PFC'd SMPS resonanting with isolation transformer leakage inductance. - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: March 13, 2023, 07:43:58 pm
content: Hi,Please confirm that this 150W offline SMPS is poor?...So, Our customer had an offline PSU designed for them and asked us to test it to check it.Their supplier has already tested it and says its fine.Spec is ……..100-265VAC input.Pout = 150W, but withstand 230W for 20 seconds at a time.Vout = 37VIsolatedIt has approx. 5vpkpk 100Hz ripple when at 150W and 115VAC…so I am guessing it’s a single stage Flyback PFC. Its very small, only some 70cm by 35cm by 60cm, enclosure is totally sealed plastic.EfficiencyTheir supplier claims efficiency of 90% at 110VAC, right through from 25W to 150W……we say that’s gotta be nonsense, would you agree?Their supplier claims efficiency of 90-92.5% at 230VAC, right through from 25W to 150W…again we say that’s nonsense….do you agree?Input current instability…From 100VAC to 240VAC the mains input current is festooned with a 1kHz oscillation as attached. They say that that only occurs when its run through an isolation transformer……..we say that just means they have messed up the input filter and PFC feedback loop…would you agree?….Above 240VAC, the mains input current has a 50% dead time...(as attached)….we say this is poor for a PFC stage…would you agree?Load transient:It immediately drops out to 0V when an 8R19 (167W) load is switched in as a step load….stays at 0V for 2 seconds then recovers…..we say this is very very poor…would you agree?PFCTheir supplier test spec claims power factor of 0.47 to 0.74 at 230VAC from 15W to 150W...we say thats not good...would you agree?Isolation transformer used:https://www.screwfix.com/p/carroll-meynell-3000va-intermittent-isolation-transformer-230v-230v/320HV?kpid=320HV&ds_rl=1249401&gclid=Cj0KCQjwk7ugBhDIARIsAGuvgPYduPhYv3FQ-PgzfM9yuM47TgR6F73jr_Jmw-os7y42bEx5rdO5BsUaAuZhEALw_wcB&gclsrc=aw.ds___---___

author: f4eru
author_rank: Super Contributor
timestamp: March 13, 2023, 08:58:28 pm
content: put a resistive load (or a R/C) on the output of your iso transformer to dampen the inductance.

author: temperance
author_rank: Super Contributor
timestamp: March 13, 2023, 09:01:13 pm
content: I think I warned you not so long ago to not connect a PFC working in discontinues mode directly to an isolation transformer because the leakage inductance of the isolation transformer will become a part of the PFC inductor. Bad bad not good.Why aren't you able to "decipher" those things by yourself?

author: Circlotron
author_rank: Super Contributor
timestamp: March 13, 2023, 09:09:21 pm
content: Back when I was working on these kind of things I had a 20uF power factor cap directly across the isolation transformer output. This helped to maintain a lower output impedance at higher frequencies. Actually the cap was across the output of the variac that was downstream of the isolation transformer, and it was a motor run capacitor, but same deal. For any measurements to be consistent you want to have an AC supply with a source impedance as low as possible at all frequencies.

author: Faringdon
author_rank: Super Contributor
timestamp: March 14, 2023, 06:33:52 am
content: Thanks yes, i kept forgetting i had the isol transformer and variac there, as before always used variable AC source or raw mains. At 240VAC it doesnt ring with the isol transformer any more, and the dead time there is 50%.This doesnt bode well.The Resonant frequency of the leakage L (read "Line inductance") and the upstream cap of the SMPS should be < 100Hz it seems.The 1kHz oscillation was one thing, and due to isol txfrmer leakage +input caps , ..though at 240vac+, it no longer oscillates with the isolation transformer, but the dead time is 50%, which isnt good.

author: temperance
author_rank: Super Contributor
timestamp: March 15, 2023, 10:32:38 am
content: A PFC with dead time?