Switching AC/DC power supply without input capacitor, what is the topology? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 14, 2023, 06:10:34 am
content: Hello.Recently got some 24V 2A switching power supplies, they work reasonably well, but when checked the PCB, found that there is no input electrolytic capacitor - only film capacitor, rated 1UF @ 450 volts. And output capacitors are too large for the given current - there are 3 pcs. 2200uf 35V capacitors and 2 pcs 35v 820uf capacitors. Also noted that there's a separate choke in series with the primary winding of transformer. All other things, like optocoupler feedback, TL431 reference voltage, are all there.So I have the question, why they're doing this, and how such circuitry is called? I'd like to read some appnotes on that...

author: CosteC
author_rank: Regular Contributor
timestamp: March 14, 2023, 06:16:59 am
content: Maybe it is PFC with 24 V output. Not standard solution yet entirely possible.Can you check controller IC model and maybe upload some photos?

author: EPAIII
author_rank: Super Contributor
timestamp: March 14, 2023, 07:16:24 am
content: The IC's spec sheet is the first place to check.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 14, 2023, 11:17:16 am
content: One of them which I still have on hands, uses SN03ACP controller, datasheet of which has no circuit like that.I'm attaching photo of complete PSU.It also does not look like APFC, because input voltage range is quite limited on the one hand, but on another hand, factor is said to be more than 0.95...

author: CosteC
author_rank: Regular Contributor
timestamp: March 14, 2023, 07:57:21 pm
content: SN03ACP is "high power factor flyback PWM controller special for lighting applications" and in practice it is (active) power factor correction unit which has flyback transformer instead of "normal" boost PFC inductor. This allows stepping voltage down and PFC in one stage.Simplicity comes at cost of large capacitance needed at secondary side as it has to be scaled for 100 Hz ripple - it is charged up with 100 Hz rectified sine from mains.Appnote: AN1059 APPLICATION NOTE from ST may be helpful.What is interesting is really rugged input protection. Could you tell something about it?

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 15, 2023, 06:17:59 am
content: I bought these on taobao, there's a bunch of sellers specifying on selling the surplus electronics. Quite often they have no idea what are they selling actually. These were sold at $2 each, so I bought 10. They were brand new, in factory packaging. Used couple of them without any issues - load regulation is good and ripple is acceptable. For a specific project, I needed 20V power supply, so I decided to mod the feedback divider of TL431 for this power supply, to get 20V at output. For some reason, power supply instantly died, and that was the moment I started investigating and found that there is no input capacitor at allMost likely, these were used in some kind of street lighting, where much stronger EMI and voltage fluctuations might present - this is why so rugged input.

author: CosteC
author_rank: Regular Contributor
timestamp: March 15, 2023, 05:05:36 pm
content: It is possible that your modification of V.out down to 20 V lowered controller voltage below UVLO and it cannot start correctly. If you change it back to 24 V doest it resurrect this PSU?I am very curious what is happening in this PSU input... GDT seems to be connected to PE (legally questionable) lot of NTCs (green?), but see only one varistor (yellow), fuse seems quite "normal" and low rupture capacity... Very peculiar.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 15, 2023, 08:01:49 pm
content: Nope, it really "died" - controller won't output anything even after I restored all original components.Since visual side of these power supplies caused that much interest, I will take some high-res photos tomorrow and post them here

author: CosteC
author_rank: Regular Contributor
timestamp: March 15, 2023, 08:10:23 pm
content: Photos - yes please.It is bit strange it died, if all you did was feedback resistor change... Unless something went not as planned by accident. -16% could cause UVLO but permanent damage is rather surprising.Can you risk another 2$ for experimentation for say 22 V output first?

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 16, 2023, 06:00:42 am
content: Well, I have only 3 of them remaining, so let's see

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 17, 2023, 12:40:26 pm
content: Here they are...Input mosfet is 12N65.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 17, 2023, 12:41:11 pm
content: Hmm...

author: CosteC
author_rank: Regular Contributor
timestamp: March 22, 2023, 07:57:32 pm
content: Thank you for photos.I wonder, could you measure AUX voltage, so pins 6-8 in unmodified PSU? Controller shall wake up between 15.5 and 17.5 V and turn off due to UVLO around 8...9.6 V. However nominal VCC starts from 11 V.12N65 (looked into UTC datasheet) MOSFET opens well around 6.5 V so well below guaranteed 8 V of controller. Design does not seem bad - I am not sure why your unit got damaged when set to 20 V output. I can easily understand unstable work, but not sure what might kill it.