No PWM output in SG6105/HS8108 chip

[Francesc V.]

Hi:

I've been trying to repair a miniATX power supply that has mentioned chip. The good news is that I have 2 for comparison (one works and the other doesn't).

The faulty one only has a stable 5Vsb (both purple and green wire have 5V). The chip is powered with the standby voltage, however when PSON is grounded, no PWM output goes in OP1 & OP2 pins. Removing Diode output filters and switching transistors doesn't make any difference.

What might be the cause for PWM circuit not switching on?

(It's not the chip).......I've changed between SMPS with same result. (the one that works, still works with a new chip).

I am adding values for all pinout both when it's in standby and switched on (these values belong to the good one). The faulty one has the same values in standby.

Regards,
Francesc.

[R_J]

Re: No PWM output in SG6105/HS8108 chip

Usually on these power supplies the standby transformer also supplies about 20v which supplies the two small drive transistors, (one is Q4,Q3) that drive the small transformer (EE-1603a) which drives the primary switching transistors
Here is a link to a schematic that might be similar https://320volt.com/wp-content/uploa...7-mje13007.png

[Francesc V.]

Re: No PWM output in SG6105/HS8108 chip

Thanks R_J.

- I checked base voltages in these transistors. About 4.5V coming from the 5Vsb I guess.
- I removed them to check that they were ok.
- I switched on the SMPS without the transistors. I wanted to see the PWM signal......but it wasn't there.
- I mounted them again..........and magically the SMPS started working

Just guessing.......but. Did I fix a cold join?

These are the times when you're not happy to see the item working.

Regards,
Francesc.

[Francesc V.]

Re: No PWM output in SG6105/HS8108 chip

Indeed it's an intermitent issue

The SMPS (indeed HS8108) turns off because it detects overvoltage. Sometimes the 3.3V becomes 4V and this causes the SMPS to switch off, I guess.

As a workarround I've disoldered pin 4 (OPP). By doing these, the voltages seem ok all the time.

What do you recommend me on this situation?

- Leave pin 4 as it is (now is on air)? Or grounded?
- Something might be wrong in in the OPP part? (I am following the schematics R_J post that are about 95% accurate)

Regards,
Francesc.

[CapLeaker]

Re: No PWM output in SG6105/HS8108 chip

I just wanted to say your supervisor is firing off for one reason or other. Since you have found the trouble on it being only the 3.3V (check the others too) I suggest looking at the capacitors and inductors, transformer and examine the feedback circuit.
I had once the same swinging voltage rail problem on some switching psu and once the voltage got too high the psu turned off and took the whole phone system with it. Took me a while to find, but I started to freeze and heat certain parts of the power supply and came to an inductor where I could repeat the scenario on demand. Resoldering the inductor made it better, but I had to desolder, clean and resolder for clean operation. It is still working today.