Help with component on PSU

[Dannyx]

Good day folks. I'm trying to figure out what's wrong with this power supply. It's from a barebone PC/POS thingy, NVM. It's supposed to be on all the times: just a single output - no ON/OFF control or anything. I tracked the problem down to the two control ICs on the back not receiving the correct supply. They're an NCP1605 (U100 on the left, for PFC I reckon) and L6599 (U300 on the right) for the main switching action....the VCC pins are common and get around 7v, which is not enough. I tracked VCC down to Q304 of which I took a close-up....DQ QZ....what could this be ? Could it be one of THESE ? At the bottom we have 2SD2211. The package and pin layout corresponds and would technically make sense: the VCC pins go to the left terminal (bottom one in the picture - the emitter, "output" let's call it) and then the tab (center contact) goes to those diodes, which would make it the "input" collector. The remaining pin (top one), is connected to that zener diode and the top optocoupler, so that would be the base.

Now the VCC for those IC (and therefore the bottom pin of the IC) jumps around between 7v and 14v, like it's trying to start up but fails. Measuring those two diodes (D309 and D308) gives me 7v, which I'm not sure is enough.

No schematic and a truck load of SMDs....it's like tough sh!t It's be really awesome if we could get it going.....any ideas ? Cheers and thanks. No blown stuff on the board BTW, of course....

[Dannyx]

Re: Help with component on PSU

UPDATE: removing the NCP ic completely results in that oscillating voltage disappearing completely for some reason...it also seemed to get quite warm...thinking of replacing it.

[R_J]

Re: Help with component on PSU

When thes supplies try and start like that usually indicates a bad cap. There should be a winding from the transformer that feeds a resistor, diode and then a capacitor that supplies the run vcc for the ic's, if this cap is bad the voltage never comes up and the supply oscillates. They are likely one of those orange caps.
The ic has a built in regulator that takes the main B+ (300v) and supplies the startup of the ic, this will pulse the transformer which will generate the run vcc to keep the ic's going
If you check the schematic for the L6599, the cap is listed as C24
When the supply is pulsing is there any secondary voltage at all?

[Dannyx]

Re: Help with component on PSU

Nope, no secondary at all, which is why I dismissed the idea of a bad cap initially, ALTHOUGH, being familiar with the operation of SMPSs and how these AUX windings operate, I did go ahead and replace one of the orange caps but didn't change anything. Why just one ? Because it was the one whose positive was connected to the VCC pins of both ICs. Granted, the cap was salvaged, not new, so that may play a role too (rhyme ), though I highly doubt it. I'll replace both of them eventually.

Seeing how the IC generates the startup, it's no wonder I get absolutely nothing without it and it might also explain the pulsing - faulty IC or faulty cap still.

EDIT: The PSU is a UMEC brand, 24v at 10A, but couldn't possibly tell you the model number right now....don't have it in front of me and can't remember it. Not that it'd matter too much - I couldn't find anything on it, unless by some freak accident someone comes in and says "ooh, I know this, it looks exactly the same a X PSU" and we're lucky to find a schematic for that one instead...

[R_J]

Re: Help with component on PSU

I see two opto couplers, one is usually for feedback to control the primary, I wonder if the second one is for turning one of the ic's on and off? standby? If there is no load on the secondary, the supply is off? maybe?

[Dannyx]

Re: Help with component on PSU

Given there's no "ON/OFF" pin, it could make sense to a degree, however how could there be any load AGAIN once the thing has shut off ? :| I also don't think I see any current sensing element on that secondary there, though this doesn't reject the idea still.

[Dannyx]

Re: Help with component on PSU

It also says the NCP starts operating at a minimum of 15v which I'm not getting....

Page 26 of NCP datasheet: an auxiliary supply must take over before VCC drops below 9v, which I'm not getting either - could be the OTHER IC is faulty - not this one. It keeps trying to start, but the other IC is not starting the AUX winding to keep it going...I could be looking in the wrong place.

[budm]

Re: Help with component on PSU

1) On page 6/36 L6599 can turn the PFC circuit on or off depends on the load.
2) Now the VCC for those IC (and therefore the bottom pin of the IC) jumps around between 7v and 14v, like it's trying to start up but fails. When you see these Voltage going up and down, do you see any output on the cold side at all? If it is trying to run and able to produce the Voltage on the AUX winding then the Voltage on the cold side will be present. Or may be then the Voltage you are seeing is from the main cap that provides the kick Voltage, see the application circuit page 31/36 on how it gets the kick start Voltage to the VCC pin.
3) What Voltage do you see on the Cathode side of the Diode that get the AC from the AUX winding?
4) What Voltage do you get on the Collector of the Transistor that being fed by the Voltage from the AUX winding?

[Dannyx]

Re: Help with component on PSU

2) When you see these Voltage going up and down, do you see any output on the cold side at all?

Nope - already answered this below No output at all.

3) What Voltage do you see on the Cathode side of the Diode that get the AC from the AUX winding?
4) What Voltage do you get on the Collector of the Transistor that being fed by the Voltage from the AUX winding?

Around 7v stable on both of them, since it's the same thing. It's not oscillating.

[budm]

Re: Help with component on PSU

Nope - already answered this below No output at all. That means the SMPS IC is not even try to run at all.

Around 7v stable on both of them, since it's the same thing. It's not oscillating.

Are you saying 7V at the Collector (input) of the Transistor or at the Emitter (Output) to the VCC pins of the ICs?

[Dannyx]

Re: Help with component on PSU

Collector, which corresponds to the diodes' cathodes, is stable at 7v.

VCC pins, corresponding to the emitter (output of the transistor), jump between 7v and 15v, presumably the NCP firing.

[budm]

Re: Help with component on PSU

Since the AUX winding is connected to the rectifier, the Cathode is connected to the Collector of the Transistor (Linear regulator set up with Zener Diode at the Base) which you indicate 7V steady, the Emitter(the transistor is connected as Linear regulator/Emitter follower) is the output which dos not make sense to be jumping between 7 and 15V when the Collector is at 7V steady.
It seems to me that the kick start Voltage of 7 ~15V is coming from somewhere else (it has to be from some where else since this SMPS IC does not have internal charge pump to give kick start Voltage).

[Dannyx]

Re: Help with component on PSU

It seems to me that the kick start Voltage of 7 ~15V is coming from somewhere else (it has to be from some where else since this SMPS IC does not have internal charge pump to give kick start Voltage).

I reckon it's coming from the NCP IC which DOES have a current source between its HV pin and VCC, which is what gets the thing going upon first startup. The HV pin sources a small current to the VCC pin which charges the VCC capacitor. Under normal circumstances, the VCC pin of the other switching IC (the L6599) is powered briefly by the same capacitor (since they're in parallel) and the IC starts operating. It drives the main FETs, which pulse the transformer, generating AUX VCC required to keep the ICs powered through that transistor, which keeps them going. The current source of the first IC shuts off when sustained operation has been achieved.

In my case, the cycle stops somewhere in the middle of the L6599 trying to start - it's either busted and doesn't pulse the main transistors at all (which would explain why we're not getting secondary voltage at all), or there's something wrong with some components in the gate drive, between the FETs and the ICs.

[Dannyx]

Re: Help with component on PSU

I removed that transistor I pictured up close and I still get the fluctuating voltage on VCC, but another thing I noticed is that without the transistor, there's no more collector voltage on the pad which connects to the tab...don't know if that's normal or not though...