How Can I test PWM DM0365R?

[chth96]

Hello
I have power adaptor(220V/input, 12V 1.2A/output) which has symptom of fluctuating output DC voltage between 11.5V-12.2V.
So I opened its plastic case and when switched it on, I heard constant tik tik sound from inside its PCB.
I measured DC voltage between negative and positive pin of the largest capacitor(400v 10uf) and it was measured contant 300V.
I also measured ESR value of all capacitors and I noticed that one of capacitor(KMG 16v 470uf), which located in the secondary side, have high ESR value(10 ohm),So I replaced it with same specification NXH capacitor.
In the vicinity of this capacitor(16v 470uf), There is ka431az regulator,I desoldered it and tested it by following below youtube guide.
https://www.youtube.com/watch?v=_BYSsQ4ODEU
I noticed that the result of diode test mode is not the same with youtube guide.So I purchased ka431az regulator which has the correct result of diode test mode, and replaced abnormal ka431az regulator with it.
But It is still fluctuating output DC voltage between 11.5V-12.2V and I still heard constant tik tik sound.
So I desoldered optocoupler and tested it with analogue multimeter by following below youtube tutorial and I also tested all of its diode with diode test mode, but It seems that there is no abnormality,
https://www.youtube.com/shorts/mDwkd3PD4sQ
I want to test PWM DM0365R which is located just above the largest capacitor(400v 10uf) But How can I test PWM DM0365R?

[R_J]

What are you using as a load on thge power supply output? if it is left open you will likely be in burst mode so the power supply is working normally, read the datasheet on the dm0365. Try using an automotive incandescent lamp similar to a 1156 as a load

6. Burst Operation : In order to minimize power dissipation
in standby mode, the FPS enters burst mode operation. As
the load decreases, the feedback voltage decreases. As
shown in Figure 8, the device automatically enters burst
mode when the feedback voltage drops below
VBURH(500mV). Switching still continues but the current
limit is set to a fixed limit internally to minimize flux density
in the transformer. The fixed current limit is larger than that
defined by VFB = VBURH and therefore, VFB is driven
down further. Switching continues until the feedback voltage
drops below VBURL(350mV). At this point switching stops
and the output voltages start to drop at a rate dependent on
the standby current load. This causes the feedback voltage to
rise. Once it passes VBURH, switching resumes. The feedback
voltage then falls and the process repeats. Burst mode
operation alternately enables and disables switching of the
Sense FET and reduces switching loss in Standby mode.

[CapLeaker]

I would suggest you do check the whole feedback circuit first. There should be more than the optocoupler and the KA431 as the later is programmable with shunt resistors. Maybe these resistors are bad or the optocoupler.
If you power up the PSU, put the negative probe of your DMM on the secondary negative and probe with the positive lead on the two legs on the optocoupler secondary side. What DC voltages do you measure?
Check out the voltage Vcc pin on the PWM and recheck the main filter capacitor and the little capacitor next to it. Check the resistor on Vstr pin.
If all capacitors are good, all components on the feedback are good, the fast recovery diode on the secondary is good, then download the spec sheet of the PWM and check the suggested voltages around its pins. If that doesn't yield any results maybe the PWM is bad, but usually they don't fail like that. What happens if you put a load on the power supply like a small 12 V light bulb?
Look at page 14. That example is very close to your PSU.
Be careful working on the primary side of the PSU. Your ground point for measuring anything after the bridge rectifier on the primary is the negative of the 400V main capacitor.

[stj]

replace the yellow electrolytics with low esr like panasonic FR or rubycon ZL

[chth96]

Thank very much for all of reply!
I measured DC voltage between each legs on the optocoupler secondary side and negative pin of capacitor(16v 470uf) in the secondary side.
It fluctuated between 11.2V and 0V, 10.5V and 0V like below dropbox links.

https://www.dropbox.com/scl/fi/jyfv2...=xdhx48j4&dl=0
https://www.dropbox.com/scl/fi/t1rrf...=yr7vnlo0&dl=0

I measured DC voltage between pin1(Ground) and pin2(Vcc) of PWM DM0365R and also measured DC voltage between pin1(Ground) and pin3(Vfb)
It fluctuated between 10.6V and 0V, 0.22V and 0V like below dropbox links.

https://www.dropbox.com/scl/fi/k0grn47uh7fgr2s11mj62/20250619_164149.mp4?rlkey=vhpgbut5sgc8jx2ppq2okaf4r&e=1&st=dd2b7vmu&dl=0
https://www.dropbox.com/scl/fi/0vef5...=m7w4uz9a&dl=0

I measured DC voltage between positive and negative of main filter capacitor(400v 10uf) and that of the little capacitor next to it.
voltage of little capacitor fluctuated between 10.7V and 0V, But filter voltage of capacitor was constant 309V like below dropbox links.

https://www.dropbox.com/scl/fi/hldem...=wkmqu8we&dl=0
https://www.dropbox.com/scl/fi/pa1o3...=8xrrexe5&dl=0
And I don't have any load like 12 V light bulb.So currently it is impossible to do load test.But Sooner or later, It would be possible to purchase MR16 12V 5w led for load.
MR 16 (12 volt) - Led verlichting shop, noodverlichting

[stj]

the transformer generates the output that gets rectified by the diode and smoothed by the 2 yellow caps and the inductor between them.
that is then fed back via the optocoupler to the control chip using the tl431 as a reference.
those are the parts that need to be checked if it's unstable.

incidentally, tl431 is very difficult to actually test - it's best if you suspect one to just swap in a fresh one and see if it changes anything.
optocouplers can slowly degrade too, but that usually causes voltages to be high or low, not unstable.

the ticking sound is disturbing,
i would desolder the transformer and examine where the windings are soldered to the legs - it could be a bad transformer connection

[chth96]

I finally measured resistance of carbon resistor..
resistance of R9 resistor is 0.99kohm and R8 is 3.904kohm

below carbon resistor is the biggest(1w) resistor in this PCB
resistance is 47.12kohm



R2 resistor is 55.06kohm and this resistor is directly connected to Vstr pin of PWM DM0365R


From the top resistor,resistance is 675.2 ohm, 0.996kohm, 6.91kohm

biggest(1w) resistor is 47.12kohm and a resistor which stands vertically is 10.6 ohm

A resistor which placed in a row direction is 1.399kohm and other resistor which placed in a column direction is 33.7ohm.

[CapLeaker]

You found your problem. VCC on pin 2 on the PWM appears then disappears. That shouldn’t be. Vcc need’s to be at least 12v for the PWM to work. If it is below a certain voltage, the PWM goes into UVLO. Read the spec sheet I posted earlier.