SMPS fluctuation, not working, no output

[Kobliha]

Hi, a switching power supply from a coffee machine came to me for repair, unfortunately I already tried everything, measured all the rectifiers, replaced the mosfets, checked the resistors and replaced the capacitors. the source is constantly referenced, the voltage on the input filter capacitor keeps fluctuate, from 320 to 250V. The voltage on the phototransistor at the optocoupler is around 12V, which also keeps fluctuating. At the same time, the IC circuit clicks and the voltage on the Vcc capacitor reaches only 10V. So I want to ask if anyone has encountered this and would advise me. there is no output voltage. Its 24 V and 5V power supply.

[DavidAlfa]

The input capacitor should be charged at ~320V all the time, it's connected straight to the mains through the rectifier.
If it's changing all the time something is bad between the mains input and the capacitor.
Either the fuse, filter, rectifier...
Measure resistance between mains plug and the rectifier ac inputs. Should be a couple ohms, not more.
Then measure the rectifier itself, maybe you have a bad diode.

[xavier60]

It's also possible that the large input capacitor is faulty.

[DavidAlfa]

It could, but looks pretty good.

[Kobliha]

I replaced all capacitors for news

[Warhawk]

Ahoj Kobliha,
please list what ICs are on the board. If there is an active PFC then the voltage on the input cap does not need to be stable. That's how active PFCs work (high DC content and low(er) AC content on the output).
We would need to draw the topology. How many switches are present on the board?
You may also try to disconnect the DCDC from everything and bring voltage to the output from a lab PSU with current limit. If there is an optocoupler, there is also likely a 431 voltage regulator. Check that one. There should be a VREF voltage at one of the pins.

Good luck, Jirka

btw: Welcome to the forum.

[MathWizard]

What chip is that in the 1st pic, and what is the yellow inductor between the chip and the blue cap ? Is that making some standby Vrail like 5V, and a low voltage Vcc for the primary side ? Or is it the main controller chip, and the yellow thing is the PFC inductor ? IDK what household voltage is in your area, but with the 400V cap in most countries w/ 110VAC , I'd expect PFC on there somewhere.

[Kobliha]

The IC chip is viper22a
, the yellow thing is transformer and in our country we have 230V input. The transformer is connected to drain pin of IC and to a Vdd pin. Output of transformer going to rectifier and 5V capacitor.

[DavidAlfa]

I'd expect PFC on there somewhere.

Nah, its' a very simple PSU, 50W at most, you rarely (If ever) see PFCs at those power levels.