Author Topic: BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)  (Read 3949 times)

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Offline MathWizard

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #25 on: December 20, 2024, 11:58:47 pm »
On U146, pins 3 and 4 are high voltage power in, then after a winding pin2 goes to the MOSFET drain. And there's an auxiliary winding between p5 and p6 on GND. Some of the energy that gets stored in the main winding, get transfered into this little winding, and then typically around 10-15V is created and rectified by D115, and smoothed by 100uF/25V near by.

I'm thinking this is Forward Converter topology, but I don't remember the details enough to explain it all, but when the average current in the primary is increasing, energy gets stored building up a magnetic field. And then when the averge current in the inductor starts decreasing, and the field starts decreasing, then you get an induced voltage on the output side and aux windings, and current flows up from ground, and out the top of the windings.
« Last Edit: December 21, 2024, 12:00:39 am by MathWizard »
 

Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #26 on: December 21, 2024, 04:14:36 am »
Read the UC3844‘s datasheet, it explains what the chip can drive.
The windings can be measured with a multimeter, the polarity only tested when desoldered (or without the transistor).
Feed a suitable (50kHz) signal in the primary and check the phase angle of the secondary voltage.
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #27 on: December 21, 2024, 05:13:08 am »
Diode D111 is in a metal casing, with marking "V5 319" visible. What type of diode is it? Zener?
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #28 on: December 21, 2024, 05:38:56 am »
On U146, pins 3 and 4 are high voltage power in,
Correct, rectified hi V from mains.

... then after a winding pin2 goes to the MOSFET drain.
Correct. So if pin 2 and pin 3 or 4 is one windings, then its on/off will be switched by the MOSFET, used to generate induced voltage on the secondaries.

And there's an auxiliary winding between p5 and p6 on GND. Some of the energy that gets stored in the main winding, get transfered into this little winding, and then typically around 10-15V is created and rectified by D115, and smoothed by 100uF/25V near by.
This is where the difficulties come from. I've had a closer look at the poles of the transformer trying to deduce the internal wiring.

I use numbers to indicate the sizes of the wires connected to the pins, the smaller the number, the thicker the wire. And I use "2x" to indicate there is two wires to one pin (of same size)

On the primary side:

Pin 2: 1
Pin 3: 1
Pin 4: 3 (2x)
Pin 5: 2
Pin 6: 2

So pin 2 and 3 has to be of one winding, so are pin 5 and 6. What's puzzling is that how can pin3 be connected to both ends of the same winding?!

On the secondary side:

Pin 7: 1
Pin 8: 1
Pin 9: 2, 3
Pin 10: 2
Pin 11: 1, 2(? not very sure)
Pin 12: 1

This is more difficult: One possibility is that pin3 7/8 are one winding, so are pins 11/23, and pins 9/10. But again, where is the single size 3 wire for?! (Unless the size 2 wire on pin 11 is actually size 3, going to pin 9.)

There is no way that one end of the winding of the primary side goes to the secondary side, that will defeat the isolation. And I checked with DMM. Absolutely both sides are isolated.

It looks like this weird thing – attached.
« Last Edit: December 21, 2024, 05:52:01 am by max.wwwang »
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #29 on: December 21, 2024, 05:43:02 am »
Read the UC3844‘s datasheet, it explains what the chip can drive.
The windings can be measured with a multimeter, the polarity only tested when desoldered (or without the transistor).
Feed a suitable (50kHz) signal in the primary and check the phase angle of the secondary voltage.

Yes, taking U146 off helps. But I'm not prepared to do that due to the likelihood of ruining the transformer and the board.
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #30 on: December 21, 2024, 05:56:41 am »
Lower section after a bit tidy-up and correction.
« Last Edit: December 21, 2024, 06:07:13 am by max.wwwang »
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #31 on: December 21, 2024, 07:01:25 am »
There is no connection between P159-3 and pin 6 of the UC3844.
Otherwise mains/dc could be switched to the output (only once).
Better move the motor speed control away as it has no connection to the power supply block.
Then there is room for the transformer on the right hand side
« Last Edit: December 21, 2024, 07:45:47 am by inse »
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #32 on: December 21, 2024, 07:59:27 am »
There is no connection between P159-3 and pin 6 of the UC3844.
Otherwise mains/dc could be switched to the output (only once).
Better move the motor speed control away as it has no connection to the power supply block.

Thank you for your eagle-eye spotting. Revised again. It is now much neater, so will make more sense. I've grouped the daughter board connector pins and motor connector pins together, which are somehow separate and independent (only somehow because, if the schematic is correct, P190-5 plays a part in the power supply).

[Correction: P190-5 does not play a part in the power supply; it only controls U145, and through which controls the motor.]

Questions:

1. When both T125 and T126 are open, there will be a short circuit across the high V DC input. This makes me wonder if there is error here. (Double checked; this path exists, so short circuit can only avoid by design that T125 and T125 will never both turned on.)

2. I recall I probably put the DMM at the Ohms setting when measuring the VCC voltage to GND (physically pin 7 of UC3844 and left hand side of R19), which appeared to cause a short circuit. R37 (1k) was violent blown. Further investigation suggests that R35 (220 Ohms) and R38 (0.56 Ohm) suffered damage, and T131 is blown. It also appears T125 is gone (and of course the fuse). The rectifier bridge is intact (thanks to the fuse). What the explanation of such damage? (All known affected parts are highlighted in dotted boxes.)
« Last Edit: December 21, 2024, 08:44:20 am by max.wwwang »
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #33 on: December 21, 2024, 08:15:50 am »
T125 and T126 must not be engaged at the same time.
T125 does the PWM for speed control, T126 is probably a motor brake shorting the motor for fast stop.
If T131 is gone and takes R38 with him, the poor UC3844 is attacked by Mains/DC from two sides via R35 and R37 - no chance…
« Last Edit: December 21, 2024, 08:20:33 am by inse »
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #34 on: December 21, 2024, 08:21:50 am »
T125 and T126 must not be engaged at the same time.
That's the only possible explanation.

T125 does the PWM for speed control, T126 is probably a motor brake shorting the motor for fast stop.
That seems to make sense – when T125 is off, motor's power is off. Turning T126 on now will short P159-1/3 with R20 (0.82 Ohm), this will brake the motor actively through the IMF before it stops. Beautiful!
« Last Edit: December 21, 2024, 08:31:35 am by max.wwwang »
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #35 on: December 21, 2024, 08:29:52 am »
Can you analyse the working around transformer U145?
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #36 on: December 21, 2024, 08:39:11 am »
U145 generates the drive signal for the highside switch T125 as it needs to be above the Mains/DC level.
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #37 on: December 21, 2024, 08:45:51 am »
U145 generates the drive signal for the highside switch T125 as it needs to be above the Mains/DC level.

Yes. So correction is made to my post above – P190-5 does not play a part in the power supply; it only feeds in U145, which controls the motor. This part is separate from power supply for the upper section.

Though I'm still very keen to understand how exactly U145 controls the motor.

Revised again based on improved understanding.
« Last Edit: December 21, 2024, 09:11:53 am by max.wwwang »
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #38 on: December 21, 2024, 09:43:27 am »
Correction: T125 is probably ok. Previous thought it was likely gone was because when measuring across B/E with the DMM diode mode, both ways seemed shorted. Now I understand this is the correct behaviour because these two poles are across not only R22 (88 Ohms) but also – and more importantly – the secondary winding of U145. This will make T125 shorted between B/E both ways.

Reorganising the schematic makes it further away from the centre of disaster. It looks like if I replace these parts, (hopefully) at least I can get back to where I was. To be safe, I will also order replacement UC3844.

This also means that, the motor is driven through U145 by input from P190-5, which much be a pulsing signal, and it's always only when the potential of the RHS of R22 is higher than LHS. This will turn on T125 and supply power (DC) to the motor. This is never a stable or sustained status, and will be an alternating on/off process. The nature of this pulse will determine the speed of the motor.
« Last Edit: December 21, 2024, 10:35:34 am by max.wwwang »
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #39 on: December 21, 2024, 10:50:48 am »
Some questions asked previously remain unsolved, such as those related to the wiring of the U146 windings. One more question –

How is the input DC voltage on pin 3 of I150 (MC78L12ACP) achieved (presumably around 25V)?
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #40 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
« Last Edit: December 21, 2024, 01:05:39 pm by inse »
 

Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #41 on: December 21, 2024, 07:48:29 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
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!
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Online inse

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #42 on: December 21, 2024, 08:07:21 pm »
My pleasure and thanks in return
 

Offline tautech

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #43 on: December 21, 2024, 08:39:20 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
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!
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.
« Last Edit: December 21, 2024, 09:00:23 pm by tautech »
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #44 on: December 21, 2024, 11:30:29 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.
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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Offline tautech

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #45 on: December 22, 2024, 12:22:26 am »
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.
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:
;D

While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #46 on: December 22, 2024, 03:19:46 am »
While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)
It has more than one Zeners. What's the best way to check them apart from DMM diode mode? There does not seem to be a way to determine with DMM their voltage spec.

What's your gut feeling about its original symptom of only not having stable 6V output (but the rest was all good)?
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Online max.wwwangTopic starter

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #47 on: December 22, 2024, 04:48:00 am »
Combined schematic tidied further based on the app notes, and it is probably close to figuring out the winding wiring, with one exception that a closed loop winding at the primary side does not make sense to me.

Parts ordered from the mighty Ali. Fingers crossed.
« Last Edit: December 22, 2024, 04:59:38 am by max.wwwang »
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Offline tautech

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #48 on: December 22, 2024, 07:45:15 am »
While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)
It has more than one Zeners. What's the best way to check them apart from DMM diode mode? There does not seem to be a way to determine with DMM their voltage spec.
Variable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.

Quote
What's your gut feeling about its original symptom of only not having stable 6V output (but the rest was all good)?
Connectivity issue somewhere or something intermittent with the diode or its snubber.
There's really not much to check in a standalone winding on the secondary side.

If nothing is obvious get out the magnifying glass.

A little story about a MIG welder I repaired years ago after it had taken a fall.....
Seemed simple enuf in that a small PCB mount transformer that provided control supply was only mounted by its through hole pins and in the fall the transformer had yanked on one and broken the fine copper to the primary.

Poor construction not having a zip tie around it so to not rely on just the pins.  |O
Anyways, seemed an easy fix as RS had the exact transformer so it was briefly tested and sent back to the customer.
A week later it was back and on further inspection a few cracked solder joints were found and repaired on the SMD part of the PCB.
All was well one would think but another week later it was back !  :palm:
After spending ages looking for cracks none were found then a EE buddy suggested I flex the PCB and listen for clicking.
Sure enuf clickity clack all day long.
I ended up hand reflowing every component on that PCB and never saw it again.  :phew:
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Offline tautech

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Re: Please help diagnose this power supply board (Switch Mode)
« Reply #49 on: December 22, 2024, 07:50:37 am »
Stick this in your datasheet records:
https://www.ti.com/lit/ds/symlink/uc3844.pdf
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