Coffee Machine Flyback Converter Problems

[LTech]

Hi EEVblog Forum, this is my first post so bear with me.
I'm having trouble repairing a power PCB for a coffee machine. Power electronics isn't my expertise so forgive me if I make any obvious mistakes. The PCB uses a flyback converter (I think, picture is attached) to convert 230V to 24V and 8V. The controller is marked with DM0265R so I assume it is this one: https://www.mouser.de/datasheet/2/308/FairchildSemiconductor_16146734254-1191933.pdf. On another board a 7805 converts the 8V to 5V.
The 24V line is always relatively stable. The 8V line is only stable when the other board is not connected, so when no load is present on this line. When the other board is attached, the 8V collapses as can be seen in the picture. This leads to the 5V converter not working and the whole machine doesn't switch on.
I already replaced the electrolytic capacitor on the 8V line because of minimal bulging and tested all the other capacitors for shorts. Does anybody have experience with this kind of error and could tell me which part of the system might be faulty or where to search for the error? Thank you.

[dzseki]

Couldn't be that the 8V line is overloaded by that other board, ie the problem is not with the power supply? Is the thing working if you run off external power supply?

[capt bullshot]

This looks like the power supply is repeatedly restarting. Without load and no output overload, this is hard to see at the output, so you should check if the supply is restarting at no load also. Connect the scope at the secondary side, directly at the transformers pins to see if it is cycling or running stable.

Restarting can be due to output overload or some protection circuit kicking in. If the power supply cycles without load, one might suspect a defective output rectifier diode, maybe just an capacitor (often a small one on the primary side) lost its capacity or an faulty optocoupler. The controller chip and the main power components appear to work.

[perieanuo]

Put dummy loads and disconnect the original loads.you may replace ALL electrolitycs before looking for other thing to repair.you have to decide first who's bad, smps or load.then,we can guide you.scope for now is useless


Envoyé de mon iPad en utilisant Tapatalk

[LTech]

This looks like the power supply is repeatedly restarting. Without load and no output overload, this is hard to see at the output, so you should check if the supply is restarting at no load also. Connect the scope at the secondary side, directly at the transformers pins to see if it is cycling or running stable.

Restarting can be due to output overload or some protection circuit kicking in. If the power supply cycles without load, one might suspect a defective output rectifier diode, maybe just an capacitor (often a small one on the primary side) lost its capacity or an faulty optocoupler. The controller chip and the main power components appear to work.

Without load the secondary side is not cycling.

Couldn't be that the 8V line is overloaded by that other board, ie the problem is not with the power supply? Is the thing working if you run off external power supply?

Put dummy loads and disconnect the original loads.you may replace ALL electrolitycs before looking for other thing to repair.you have to decide first who's bad, smps or load.then,we can guide you.scope for now is useless


Envoyé de mon iPad en utilisant Tapatalk

The other board seems to be working with a bench power supply and draws an appropiate 60mA. I put a similar resistive load on the 8V line and it was very stable. My thinking right now is that the MCU, which is on the 5V line, switches on normally and then after about 150ms switches on some other external circuitry which triggers the over load protection of the smps but not of the bench power supply.
I am now going ahead and replace all electrolytics. Do you have any other ideas what could be causing this behaviour?

[perieanuo]

You need to put loads on every voltage lines to trigger the current protection which may be global,if I find time I'll look into chip specs tomorrow.in my experience, it may be C or smps transformer, but the transf is rare to dye.if you're shure it's protection triggering,it's that or why not a simple solder problem


Envoyé de mon iPad en utilisant Tapatalk

[perieanuo]

Replace especially cap from Vcc pin of your pwm controller.don't think, just change it
The one between the main fuse, R39 and chip...


Envoyé de mon iPad en utilisant Tapatalk

[LTech]

Replace especially cap from Vcc pin of your pwm controller.don't think, just change it
The one between the main fuse, R39 and chip...


Envoyé de mon iPad en utilisant Tapatalk

That did it and it blinks again . Thanks a lot.

[perieanuo]

Welcome, glad that helped


Envoyé de mon iPad en utilisant Tapatalk

[toyboychen]

Hi LTech,

I have the same problem when I repaired a ATX power supply with a DM0265R recently. The sb5V is stable when it's free load, however which begin to binks once it connect to load. I spend two days to check it and fixed it at last. The DM0265R is protection due to Vcc (pin2) over peak voltage (although 13-18V on multimeter). When I added a 1k resistor between Vcc and GND to absorb the peak voltage, it working great. Normally here is a ca. 33-43v voltage-regulator diode to absorb peak voltage. Good luck!

Short: Active the chip protection by peak overvoltage on Vcc.