Universal SMPS Schematic Comments  - Page 1
Forum section: electronics / projects

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 09:37:53 am
content: Hello,I reverse engineered one flyback SMPS universal power supply. Output power is 6,75VDC - 4W. I think system works in 100kHz. I haven't measured it yet.Do you have any comment on this circuit? I am not sure if I could make the schematic write. I learned a lot of electronic stuff while reversing this but there is two points that I didnt understand.1. Why do they use XY capacitors so in the schematic?2. TL431 feedback mechanism. Because there are a lot of resistors and a cap. Besides that opto bias resistor is connected top of 180Ohm. This is also interesting for me.

author: Martinn
author_rank: Frequent Contributor
timestamp: November 03, 2023, 10:25:44 am
content: Quote from: electrovanga on November 03, 2023, 09:37:53 amWhat do you mean by "XY capacitors"? Do you know that in PSU terms, X and Y capacitors have a very specific meaning? Or did you want to write "many capacitors"?BTW C4 and C5 could be a cheap emulation of a Y capacitor (but it would be drawn incorrectly, see C5 figure 14 in the datasheet).You write "flyback", but the converter as shown (dots on transformer and output choke) look more like a forward. Are you sure the dots are correct? Again look at figure 14.R/C around TL431 are typically loop compensation (for stability).Schematic: Primary looks OK, secondary still has some bugs. Maybe post a photo of the PCB?

author: wraper
author_rank: Unknown
timestamp: November 03, 2023, 10:44:57 am
content: I'm pretty sure you reverse engineered it wrong. You have output shorted and L2 in the wrong place. Not to say most likely it's not an inductor but ferrite bead. C4 and C5 are shorted and doing nothing. Circuit around TL431 and optocoupler is totally wrong too.

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 10:48:34 am
content: Hi,There is XY sign on the capacitor. That's why I named it as XY capacitor.I also updated the drawing with some findings but I am still not sure why 1kOhm Rbias for opto placed like this.

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 10:51:53 am
content: Quote from: wraper on November 03, 2023, 10:44:57 amAny help for me to learn how can I correct it?

author: wraper
author_rank: Unknown
timestamp: November 03, 2023, 10:53:10 am
content: As of second circuit, TL431 will emit magic smoke and output voltage is not limited as optocoupler will never turn on. C4/C5 must be between primary and secondary side.

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 10:58:02 am
content: Quote from: wraper on November 03, 2023, 10:53:10 amHi Wraper,I have already mentioned in my first post about that short circuit.I have realised it during the post. Circuitmaker connected it automatically. The circuit maker is connecting some componnets if you drag some parts.Could you judge this schematic below, please?

author: wraper
author_rank: Unknown
timestamp: November 03, 2023, 11:07:11 am
content: Circuit should be similar to this.

author: Martinn
author_rank: Frequent Contributor
timestamp: November 03, 2023, 11:17:52 am
content: If you want to learn: Explain to us why the dots on the transformer are wrong and why they are correct in wraper's schematic.

author: Martinn
author_rank: Frequent Contributor
timestamp: November 03, 2023, 11:21:41 am
content: Quote from: electrovanga on November 03, 2023, 10:48:34 amIt says X1Y1.Read about safety capacitor classifications:https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/As for why there are two of them:https://electronics.stackexchange.com/questions/662179/why-two-emi-y-capacitors-in-series-between-primary-and-secondary

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 11:25:16 am
content: Quote from: wraper on November 03, 2023, 11:07:11 amThanks a lot. I will dig into it.Quote from: Martinn on November 03, 2023, 11:21:41 amI thought that those are just a sign | .I will check those links. Thanks for support.

author: tom66
author_rank: Super Contributor
timestamp: November 03, 2023, 12:02:26 pm
content: Both anode and cathode of the optocoupler are effectively connected to the same node; how does it turn on?

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 12:08:40 pm
content: Quote from: tom66 on November 03, 2023, 12:02:26 pmHi,Where is the same node? Could you show, please?

author: electrovanga
author_rank: Contributor
timestamp: November 03, 2023, 12:23:01 pm
content: Quote from: Martinn on November 03, 2023, 11:17:52 amHi Martin,I thought about it a little bit and had an explaination and question in my mind.My referance is output. I want to have upper side positive voltage and lower side negative voltage of the coil. This is fine. After that when I think about magnetic flux direction, it is going a circle inside of the core. As it is so, positive flows through the negative makes the primary side reversed like in the schematic.Question here will be when we rectify the AC voltage wave, upper side of the primary will be positive all the time as the capacitors' positive is also looking upwards. Can the answer be as I connect the dot of primary to "ground", I make the upper side -Vprimary and the Vsecondary becomes positive and let the current flow over schottky diode?Another quesiton will be how I can understand in real life the dots of the transformer in my hand.

author: tom66
author_rank: Super Contributor
timestamp: November 03, 2023, 01:37:19 pm
content: See R7 and R8. Anode of the optocoupler is connected to +Vout via 180 ohm resistor, and cathode is connected to Vout directly. How can the optocoupler ever light? The TL431 will probably just end up smoking.

author: Martinn
author_rank: Frequent Contributor
timestamp: November 03, 2023, 02:33:48 pm
content: Quote from: electrovanga on November 03, 2023, 12:23:01 pmThe truth be told I am also constantly struggling with the dots.https://circuitdigest.com/article/understanding-dot-convention-in-transformersFor measuring, you could connect some sine wave at one winding and observe the polarity in the other to find out winding polarity. When the transformer is connected to the rest of the (unpowered!) circuit, this might create unwanted interference. Maybe try injecting a very low voltage (e.g. 100 mV sine, 10-100 kHz) so the diodes are not conducting.An important thing to realize is that in a flyback converter, current flows in primary and secondary alternatingly, which also means 100% of the energy you get out of the secondary was stored in the core when switching from primary to secondary conduction.In contrast, in a forward converter conduction pri/sec is simultaneous and there is no (intentional) energy storage in the core. See herehttps://www.ti.com/lit/an/snva603/snva603.pdfand compare p. 3 and 4.

author: electrovanga
author_rank: Contributor
timestamp: November 06, 2023, 09:43:25 am
content: Hi Martinn,I will test it with this way.One more question will be how can I measure switching frequency of this product? What is the safe way as there is main line on the board I can't connect the oscilloscope to any point?

author: MathWizard
author_rank: Super Contributor
timestamp: November 07, 2023, 04:39:12 am
content: If it's a forward converter, and I remember correctly, when current is entering the top of main primary inductor, energy gets stored in the magnetic field, and when the mosfet in the chip shuts off, and most the current in the primary side stops, then the stored energy should make a current flow out of the top of the secondary side. If it was a normal transformer, the energy gets transferred pretty much right away, from 1 side to the other.The final output inductor should help keep current flowing and charging the output cap, when the main inductor is not supplying energy.Something like that, but I haven't gotten much into the equations for these yet.Nope I'm wronghttps://en.wikipedia.org/wiki/Forward_converterQuote