Electronics > Repair
BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)
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inse:
There is an auxiliary winding between U146-5 and -6 that is generating the supply voltage on the primary side, just like the secondary windings except that it‘s tied to primary GND.
There is an example circuit of a flyback converter in the Onsemi datasheet of UC3844 which shows a lot of the features of your patient.
Except its using the aux winding for both supply and feedback, not an optocoupler.
Or look at this app note if you really want to dive deeper: https://ww1.microchip.com/downloads/en/Appnotes/AN18-APID.pdf
max.wwwang:

--- Quote from: inse on December 21, 2024, 12:29:42 pm ---There is an auxiliary winding between U146-5 and -6 that is generating the supply voltage on the primary side, just like the secondary windings except that it‘s tied to primary GND.
There is an example circuit of a flyback converter in the Onsemi datasheet of UC3844 which shows a lot of the features of your patient.
Except its using the aux winding for both supply and feedback, not an optocoupler.
Or look at this app note if you really want to dive deeper: https://ww1.microchip.com/downloads/en/Appnotes/AN18-APID.pdf

--- End quote ---
Yes! And the app note is very helpful after a quick look. Will read it closer. This discussion has been very enlightening. Thank you very much. I wish you a super merry Christmas and a very fulfilling new year!
inse:
My pleasure and thanks in return
tautech:

--- Quote from: max.wwwang on December 21, 2024, 07:48:29 pm ---
--- Quote from: inse on December 21, 2024, 12:29:42 pm ---There is an auxiliary winding between U146-5 and -6 that is generating the supply voltage on the primary side, just like the secondary windings except that it‘s tied to primary GND.
There is an example circuit of a flyback converter in the Onsemi datasheet of UC3844 which shows a lot of the features of your patient.
Except its using the aux winding for both supply and feedback, not an optocoupler.
Or look at this app note if you really want to dive deeper: https://ww1.microchip.com/downloads/en/Appnotes/AN18-APID.pdf

--- End quote ---
Yes! And the app note is very helpful after a quick look. Will read it closer. This discussion has been very enlightening. Thank you very much. I wish you a super merry Christmas and a very fulfilling new year!

--- End quote ---
So very often use of a SMPS controller IC very closely mirrors App notes and/or datasheets.
They should be your first port of call when troubleshooting SMPS.

Typical failure points are the controller IC Vcc cap (high ESR and/or diminished uF capacity), dropper resistors from HVDC and sometimes a flyback diode.
max.wwwang:

--- Quote from: tautech on December 21, 2024, 08:39:20 pm ---So very often use of a SMPS controller IC very closely mirrors App notes and/or datasheets.
They should be your first port of call when troubleshooting SMPS.

Typical failure points are the controller IC Vcc cap (high ESR and/or diminished uF capacity), dropper resistors from HVDC and sometimes a flyback diode.

--- End quote ---
Thank you sir. Very true, the circuits using the chips are typically very similar, or even identical to the examples in the corresponding app notes. This is demonstrated even in my limited experience.

I will take note of your nice summary of the common failures of these things. Probably you said the same in one of my previous repair projects asking for help here. And I vividly remember in one previous project, another member, seeing through the problem of the circuit I was working on, was almost screaming out --- just replace that silly cap!  :-DD And it turned out he was exactly right.

My problem is, probably due to my relatively low frequency of exposing to these projects, things learned at one point easily fades away from memory. This is something I feel very frustrated about.  :palm:
 
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