Help to repair Meanwell ELG-240-48 CC-CV with PFC power supply.

[N2F]

Hello everyone.
First of all, thank you for this educational platform.

The question: I have many broken Meanwell ELG-240-48A power supplies. I have deduced the AC stage circuit a bit to determine the fault, but I need help (I only have basic knowledge of electronics). The power supply is a CC-CV with PFC based on an NCP1399 and NCP1611 (for PFC).

I have changed the 20N60S1 power mosfets because one was broken. It hasn't solved the problem.

I think the optocoupler going to pin 7 of the NCP1399 (OVP/OTP) is open. Measuring the transistor with the multimeter in diode mode it gives me 0L in both directions (without disassembling). I plan to change it when I get a replacement, but I don't understand how that optocoupler works. The OTP/OVP pin is supposed to be connected to a thermistor but there isn't one (I think).

Attached capture and thermal image. In the thermal image it can be seen that the NCP1399 receives current through the HV pin through two 2.7 KOhm resistors (in good condition). There are NO more hot spots on the plate.

What should I look at?
Where do I begin?

Thanks for the answers and the time invested.

[fenugrec]

It's not clear which schematic is closest to what you have, until you've traced yours (you've done most of the work already on the images).
Is the PFC operating normally (probably around 400V on the main caps) ?
A shorted mosfet could have applied the high voltage rails to other parts and damaged them (e.g. the controller IC)
- is the controller IC powered ?
- does it oscillate or do anything at all ?
- optos, you could test at low current (5-10mA at their usual Vf between 1 and 2 volts, should "probably" not damage anything), in case your initial measurement was not conclusive. Obviously there will be one opto for the feedbackloop, some of your schems show 2 but it would be important to know exactly how your circuit is built. I hope you looked really hard for schematics online ?

[Vovk_Z]

I got a bunch of MDR-100 (with APFC) repaired, and the most popular problem was the dead MOSFER+PWM controller. Then some of them had opamp at secondary circuits dead. And some of them had just dead APFC-controller. Some had faulty ceramic SMD caps (both in primary and in secondary).
If there is some small voltage somewhere in secondary circuits then possibly it's a sign that primary circuits (PWM controller etc) work.
If there isn't any voltage anywhere - it possibly means you have a primary circuits problem, like a dead PWM-controller.
It can be easier if you have several broken ones - you have to find the easiest one to repair and then use it as an example.

[Jwillis]

I think the optocoupler going to pin 7 of the NCP1399 (OVP/OTP) is open. Measuring the transistor with the multimeter in diode mode it gives me 0L in both directions (without disassembling). I plan to change it when I get a replacement, but I don't understand how that optocoupler works. The OTP/OVP pin is supposed to be connected to a thermistor but there isn't one (I think).

The optocoupler is likely a PC817. Very common in most SMPS's.  The optocouple consists of a LED between pin 1 (Anode) and pin 2 (Cathode) and a  Phototransistor between pin 3 (Emitter) and pin 4 (Collector). When the LED lights up it emits photons that activates the Base of the Phototransistor. This creates a current that "turns on" the transistor allowing a current to pass from emitter to collector. Since no current can pass across light the optocouple keeps the High voltage side of the power supply isolated from the Low voltage side. By flashing the LED at specific rates, the current across the colllector and emitter can be controlled. This controls the feed back signal from the low voltage side to the NCP1399 which uses that signal to control the voltage and current of the SMPS.This is a simplified explanation of the function of the optocoupler.

It's not uncommon for the  OTP/OVP  not to be used.  In many cases only the over voltage is used. For Over Temp a  thermistor will look  like a resistor if one is used . A diode will also be connected for Over voltage.