown air over it all the time..There is no more room for any more input electrolytics.You say the ratings are out by 100 times.....i see nothing like this.....the sim runs well....and is only a representation of the actual LNK3902.Do you agree, the 2R2 inrush res is needed?....and better than an NTC, since NTCs dont work when hot.

author: wrangler
author_rank: Newbie
timestamp: April 29, 2023, 10:22:24 pm
content: Look at PTC inrush limiter, not NTC. PTC: high resistance when cool, low resistance when hot.I think your sim is decent, but I would put a load on there. You can dump the switch and use the Phi setting in the voltage source, I see you're trying to simulate a "plug in" on the top of the AC cycle. Poke around a bit, you'll see that the inductors have some effect on this. Look up "remnant magnetization."Understand for yourself what half vs full-bridge rectification means for your ripple current.From my experience, common mode noise in flybacks is usually much more significant than differential. Go understand how common mode chokes work, and pay special attention to what the leakage inductance means... you'll be pleasantly surprised about what the leakage inductance does as a "hidden" part of your schematic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 30, 2023, 02:01:04 am
content: Cap ESR is about 10x too low.Inductor is more ESR than that, too.Why is MOV at the rect?Why is rect 1200V? (It's a fine part, but a 600V one would perform a little better, or SiC for that matter)Is this PFC boost? Where is switch? No bypass diode?If mains rect, why half wave?Where is EMI filter?Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: April 30, 2023, 02:02:23 am
content: Quote from: wrangler on April 29, 2023, 10:22:24 pmPTC = positive temperature coefficient. Resistance goes up with temperature.

author: Faringdon
author_rank: Super Contributor
timestamp: April 30, 2023, 07:27:50 pm
content: QuoteThanks....Quote....Yes its not accurate, but i know the ripple rating...and this....The input 22uF caps suffer ripple current which is just over twice their ripple current rating, however, the product is only used 20mins per day, and there is serious blown air over all of it, all the time.Theres no actual way of calculating what is the ESR....but you dont need it....you have the ripple current rating...ESR coulD be calculated with the DF...but they give DF only at 830MHZ, so its not useful for calculating ESR.QuoteMOV is just after input fuse, as it should beQuoteayk, it needs to be at least twice mains peak, due to Half wave rect, but due to transients, we make it higher.QuoteNo its Buck....24vout and 17woutQuoteGood qu, sorry i didnt explain...this product powers a TRIAC driver, which is referenced to mains neutral, so mains neutral is the entire circuit ground, as in the attached.....as i am a loaf-head, i stupidly put an inductor in the cct ground in the above, so i corrected it as in the attached...so yes, it has to carry neutral as its ground, right the way through to the output...so only half wave rect will do.QuoteWhat you see in the attached is it....nothing more will fit on the PCB.....but theres enough anyway for a non isolated buck that takes neutral right through to output so doent need common mode chokes or y caps.......the diff mode is dealt with by 44uf's worth of 'lytics....plus the 680n X2 cap...and then theres the 100uH inductor.....and Thanks....because you just reminded me...100uH is too big...it should be more like 22uF...because that will well smooth the mains input currentSo in summary....no transformer, and non-isolated design, means no common mode filter is necessary.

author: mzzj
author_rank: Super Contributor
timestamp: April 30, 2023, 08:21:42 pm
content: Quote from: Faringdon on April 30, 2023, 07:27:50 pmCapacitor ESR has similar effect as R28 limiting resistor. Your cap ESR is probably ballpark 1-2 ohms instead of 0.1 ohms but it won't solve the problem in simulation altogether as you would need 4 ohm ESR.Current trough the L1 L2 coils is excess of 130 Amps yet datasheet shows that the particular model saturates at 1.3 amps.What would you think happens to inductance when you saturate your coils?What would you think happens to the voltage spike with the "new" inductance value?Also the datasheet values for electrolytic cap DF at 830Mhz have to be some sort of misprint or misunderstanding. Did forum user "treez" freelance by a change at Wuerth?

author: Faringdon
author_rank: Super Contributor
timestamp: April 30, 2023, 08:25:16 pm
content: QuoteThanks, yes you are correct.I now need a saturating inductor model.Though anyway.......ayk, mains cable is about 1uH per metre...and that won't saturate, and the stray input inductance could be 5uH total.....and so 1R at least is needed as an inrush limiting resistor.